LA Course #GCE-6-1407
About Stormwater Management
Edited by Len Phillips, Updated in January 2023
Sections
About Stormwater Management
Edited by Len Phillips, Updated in January 2023
Sections
The treatment and storage of urban stormwater using trees and plants is a practice that is becoming well known and often required on new construction projects. Instead of building larger storm drains, holding tanks, and water treatment plants, trees are now seen as a critical part of the process for keeping stormwater runoff under control. The details of bringing together trees, urban soils, and highly controlled water flows, require training and experience that is not readily available. This Topic will assist in providing an overview of this important trend for dealing with stormwater in new construction and for fixing problems after flood damage to prevent a re-occurrence.
Dealing with Stormwater Runoff
Soil Infiltration
Stormwater runoff is not important in a forest because about 80% to 95% of the annual precipitation in a forest is captured by soil infiltration and the remaining 5% to 20% is captured by the tree’s bark and leaves where it slowly flows down the tree after the rain is over or is evaporated in the process of flowing down the tree. Native forest soils with ample organic layers are the “sponges” of healthy ecosystems. Soil pore spaces store water, move it in all directions, and interact with root’s and microorganism’s need for water. The rainwater stored in the soil during and after a storm is made available for plant growth while at the same time reducing the stormwater runoff rate.
However, in disturbed soils, rainfall runoff from hard impervious (non-porous) surfaces will rapidly flow off the land unless it can be directed into the soil through pervious (porous) areas such as forests, lawns, mulched areas, crushed stone or gravel areas, as well as by an underground perforated pipe flowing to a brook.
A typical tree growing in 1,000 cubic feet (28 m3) of pervious soil can hold the rainwater runoff from a 1 inch (2.5 cm), 24 hour storm event flowing from 2,400 square feet (223 m2) of an impervious surface area. This is a significantly greater area than just the area under the tree canopy. This calculation accounts only for soil storage, not for interception and evapotranspiration which are two other ways for trees to capture and control rainwater runoff.
The forest duff (dead leaves) or mulch layer on the surface of the soil also stores and transmits water and protects the underlying soil from erosion. It is estimated that duff will absorb 2% to 4% of the annual precipitation.
Interception
Interception is the amount of rainfall temporarily held on tree leaves, bark, and stem surfaces. The volume of rain intercepted depends on the duration and rate of the rainfall event. Rain can be intercepted by the tree architecture (e.g. leaf, bark, and stem surface area, roughness, the density of the crown, tree size, and foliation period). Interception is not typically included in stormwater calculations but it can nonetheless provide additional stormwater retention benefits beyond storage in the soil. In conjunction with interception, evaporation occurs when rainfall on the leaves and bark evaporates after the storm.
The bark, leaves, and limbs of trees and large shrubs act like umbrellas or shallow cups that intercept and evaporate the rainfall. They provide year-round capture and delays in peak flows of rainwater and typically capture about 10% to 15% of the total annual precipitation in a healthy forest. This function varies according to whether the trees are deciduous or evergreen and whether the deciduous trees are dormant. This rainwater evaporates or slowly drips from leaf surfaces and trickles down the limbs and trunks of trees and shrubs to the ground. For example, an evergreen can intercept more than 4,000 gallons of rainfall a year.
Since larger trees have more leaves to intercept rain, they intercept significantly more rain than small leaves or young trees. The interception rate will increase at a faster rate than the tree’s size increases every year. For example, a typical hackberry tree in the U.S. Midwest has been measured at three different ages to determine the amount of interception that occurs.
Dealing with Stormwater Runoff
Soil Infiltration
Stormwater runoff is not important in a forest because about 80% to 95% of the annual precipitation in a forest is captured by soil infiltration and the remaining 5% to 20% is captured by the tree’s bark and leaves where it slowly flows down the tree after the rain is over or is evaporated in the process of flowing down the tree. Native forest soils with ample organic layers are the “sponges” of healthy ecosystems. Soil pore spaces store water, move it in all directions, and interact with root’s and microorganism’s need for water. The rainwater stored in the soil during and after a storm is made available for plant growth while at the same time reducing the stormwater runoff rate.
However, in disturbed soils, rainfall runoff from hard impervious (non-porous) surfaces will rapidly flow off the land unless it can be directed into the soil through pervious (porous) areas such as forests, lawns, mulched areas, crushed stone or gravel areas, as well as by an underground perforated pipe flowing to a brook.
A typical tree growing in 1,000 cubic feet (28 m3) of pervious soil can hold the rainwater runoff from a 1 inch (2.5 cm), 24 hour storm event flowing from 2,400 square feet (223 m2) of an impervious surface area. This is a significantly greater area than just the area under the tree canopy. This calculation accounts only for soil storage, not for interception and evapotranspiration which are two other ways for trees to capture and control rainwater runoff.
The forest duff (dead leaves) or mulch layer on the surface of the soil also stores and transmits water and protects the underlying soil from erosion. It is estimated that duff will absorb 2% to 4% of the annual precipitation.
Interception
Interception is the amount of rainfall temporarily held on tree leaves, bark, and stem surfaces. The volume of rain intercepted depends on the duration and rate of the rainfall event. Rain can be intercepted by the tree architecture (e.g. leaf, bark, and stem surface area, roughness, the density of the crown, tree size, and foliation period). Interception is not typically included in stormwater calculations but it can nonetheless provide additional stormwater retention benefits beyond storage in the soil. In conjunction with interception, evaporation occurs when rainfall on the leaves and bark evaporates after the storm.
The bark, leaves, and limbs of trees and large shrubs act like umbrellas or shallow cups that intercept and evaporate the rainfall. They provide year-round capture and delays in peak flows of rainwater and typically capture about 10% to 15% of the total annual precipitation in a healthy forest. This function varies according to whether the trees are deciduous or evergreen and whether the deciduous trees are dormant. This rainwater evaporates or slowly drips from leaf surfaces and trickles down the limbs and trunks of trees and shrubs to the ground. For example, an evergreen can intercept more than 4,000 gallons of rainfall a year.
Since larger trees have more leaves to intercept rain, they intercept significantly more rain than small leaves or young trees. The interception rate will increase at a faster rate than the tree’s size increases every year. For example, a typical hackberry tree in the U.S. Midwest has been measured at three different ages to determine the amount of interception that occurs.
- 5 year old hackberry intercepts 133 gal (0.5m3) rainfall per year
- 20 year old hackberry intercepts 1,394 gal (5.3 m3) rainfall per year
- 40 year old hackberry intercepts 5,387 gal (20.4 m3) rainfall per year
A study of New York City street trees determined that stormwater runoff amounted to 890.6 million gallons annually with a value of $35.6 million in stormwater management costs. The average street tree intercepted 1,432 gallons of rainfall annually, while larger trees like London plane tree intercepted almost 3,000 gallons.
Evapotranspiration
As previously mentioned, total rainfall runoff is significantly less in a forested watershed compared to an urbanized one. Another reason for this is that trees, shrubs, and forbs release large amounts of water vapor through their leaves during photosynthesis. Evapotranspiration is the sum of water evaporated from soil and plant surfaces and the water lost as a result of transpiration. Transpiration is a process in which trees absorb water through their roots and transfer it up to the leaves where it participates in photosynthesis before it evaporates into the environment through leaf pores.
Evapotranspiration also reduces pollutant loadings as it cools and cleanses the air. Evapotranspiration continues to reduce the volume of stormwater stored in the soil long after a rainfall event ends. Evapotranspiration from plants, along with evaporation from water surfaces, accounts for two-thirds of the rain falling on the lower 48 States. The transpiration rate is influenced by factors such as tree species, size, soil moisture, increasing sunlight duration and intensity, air temperature, wind speed, and the relative humidity. A single mature oak tree can transpire over 40,000 gallons of water per year.
After a rain storm, the rate of evapotranspiration declines at the same rate as that of the soil drying out.
With further soil drying, the relative transpiration rates remains between 10% and 20% of that observed at the very end of the rain storm.
Transpiration uses heat from the air to change the water in the vegetation into water vapor, so in addition to providing stormwater benefits, transpiration also decreases ambient air temperature and reduces the urban heat island effect. Hydrologic studies have shown that an average of 60% of the rainfall in a forest is taken up by trees and transpired back into the atmosphere. Large amounts of water vapor will become a cloud that will result in additional rain downwind from the first rain.
Groundwater Recharge
Recharge refers to replenishment of both groundwater levels and normal weather stream flows. In the soil, the rainwater is filtered and slowly moves to streams as subsurface flows. In forest soils, infiltration rates can range from as much as 10 to 18 inches per hour depending on soil composition.
Tree roots in symbiosis with fungi enlarge soil pore spaces and fissures in bedrock, increasing porosity and groundwater recharge capacities. Heavy rainstorms saturate the soil layer and then cause the shallow subsurface water flow.
Stormwater Management
There are several types of soil that use large pore storage as a means of on-site stormwater management. Three of the primary ones are:
Evapotranspiration
As previously mentioned, total rainfall runoff is significantly less in a forested watershed compared to an urbanized one. Another reason for this is that trees, shrubs, and forbs release large amounts of water vapor through their leaves during photosynthesis. Evapotranspiration is the sum of water evaporated from soil and plant surfaces and the water lost as a result of transpiration. Transpiration is a process in which trees absorb water through their roots and transfer it up to the leaves where it participates in photosynthesis before it evaporates into the environment through leaf pores.
Evapotranspiration also reduces pollutant loadings as it cools and cleanses the air. Evapotranspiration continues to reduce the volume of stormwater stored in the soil long after a rainfall event ends. Evapotranspiration from plants, along with evaporation from water surfaces, accounts for two-thirds of the rain falling on the lower 48 States. The transpiration rate is influenced by factors such as tree species, size, soil moisture, increasing sunlight duration and intensity, air temperature, wind speed, and the relative humidity. A single mature oak tree can transpire over 40,000 gallons of water per year.
After a rain storm, the rate of evapotranspiration declines at the same rate as that of the soil drying out.
With further soil drying, the relative transpiration rates remains between 10% and 20% of that observed at the very end of the rain storm.
Transpiration uses heat from the air to change the water in the vegetation into water vapor, so in addition to providing stormwater benefits, transpiration also decreases ambient air temperature and reduces the urban heat island effect. Hydrologic studies have shown that an average of 60% of the rainfall in a forest is taken up by trees and transpired back into the atmosphere. Large amounts of water vapor will become a cloud that will result in additional rain downwind from the first rain.
Groundwater Recharge
Recharge refers to replenishment of both groundwater levels and normal weather stream flows. In the soil, the rainwater is filtered and slowly moves to streams as subsurface flows. In forest soils, infiltration rates can range from as much as 10 to 18 inches per hour depending on soil composition.
Tree roots in symbiosis with fungi enlarge soil pore spaces and fissures in bedrock, increasing porosity and groundwater recharge capacities. Heavy rainstorms saturate the soil layer and then cause the shallow subsurface water flow.
Stormwater Management
There are several types of soil that use large pore storage as a means of on-site stormwater management. Three of the primary ones are:
- High quality loam with low compaction (less than 85% compaction) has about 20% stormwater storage capacity.
- Large-grained clean sand between 1/20 and 1/10 inch (0.5mm to 2.0mm) diameter with low compaction (less than 85%) has about 30% stormwater storage capacity.
- Clear angular rock between 0.75” to 1.5” (2 to 4 cm) diameter with high compaction (90%) has about 40% stormwater storage capacity
- The runoff rate is also very important. If rainwater can’t be absorbed or contained quickly enough, then flooding and pollution will be a problem.
- Water separation refers to preventing stormwater from entering sanitary sewers. This can help avoid flooding and the spread of non-point source pollution.
Riparian Stabilizers
The riparian forest along the edge of streams provides a buffer during storm events. The buffer vegetation removes nitrogen and phosphorus leached from adjacent forest, turf, or agricultural lands and provides stability to the stream banks. The riparian vegetation also provides shade that cools the stream water temperature and provides aquatic and wildlife habitat for many species, while reducing stream velocity and downstream flooding.
Solutions
Looking at all the information above, arborists and landscape architects/designers must consider options that use trees and soil or aggregate to provide the best means to reduce the stormwater volume and the runoff rate. Trees and soil provide the additional benefit of significantly improving water quality by removing pollutants like total suspended solids, metals, nitrogen, phosphorous, as well as pathogens and hydrocarbons. Additional benefits include considering options such as abating soil compaction, conservation landscaping, and bioretention projects as well as designs for shade trees, and forest protection and reforestation.
Vegetative solutions have another stormwater treatment feature that doesn’t factor into most calculations and that is the plants that grow in them. Through interception and evapotranspiration, trees can prevent a significant amount of rainfall from ever reaching the ground and as the trees get larger, they can intercept and transpire greater quantities of rainfall and efficiently retain dissolved nutrients. This means that vegetated stormwater management solutions should, with proper maintenance, actually improve over time.
The riparian forest along the edge of streams provides a buffer during storm events. The buffer vegetation removes nitrogen and phosphorus leached from adjacent forest, turf, or agricultural lands and provides stability to the stream banks. The riparian vegetation also provides shade that cools the stream water temperature and provides aquatic and wildlife habitat for many species, while reducing stream velocity and downstream flooding.
Solutions
Looking at all the information above, arborists and landscape architects/designers must consider options that use trees and soil or aggregate to provide the best means to reduce the stormwater volume and the runoff rate. Trees and soil provide the additional benefit of significantly improving water quality by removing pollutants like total suspended solids, metals, nitrogen, phosphorous, as well as pathogens and hydrocarbons. Additional benefits include considering options such as abating soil compaction, conservation landscaping, and bioretention projects as well as designs for shade trees, and forest protection and reforestation.
Vegetative solutions have another stormwater treatment feature that doesn’t factor into most calculations and that is the plants that grow in them. Through interception and evapotranspiration, trees can prevent a significant amount of rainfall from ever reaching the ground and as the trees get larger, they can intercept and transpire greater quantities of rainfall and efficiently retain dissolved nutrients. This means that vegetated stormwater management solutions should, with proper maintenance, actually improve over time.
Recycling Rainwater
The chemicals used to treat municipal water and the dissolved minerals found naturally in many of our water systems can produce an imbalance in the soil when this water is used for irrigating trees and plants. Chemical fertilizers, fungicides, and pesticides in the soil, along with drought, can all disrupt the balance and harmony of the soil. This imbalance causes trees and plants to weaken and make them more susceptible to disease and pest attacks.
To prevent these concerns, consider recycling rainwater. When rainwater is collected from the roofs of structures, it picks up very little contamination. The rainwater should be stored in rain barrels, cisterns, or other reservoirs and used specifically for watering landscapes and trees. This effort can help to improve the health of the plants, lawns, and trees. Rain is soft water that is devoid of minerals, chlorine, fluoride, and other metallic contaminants. Furthermore, by diverting water from flowing into storm drains or flowing over the land, it helps to decrease the impact of excessive runoff on the local waterways.
In addition to efficient watering practices, watering a landscape with rainwater, or reused water can help relieve the strain on the local municipal water supply. However, rainwater and gray water reuse should not be used as a replacement for water-efficient landscape plants and irrigation practices. Be mindful that the first steps to water-efficiency outdoors should be drought tolerant, low water use of landscape plants and wise watering.
Water Collection
Commercial rooftop collection systems are available and simply divert roof downspouts into a covered barrel as an easy, low-cost approach. Some states have laws that prohibit collection of rainwater, and other states that encourage it, and still others that require it, so learn the local state's regulations before implementing any rainwater collection system.
Recycled Water
Water recycling is using treated water from a wastewater treatment facility for non-potable beneficial purposes such as agricultural and landscape irrigation, public parks and golf course irrigation, industrial processes, toilet flushing, and replenishing ground water. Other non-potable applications include cooling water for power plants and oil refineries, industrial process water for such facilities as paper mills, dust control, construction activities, concrete mixing, and artificial lakes.
Gray Water
Household wastewater from bathroom sinks, showers, bathtubs, and clothes washers, is called "gray water" and it can be used for landscape irrigation without any treatment. (The use of non-toxic and low-sodium soap and personal care products is required to protect vegetation when using gray water for irrigation.) Gray water systems divert used water to a storage tank for landscape irrigation when this water is needed. These systems might require the use of extensive plumbing in the home or building, as well as a means of getting the water out of the structure and into the outdoor landscape.
The National Science Foundation (NSF) International has developed a standard called “NSF 350 – Onsite Residential and Commercial Reuse Treatment Systems”. This standard encompasses residential wastewater treatment systems along with systems that treat only the gray water portion.
Benefits
Recycled water can satisfy most landscape water demands, as long as it is adequately treated to ensure water quality appropriate for the use. In uses where there is a greater chance of human exposure to the water, more treatment is required. As for any water source that is not properly treated, health problems could arise from drinking or being exposed to recycled water if it contains disease-causing organisms or other contaminants.
The United States Environmental Protection Agency (EPA) developed a technical document entitled “Guidelines for Water Reuse” which contains a summary of local state requirements, and guidelines for the treatment and uses of recycled water. State and Federal regulatory oversight has successfully provided a framework to ensure the safety of the many water recycling projects that have been developed in the United States.
Using Recycled Water
Although most water recycling projects have been developed to meet non-potable water demands, a number of projects use recycled water indirectly for potable purposes. These projects include recharging ground water aquifers and augmenting surface water reservoirs with recycled water. In ground water recharge projects, recycled water can be spread or injected into ground water aquifers to augment ground water supplies.
In the U.S., numerous successful large area, ground water recharge projects have been operating for many years. Planned augmentation of surface water reservoirs has been less common. However, there are currently several existing projects that have been completed and several others in the planning stages. The use of gray water for landscape irrigation and toilet flushing reduces the amount of potable water distributed to these sites, the amount of fertilizer needed, and the amount of wastewater generated, transported, and treated at wastewater treatment facilities. In other words, recycled water saves water, energy, and money. Successful gray water systems have been operating for many years and they meet up to 50% of a property's water needs by supplying water for landscaping. Recycling gray water saves fresh potable water for other uses, reduces the volume of wastewater going to septic systems and wastewater treatment plants, and increases infrastructure capacity for new users.
Benefits of Water Recycling
In addition to providing a dependable, locally controlled recycled water supply, water recharging provides tremendous environmental benefits. By providing an additional source of water, water recycling can help find ways to decrease the diversion of water from sensitive ecosystems. Other benefits include decreasing wastewater discharges and reducing and preventing pollution. Recycled water can also be used to create or enhance wetlands and riparian habitats.
The chemicals used to treat municipal water and the dissolved minerals found naturally in many of our water systems can produce an imbalance in the soil when this water is used for irrigating trees and plants. Chemical fertilizers, fungicides, and pesticides in the soil, along with drought, can all disrupt the balance and harmony of the soil. This imbalance causes trees and plants to weaken and make them more susceptible to disease and pest attacks.
To prevent these concerns, consider recycling rainwater. When rainwater is collected from the roofs of structures, it picks up very little contamination. The rainwater should be stored in rain barrels, cisterns, or other reservoirs and used specifically for watering landscapes and trees. This effort can help to improve the health of the plants, lawns, and trees. Rain is soft water that is devoid of minerals, chlorine, fluoride, and other metallic contaminants. Furthermore, by diverting water from flowing into storm drains or flowing over the land, it helps to decrease the impact of excessive runoff on the local waterways.
In addition to efficient watering practices, watering a landscape with rainwater, or reused water can help relieve the strain on the local municipal water supply. However, rainwater and gray water reuse should not be used as a replacement for water-efficient landscape plants and irrigation practices. Be mindful that the first steps to water-efficiency outdoors should be drought tolerant, low water use of landscape plants and wise watering.
Water Collection
Commercial rooftop collection systems are available and simply divert roof downspouts into a covered barrel as an easy, low-cost approach. Some states have laws that prohibit collection of rainwater, and other states that encourage it, and still others that require it, so learn the local state's regulations before implementing any rainwater collection system.
Recycled Water
Water recycling is using treated water from a wastewater treatment facility for non-potable beneficial purposes such as agricultural and landscape irrigation, public parks and golf course irrigation, industrial processes, toilet flushing, and replenishing ground water. Other non-potable applications include cooling water for power plants and oil refineries, industrial process water for such facilities as paper mills, dust control, construction activities, concrete mixing, and artificial lakes.
Gray Water
Household wastewater from bathroom sinks, showers, bathtubs, and clothes washers, is called "gray water" and it can be used for landscape irrigation without any treatment. (The use of non-toxic and low-sodium soap and personal care products is required to protect vegetation when using gray water for irrigation.) Gray water systems divert used water to a storage tank for landscape irrigation when this water is needed. These systems might require the use of extensive plumbing in the home or building, as well as a means of getting the water out of the structure and into the outdoor landscape.
The National Science Foundation (NSF) International has developed a standard called “NSF 350 – Onsite Residential and Commercial Reuse Treatment Systems”. This standard encompasses residential wastewater treatment systems along with systems that treat only the gray water portion.
Benefits
Recycled water can satisfy most landscape water demands, as long as it is adequately treated to ensure water quality appropriate for the use. In uses where there is a greater chance of human exposure to the water, more treatment is required. As for any water source that is not properly treated, health problems could arise from drinking or being exposed to recycled water if it contains disease-causing organisms or other contaminants.
The United States Environmental Protection Agency (EPA) developed a technical document entitled “Guidelines for Water Reuse” which contains a summary of local state requirements, and guidelines for the treatment and uses of recycled water. State and Federal regulatory oversight has successfully provided a framework to ensure the safety of the many water recycling projects that have been developed in the United States.
Using Recycled Water
Although most water recycling projects have been developed to meet non-potable water demands, a number of projects use recycled water indirectly for potable purposes. These projects include recharging ground water aquifers and augmenting surface water reservoirs with recycled water. In ground water recharge projects, recycled water can be spread or injected into ground water aquifers to augment ground water supplies.
In the U.S., numerous successful large area, ground water recharge projects have been operating for many years. Planned augmentation of surface water reservoirs has been less common. However, there are currently several existing projects that have been completed and several others in the planning stages. The use of gray water for landscape irrigation and toilet flushing reduces the amount of potable water distributed to these sites, the amount of fertilizer needed, and the amount of wastewater generated, transported, and treated at wastewater treatment facilities. In other words, recycled water saves water, energy, and money. Successful gray water systems have been operating for many years and they meet up to 50% of a property's water needs by supplying water for landscaping. Recycling gray water saves fresh potable water for other uses, reduces the volume of wastewater going to septic systems and wastewater treatment plants, and increases infrastructure capacity for new users.
Benefits of Water Recycling
In addition to providing a dependable, locally controlled recycled water supply, water recharging provides tremendous environmental benefits. By providing an additional source of water, water recycling can help find ways to decrease the diversion of water from sensitive ecosystems. Other benefits include decreasing wastewater discharges and reducing and preventing pollution. Recycled water can also be used to create or enhance wetlands and riparian habitats.
Bioretention Basins
Bioretention basins are small areas of land with landscaped depressions or shallow basins used to slow and treat on-site stormwater runoff. Bioretention basins are one of the most effective ways for removal of suspended solids, heavy metals, hydrocarbons, organic compounds, and dissolved nutrients. Healthy trees have been found to be especially good for removal of dissolved nitrogen and phosphorus. Not only have the trees been shown to significantly improve the nutrient removal process to clean the water, but many trees seem to benefit from the nutrients in the stormwater.
Basin Components
Bioretention basins consist of a grass buffer strip, a ponding area, planting soil, a sand bed, and plants including trees. Each of the components of the bioretention basin is designed to perform a specific function. The basin components contain the following features:
1. Grass swales or filter strip - reduces incoming stormwater runoff velocity and catches suspended solids.
The grass should remove 25% to 30% of the sediment load. The addition of a pea gravel flow spreader in the design of the basin also helps capture sediments.
2. Ponding area - provides storage of excess stormwater flows and its subsequent evaporation or infiltration. It
also aids in the additional settlement of fine particulate matter.
3. Vegetation - helps remove water through evapotranspiration and removal of nutrients by absorption into
plant roots.
1.
4. Mulch - an organic layer that encourages micro biological degradation of petroleum-based pollutants, aids
in pollutant filtration and reduces soil erosion. The mulch layer should be composed of 1 to 2 inch (3 to 5 cm) sized shredded hardwood or wood chips laid to a depth of 2 to 4 inches (5 to 10 cm). The mulch layer reduces erosion, helps maintain moisture levels for plant roots and aids in filtration and decomposition of organic materials. This layer acts in a similar way to the leaf litter in a forest and prevents the erosion and the drying of underlying soils.
5. Soil - supports vegetation growth along with nutrient uptake and provisions for water storage. Soils should
include some clay loam to absorb pollutants such as hydrocarbons, heavy metals, and nutrients.
6. Sand bed - provides drainage and aeration of planting soil as well as an aid in flushing pollutants. The sand
bed also reduces the water flow velocity, filters particulates, and spreads the flow over the length of the bioretention area.
7. Under-drain basin - a perforated pipe removes excess treated water to a storm drain basin or receiving
waters.
Design and Construction
Bioretention basins can be adapted through minor design adjustments to meet a wide range of climate and geological conditions found in the United States. Typically, bioretention practices are best suited to small sites and highly urbanized spaces. Bioretention practices are used in:
The layout of the bioretention area is determined after site constraints such as location of utilities, underlying soils, existing vegetation, and drainage are considered. Sites with loamy sand soils are especially appropriate for bioretention because the excavated soil can be backfilled and used as the planting soil, thus eliminating the cost of importing planting soil. An unstable surrounding soil stratum and soils with a clay content of greater than 25% may preclude the use of bioretention, as would a site with slopes greater than 20%, or a site with mature trees that would be removed to allow construction.
Research and experience have resulted in the development of the following requirements necessary for the construction of a properly functioning bioretention basin:
1. Approximately 5% of the impervious area to be drained must be dedicated to bioretention basin
development.
2. Drainage areas should not exceed five acres. One-half to two acres is preferred to avoid rapid clogging of the
filter. Multiple bioretention basins are recommended for larger sites.
3. A minimum area of 200 square feet is required for a properly functioning bioretention basin. The minimum
dimensions for a bioretention basin is 10 feet wide by 20 feet long (equal to one parking space). A length to width ratio of at least 2:1 is the best.
4. The slope of the landscape area should be a maximum of 5% - 6% and sufficient to allow filtered
stormwater to flow to the discharge. The maximum sheet flow velocity to prevent erosive conditions is 1 foot per second (0.3 meters per second) for planted groundcover and 3 feet per second (0.9 meters per second) for mulch.
5. Minimum elevation or depth from the point of inflow into the basin, through the filter material and to the
outflow should be 5 feet.
6. A minimum separation distance of 3 feet between the bottom of the bioretention basin and the elevation of
the seasonally high water table is required.
7. Depth to bedrock needs to be surveyed to establish and facilitate fitting of basins within required dimensions
previously mentioned.
8. Bioretention basins need to drain reasonably fast to function correctly. They should not be sited in areas
where there is continuous flow from groundwater, sump pumps, or other water sources.
9. Bioretention basins need to be integrated into a site plan to ensure that their full aesthetic potential is
captured and they are located correctly within the site's elevation plan to function properly.
10. The design of the bioretention basins can vary widely due to site conditions and the wishes of the
neighborhood where the basin is being installed. Some design variations will also increase effectiveness of the basins. The design should consider water conveyance to ensure that stormwater flows do not cause erosion prior to or after treatment and if possible be subject to other treatment practices during conveyance.
11. If treated water is not to be allowed to infiltrate into native soils, the use of an under-drain with perforated
pipe is needed to convey treated water to the stormwater drainage system. Partial exfiltration can be used to recharge groundwater. This calls for the under-drain to be installed in only part of the basin. Some level of
infiltration occurs throughout the remainder of the basin, recharging the ground water. The partial under- drain acts more as an overflow. The variation is only suitable to apply if soils and other conditions are appropriate to encourage infiltration.
12. An overflow structure to the storm drainage system is needed to convey storm flows larger than can be
treated by the designed basin.
13. Maintenance design should ensure that easy access is possible for maintenance personnel and any
associated machinery.
14. The choice of correct landscaping materials is critical to functioning and aesthetics of bioretention basins.
Plants help reduce water quantities through evapotranspiration, removal of pollutants and nutrients, and their root systems increase water percolation. Utilize native plants or cultivars of native plants where possible because they may better tolerate the climatic conditions and extremes of the bioretention basin area.
15. Include a mixture of trees, shrubs and herbaceous materials. Such combinations are more visually pleasing
and provide a variety of habitats for wildlife. Edge plants may experience longer periods of dryness.
Ponding Area
The water is ponded in the bioretention area to a depth of 6 to 9 inches (15 – 23 cm). This area allows for surface storage of stormwater before filtration occurs as well as some evaporation and settling out of heavy sediments. Stormwater storage is also provided by the voids in the soil. The stored water and nutrients in the water and soil are then available to the plants for uptake.
The bioretention area is graded to divert excess stormwater from large storms into stormwater drains once the basin’s ponding area is full of stormwater runoff. The water that is stored in the bioretention basin drains over a period of days into the underlying soils where it is treated by a number of physical, chemical, and biological processes. The slowed, cleaned water is allowed to infiltrate native soils or it is directed to nearby stormwater drains or receiving waters.
Soil
An engineered soil bed containing a sand-soil matrix provides most of the basins filtration capacity as well as providing water, nutrients and support for the plant community. The soil and its microorganisms work together with trees and plants as a powerful system to improve the quality of stormwater as the water is filtered through the soil. Some clay in the planting soil provides adsorption sites for hydrocarbons, heavy metals, nutrients and other pollutants. Some pollutants are also held by the soil, others are taken up and transformed by plants or microbes into food, and still other pollutants are first held by soil and then taken up by vegetation or degraded by bacteria.
Climates
In arid climates plant selection should focus on choosing drought-tolerant species. In cold climates bioretention basins can be used for snow storage. However, if the use of sodium chloride and calcium-based deicers and sand is not managed, maintenance will be complicated and more costly. Also, the plant material selection must be limited to non-woody, salt tolerant species.
Bioretention Basin Maintenance
Routine inspection and attention to maintenance needs are required if bioretention basins are to continue to function correctly. High maintenance levels are required for new basins, but once established and correctly operating maintenance requirements are expected to decline. The property’s normal landscaping contractor, when provided with appropriate training, can be expected to successfully maintain an established bioretention basin. Scheduled maintenance tasks include:
Nitrogen Cycle in Bioretention Soils
Nitrogen occurs in several different forms in typical bioretention soils as:
- particulate organic nitrogen.
- dissolved organic nitrogen.
- ammonium (NH4+).
- nitrate (NO3-).
Nitrate is the most common dissolved form of nitrogen because it has a negative charge and typical bioretention soil is also negatively charged. This means that nitrate is not adsorbed into the soil and often leaches out. Improving nitrogen performance of the bioretention basin therefore requires adequate time for biological processes to occur and the presence of plants.
The Role of Plants in Nitrogen Removal Soils with plants have higher microbial populations than barren soils and there is a rapid nitrogen uptake by microbes. The microbial immobilization (uptake) of nutrients occurs 30-100 times faster than uptake by plants and this is the initial pathway in which nitrate is taken up from stormwater in bioretention basins. However, the presence of plants is crucial to capitalizing on this microbial nitrate uptake in two ways:
1. Plant roots release carbon exudates into the surrounding soil, and microbes use this carbon as an energy
source. As a result, soils with plants have been found to have much higher microbial populations than those without plants. Bacteria and fungi are 20-50 times more abundant in the rhizosphere of plant roots.
2. Microbial lifespans are short and nutrients are not retained for very long in microbes. If plants are present,
they can take up the nitrogen immobilized by microbes. Without plants, much of the nitrogen taken up by the microbes is eventually flushed out of the soil, which causes problems downstream.
Phosphorus Removal Plants affect phosphorus removal from bioretention basins on the same time scales as nitrogen removal.
1. Soon after stormwater enters the basin, soils with plants, which have higher microbial populations than barren soils, result in a rapid phosphorus uptake by microbes.
2. A short time later, plants then take up the microbes’ phosphorus.
3. Finally, the phosphorus taken up by plants is converted into recalcitrant soil organic matter.
Healthy vegetation is essential to maximizing phosphorus removal in bioretention basins and loamy sand produced the healthiest vegetation and greatest nutrient removal. The greater water holding capacity of the finer grained media such as loamy sand and sand gave the vegetation in these basins a longer period of time to access moisture and nutrients and thus facilitated greater nutrient removal.
Street Tree Planters
Once seen as highly problematic for many reasons, street trees are proving to be a great value to people living, working, shopping, sharing, walking, and motoring in and through urban places.
For a planting cost of approximately US $250 – $600 (including the first 3 years of maintenance) a single street tree returns over US $90,000 of direct benefits (not including aesthetic, social, and natural benefits) in the lifetime of the tree. Street trees are generally planted from 4 feet to 12 feet from the curbs. These trees provide so many benefits to those streets they occupy that they should always be considered as infrastructure.
With new attentions being paid to global warming causes and impacts, more is becoming known about negative environmental impacts of treeless urban streets. Arborists, landscape architects, and designers are well on the way to recognizing the need for urban street trees to be the preferred urban design amenity, rather than luxury items that were tolerated by traffic engineering and budget conscious city administrators.
The many identified problems of street trees are overcome with care by the designers. Generally street trees are planted every 15 – 30 feet along the street or sidewalk. These trees are carefully positioned to allow adequate sight triangles at intersections and driveways, to not block street lights, and to not impact utility lines above or below ground. Street trees of various varieties are used in all climates, including high altitude, semi-arid, and even arid urban places.
Rooting Space
Unfortunately, planting success in a tree pit often follows what might be called the “Rule of Four”: the roots of a tree with a 4 inch (10 cm) trunk caliper will fill up a 4x4x4foot (1.2x1.2x1.2 m) soil pit within 4 years. This usually results in a growth slowdown or stoppage and tree death. A planned street or sidewalk reconstruction offers a prime opportunity to build better tree planting sites. The following strategies are the best bets for ensuring tree survival and green streets.
Tree planting sites should be large enough to accommodate the tree's roots at maturity. There should be four square feet (0.4 sq. m) of soil surface area for every one inch (2.5 cm) of trunk caliper the tree is expected to attain, or two cubic feet (0.2 sq. m) of soil for every square foot of the future crown projection or spread (which is the area under the drip line and called the tree spread by most nurseries). The soil should be a minimum three feet (90 cm) deep for normal growth and vigor.
Although care and maintenance of trees in urban places is a costly task, the value in returned benefits is so great that a sustainable community cannot be imagined without these important green features.
Bioretention basins are small areas of land with landscaped depressions or shallow basins used to slow and treat on-site stormwater runoff. Bioretention basins are one of the most effective ways for removal of suspended solids, heavy metals, hydrocarbons, organic compounds, and dissolved nutrients. Healthy trees have been found to be especially good for removal of dissolved nitrogen and phosphorus. Not only have the trees been shown to significantly improve the nutrient removal process to clean the water, but many trees seem to benefit from the nutrients in the stormwater.
Basin Components
Bioretention basins consist of a grass buffer strip, a ponding area, planting soil, a sand bed, and plants including trees. Each of the components of the bioretention basin is designed to perform a specific function. The basin components contain the following features:
1. Grass swales or filter strip - reduces incoming stormwater runoff velocity and catches suspended solids.
The grass should remove 25% to 30% of the sediment load. The addition of a pea gravel flow spreader in the design of the basin also helps capture sediments.
2. Ponding area - provides storage of excess stormwater flows and its subsequent evaporation or infiltration. It
also aids in the additional settlement of fine particulate matter.
3. Vegetation - helps remove water through evapotranspiration and removal of nutrients by absorption into
plant roots.
1.
4. Mulch - an organic layer that encourages micro biological degradation of petroleum-based pollutants, aids
in pollutant filtration and reduces soil erosion. The mulch layer should be composed of 1 to 2 inch (3 to 5 cm) sized shredded hardwood or wood chips laid to a depth of 2 to 4 inches (5 to 10 cm). The mulch layer reduces erosion, helps maintain moisture levels for plant roots and aids in filtration and decomposition of organic materials. This layer acts in a similar way to the leaf litter in a forest and prevents the erosion and the drying of underlying soils.
5. Soil - supports vegetation growth along with nutrient uptake and provisions for water storage. Soils should
include some clay loam to absorb pollutants such as hydrocarbons, heavy metals, and nutrients.
6. Sand bed - provides drainage and aeration of planting soil as well as an aid in flushing pollutants. The sand
bed also reduces the water flow velocity, filters particulates, and spreads the flow over the length of the bioretention area.
7. Under-drain basin - a perforated pipe removes excess treated water to a storm drain basin or receiving
waters.
Design and Construction
Bioretention basins can be adapted through minor design adjustments to meet a wide range of climate and geological conditions found in the United States. Typically, bioretention practices are best suited to small sites and highly urbanized spaces. Bioretention practices are used in:
- areas where few pervious surfaces exist, such as near parking lots, streets, or large buildings. These basins are often fit into existing parking lot islands and adjoining landscaped areas.
- areas with highly contaminated runoff, like gas stations and parking lots. These locations must have the bottom of the bioretention basin lined with an impermeable liner to prevent the flow of contaminated water to nearby stormwater drains, groundwater sources, and receiving waters.
- areas where existing developments are being required to retrofit with stormwater management practices. Bioretention basins are a suitable option that can be implemented by modifying an existing landscape or adding one to a parking lot that is being redesigned or resurfaced.
- areas near cold water streams where ponding water exists for only short periods and are not likely to warm up.
The layout of the bioretention area is determined after site constraints such as location of utilities, underlying soils, existing vegetation, and drainage are considered. Sites with loamy sand soils are especially appropriate for bioretention because the excavated soil can be backfilled and used as the planting soil, thus eliminating the cost of importing planting soil. An unstable surrounding soil stratum and soils with a clay content of greater than 25% may preclude the use of bioretention, as would a site with slopes greater than 20%, or a site with mature trees that would be removed to allow construction.
Research and experience have resulted in the development of the following requirements necessary for the construction of a properly functioning bioretention basin:
1. Approximately 5% of the impervious area to be drained must be dedicated to bioretention basin
development.
2. Drainage areas should not exceed five acres. One-half to two acres is preferred to avoid rapid clogging of the
filter. Multiple bioretention basins are recommended for larger sites.
3. A minimum area of 200 square feet is required for a properly functioning bioretention basin. The minimum
dimensions for a bioretention basin is 10 feet wide by 20 feet long (equal to one parking space). A length to width ratio of at least 2:1 is the best.
4. The slope of the landscape area should be a maximum of 5% - 6% and sufficient to allow filtered
stormwater to flow to the discharge. The maximum sheet flow velocity to prevent erosive conditions is 1 foot per second (0.3 meters per second) for planted groundcover and 3 feet per second (0.9 meters per second) for mulch.
5. Minimum elevation or depth from the point of inflow into the basin, through the filter material and to the
outflow should be 5 feet.
6. A minimum separation distance of 3 feet between the bottom of the bioretention basin and the elevation of
the seasonally high water table is required.
7. Depth to bedrock needs to be surveyed to establish and facilitate fitting of basins within required dimensions
previously mentioned.
8. Bioretention basins need to drain reasonably fast to function correctly. They should not be sited in areas
where there is continuous flow from groundwater, sump pumps, or other water sources.
9. Bioretention basins need to be integrated into a site plan to ensure that their full aesthetic potential is
captured and they are located correctly within the site's elevation plan to function properly.
10. The design of the bioretention basins can vary widely due to site conditions and the wishes of the
neighborhood where the basin is being installed. Some design variations will also increase effectiveness of the basins. The design should consider water conveyance to ensure that stormwater flows do not cause erosion prior to or after treatment and if possible be subject to other treatment practices during conveyance.
11. If treated water is not to be allowed to infiltrate into native soils, the use of an under-drain with perforated
pipe is needed to convey treated water to the stormwater drainage system. Partial exfiltration can be used to recharge groundwater. This calls for the under-drain to be installed in only part of the basin. Some level of
infiltration occurs throughout the remainder of the basin, recharging the ground water. The partial under- drain acts more as an overflow. The variation is only suitable to apply if soils and other conditions are appropriate to encourage infiltration.
12. An overflow structure to the storm drainage system is needed to convey storm flows larger than can be
treated by the designed basin.
13. Maintenance design should ensure that easy access is possible for maintenance personnel and any
associated machinery.
14. The choice of correct landscaping materials is critical to functioning and aesthetics of bioretention basins.
Plants help reduce water quantities through evapotranspiration, removal of pollutants and nutrients, and their root systems increase water percolation. Utilize native plants or cultivars of native plants where possible because they may better tolerate the climatic conditions and extremes of the bioretention basin area.
15. Include a mixture of trees, shrubs and herbaceous materials. Such combinations are more visually pleasing
and provide a variety of habitats for wildlife. Edge plants may experience longer periods of dryness.
Ponding Area
The water is ponded in the bioretention area to a depth of 6 to 9 inches (15 – 23 cm). This area allows for surface storage of stormwater before filtration occurs as well as some evaporation and settling out of heavy sediments. Stormwater storage is also provided by the voids in the soil. The stored water and nutrients in the water and soil are then available to the plants for uptake.
The bioretention area is graded to divert excess stormwater from large storms into stormwater drains once the basin’s ponding area is full of stormwater runoff. The water that is stored in the bioretention basin drains over a period of days into the underlying soils where it is treated by a number of physical, chemical, and biological processes. The slowed, cleaned water is allowed to infiltrate native soils or it is directed to nearby stormwater drains or receiving waters.
Soil
An engineered soil bed containing a sand-soil matrix provides most of the basins filtration capacity as well as providing water, nutrients and support for the plant community. The soil and its microorganisms work together with trees and plants as a powerful system to improve the quality of stormwater as the water is filtered through the soil. Some clay in the planting soil provides adsorption sites for hydrocarbons, heavy metals, nutrients and other pollutants. Some pollutants are also held by the soil, others are taken up and transformed by plants or microbes into food, and still other pollutants are first held by soil and then taken up by vegetation or degraded by bacteria.
Climates
In arid climates plant selection should focus on choosing drought-tolerant species. In cold climates bioretention basins can be used for snow storage. However, if the use of sodium chloride and calcium-based deicers and sand is not managed, maintenance will be complicated and more costly. Also, the plant material selection must be limited to non-woody, salt tolerant species.
Bioretention Basin Maintenance
Routine inspection and attention to maintenance needs are required if bioretention basins are to continue to function correctly. High maintenance levels are required for new basins, but once established and correctly operating maintenance requirements are expected to decline. The property’s normal landscaping contractor, when provided with appropriate training, can be expected to successfully maintain an established bioretention basin. Scheduled maintenance tasks include:
- watering plants daily for at least the first two weeks after installation.
- re-mulching bare areas.
- mowing turf areas.
- treating plant diseases.
- watering plants throughout periods of persistent drought.
- When the ponding of water lasts for more than 48 hours, remove the top 2" - 3" of discolored planting medium and replace it with fresh material.
- a monthly inspection of the basin to evaluate the condition and problems needing maintenance attention, removing litter and plant debris and repairing eroded areas.
- removing and replacing dead and diseased plants twice per year.
- adding new mulch once per year.
- removing all tree stabilizers and wires at the end of the first growing season, if any were installed at the time of installation. Root stabilizers do not need to be removed.
Nitrogen Cycle in Bioretention Soils
Nitrogen occurs in several different forms in typical bioretention soils as:
- particulate organic nitrogen.
- dissolved organic nitrogen.
- ammonium (NH4+).
- nitrate (NO3-).
Nitrate is the most common dissolved form of nitrogen because it has a negative charge and typical bioretention soil is also negatively charged. This means that nitrate is not adsorbed into the soil and often leaches out. Improving nitrogen performance of the bioretention basin therefore requires adequate time for biological processes to occur and the presence of plants.
The Role of Plants in Nitrogen Removal Soils with plants have higher microbial populations than barren soils and there is a rapid nitrogen uptake by microbes. The microbial immobilization (uptake) of nutrients occurs 30-100 times faster than uptake by plants and this is the initial pathway in which nitrate is taken up from stormwater in bioretention basins. However, the presence of plants is crucial to capitalizing on this microbial nitrate uptake in two ways:
1. Plant roots release carbon exudates into the surrounding soil, and microbes use this carbon as an energy
source. As a result, soils with plants have been found to have much higher microbial populations than those without plants. Bacteria and fungi are 20-50 times more abundant in the rhizosphere of plant roots.
2. Microbial lifespans are short and nutrients are not retained for very long in microbes. If plants are present,
they can take up the nitrogen immobilized by microbes. Without plants, much of the nitrogen taken up by the microbes is eventually flushed out of the soil, which causes problems downstream.
Phosphorus Removal Plants affect phosphorus removal from bioretention basins on the same time scales as nitrogen removal.
1. Soon after stormwater enters the basin, soils with plants, which have higher microbial populations than barren soils, result in a rapid phosphorus uptake by microbes.
2. A short time later, plants then take up the microbes’ phosphorus.
3. Finally, the phosphorus taken up by plants is converted into recalcitrant soil organic matter.
Healthy vegetation is essential to maximizing phosphorus removal in bioretention basins and loamy sand produced the healthiest vegetation and greatest nutrient removal. The greater water holding capacity of the finer grained media such as loamy sand and sand gave the vegetation in these basins a longer period of time to access moisture and nutrients and thus facilitated greater nutrient removal.
Street Tree Planters
Once seen as highly problematic for many reasons, street trees are proving to be a great value to people living, working, shopping, sharing, walking, and motoring in and through urban places.
For a planting cost of approximately US $250 – $600 (including the first 3 years of maintenance) a single street tree returns over US $90,000 of direct benefits (not including aesthetic, social, and natural benefits) in the lifetime of the tree. Street trees are generally planted from 4 feet to 12 feet from the curbs. These trees provide so many benefits to those streets they occupy that they should always be considered as infrastructure.
With new attentions being paid to global warming causes and impacts, more is becoming known about negative environmental impacts of treeless urban streets. Arborists, landscape architects, and designers are well on the way to recognizing the need for urban street trees to be the preferred urban design amenity, rather than luxury items that were tolerated by traffic engineering and budget conscious city administrators.
The many identified problems of street trees are overcome with care by the designers. Generally street trees are planted every 15 – 30 feet along the street or sidewalk. These trees are carefully positioned to allow adequate sight triangles at intersections and driveways, to not block street lights, and to not impact utility lines above or below ground. Street trees of various varieties are used in all climates, including high altitude, semi-arid, and even arid urban places.
Rooting Space
Unfortunately, planting success in a tree pit often follows what might be called the “Rule of Four”: the roots of a tree with a 4 inch (10 cm) trunk caliper will fill up a 4x4x4foot (1.2x1.2x1.2 m) soil pit within 4 years. This usually results in a growth slowdown or stoppage and tree death. A planned street or sidewalk reconstruction offers a prime opportunity to build better tree planting sites. The following strategies are the best bets for ensuring tree survival and green streets.
Tree planting sites should be large enough to accommodate the tree's roots at maturity. There should be four square feet (0.4 sq. m) of soil surface area for every one inch (2.5 cm) of trunk caliper the tree is expected to attain, or two cubic feet (0.2 sq. m) of soil for every square foot of the future crown projection or spread (which is the area under the drip line and called the tree spread by most nurseries). The soil should be a minimum three feet (90 cm) deep for normal growth and vigor.
Although care and maintenance of trees in urban places is a costly task, the value in returned benefits is so great that a sustainable community cannot be imagined without these important green features.
Bioswales
Bioswales are landscape elements designed to remove silt and pollution from surface water runoff. A bioswale is basically a drainage ditch with gently sloped sides and it is filled with vegetation growing in compost or coarse soil. Bioswales go by many names including: vegetated swales, grassy swales, bioretention areas, and filter strips. In their simplest forms, bioswales are linear rain gardens planted with native vegetation that receive and absorb stormwater runoff from impervious surfaces.
The majority of annual precipitation comes from frequent, small rain events. Much of the value of bioswales comes from infiltrating and filtering nearly all of this water. The water flows along the wide and shallow ditch that is designed to maximize the time water spends in the swale. This delay traps pollutants and silt. Depending upon the geometry of the available land, a bioswale may have anything from a meandering to an almost straight channel alignment.
Bioswale Locations
A common application for a bioswale is around parking lots, where substantial automotive pollution is collected by the pavement and then flushed off the surface by the rain. The bioswale wraps around the parking lot and treats the runoff before releasing it to the watershed or storm sewer.
More complex bioswales are installed with under-drains and infiltration trenches to manage and treat stormwater runoff from large developments. These complex bioswales may be useful in industrial parks, office complexes, retail centers and high-density apartment projects. These larger bioswales are planted with native or cultivars of native flowering perennials that can have attractive landscaping features and also provide food and shelter for birds and butterflies.
Trapped Pollutants
Studies have found that properly designed and constructed bioswales are able to achieve excellent removal of heavy metals, total suspended solids, as well as oil and grease. Bioswales remove suspended solids through settling and filtration. Dissolved pollutants are removed and transformed as runoff infiltrates into the ground.
There are several water pollutants that may be captured with a bioswale. They fall into four categories:
1. Silt – is composed of soil particles picked up by the water runoff and if untreated, they cause turbidity to the receiving waters.
2. Inorganic compounds – include lead, chromium, cadmium and other heavy metals washing off vehicles in a parking lot. Lead is the most prevalent chemical of this class, deriving from automotive residue and spillage of leaded gasoline. Other common inorganic compounds are phosphates and nitrates. The source of these nutrients is usually excess fertilization. All of these nutrients can cause eutrophication in the receiving waters.
3. Organic chemicals – are from pesticides that were over applied on agricultural land and urban landscapes. These chemicals can lead to a variety of organism poisoning and aquatic ecosystem disturbance.
4. Pathogens – are typically derived from surface runoff containing animal wastes and can lead to a variety of diseases in humans and aquatic organisms.
Note: many of these compounds, chemicals, and pathogens will be degraded if the right plants are used. See the section below pertaining to phytoremediation for more information.
Thermal Pollution
Bioswales can also reduce thermal pollution. Stormwater can rise in temperature as it flows across impervious surfaces; hot parking lots in summer for example. Heated stormwater flowing into streams can impact fish and other wildlife that depend on cold water streams to live and breed. Heated runoff from impervious surfaces can be cooled by as much as 19°F (12°C) between stormwater entering a bioswale and stormwater filtering out of the bioswale.
Designing a Bioswale
For best results, enhance and utilize existing natural drainage swales whenever possible. Existing swales can be enhanced with native plants or cultivars of native plants. The thicker and heavier the grasses and other vegetation, the better the swale can filter out the contaminants. Keep in mind that subgrade drains and amended soils may be needed to facilitate infiltration.
Other considerations when designing or maintaining bioswales:
Bioswales are most typically viewed as intermittent streams, unless local precipitation is very high, or other continuous sources of surface runoff are present. These include persistent over-irrigation or natural springs. Typical bioswale design calls for water retention on the order of 60 to 120 hours following a storm event. In any case the flow depth is normally quite shallow in order to maximize the ratio of land surface to water volume, and hence produce the highest possible capture of infiltration and pollutant trapping.
Channel gradients are typically in the range of 1% to 3% to create a slow to moderate stream velocity. Cross gradient slopes are typically steeper than the channel gradient. A typical design standard for the cross-section gradient slope is 5% to 33%. The overall channel flow capacity is often sized to a given flood capacity standard. The ability to properly process a ten-year flood is a common standard. Often the mid-channel bottom has an exfiltration trench that may contribute to total infiltration by lining the bottom of the trench with rock and sand; such a design feature is particularly useful in cases where the characteristic bioswale soil is rather impermeable.
Maintaining A Bioswale
Once established, bioswales require less maintenance than turf grass because they need less water and no mowing or fertilizer. Native grasses and forbs are adapted to local rainfall patterns. Natives and cultivars of natives also resist local pests and disease, but do encourage local wildlife.
Bioswales are landscape elements designed to remove silt and pollution from surface water runoff. A bioswale is basically a drainage ditch with gently sloped sides and it is filled with vegetation growing in compost or coarse soil. Bioswales go by many names including: vegetated swales, grassy swales, bioretention areas, and filter strips. In their simplest forms, bioswales are linear rain gardens planted with native vegetation that receive and absorb stormwater runoff from impervious surfaces.
The majority of annual precipitation comes from frequent, small rain events. Much of the value of bioswales comes from infiltrating and filtering nearly all of this water. The water flows along the wide and shallow ditch that is designed to maximize the time water spends in the swale. This delay traps pollutants and silt. Depending upon the geometry of the available land, a bioswale may have anything from a meandering to an almost straight channel alignment.
Bioswale Locations
A common application for a bioswale is around parking lots, where substantial automotive pollution is collected by the pavement and then flushed off the surface by the rain. The bioswale wraps around the parking lot and treats the runoff before releasing it to the watershed or storm sewer.
More complex bioswales are installed with under-drains and infiltration trenches to manage and treat stormwater runoff from large developments. These complex bioswales may be useful in industrial parks, office complexes, retail centers and high-density apartment projects. These larger bioswales are planted with native or cultivars of native flowering perennials that can have attractive landscaping features and also provide food and shelter for birds and butterflies.
Trapped Pollutants
Studies have found that properly designed and constructed bioswales are able to achieve excellent removal of heavy metals, total suspended solids, as well as oil and grease. Bioswales remove suspended solids through settling and filtration. Dissolved pollutants are removed and transformed as runoff infiltrates into the ground.
There are several water pollutants that may be captured with a bioswale. They fall into four categories:
1. Silt – is composed of soil particles picked up by the water runoff and if untreated, they cause turbidity to the receiving waters.
2. Inorganic compounds – include lead, chromium, cadmium and other heavy metals washing off vehicles in a parking lot. Lead is the most prevalent chemical of this class, deriving from automotive residue and spillage of leaded gasoline. Other common inorganic compounds are phosphates and nitrates. The source of these nutrients is usually excess fertilization. All of these nutrients can cause eutrophication in the receiving waters.
3. Organic chemicals – are from pesticides that were over applied on agricultural land and urban landscapes. These chemicals can lead to a variety of organism poisoning and aquatic ecosystem disturbance.
4. Pathogens – are typically derived from surface runoff containing animal wastes and can lead to a variety of diseases in humans and aquatic organisms.
Note: many of these compounds, chemicals, and pathogens will be degraded if the right plants are used. See the section below pertaining to phytoremediation for more information.
Thermal Pollution
Bioswales can also reduce thermal pollution. Stormwater can rise in temperature as it flows across impervious surfaces; hot parking lots in summer for example. Heated stormwater flowing into streams can impact fish and other wildlife that depend on cold water streams to live and breed. Heated runoff from impervious surfaces can be cooled by as much as 19°F (12°C) between stormwater entering a bioswale and stormwater filtering out of the bioswale.
Designing a Bioswale
For best results, enhance and utilize existing natural drainage swales whenever possible. Existing swales can be enhanced with native plants or cultivars of native plants. The thicker and heavier the grasses and other vegetation, the better the swale can filter out the contaminants. Keep in mind that subgrade drains and amended soils may be needed to facilitate infiltration.
Other considerations when designing or maintaining bioswales:
- Costs vary greatly depending on size, plant material, and site considerations. Bioswales are generally less expensive when used in place of underground piping.
- Deep-rooted native plants or cultivars of native plants are preferred for infiltration and reduced maintenance.
- Soil infiltration rates should be greater than one-half inch per hour.
- A parabolic or trapezoidal cross-section shape is recommended with side slopes no steeper than 3:1.
- Avoid soil compaction during construction and plant installation.
- Swales should be sized to convey at least a 10-year storm or about 4 inches (10 cm) of rainfall in 24 hours.
Bioswales are most typically viewed as intermittent streams, unless local precipitation is very high, or other continuous sources of surface runoff are present. These include persistent over-irrigation or natural springs. Typical bioswale design calls for water retention on the order of 60 to 120 hours following a storm event. In any case the flow depth is normally quite shallow in order to maximize the ratio of land surface to water volume, and hence produce the highest possible capture of infiltration and pollutant trapping.
Channel gradients are typically in the range of 1% to 3% to create a slow to moderate stream velocity. Cross gradient slopes are typically steeper than the channel gradient. A typical design standard for the cross-section gradient slope is 5% to 33%. The overall channel flow capacity is often sized to a given flood capacity standard. The ability to properly process a ten-year flood is a common standard. Often the mid-channel bottom has an exfiltration trench that may contribute to total infiltration by lining the bottom of the trench with rock and sand; such a design feature is particularly useful in cases where the characteristic bioswale soil is rather impermeable.
Maintaining A Bioswale
Once established, bioswales require less maintenance than turf grass because they need less water and no mowing or fertilizer. Native grasses and forbs are adapted to local rainfall patterns. Natives and cultivars of natives also resist local pests and disease, but do encourage local wildlife.
Rain Gardens
The rain garden is designed with a shallow basin so stormwater runoff from impervious areas, like roofs, driveways, walkways, parking lots, and compacted lawn areas, can be absorbed into the soil instead of flowing into storm drains and surface waters which cause erosion, water pollution, flooding, and diminished groundwater availability. The purpose of a rain garden is to improve water quality in the soil and they can cut down on the amount of pollution reaching streams and lakes by up to 30%.
Rain gardens might be considered small bioretention areas or bioswale gardens and can be designed with plants that attract local wildlife. Rain gardens differ from retention basins because the water will infiltrate the ground within a day or two. This creates the advantage that the rain garden does not allow mosquitoes to breed. Vegetated roadside swales, now promoted as “bioswales”, remain the conventional drainage system in many parts of the world. Rain gardens are at times confused with bioswales. Swales slope to a destination, while rain gardens do not. However, a bioswale may end with a rain garden. Rain gardens are also referred to as Water Sensitive Urban Design (WSUD) in Australia and Low Impact Development (LID) in the United States. They allow new construction to deal with excessive rainwater runoff without burdening the public stormwater systems.
Native plants and cultivars of native plants are recommended for rain gardens because they generally do not require fertilizer and are more tolerant of the local climate, soil, and water conditions. Native plant root systems enhance infiltration, maintain soil permeability, provide moisture redistribution, and sustain diverse microbial populations. Also, through the process of transpiration, rain garden plants return water vapor to the atmosphere, which accelerates drying the soil between storms.
Benefits
Rain gardens are beneficial for many reasons. They:
Design
Rain gardens are designed to capture the initial flow of stormwater and reduce the accumulation of toxins flowing directly into natural waterways through ground filtration. Most rain gardens are designed to be an endpoint of drainage with a capacity to percolate all incoming water through a series of soil or gravel layers beneath the surface plantings. A French drain may be used to direct a portion of the rainwater to an overflow location during heavier rainfall events.
Soil and Drainage Improvement
When a proposed garden’s soil is not permeable enough to allow water to drain and filter properly, the soil should be replaced and an under-drain installed. The ideal rain garden soil should typically contain 60% sand, 20% organic material, and 20% topsoil. There is a current trend to use biochar or compost as the organic component of soil. Combine the sandy soil mixture with a surrounding soil if it also has high sand content.
The more polluted the water, the longer it must be retained in the soil for purification. This is often achieved by building several smaller rain garden basins in a row with soil that is deeper than the seasonal high-water table.
Dealing with Pollutants
A rain garden requires an area where water can collect and infiltrate the soil, and then use plants to absorb the pollutants, maintain infiltration rates, diversify microbe communities, and increase the soil’s water holding capacity. This includes any adjacent trees or plants extending roots into the rain garden area.
The primary challenge of rain garden design focuses on determining the types of pollutants flowing into the garden and the acceptable loads of pollutants the rain garden's filtration system can handle during storm-water events. This challenge is specifically acute when a rain event occurs after a long dry period. The initial stormwater is often highly contaminated with the accumulated pollutants.
Nitrogen and phosphorus levels and overall sediment loads in the stormwater are reduced by the action the plants and microorganisms have on the water. Multiple rain gardens over an area will have a positive cumulative effect on both the volume and quality of groundwater leaving the site.
The alternative to a rain garden is simply adjusting the landscape so that downspouts and paved surfaces drain into existing gardens. That may be all that is needed if the soil has been well loosened and plants are well established. However, many plants do not tolerate saturated roots for long and often more water runs off a roof than a garden can handle. Often the required location and storage capacity of the garden area must be determined first. If it is not sufficient in area, a rain garden should be constructed in place of the conventional garden.
Types of Rain Gardens
The two basic types of rain gardens are under-drained and self-contained. Both types are used to improve groundwater quality, reduce runoff volumes and generally facilitate infiltration of cleaned water. Which type of garden is selected to be built has to be a balance of volumes of water to be treated, existing soil conditions, available space, and the budget Add Ethan's graphic for the project.
In some cases, infiltration is not desired. For example,
In these cases, the under-drain system can be used to move excess water into a conventional storm sewer pipe system.
Drainage Rates
Rain gardens are designed to be drained within 4 hours after a 1 inch (2.5 cm) rain event. Under-drained rain gardens typically are designed to drain within 2 hours of the design storm event. This is achieved through the use of highly porous planting media and under-drains which are pervious pipes that carry the cleaned rainwater away from the garden. As a result, the plants selected for the rain garden need to be able to withstand both the extremes of flooding and drought. Plants on the upper edges of the garden are often tolerant of drought in their cultural requirements with plants lower in the garden being more adapted to floodplain conditions. Many riparian edge species are particularly well suited to the extreme environments of rain gardens.
Rain gardens with no under-drain typically hold moisture longer, particularly in the lower areas of the garden. Plants selected for this garden should be able to tolerate inundation for a more extended period of time. However, as in the case of the under-drained rain garden, the surface is drained within four hours, although the soil may be saturated. As with the bioretention cell, soils of the rain garden must be amended with a very porous soil. The depth of the soil should be a minimum of 8” deep (20 cm) and ideally to a depth of 2’- 3’ (60 – 90 cm). The lower the amount of soil amendment added when the garden is built, the more necessary it is to have plants adapted to prolonged periods of wetness. As with the under-drained rain garden, the plants on the upper edges of the garden will need to be more drought-tolerant in their cultural requirements than the plants in the lower areas.
In both types of gardens, the ground is excavated and the planting media is imported to the site. The imported planting media should be clean and weed seed free. A liner may or may not be used, depending on the design, water retention requirements, and local soil conditions, as mentioned above.
Plant Selection
Plants selected for use in a rain garden should tolerate both saturated and dry soil. Using native plants and cultivars of native plants is generally encouraged. This way the rain garden may contribute to urban habitats for native butterflies, birds, and beneficial insects. Well planned plantings require minimal maintenance to survive and are compatible with adjacent land use.
Trees generally contribute most when located close enough to tap moisture from the rain garden, yet do not excessively shade the garden. Some shading of open surface waters can be desired to keep the water temperature cool. Plants will not tolerate inundation with warm water because the heat drives out the dissolved oxygen. This means a plant tolerant of early spring, cold water flooding may not survive a summer inundation of warm water.
Adding Plants to the Garden
In both the under-drained and self-contained rain gardens, the success of the garden is greater when starting with healthy and smaller plants, rather than larger plants. Some plants are successful in rain gardens only when they are installed small and have a chance to adapt to the local conditions as they grow.
Plants and trees with deep fibrous roots tend to have a competitive advantage in a rain garden and provide the most cleaning and filtration benefits to the environment. Typical rain gardens are populated with natives or native cultivars because they are well adapted to a locality, but other ornamental plants that are non-invasive but able to grow in the garden conditions can also be excellent choices.
Successful Rain Gardens
Most of the examples of successful rain gardens are populated with herbaceous perennials, woody shrubs, or trees. This does not mean that annuals are not a possible choice for such gardens; rather it means that the successful gardens have been designed for habitat and low maintenance goals rather than purely seasonal aesthetics and color effects. Some annuals are good candidates for a higher maintenance version of a rain garden.
The rain garden is designed with a shallow basin so stormwater runoff from impervious areas, like roofs, driveways, walkways, parking lots, and compacted lawn areas, can be absorbed into the soil instead of flowing into storm drains and surface waters which cause erosion, water pollution, flooding, and diminished groundwater availability. The purpose of a rain garden is to improve water quality in the soil and they can cut down on the amount of pollution reaching streams and lakes by up to 30%.
Rain gardens might be considered small bioretention areas or bioswale gardens and can be designed with plants that attract local wildlife. Rain gardens differ from retention basins because the water will infiltrate the ground within a day or two. This creates the advantage that the rain garden does not allow mosquitoes to breed. Vegetated roadside swales, now promoted as “bioswales”, remain the conventional drainage system in many parts of the world. Rain gardens are at times confused with bioswales. Swales slope to a destination, while rain gardens do not. However, a bioswale may end with a rain garden. Rain gardens are also referred to as Water Sensitive Urban Design (WSUD) in Australia and Low Impact Development (LID) in the United States. They allow new construction to deal with excessive rainwater runoff without burdening the public stormwater systems.
Native plants and cultivars of native plants are recommended for rain gardens because they generally do not require fertilizer and are more tolerant of the local climate, soil, and water conditions. Native plant root systems enhance infiltration, maintain soil permeability, provide moisture redistribution, and sustain diverse microbial populations. Also, through the process of transpiration, rain garden plants return water vapor to the atmosphere, which accelerates drying the soil between storms.
Benefits
Rain gardens are beneficial for many reasons. They:
- improve water quality by filtering water in the garden soil,
- provide localized flood control,
- are aesthetically pleasing,
- provide interesting planting opportunities,
- encourage wildlife and biodiversity,
- tie together buildings and their surrounding environments in attractive and environmentally advantageous ways,
- provide significant solutions to environmental and ground water problems.
Design
Rain gardens are designed to capture the initial flow of stormwater and reduce the accumulation of toxins flowing directly into natural waterways through ground filtration. Most rain gardens are designed to be an endpoint of drainage with a capacity to percolate all incoming water through a series of soil or gravel layers beneath the surface plantings. A French drain may be used to direct a portion of the rainwater to an overflow location during heavier rainfall events.
Soil and Drainage Improvement
When a proposed garden’s soil is not permeable enough to allow water to drain and filter properly, the soil should be replaced and an under-drain installed. The ideal rain garden soil should typically contain 60% sand, 20% organic material, and 20% topsoil. There is a current trend to use biochar or compost as the organic component of soil. Combine the sandy soil mixture with a surrounding soil if it also has high sand content.
The more polluted the water, the longer it must be retained in the soil for purification. This is often achieved by building several smaller rain garden basins in a row with soil that is deeper than the seasonal high-water table.
Dealing with Pollutants
A rain garden requires an area where water can collect and infiltrate the soil, and then use plants to absorb the pollutants, maintain infiltration rates, diversify microbe communities, and increase the soil’s water holding capacity. This includes any adjacent trees or plants extending roots into the rain garden area.
The primary challenge of rain garden design focuses on determining the types of pollutants flowing into the garden and the acceptable loads of pollutants the rain garden's filtration system can handle during storm-water events. This challenge is specifically acute when a rain event occurs after a long dry period. The initial stormwater is often highly contaminated with the accumulated pollutants.
Nitrogen and phosphorus levels and overall sediment loads in the stormwater are reduced by the action the plants and microorganisms have on the water. Multiple rain gardens over an area will have a positive cumulative effect on both the volume and quality of groundwater leaving the site.
The alternative to a rain garden is simply adjusting the landscape so that downspouts and paved surfaces drain into existing gardens. That may be all that is needed if the soil has been well loosened and plants are well established. However, many plants do not tolerate saturated roots for long and often more water runs off a roof than a garden can handle. Often the required location and storage capacity of the garden area must be determined first. If it is not sufficient in area, a rain garden should be constructed in place of the conventional garden.
Types of Rain Gardens
The two basic types of rain gardens are under-drained and self-contained. Both types are used to improve groundwater quality, reduce runoff volumes and generally facilitate infiltration of cleaned water. Which type of garden is selected to be built has to be a balance of volumes of water to be treated, existing soil conditions, available space, and the budget Add Ethan's graphic for the project.
In some cases, infiltration is not desired. For example,
- if the bottom of the garden has less than 4 feet (1.2 m) of clearance to the seasonal mean high-water table.
- if the adjacent soils are contaminated and the cleaned water from the garden would become re-contaminated by coming in contact with the adjacent native soils.
In these cases, the under-drain system can be used to move excess water into a conventional storm sewer pipe system.
Drainage Rates
Rain gardens are designed to be drained within 4 hours after a 1 inch (2.5 cm) rain event. Under-drained rain gardens typically are designed to drain within 2 hours of the design storm event. This is achieved through the use of highly porous planting media and under-drains which are pervious pipes that carry the cleaned rainwater away from the garden. As a result, the plants selected for the rain garden need to be able to withstand both the extremes of flooding and drought. Plants on the upper edges of the garden are often tolerant of drought in their cultural requirements with plants lower in the garden being more adapted to floodplain conditions. Many riparian edge species are particularly well suited to the extreme environments of rain gardens.
Rain gardens with no under-drain typically hold moisture longer, particularly in the lower areas of the garden. Plants selected for this garden should be able to tolerate inundation for a more extended period of time. However, as in the case of the under-drained rain garden, the surface is drained within four hours, although the soil may be saturated. As with the bioretention cell, soils of the rain garden must be amended with a very porous soil. The depth of the soil should be a minimum of 8” deep (20 cm) and ideally to a depth of 2’- 3’ (60 – 90 cm). The lower the amount of soil amendment added when the garden is built, the more necessary it is to have plants adapted to prolonged periods of wetness. As with the under-drained rain garden, the plants on the upper edges of the garden will need to be more drought-tolerant in their cultural requirements than the plants in the lower areas.
In both types of gardens, the ground is excavated and the planting media is imported to the site. The imported planting media should be clean and weed seed free. A liner may or may not be used, depending on the design, water retention requirements, and local soil conditions, as mentioned above.
Plant Selection
Plants selected for use in a rain garden should tolerate both saturated and dry soil. Using native plants and cultivars of native plants is generally encouraged. This way the rain garden may contribute to urban habitats for native butterflies, birds, and beneficial insects. Well planned plantings require minimal maintenance to survive and are compatible with adjacent land use.
Trees generally contribute most when located close enough to tap moisture from the rain garden, yet do not excessively shade the garden. Some shading of open surface waters can be desired to keep the water temperature cool. Plants will not tolerate inundation with warm water because the heat drives out the dissolved oxygen. This means a plant tolerant of early spring, cold water flooding may not survive a summer inundation of warm water.
Adding Plants to the Garden
In both the under-drained and self-contained rain gardens, the success of the garden is greater when starting with healthy and smaller plants, rather than larger plants. Some plants are successful in rain gardens only when they are installed small and have a chance to adapt to the local conditions as they grow.
Plants and trees with deep fibrous roots tend to have a competitive advantage in a rain garden and provide the most cleaning and filtration benefits to the environment. Typical rain gardens are populated with natives or native cultivars because they are well adapted to a locality, but other ornamental plants that are non-invasive but able to grow in the garden conditions can also be excellent choices.
Successful Rain Gardens
Most of the examples of successful rain gardens are populated with herbaceous perennials, woody shrubs, or trees. This does not mean that annuals are not a possible choice for such gardens; rather it means that the successful gardens have been designed for habitat and low maintenance goals rather than purely seasonal aesthetics and color effects. Some annuals are good candidates for a higher maintenance version of a rain garden.
Infiltration Trenches
Infiltration trenches, also called percolation trenches, are linear ditches that collect rainwater from adjacent surfaces. Unlike the common roadside ditch, these trenches contain highly permeable soils that allow the water to quickly seep into the ground. An infiltration trench is similar in concept to a dry well, which is typically an excavated hole filled with gravel. Another similar drainage structure is a French drain, which directs water away from a building foundation, but is usually not designed to improve water quality.
Purpose
The primary purpose of an infiltration trench is to treat stormwater quality. As rain falls on impervious surfaces, it flows downhill across the surface of a street, sidewalk, or parking lot collecting pollutants present on the surface. Infiltration trenches are dug in areas where they can intercept this surface flow. Because they are linear ditches, they are very practical to install parallel to roadsides, or around the perimeter of parking lots.
The primary benefits of the infiltration trench are basic water quality treatment, reduction of peak flows in sewer systems, and groundwater recharge. Also, since the peak flow across the surface is reduced, the amount of surface water entering waterways is slowed down and this reduces channel erosion.
Infiltration trenches also reduce the amount of stormwater that would enter the storm drainage system and ultimately go to a body of water without any treatment. The trenches allow water treatment by storing the water in the soil, which acts as an underground reservoir, until it can percolate down and recharge the water table.
Infiltration trenches should not be used near farms or industrial complexes because the chemicals usually found in the runoff water could contaminate the groundwater. Also, the pollutants that sit on industrial and agricultural surfaces are more toxic than those on commercial or residential areas. These chemicals require special treatment in the soil before entering the nearest water body. The special treatments are offered by the construction of bioswales, rain gardens, or bioretention basins. The worst cases could be handled with a phytoremediation plan.
Trench Design
Elevated embankments are usually included on the sides of the trench to allow the water to pool, but not overflow. Once the water is collected, it begins seeping into the highly porous sand in the trench. The soil particles act like a sieve, preventing larger particles from moving downward with the water. Simultaneously, microorganisms in the soil digest the organic pollutants. Proximity to trees will also enrich the soil microorganisms and increase water absorption.
There are a number of important things to consider when installing a sand infiltration trench. Filtration systems such as this are prone to clogging with sediments suspended in the stormwater. They are effective at treating stormwater only if the soil has sufficient porosity. To function properly, a trench must be designed with a pretreatment structure such as a grass channel or swale that will capture some of the particles likely to clog the trench.
A second option is to include a small pond or some other type of miniature sedimentation tank that will store the water runoff long enough to let particles sink to the bottom before it flows to the infiltration trench. The soil in the drainage area should not have large quantities of fine particles like silt and clay because they are likely to cause clogging in the filter. Even though the first few feet of soil is engineered to allow water to flow through quickly, the soil type beyond the trench needs to allow water to travel at a rate of at least ½ inch/hour.
Sometimes the water flows through the loose soil into groundwater so quickly that pollutants are not removed. For this reason, infiltration trenches should not be constructed anywhere near public or private wells. Also, these trenches should not be placed too close to buildings as excess groundwater can cause basement flooding.
Infiltration trenches, also called percolation trenches, are linear ditches that collect rainwater from adjacent surfaces. Unlike the common roadside ditch, these trenches contain highly permeable soils that allow the water to quickly seep into the ground. An infiltration trench is similar in concept to a dry well, which is typically an excavated hole filled with gravel. Another similar drainage structure is a French drain, which directs water away from a building foundation, but is usually not designed to improve water quality.
Purpose
The primary purpose of an infiltration trench is to treat stormwater quality. As rain falls on impervious surfaces, it flows downhill across the surface of a street, sidewalk, or parking lot collecting pollutants present on the surface. Infiltration trenches are dug in areas where they can intercept this surface flow. Because they are linear ditches, they are very practical to install parallel to roadsides, or around the perimeter of parking lots.
The primary benefits of the infiltration trench are basic water quality treatment, reduction of peak flows in sewer systems, and groundwater recharge. Also, since the peak flow across the surface is reduced, the amount of surface water entering waterways is slowed down and this reduces channel erosion.
Infiltration trenches also reduce the amount of stormwater that would enter the storm drainage system and ultimately go to a body of water without any treatment. The trenches allow water treatment by storing the water in the soil, which acts as an underground reservoir, until it can percolate down and recharge the water table.
Infiltration trenches should not be used near farms or industrial complexes because the chemicals usually found in the runoff water could contaminate the groundwater. Also, the pollutants that sit on industrial and agricultural surfaces are more toxic than those on commercial or residential areas. These chemicals require special treatment in the soil before entering the nearest water body. The special treatments are offered by the construction of bioswales, rain gardens, or bioretention basins. The worst cases could be handled with a phytoremediation plan.
Trench Design
Elevated embankments are usually included on the sides of the trench to allow the water to pool, but not overflow. Once the water is collected, it begins seeping into the highly porous sand in the trench. The soil particles act like a sieve, preventing larger particles from moving downward with the water. Simultaneously, microorganisms in the soil digest the organic pollutants. Proximity to trees will also enrich the soil microorganisms and increase water absorption.
There are a number of important things to consider when installing a sand infiltration trench. Filtration systems such as this are prone to clogging with sediments suspended in the stormwater. They are effective at treating stormwater only if the soil has sufficient porosity. To function properly, a trench must be designed with a pretreatment structure such as a grass channel or swale that will capture some of the particles likely to clog the trench.
A second option is to include a small pond or some other type of miniature sedimentation tank that will store the water runoff long enough to let particles sink to the bottom before it flows to the infiltration trench. The soil in the drainage area should not have large quantities of fine particles like silt and clay because they are likely to cause clogging in the filter. Even though the first few feet of soil is engineered to allow water to flow through quickly, the soil type beyond the trench needs to allow water to travel at a rate of at least ½ inch/hour.
Sometimes the water flows through the loose soil into groundwater so quickly that pollutants are not removed. For this reason, infiltration trenches should not be constructed anywhere near public or private wells. Also, these trenches should not be placed too close to buildings as excess groundwater can cause basement flooding.
Phytoremediation
Trees are very good at removing and using nitrates, phosphates, and other nutrients and contaminates such as heavy metals, pesticides, solvents, oils, and hydrocarbons from the soil and water. This process is called phytoremediation. A single maple growing along a road can remove 60 mg of cadmium, 140 mg of chromium, 820 mg of nickel, and 5200 mg of lead in a single growing season. All these pollutants are stored in the tree’s wood.
When it rains, pure water hits the ground and flows toward a lower elevation. As it flows the water molecules pick up pieces of dirt and pollutants which then contaminate the water quality. The pollutant loading found in urban runoff can have a detrimental effect on water quality and stormwater runoff. Pollutant effects can include oxygen depletion, eutrophication (the enrichment of an ecosystem with chemical compounds containing nitrogen, phosphorus, or both), and toxicity. The impact of phosphorus and nitrogen on water quality is of particular concern, because nutrients in runoff can cause eutrophication, deplete dissolved oxygen levels, and increase turbidity in the receiving waters.
Nutrients in stormwater runoff can be contributed by fertilizers, atmospheric deposition, soil erosion, animal wastes, and detergents. Phosphorus can exist as both dissolved and particulate forms in runoff and include organic and inorganic components. Nitrogen can exist in both organic and inorganic forms such as ammonia, nitrate, and nitrite. Heavy metals such as copper, lead, zinc, and cadmium are also carried in stormwater runoff and can accumulate in aquatic systems because they cannot be broken down into less toxic forms. Sources of these heavy metals are present almost everywhere and include car brake pads, building siding and roofs, tires, and atmospheric deposition.
Phytotechnology
The discussion on phytoremediation must begin with a review of phytotechnology. Phytotechnology is a broad term that focuses on prevention of ecological problems before they actually occur. Phytotechnology includes the construction of wetlands, bioswales, rain gardens, bioretention basins, and the use of phytoremediation. Phytoremediation is defined as the use of vegetation and their associated microorganisms to remediate or prevent contaminants in soils, sediments, and groundwater.
Phytoremediation
Phytoremediation is the use of living green plants for on-site removal of pollutants from contaminated soil, water, sediments, and air. Phytoremediation was formally established in the 1980’s and 1990’s when a large number of plants were selected for their potential to hyperaccumulate metals in their tissues. (A hyperaccumulator plant is one that can absorb toxins to a greater concentration than the soil in which it is growing.) The research at that time did not apply the studies to actual testing in the field and as a result, the plantings to capture metal pollutants were not successful and confusion about what is possible for quality phytoremediation still exists today. However, projects to mitigate organic contaminants such as fuel and solvent spills have been successful. Poplars (Populus spp.) for example have been very successful in bioswales to mitigate gas and oil spills at gas stations and refinery sites. Other trees have been used to stop plumes of dry-cleaning solvents at the appropriate locations.
Plants have also been successful in filtering pollutants from stormwater runoff in bioswales and bioretention basins. Phytoremediation was originally supposed to use specially selected plants to eliminate contaminants such as metals, pesticides, explosives, oil, excess nutrients, and pathogens from soil and water. Risk reduction was to be through a process of removal, degradation, or containment of a pollutant or a combination of any of these processes and the contaminated plant was destroyed and the pollutant recovered. Phytoremediation was identified as a more cost effective and publicly acceptable method of removing environmental contaminants than other methods. However, often the laboratory results could not be replicated in the field and phytoremediation was given a bad reputation.
Phytoremediation has since become refined to dealing with rainwater runoff and it is now recognized as an energy efficient, aesthetically pleasing method of remediating sites with low to moderate levels of contamination and it can be used in conjunction with other remedial methods as a finishing step to the remedial process.
One of the main advantages of phytoremediation is its relatively low cost compared to other remedial methods such as excavation and chemical processing. In many cases phytoremediation has been found to be less than half the price of alternative methods. Phytoremediation also offers a permanent on site remediation. However, phytoremediation is a process which is dependent on the depth of the roots and the tolerance of the plant to the pollutant.
Phytoremediation Technology
The best way to decide if phytotechnology systems may be applicable for dealing with pollutants is to identify the exact contaminant and then select the best means to deal with it. For example, both nitrogen (N) and phosphorus (P) are macronutrients needed for plant growth and are components of all complete fertilizers. Excess fertilizer application to residential, commercial, and municipal lawns and landscapes becomes a major source of pollution with the potential for reduction via phytoremediation. Runoff from urban and suburban areas continues to increase as more land is developed and more hardscape areas are installed.
The heavy metals copper (Cu) and zinc (Zn) are washed off houses and vehicles as well as being components of fertilizer. They are micronutrients for plants and accumulate in the plant tissue. Other metals such as cadmium (Cd) and lead (Pb) accumulate in the plant roots since they are not as mobile.
Environmental Value
Ideally, if commonly used landscape trees, shrubs, and herbaceous plants could be used for stormwater phytoremediation, our plants would have an additional environmental value. It is therefore important to screen commonly available landscape trees for their potential use in these systems. Furthermore, if the pollutants can be degraded, broken down into smaller, less toxic components that plants and microorganisms can process, then phytotechnology systems can be an ideal remediation option where the pollutant is degraded and disappears, and there is no need to harvest the plants.
Beyond the scope of this article are situations where scientists are using phytoremediation to clean up hazardous waste sites and brownfields. In these cases specific plants are used to absorb specific heavy metals. A complete list of hyperaccumulators (trees and shrubs that absorb heavy metals) are indicated at the end of this Section. The hyperaccumulators are then harvested and the metals are extracted and recovered, removed, or recycled. However, this effort is extremely difficult and has only moderate success at this time.
Pros & Cons
The low cost of phytoremediation (up to 1000 times cheaper than excavation and reburial) is the main advantage of phytoremediation. However many of the pros and cons depend on the location of the polluted site, the contaminants in question, and the application of phytoremediation.
When compared to other more traditional methods of environmental remediation it becomes clear what the detailed advantages and disadvantages of phytoremediation actually are. In general, the following pros and cons would apply.
Advantages:
Disadvantages:
Tree Selection
Pollutant availability and uptake by plants is very much dependent upon the mycorrhizal fungi which form a symbiotic relationship with plant roots. The soil’s mycorrhizae help transfer nutrients and metals to the plant roots. These mycorrhizae are located in the root’s rhizosphere where they will biologically transform pollutants into less toxic forms through enzymatic detoxification, thus making them available for plant uptake via the mycorrhizae and root association.
Phytoremediation research with woody trees and shrubs is ongoing with the poplars, mentioned above, and willows (Salix sp.) having been identified as a significant accumulator of pollutants. Willows provide a significant contribution of heavy metal uptake in their leaves, with concentrations highest in the autumn. Willows are also able to tolerate high metal concentrations in their biomass through the use of sulfur-rich proteins.
Identifying other trees and shrubs that can be used for phytoremediation are increasing the value of landscape plants, and are an additional marketing tool available to nurseries. New public stormwater runoff systems incorporate trees into the systems that are being designed for streetscapes and landscapes. Incorporation of these plants into these new landscapes are intended to improve the runoff water and ground water quality. Many of these landscape plants might be appropriate to use in stormwater treatment systems.
The majority of plants currently used in phytoremediation applications are called hyperaccumulators. They readily absorb heavy metals and other pollutants and are used in stormwater ponds, riparian buffers, rain gardens, green roofs, constructed wetlands, and bioretention projects, and all the other methods for dealing with stormwater that have been mentioned in this article. They are usually herbaceous or non-woody.
A typical plant may accumulate about 100 parts per million (ppm) zinc and 1 ppm cadmium. A normal plant can be poisoned with as little as 1,000 ppm of zinc or 20 to 50 ppm of cadmium in its shoots. Hyperaccumulators can accumulate up to 30,000 ppm zinc and 1,500 ppm cadmium in its shoots, while exhibiting few or no toxicity symptoms.
Hyperaccumulator Plants Suitable for Phytoremediation
Herbaceous Plants suitable for phytoremediation are:
thale cress (Arabidopsis thaliana)
hairy goldenrod (Solidago canadensis)
alpine pennycress (Thlaspi caerulescens)
violets (Viola spp.)
Shrubs (and their cultivars) suitable for phytoremediation are:
abelia (Abelia x grandiflora)
hydrangea (Hydrangea spp.
inkberry (Ilex glabra)
anise (Illicium floridanum)
cherry laurel (Prunus laurocerasus)
Paul’s scarlet rose (Rosa spp.)
Scarlet Curls willow (Salix x ‘Scarlet Curls’)
vitex (Vitex agnus-castus)
Trees (and their cultivars) suitable for phytoremediation are:
amur maple (Acer ginnala)
red maple (Acer rubrum)
honeylocust (Gleditsia triacanthos)
Little Gem magnolia (Magnolia grandiflora ‘Little Gem’)
aspen (Populus tremula)
corkscrew willow (Salix matsudana ‘Tortuosa’)
For an up-to-date and complete list of hyperaccumulators visit Wikipedia. Their list is according to the pollutants that each plant absorbs.
Trees are very good at removing and using nitrates, phosphates, and other nutrients and contaminates such as heavy metals, pesticides, solvents, oils, and hydrocarbons from the soil and water. This process is called phytoremediation. A single maple growing along a road can remove 60 mg of cadmium, 140 mg of chromium, 820 mg of nickel, and 5200 mg of lead in a single growing season. All these pollutants are stored in the tree’s wood.
When it rains, pure water hits the ground and flows toward a lower elevation. As it flows the water molecules pick up pieces of dirt and pollutants which then contaminate the water quality. The pollutant loading found in urban runoff can have a detrimental effect on water quality and stormwater runoff. Pollutant effects can include oxygen depletion, eutrophication (the enrichment of an ecosystem with chemical compounds containing nitrogen, phosphorus, or both), and toxicity. The impact of phosphorus and nitrogen on water quality is of particular concern, because nutrients in runoff can cause eutrophication, deplete dissolved oxygen levels, and increase turbidity in the receiving waters.
Nutrients in stormwater runoff can be contributed by fertilizers, atmospheric deposition, soil erosion, animal wastes, and detergents. Phosphorus can exist as both dissolved and particulate forms in runoff and include organic and inorganic components. Nitrogen can exist in both organic and inorganic forms such as ammonia, nitrate, and nitrite. Heavy metals such as copper, lead, zinc, and cadmium are also carried in stormwater runoff and can accumulate in aquatic systems because they cannot be broken down into less toxic forms. Sources of these heavy metals are present almost everywhere and include car brake pads, building siding and roofs, tires, and atmospheric deposition.
Phytotechnology
The discussion on phytoremediation must begin with a review of phytotechnology. Phytotechnology is a broad term that focuses on prevention of ecological problems before they actually occur. Phytotechnology includes the construction of wetlands, bioswales, rain gardens, bioretention basins, and the use of phytoremediation. Phytoremediation is defined as the use of vegetation and their associated microorganisms to remediate or prevent contaminants in soils, sediments, and groundwater.
Phytoremediation
Phytoremediation is the use of living green plants for on-site removal of pollutants from contaminated soil, water, sediments, and air. Phytoremediation was formally established in the 1980’s and 1990’s when a large number of plants were selected for their potential to hyperaccumulate metals in their tissues. (A hyperaccumulator plant is one that can absorb toxins to a greater concentration than the soil in which it is growing.) The research at that time did not apply the studies to actual testing in the field and as a result, the plantings to capture metal pollutants were not successful and confusion about what is possible for quality phytoremediation still exists today. However, projects to mitigate organic contaminants such as fuel and solvent spills have been successful. Poplars (Populus spp.) for example have been very successful in bioswales to mitigate gas and oil spills at gas stations and refinery sites. Other trees have been used to stop plumes of dry-cleaning solvents at the appropriate locations.
Plants have also been successful in filtering pollutants from stormwater runoff in bioswales and bioretention basins. Phytoremediation was originally supposed to use specially selected plants to eliminate contaminants such as metals, pesticides, explosives, oil, excess nutrients, and pathogens from soil and water. Risk reduction was to be through a process of removal, degradation, or containment of a pollutant or a combination of any of these processes and the contaminated plant was destroyed and the pollutant recovered. Phytoremediation was identified as a more cost effective and publicly acceptable method of removing environmental contaminants than other methods. However, often the laboratory results could not be replicated in the field and phytoremediation was given a bad reputation.
Phytoremediation has since become refined to dealing with rainwater runoff and it is now recognized as an energy efficient, aesthetically pleasing method of remediating sites with low to moderate levels of contamination and it can be used in conjunction with other remedial methods as a finishing step to the remedial process.
One of the main advantages of phytoremediation is its relatively low cost compared to other remedial methods such as excavation and chemical processing. In many cases phytoremediation has been found to be less than half the price of alternative methods. Phytoremediation also offers a permanent on site remediation. However, phytoremediation is a process which is dependent on the depth of the roots and the tolerance of the plant to the pollutant.
Phytoremediation Technology
The best way to decide if phytotechnology systems may be applicable for dealing with pollutants is to identify the exact contaminant and then select the best means to deal with it. For example, both nitrogen (N) and phosphorus (P) are macronutrients needed for plant growth and are components of all complete fertilizers. Excess fertilizer application to residential, commercial, and municipal lawns and landscapes becomes a major source of pollution with the potential for reduction via phytoremediation. Runoff from urban and suburban areas continues to increase as more land is developed and more hardscape areas are installed.
The heavy metals copper (Cu) and zinc (Zn) are washed off houses and vehicles as well as being components of fertilizer. They are micronutrients for plants and accumulate in the plant tissue. Other metals such as cadmium (Cd) and lead (Pb) accumulate in the plant roots since they are not as mobile.
Environmental Value
Ideally, if commonly used landscape trees, shrubs, and herbaceous plants could be used for stormwater phytoremediation, our plants would have an additional environmental value. It is therefore important to screen commonly available landscape trees for their potential use in these systems. Furthermore, if the pollutants can be degraded, broken down into smaller, less toxic components that plants and microorganisms can process, then phytotechnology systems can be an ideal remediation option where the pollutant is degraded and disappears, and there is no need to harvest the plants.
Beyond the scope of this article are situations where scientists are using phytoremediation to clean up hazardous waste sites and brownfields. In these cases specific plants are used to absorb specific heavy metals. A complete list of hyperaccumulators (trees and shrubs that absorb heavy metals) are indicated at the end of this Section. The hyperaccumulators are then harvested and the metals are extracted and recovered, removed, or recycled. However, this effort is extremely difficult and has only moderate success at this time.
Pros & Cons
The low cost of phytoremediation (up to 1000 times cheaper than excavation and reburial) is the main advantage of phytoremediation. However many of the pros and cons depend on the location of the polluted site, the contaminants in question, and the application of phytoremediation.
When compared to other more traditional methods of environmental remediation it becomes clear what the detailed advantages and disadvantages of phytoremediation actually are. In general, the following pros and cons would apply.
Advantages:
- It is more economically viable only when using the same tools and supplies as agriculture.
- It is less disruptive to the environment and does not involve waiting for new plant communities to recolonize the site.
- Disposal sites are not needed.
- It is more likely to be accepted by the public and it is more aesthetically pleasing.
- It avoids excavation and transport of polluted material.
- It can treat sites polluted with more than one type of pollutant.
Disadvantages:
- It is dependent on the climate, geology, altitude, and temperature requirements of the plant.
- Large scale operations require access to agricultural equipment and knowledge.
- Success is dependent on the tolerance of the plant to the pollutant.
- Some contaminants collected in plant leaves may be released back into the environment in autumn.
- Other contaminants may be collected in woody tissues and used as fuel.
- Time taken to remediate sites far exceeds that of other technologies.
- Pollutant solubility may result in increased environmental damage and leaching.
Tree Selection
Pollutant availability and uptake by plants is very much dependent upon the mycorrhizal fungi which form a symbiotic relationship with plant roots. The soil’s mycorrhizae help transfer nutrients and metals to the plant roots. These mycorrhizae are located in the root’s rhizosphere where they will biologically transform pollutants into less toxic forms through enzymatic detoxification, thus making them available for plant uptake via the mycorrhizae and root association.
Phytoremediation research with woody trees and shrubs is ongoing with the poplars, mentioned above, and willows (Salix sp.) having been identified as a significant accumulator of pollutants. Willows provide a significant contribution of heavy metal uptake in their leaves, with concentrations highest in the autumn. Willows are also able to tolerate high metal concentrations in their biomass through the use of sulfur-rich proteins.
Identifying other trees and shrubs that can be used for phytoremediation are increasing the value of landscape plants, and are an additional marketing tool available to nurseries. New public stormwater runoff systems incorporate trees into the systems that are being designed for streetscapes and landscapes. Incorporation of these plants into these new landscapes are intended to improve the runoff water and ground water quality. Many of these landscape plants might be appropriate to use in stormwater treatment systems.
The majority of plants currently used in phytoremediation applications are called hyperaccumulators. They readily absorb heavy metals and other pollutants and are used in stormwater ponds, riparian buffers, rain gardens, green roofs, constructed wetlands, and bioretention projects, and all the other methods for dealing with stormwater that have been mentioned in this article. They are usually herbaceous or non-woody.
A typical plant may accumulate about 100 parts per million (ppm) zinc and 1 ppm cadmium. A normal plant can be poisoned with as little as 1,000 ppm of zinc or 20 to 50 ppm of cadmium in its shoots. Hyperaccumulators can accumulate up to 30,000 ppm zinc and 1,500 ppm cadmium in its shoots, while exhibiting few or no toxicity symptoms.
Hyperaccumulator Plants Suitable for Phytoremediation
Herbaceous Plants suitable for phytoremediation are:
thale cress (Arabidopsis thaliana)
hairy goldenrod (Solidago canadensis)
alpine pennycress (Thlaspi caerulescens)
violets (Viola spp.)
Shrubs (and their cultivars) suitable for phytoremediation are:
abelia (Abelia x grandiflora)
hydrangea (Hydrangea spp.
inkberry (Ilex glabra)
anise (Illicium floridanum)
cherry laurel (Prunus laurocerasus)
Paul’s scarlet rose (Rosa spp.)
Scarlet Curls willow (Salix x ‘Scarlet Curls’)
vitex (Vitex agnus-castus)
Trees (and their cultivars) suitable for phytoremediation are:
amur maple (Acer ginnala)
red maple (Acer rubrum)
honeylocust (Gleditsia triacanthos)
Little Gem magnolia (Magnolia grandiflora ‘Little Gem’)
aspen (Populus tremula)
corkscrew willow (Salix matsudana ‘Tortuosa’)
For an up-to-date and complete list of hyperaccumulators visit Wikipedia. Their list is according to the pollutants that each plant absorbs.
Storm Risk Management Plan
Of all the rain storms that occur in any area, 99% of them can be handled with rain gardens, bioswales, and other forms of rainwater runoff systems that will allow the water to soak into the soil and not cause any serious problems. Then along comes that 1% storm that causes flooding or tree damage that overwhelms all the efforts to minimize storm damage.
When this happens, a storm risk management plan should be in place that will provide guidance in dealing with excess water and tree damage. Before any storms come your way, a risk management plan should be prepared and available to address mitigation and prevention strategies, preparedness planning, and warning systems that predict the potential of a disaster. Implementing effective, efficient, equitable methods to assess the severity of tree damage and estimating the amount of tree debris is an important component of the response and recovery process.
Before The Storm
A report should be prepared that describes observations, an inventory of the plants and trees, equipment, and labor availability and anything else needed for dealing with a storm emergency on all the local properties. This report should provide the necessary information to develop a base plan before the storm.
Tree Damage Assessment
Tree damage assessment and debris management are some of the most challenging events following a major storm. Assessing tree damage requires trained people to evaluate the damage, determine corrective actions, and estimate woody debris volumes. A plan should be prepared to evaluate the debris estimation and approaches to be considered in dealing with the cleanup process. The plan will vary in complexity, the time needed to implement, and the required skill levels of the people involved in the process.
If the roads are impassible because of the storm damage and debris, other options to evaluate the extent of storm damage must be considered. Remote sensing techniques using aerial sketch mapping, airborne videography, aerial photography, drone surveillance, and satellite images are several ways to estimate damage following the storm. These approaches can be used to estimate damage at regional, local, and individual tree levels. Estimates are based on the percent reduction in canopy cover, changes in the tree stand composition, difference in vegetation vigor, basal area change, and comparison of pre- and post-storm inventories, and formulas. A rapid estimate within one day and ideally within 12 hours after the storm has ended is important for emergency planners to rate recovery needs and mobilize fiscal, human, and equipment resources as necessary.
Damage Assessment
Rapid assessment approaches support quick evaluation to determine if state and federal disaster declarations are appropriate. Estimates of debris volumes following the storm can be generated using a street-segment based approach. This approach can be performed either by collecting a statistical sample in the field or estimating with i-Tree, using the Storm Damage Assessment Protocol. Ideally, the i-Tree approach uses pre-storm sample plots to predict tree debris. The United States Army Corps of Engineers has also generated storm tree debris estimates using methods adapted from hurricane tree debris estimation models.
Whether localized or widespread, damage to electric distribution systems, blocked roadways, and property damage from fallen trees and limbs pose significant safety concerns and disrupt normal community functions. The time to recover from storms may take longer in more rural areas, especially the repair of downed electrical systems.
Data Collection
Once damage assessment reports are received, any missing information will be sought. Cities are first contacted for clarification of data or asked directly for missing data. Further, missing city data can be located online through the official city website or other online means. Missing weather information can be located principally through collected data of the National Oceanic and Atmospheric Administration. Various other websites can be searched for data and clarification on specific storms. Additional data developed from the 30m resolution 2001 National Land Cover Database (NLCD) for tree canopy, population, and land area provide the urban forest data interface, from the USDA Northern Research Station. Debris eligible for FEMA reimbursement is that from the public right-of-way with some private debris collected when brought and dumped on the public right-of-way.
Sources
Of all the rain storms that occur in any area, 99% of them can be handled with rain gardens, bioswales, and other forms of rainwater runoff systems that will allow the water to soak into the soil and not cause any serious problems. Then along comes that 1% storm that causes flooding or tree damage that overwhelms all the efforts to minimize storm damage.
When this happens, a storm risk management plan should be in place that will provide guidance in dealing with excess water and tree damage. Before any storms come your way, a risk management plan should be prepared and available to address mitigation and prevention strategies, preparedness planning, and warning systems that predict the potential of a disaster. Implementing effective, efficient, equitable methods to assess the severity of tree damage and estimating the amount of tree debris is an important component of the response and recovery process.
Before The Storm
A report should be prepared that describes observations, an inventory of the plants and trees, equipment, and labor availability and anything else needed for dealing with a storm emergency on all the local properties. This report should provide the necessary information to develop a base plan before the storm.
Tree Damage Assessment
Tree damage assessment and debris management are some of the most challenging events following a major storm. Assessing tree damage requires trained people to evaluate the damage, determine corrective actions, and estimate woody debris volumes. A plan should be prepared to evaluate the debris estimation and approaches to be considered in dealing with the cleanup process. The plan will vary in complexity, the time needed to implement, and the required skill levels of the people involved in the process.
If the roads are impassible because of the storm damage and debris, other options to evaluate the extent of storm damage must be considered. Remote sensing techniques using aerial sketch mapping, airborne videography, aerial photography, drone surveillance, and satellite images are several ways to estimate damage following the storm. These approaches can be used to estimate damage at regional, local, and individual tree levels. Estimates are based on the percent reduction in canopy cover, changes in the tree stand composition, difference in vegetation vigor, basal area change, and comparison of pre- and post-storm inventories, and formulas. A rapid estimate within one day and ideally within 12 hours after the storm has ended is important for emergency planners to rate recovery needs and mobilize fiscal, human, and equipment resources as necessary.
Damage Assessment
Rapid assessment approaches support quick evaluation to determine if state and federal disaster declarations are appropriate. Estimates of debris volumes following the storm can be generated using a street-segment based approach. This approach can be performed either by collecting a statistical sample in the field or estimating with i-Tree, using the Storm Damage Assessment Protocol. Ideally, the i-Tree approach uses pre-storm sample plots to predict tree debris. The United States Army Corps of Engineers has also generated storm tree debris estimates using methods adapted from hurricane tree debris estimation models.
Whether localized or widespread, damage to electric distribution systems, blocked roadways, and property damage from fallen trees and limbs pose significant safety concerns and disrupt normal community functions. The time to recover from storms may take longer in more rural areas, especially the repair of downed electrical systems.
Data Collection
Once damage assessment reports are received, any missing information will be sought. Cities are first contacted for clarification of data or asked directly for missing data. Further, missing city data can be located online through the official city website or other online means. Missing weather information can be located principally through collected data of the National Oceanic and Atmospheric Administration. Various other websites can be searched for data and clarification on specific storms. Additional data developed from the 30m resolution 2001 National Land Cover Database (NLCD) for tree canopy, population, and land area provide the urban forest data interface, from the USDA Northern Research Station. Debris eligible for FEMA reimbursement is that from the public right-of-way with some private debris collected when brought and dumped on the public right-of-way.
Sources
- Arthur, E.L. et al, “Phytoremediation – An overview”, Critical Reviews in Plant Sciences, 24:109-122, 2005.
- “Bioswales”, USDA Natural Resources Conservation Service, 2005.
- Coyman, Sandy; Keota Silaphone. "Rain Gardens in Maryland's Coastal Plain". p. 2. Retrieved 11 October 2011.
- Denman, L.; May, P.; and P. Breen, “An investigation of the potential to use street trees and their root zone soils to remove nitrogen from urban stormwater”, Australian Journal of Water Resources: 1 (3): 303-311, 2006.
- Dietz, Michael E.; Clausen, John C. "A Field Evaluation of Raingarden Flow and Pollutant Treatment". Water, Air, and Soil Pollution 167 (1–4): 123–138. doi:10.1007/s11270-005-8266-8, 2005.
- Dussaillant, A. Ph.D., et al, “Water Science & Technology”, Water Supply Journal, Vol. 5, pp. 173-179, 2005.
- Endreny, Ted, “Storm Water Management”, The SUNY College of Environmental Science and Forestry (ESF) Department of Environmental Resource Engineering, 2014.
- Environmental Protection Agency, “Stormwater Case Studies”, 2013.
- Environmental Protection Agency, “Water Sense”, Office of Wastewater Management, 2014.
- Fox, L.J., Struik, P.C., Appleton, B.L. and Rule, J.H. “Nitrogen phytoremediation by water hyacinths (EIchhornia crassipes)”, Water, Air, and Soil Pollution. DOI 10.1007/s11270-008-9708-x, 2008.
- Hauer, Richard J., Angela J. Hauer, Dudley R. Hartel, and Jill R. Johnson, “Rapid Assessment of Tree Debris Following Urban Forest Ice Storms”, Arboriculture & Urban Forestry, 37(5): September 2011.
- Henderson, C.F.K. “The Chemical and Biological Mechanisms of Nutrient Removal from Stormwater in Bioretention basins”. Thesis. Griffith School of Engineering, Griffith University. 2009.
- Hsieh, C., Davis, A., Needelman, B., “Bioretention column studies of phosphorus removal from urban stormwater runoff”, Water Environment Research, 79(2): 177-184, 2007.
- Kennon, Kate, “Pollutant Purging Plants”, BSLA Fieldbook, 2014.
- Kochian, Leon V., “Phytoremediation: Using Plants To Clean Up Soils”, USDA Agricultural Research Service, March 2014.
- Loehrlein, Marietta, “Bioswale”, Encyclopedia of Earth, 2011.
- Lucas, W. C.; Greenway, M., “Hydraulic response and nitrogen retention in bioretention mesocosms with regulated outlets: part II–nitrogen retention”, Water Environ Res. Aug;83(8):703-13, 2011.
- Lucas, W. C.; Greenway, M. “Nutrient Retention in Vegetated and Non-vegetated Bioretention Mesocosms”, Journal of Irrigation Drainage, E-ASCE, 134 (5): 613-623. 2008.
- Marritz, Leda, “Stormwater Quantity and Rate Control Benefits of Trees in Uncompacted Soil”, and related blogs, 2011-2014.
- Metropolitan Council, St. Paul, MN, "Minnesota Urban Small Sites Best Management Practice Manual –Infiltration Trenches", July 2001.
- “Rain Gardens”, University of Rhode Island, Healthy Landscapes Program, 2014.
- “Rain Gardens Made One Maryland Community Famous”, Wisconsin Natural Resources, February 2003.
- Read, J., Fletcher, T. D., Wevill, T., Deletic, A., “Plant Traits that Enhance Pollutant Removal from Stormwater in Biofiltration Systems”. International Journal of Phytoremediation, 12, 34–53. 2010.
- Read, J.; Wevill, T.; Fletcher, T. D.; Deletic, A., “Variation Among Plant Species in Pollutant Removal from Stormwater in Biofiltration Systems”, Water Resources, 42, 893–902. 2008.
- Ruby, Mindy and Dr. Bonnie Appleton, “Using Landscape Plants for Phytoremediation”, Hampton Roads Agricultural Research and Extension Center, Virginia Tech University, 2009.
- “Stormwater Management, Bioretention Systems”, Lake Superior Streams, 2005.
- Strassberg, Valerie and Brad Lancaster, "Fighting water with water: Behavioral change versus climate change", Environmental Issues, 103 (6): 59, Retrieved 29 March 2012.
- Tuncsiper, B.; Ayaz, S.C; Akca, L., “Modeling and Evaluation of Nitrogen Removal Performance on Subsurface Flow and Free Water Surface Constructed Wetlands”, Water Science Technology, 53 (12), 111-120, 2006.
- Upper Des Plaines River Ecosystem Partnership, “Bioswales”, 2012.
- U.S. Environmental Protection Agency, “Urban Runoff”, Nonpoint Source News-Notes, Washington, D.C., Issue #42, August/September 1995.
- Ward, Laura, “Benefits of Recycling Rainwater”, University of Missouri, 2009.
- Wolverton, B.C. Ph.D., R.C. McDonald-McCaleb, “Biotransformation of Priority Pollutants Using Biofilms and Vascular Plants”, Journal of the Mississippi Academy Of Sciences, Vol. XXXI, pp. 79-89, 1986.
The test that follows contains 80 questions. Before taking the test be sure you have read the article carefully. The passing grade is 80% on the entire test.
LA CES will award 6.0 PDH (HSW) credits for a passing grade. North Carolina Board of LA and New Jersey Board of Architects have approved this course for 6.0 credits.
The cost for taking this test is $20 per credit. If you purchase an annual subscription for 12 credits, the cost per credit is reduced by 50% (see Annual Subscription link below). We will report your passing test score to LA CES. If you are also ISA* certified we will report your passing score to ISA for no additional cost. Please be sure to add your ISA Cert. number when you sign in. Tests with passing scores may be submitted only once to each organization.
*ISA has approved this course for 4.0 CEUs which may be applied toward Certified Arborist, Municipal Specialist, or BCMA science credits.
To take the test by the pay per test option, click on the 'Pay Now' button below where you can send payment online securely with your credit card or Pay Pal account. After your payment is submitted, click on ‘Return to gibneyCE.com’. That will take you to the test sign in page followed by the test. If you are an ISA and/or CLARB member, please be sure to include your certification/member number(s) along with your LA license and ASLA numbers.
To take the test as an annual subscriber with reduced rates, click on Password and enter your test password which will take you to the test sign in page. If you would like to become a subscriber see our Annual Subscription page for details.
When you have finished answering all questions you will be prompted to click ‘next’ to send your answers to gibneyCE.com. You can then click ‘next’ to view your test summary. A test review of your answers is available upon request. You can spend as much time as you would like to take the test but it is important not to leave the test site until you have answered all the questions and see the 'sending your answers' response.
Test re-takes are allowed, however you will have to pay for the retake if you are using the pay per test option.
All passing test scores are sent from gibneyCE.com to your organization(s) at the end of every month and they will appear on your certification record 4 to 6 weeks after that.
LA CES maintains a record of earned PDH credits on their website http://laces.asla.org/
ISA maintains a record of earned CEU credits on their website http://www.isa-arbor.com/