Impacts of Stormwater

Sep 18, 2013

Impacts of Stormwater Management

Stormwater management ordinances that have been adopted by counties have been effective at controlling the rate of stormwater runoff, even as the construction of impervious surfaces is constantly creating larger and larger volumes of stormwater runoff. The ordinances in place now generally reduce the peak flow rate from a developed site, but have a more limited impact on the total volume of runoff.

The common method for Transporting stormwater is routing it through storm sewers and eventually out to a receiving body of water. Under this scenario, stormwater is not easily able to infiltrate through soil and recharge aquifers, which may lead to lower groundwater levels. The stormwater will also travel faster through the smoother surfaces of the storm sewers then compared to stormwater traveling over natural surfaces. As mentioned previously, urban stormwater runoff is known to contain high amounts of pollutants. These factors combine and may result in stream degradation and damage the ecosystem. A publication recently released by the USEPA entitled Urban Stormwater Management in the United States as part of the National Pollutant Discharge Elimination System (NPDES) addressed these specific issues. The publication discusses the current state of urban stormwater runoff and the affects on downstream bodies of water. The committee states "the creation of impervious surfaces that accompanies urbanization profoundly affects how water moves both above and below ground during and following storm events, the quality of that stormwater, and the ultimate condition of nearby rivers, lakes, and estuaries" (Committee on Reducing Stormwater Discharge Contributions to Water Pollution).

Although the current means of stormwater management have been successful, there are a number of strategies that can improve water quality and further reduce runoff volume. These strategies are detailed below. Their advantages are identified as well as any drawbacks or limitations.

Impervious Surface Limitation

The increase in urbanization brings with it additional impervious surfaces. Parking lots, sidewalks, rooftops and roadways are all examples of the impervious surfaces that are common in developed areas. These surfaces have limited infiltration capacity, causing all the precipitation on the surface to become runoff. As discussed earlier the water quality of stormwater runoff is very poor, especially in highly urbanized areas.

There are a number of ways to reduce the impervious area on site. Many of these specific techniques are discussed in the following sections of the paper. The effectiveness of these techniques are site specific and depend on a number of factors such as underlying soil type, local groundwater levels, rainfall intensity and current soil moisture.

Green Infrastructure

Green infrastructure can be used to describe a number of different practices. It can refer to site specific Best Management Practices (BMPs) that have environmentally friendly impacts. Green infrastructure can also refer to connecting open areas for wildlife to travel and to effectively convey runoff, as in the Green Infrastructure Vision. The following sections focus on types of Green Infrastructure BMPs and their effectiveness and any concerns regarding their implementation.

Permeable Pavements and Surfaces

Pervious concrete and asphalt can be used as an alternative to traditional methods. Pervious pavement and concrete are very similar to the traditional variety in their composition except they have more void space to allow the water to infiltrate. (Illinois Urban Manual: Practice Standard Permeable Pavement, 1999) Another alternative for replacing traditional pavement and concrete, are precast concrete blocks and plastic grids. The concrete blocks or pavers are placed above layers of porous materials like sand and gravel. The pavers are laid so there are gaps to further allow infiltration. The plastic grid can be laid over an already pervious material like grass and help provide structural support for driving and parking. All of these techniques increase the infiltration capacity of the surface.

The actual reduction in stormwater runoff depends on a number of factors including rainfall intensity and underlying soil conditions. Using traditional runoff calculations the average reduction in runoff volume can be estimated at 75%. Site specific calculations should be completed to properly quantify the potential reduction in stormwater volume. It is also worth noting that permeable pavements tend to perform best during low intensity storm events and may not have high reduction rates for large storm events.

There are a few drawbacks to using the pervious surfaces mentioned above. First, the current permeable mixtures do not always have the same structural capacity as the traditional variety. Pervious surfaces may be used for low traffic streets, sidewalks, driveways and parking lots but not loading docks, major highways or high traffic streets. The precast blocks or pavers may also work very well for driveways or sidewalks, especially because they are available in a variety of styles and shapes. However, if pervious materials are used for sidewalks or other pedestrian facilities, special consideration must be given to ensure that accessibility for the disabled is maintained. A plastic grid system is useful for temporary drives or emergency access drives. These applications all have their benefits but cannot supplant traditional paving materials used for high traffic streets and highways.

There are a few maintenance issues associated with permeable pavements that are unique to the permeable surfaces. The material located in the voids may be washed away over time and need to be replaced. An example of maintenance is the reapplication of sand or other permeable materials to the void space in the permeable pavers. That being said, impermeable surfaces also require regular maintenance such as repaving and resurfacing.The City of Chicago began its Green Alley Program in the summer of 2006 by installing permeable pavements in five alleys that were in need of repair. The alleys were re-graded so that the stormwater would properly drain into the sewer system. Other green improvements were made to the lighting and the color of the surface. The older street lights were replaced with "dark sky" street lamps which project the light directly to street level, only projecting light to the alley. Lighter colored surfaces reflect more sunlight back to the atmosphere, thereby lowering the temperature of the surface. Stormwater volume has been decreased though the program.

Figure 4: Before green alley was installed ...and after the green alley was installed.

In this case the alley is not only more aesthetically pleasing it also has green benefits.

Green Roofs

A green roof is a roof either above, below or at grade that is commonly covered with a carpet of plants. There are three different types of green roofs, intensive, extensive and semi-intensive. The main difference between the three types is the weight of the fully saturated media. For example an intensive roof is one that has more than 6" of growing medium depth and has a fully saturated weight of 50-300 lb/square foot. An extensive roof on the other hand has 6" or less of growing medium and only has a saturated weight of 10-35 lb/square foot. The type of surface chosen really depends on a few factors like the structural capacity of the roof and the available funds for the project (Peck and Bruce, 2007).

There are a number of layers involved in the green roof under the vegetation. These include the growing medium, drainage layer, roof barrier, insulation, root barrier, roofing membrane and structural support. Figure 5 is a cross section of a typical green roof. The exact configuration of a green roof will vary depending on the site specific conditions and the type of roof chosen.

Figure 5: Green Roof Cross Section (American Wick Drain Corp.)

Green roofs help to decrease stormwater runoff volume as well as increase the quality of the water. Rain will fall on the roofs and the plants will absorb the water and either Transpire back to the atmosphere or release the water at a much slower rate than if it had been a regular roof. A drainage system needs to be in place to allow the water that percolates through the green roof to be released off of the roof. The water that is released will have high water quality because not only is it rain water, it has also gone through the roof's plant system which helps to filter the water.

There are a few things to keep in mind when constructing a green roof. One that has already been mentioned is the structural capacity of the roof. Installing a green roof to the surface of an existing roof will add loading to the structure below. The majority of the loading will be "dead loading" due to the actual weight of the roof but it is also important to remember the live loads that will occur when installing the roof and performing maintenance. It is recommended that an analysis be preformed to determine exactly how much the existing structure can handle. Another important aspect to remember is insuring the integrity of your building by properly waterproofing the roof. This step will require contacting a roofer with green roof experience.

The intensity of a rainfall event will have an impact on the amount of water the green roof is able to collect. A low intensity storm event will allow the soil to absorb the precipitation and release it at a slower rate. The trouble arises during high intensity storm events when the roof cannot accommodate the precipitation. In this case, it is important to design the roof to carry the excess water safely off the roof.

Rain Barrels and Gardens

Rain Barrels

Rain barrels collect rainwater so the water can be reused for watering or other landscaping purposes. There are a number of organizations that provide rain barrels at a discounted price. MWRDGC has a program that supplies each resident of Cook County 2 rain barrels for $40 each. The benefit of rain barrels is that they hold water that would otherwise become stormwater runoff and allow it to be used later. When this water is later used, it will more than likely be infiltrated into the then dry soil or be transpired through the plant into the atmosphere.

Figure 6: Rain Barrel (City of Chicago Water Management)

A typical rain barrel is approximately 55 gallons which is approximately 7.35 ft3 of water. For a 1000 square foot regular roof during a 1" storm event, approximately 9.5 rain barrels would be necessary to collect all of the runoff. Although rain barrels do not make a significant impact during large storm events, they can greatly reduce the runoff from small seasonal events. They must be drained during the winter months, to prevent damage due to freezing. A typical rain barrel configuration can be seen in Figure 6.

Rain Cisterns are typically larger than the average rain barrel. They are available in a number of different sizes and shapes. At Chicago's Center for Green Technology there are four 3,000 gallon cisterns used to collect the rainwater and reuse it for landscaping purposes. The same concerns apply for cisterns as rain barrels. They also should be properly drained during the winter season.

Rain Gardens

Rain gardens are areas created in naturally wet areas to help slow runoff and increase infiltration. They are usually planned in areas where water naturally flows or is forced to flow. Examples of locations would at the downgrade of a downspout or a parking lot. Rain gardens can be a great way to reduce stormwater volume and improve the water quality. Rain gardens are typically constructed from natural plants that have deeper root systems to absorb the water and filter the pollutants. Pollutants that are commonly found on surfaces before a rain storm are called first flush pollutants. Rain gardens are especially effective at removing these pollutants.

Figure 7: Village of Bellwood Rain Gardens (CNT)

The Center for Neighborhood Technology (CNT) partnered with the Village of Bellwood to demonstrate the positive affects of a rain garden. Over 40% of the village is in the floodplain of Addison Creek, which leads to shallow flooding throughout much of the area during large storm events. The village is also on a combined sewer system and therefore experiences a high amount of CSO events. The ultimate goal of the test site is to show residents that they can disconnect their downspout from their sump pump and avoid ponding in their yard. Rain gardens were constructed at the Bellwood Water Department to absorb runoff from an adjacent roof. One rain garden was planted with native plant species, while the other one was typical turf. A monitoring system was installed and a firm was hired to collect and analyze data obtained from the test sites. The data is still being obtained and analyzed. Figure 7 shows both of the test sites, the plot on the left has natural plantings and the one of the right has turf grass. (Center for Neighborhood Technology) A similar study preformed in Connecticut found that 98.8% of runoff was infiltrated through the rain garden over the 56 week observation period (Dietz and Clausen). It is important to reemphasize that the effectiveness of a particular BMP is dependent on a number of site specific factors, such as soil conditions and site design.

The US Fish and Wildlife has recognized the importance of rain gardens and has awarded the State of Illinois $10,000 annually since 2005. These funds have been conferred in $500 grants to community organizations and schools to build rain gardens. This program is called the Rain Garden Initiative.

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