9. The Impervious Surface Budget Parking Lots Roads Driveways Sidewalks Derived from the City of Olympia, WA ISRS Final Report Offices Stores Houses Patios 65% transportation 35% structures
29. Pervious Surfaces Pringle Creek, Salem, OR Pervious pavements remove 50-98% of suspended sediments and associated particulate metals, as well as 97-99% of motor oil. Rogue Valley Sewer Services
35. Rainwater Harvesting No permit is required to harvest up to 5,000 gallons of rainwater and use it for outdoor irrigation w/o connecting to potable plumbing.
37. Kensington Estates Total acres: 23.92 Lots: 103 (4,143 ft 2 ave.) Open space: 15 acres (63%) Effective impervious area: ~ 0 %
38. Kensington Estates Cost Comparison Conventional Low Impact Site Prep $220,000 $150,000 Erosion Control $75,000 $25,000 Storm drainage $430,000 $ 150,000 Utilities $650,000 $625,000 Road Construction $250,000 $275,000 Total $1,625,000 $1,225,000 Unit Cost $15,777 $11,893
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41. Contact Information Robert Emanuel, Ph.D. OSU Extension Service Faculty Water Resources & Community Development 2204 Fourth Street Tillamook, OR 97141 (503) 842-3433 [email_address]
Editor's Notes
What you see here is the typical way that water moves after it rains or snows on an undeveloped Pacific Northwest landscape. Low Impact Development practices can help protect the natural hydrology of watersheds. Under natural conditions in the Pacific Northwest, about 75 percent of the water from each rainfall event is either intercepted by the forest and returned to the atmosphere through evapo-transpiration or trapped on the forest floor, where it slowly soaks, or infiltrates, into the ground. There is very little surface runoff. The water that infiltrates is critical to maintaining the base flows of streams for fish and other aquatic life. The temperature, volume and quality of this base flow are crucial to maintaining habitat for sensitive and endangered species, such as salmon.
When we build upon the land, we typically channel the water that previously fell on the forest, concentrating it into gutters and storm drains. This approach made sense when the primary goal was to create a system that is highly efficient at getting water out of the way – off of the land and into streams.
When the water reaches a storm drain, it is usually piped directly to a stream, with no treatment, carrying any pollutants it picked up along the way.
Or are we connected to, and the perpetrators of, something much more ugly and dangerous?
When land is developed, the frequency, volume and rate of flow of surface runoff increases dramatically – from 0.3% before, to 30% after development (100 times more runoff!) This is because of increased impervious areas, such as roads, driveways and buildings. The reduction of vegetation from development also decreases the amount of rainfall returning to the atmosphere through evapo-transpiration and the amount that infiltrates to the ground and recharges aquifers.
Graph: Relationship between % of impervious cover and water quality degradation Impervious cover is integrative, in that it indicates overall water quality impacts of urbanization without regard to specific pollutants or resources. Study after study points to common thresholds for water quality degradation at 10 & 25%. Where impervious cover is below 10%, water quality is pretty well protected. (Some studies show that more sensitive habitats may be impacted at lower levels of 7-8%.) From 10% to about 25%, we see impacted water quality. And above about 25%, degradation is unavoidable. Certain measures can be taken to reduce impacts, but water quality is still going to suffer some level of degradation.
Pie Chart: Impervious Surface budget (Structures and Transportation) Look at how you can reduce impervious surface. Roughly 65% of our total impervious surface is for transportation (parking, driveways, roads), while 35% of the total is for structures (e.g. houses, stores, patios). It therefore often makes sense to focus on the transportation element, looking at where impervious surface can be reduced and where alternatives can be used.
Want to avoid this situation!!! Conventional stormwater management and urban development often causes localized flooding, creating hazards for city dwellers and nightmares for public works officials.
The unnaturally high volumes and rates of urban stormwater runoff cause what is now diagnosed as “urban stream syndrome”. Eroded stream banks, poor wildlife habitat, elevated stream temperatures and increased sediment are some symptoms.
Anything that touches the land can end up being carried into our rivers by stormwater runoff. Cars, roads, lawns, and rooftops are sources of many storm water pollutants. Even higher pollutant loads can be found in the runoff from some industrial land uses. While you may not think of it as a pollutant, high temperature is one of the most common problems in Oregon’s rivers and streams, making it tough for our native cold-water fish to survive. Urban runoff is one factor contributing to unnaturally warm waters.
The challenge is how to make a residential subdivision, or a commercial property, function hydrologically like the wooded area shown. Because of the amount of directly connected impervious area (streets, driveways, rooftops and sidewalks), the highly compacted soils and the efficient storm drainage pipe collection systems, there is a huge increase in the volume, frequency and rate of stormwater runoff, as well as a decrease in infiltration.
Low Impact Development can make developed areas function more like natural areas by mimicking natural hydrology and harnessing the power of plants and soil. We must change the way we build our cities because current development practices have pushed our natural resources to a critical point – endangered salmon are one example. These new development practices better balance economic growth and environmental integrity. LID practices have the added benefits of reducing infrastructure costs and using greenspaces, trees and plants to make communities more attractive.
The first step in low impact development is to conserve natural drainages, trees and other vegetation, and soils. Trees and natural forest cover in the Pacific Northwest are terrific “sponges” for storing and slowly releasing stormwater. Comprehensive land use planning, watershed or basin planning, habitat conservation plans, and stream and wetland buffers are good tools to identify and set aside natural areas within a community and on an individual site. A significant portion of trees and other vegetation should be left in a natural state and not developed. Once conservation areas are established for each site, the designer can then work within the developable area and evaluate the effects of design options on these areas. Next we’ll look at some examples of on-site facilities that can be used to manage stormwater more sustainably.
LID practices can be used at multiple scales: -on individual sites or for multi-acre developments, -for new development, redevelopment and retrofits -on residential, commercial, public, and even some industrial properties
Conventional development collects water by series of pipes and sends water directly to streams. Larger urban areas collect stormwater in detention ponds. These detention ponds meter the water out more slowly, but they do not reduce the high volume of runoff. This system concentrates runoff and pollutants, and relies on large infrastructure investments.
Low Impact Development allows water to infiltrate on site, as close to the source as possible. You want to spread the water out, not concentrate it. Allow the water to infiltrate and be soaked up by plants in small, dispersed bioswales, raingardens, open grassways, etc. You are trying to create an urbanized system that functions more like a natural system, by harnessing the power of soils and plants.
Swales and rain gardens both use a process called “bioretention,” where the microbes in the soil and on plant roots actually break down pollutants that filter into these systems. Some of the water soaks into the ground and some is taken up by the plants. Swales are long and narrow and they are designed to move water from one end to the other. Rain gardens are shallow basins that capture runoff and let it soak into the ground. They have an overflow for large storms, but they are not designed to convey water from one point to another. Examples of swales and rain gardens range from a small, front yard rain garden to neighborhood-level facilities. Residential, commercial, and public. Schools are terrific sites for demonstration projects. Boardman is an interesting example of a small community modifying swales for their local climate and adopting them broadly in all new residential construction. Their sidewalks are graded to slope away from the street, toward the shallow, grassy basin, which is actually part of the right of way but it is designed to blend in with the front yard. What examples are there in your area?
The same approach can be used to treat runoff from streets and parking lots. It can be used in many different types of areas, from urban to rural. Curb cuts allow the water to enter the swale, where much of it soaks in. Another curb cut on the downhill side allows the water to overflow in large storms, and enter the pre-existing stormdrain or a neighborhood stormwater facility.
Bioretention can be used almost anywhere. Here a bioretention system has been designed into a retaining wall between a parking lot and a building. Native plants were used as part of the design. Bioretention systems provide habitat for wildlife and make communities more attractive and livable.
Planters can be incorporated into buildings or placed on sidewalks to help store and/or filter runoff. Each tree is worth about 80,000 gallons of stormwater treatment a year (for a large diameter, mature street tree).
This is a planter.
This is a planter
Infiltration planter?
Pervious concrete and pervious asphalt are mixed without the “fines” or small particles, so there are gaps between the stones, allowing spaces for water to pass through – think of a rice krispy treat. The material is not compacted as much as conventional concrete or asphalt. Pervious pavers are usually made with conventional concrete and designed so there will always be spaces between the pavers where water can infiltrate. Grassy pavers use a recycled plastic structure that can support very heavy loads and can either be filled with gravel or with soil and then planted with grass. Pringle Creek has the largest application of pervious pavement in the state. In the photo, the foreground is a city street with conventional asphalt, and you can see the pervious asphalt in the entryway. Notice how there is no water on the surface of the pervious asphalt on this rainy day. With no standing water, there is less road spray and icy conditions. Pervious asphalt and concrete not only infiltrate water. Microbial activity also treats it.
Conventional Lot v. Turf Lot Use of alternative surfaces can be explored, encouraged, and required, primarily for low traffic and overflow areas. The example here at a major mall in Connecticut which has recently expanded shows both the traditional paved parking area and the new turf parking area for overflow parking. The turf lot is not simply grass, but an engineered design including a proper subbase, an interlocking ring system for structural support, and turf grass. [Water infiltrates rapidly, so during dry periods the lot is irrigated with runoff from the paved areas that has been held in a preexisting detention basin.] Although the long-term effectiveness and integrity of the system--both in protecting water quality and meeting parking needs--has yet to be seen, this particular project is a good example of a an instance where the local land use boards had both ideas and regulations guiding what they wanted for their community and the developer and property owner were willing to take on some risks in experimenting with a new approach.
There is an impermeable barrier, and then a layer of a lightweight soil mixture, and plants that are adapted to the harsh conditions of a rooftop – sedums are commonly used. Much of the rainfall is evaporated, leaving much less runoff than you would see from a typical roof. The structure must be strong enough to support the weight of wet soil. More expensive to install than conventional roofs, but have reduced maintenance and replacement costs over the roof’s typical 40-year lifetime. They also provide excellent insulation.
By harvesting the rainwater that falls on your rooftop, you can reduce the flashy flows caused by urban runoff and get the second benefit of reducing the need for treated municipal water. Harvested rainwater can be used for irrigation, for toilet flushing indoors, or it can even be treated for drinking water. These systems should always have an overflow for large storms. Larger cisterns are more effective than small (typically 55-gallon) rain barrels in our wet winter/dry summer climate because rain barrels fill up so quickly and the small quantity of water they hold will likely be used to water your garden for just a few summer days. But they are an easy, do-it-yourself introduction to rainwater harvesting. You would be amazed at how much water comes off of your rooftop in one rainstorm. (If people are curious, you can note in the rainbarrel photo that the rainbarrels are raised on cement blocks in order to create more water pressure coming out of the spigot at the bottom. It also makes it easier to attach a hose or place a watering can under the spigot.
This cost estimate shows that the price to develop each lot is significantly less for the LID subdivision. Key reasons for this include the elimination of stormwater ponds, roadway curbs and gutters, and much of the storm drainage infrastructure. The individual lot sizes decreased from 7200 to 4100 sq feet. The lot yield was maintained, but the LID design allowed for preservation of 62% of the area as open space.
Stormwater Solutions workshops are tailored to small and mid-sized Oregon communities. To find OSU Extension’s LID website, you can also go to http://extension.oregonstate.edu/watershed and click on “Sustainable Development.” SWAMP = S torm W ater A ssessment and M anagement decision-support P rocess. SWAMP is an open-source, web-based tool designed to assist local governments and developers in streamlining adoption of low impact development practices. SWAMP will serve communities in Oregon’s coastal watersheds, including the inland communities in the Rogue and Umpqua watersheds.
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