Credit 3.2: Reduce and Treat Runoff from Impervious Surfaces
This credit applies to actions that minimize the amount of impervious surface area and encourages the use of low impact development measures where feasible to further reduce the quantity and improve the quality of surface runoff.
The runoff associated with rain events is typically referred to as “stormwater” but it is important to recognize that the majority of surface runoff is from light, steady rainfall and not just occasional storms. Therefore, in this section, we use the term “rainwater management” in addition to stormwater to refer to the runoff from regular rainfalls as well as bigger storm.
Low impact development (LID) is an approach to site development and rainwater management that aims to mitigate the impacts of surface runoff to water bodies by:
- Reducing the amount of impervious surface area and thereby reducing the quantity of surface runoff; and
- Treating the runoff generated by any impervious areas that do occur by using measures such as absorbent landscape (deep, organic soils), rain gardens, green roofs and permeable paving.
Impervious surface area (ISA) is the area of a given lot or property that is covered by man-made surfaces that do not allow water to transmit or filter through them; instead, the water runs off the surface. Impervious surfaces include rooftops, roads, sidewalks, driveways, and parking lots that are covered by impenetrable materials such as shingles, asphalt, concrete, plastic, brick, and stone. The ISA is often referred to as the “built footprint” and as this credit emphasizes, a smaller built footprint is good for the environment and the pocketbook.
Effective impervious area (EIA) is the impervious surface area on a site that drains into a conveyance system (ditch or pipe) without any treatment to reduce flows or improve quality. The objective is to reduce EIA as much as possible. To reduce EIA, runoff can be directed to LID features such as absorbent landscaping (characterized by deeper, organic soils), rain gardens, green roofs, and permeable paving. The ultimate goal is to avoid using only pipes, culverts, and ditches that move rainwater offsite, and instead use “natural” drainage systems that slow, absorb, and filter water through vegetation and soil just as forests, wetlands, grasslands, and other natural ecosystems do.
Where this credit applies
This credit applies to any development that alters the amount of impervious surface on a site. This credit is particularly important for sloping sites, sites with runoff from upland areas, sites with highly erodible materials, bluff properties where slope failure risk is high, and sites that are adjacent or connected to water bodies that receive rainwater runoff.
This credit offers up to 6 base points plus up to 2 bonus points
|Reduce and treat runoff from impervious surfaces||Base Points|
|< ¼ acre||¼ – ½ acre||> ½ acre|
|Impervious Surface Area (ISA)* as % of lot area||< 25%||< 15%||—||
|Effective Impervious Surface Area (EIA)** as % of lot area||< 10%||< 5%||—||
|Bonus (available once any of the above conditions have been met)||
|Detain at least 1000 US gallons (3800 litres) of rainwater||
|Use detained rainwater for household and/or landscape use||
*ISA includes all artificial hard surfaces, including those that use LID measures to reduce and/or treat runoff. The smaller the total ISA, the more points are available.
**EIA is the ISA minus the area of surfaces that are treated with some LID measure such as a green roof, rain garden, permeable pavement or pavers, or absorbent landscape that receives drainage from a roof, deck or other impervious surface. Use the EIA Calculation Table later in this section to determine the EIA on your site. If no ISA is treated, EIA = ISA.
How to proceed
Right from the start, incorporate LID techniques in the design of your project to reduce the ISA and EIA. Problems often arise when building and site design plans are completed only to discover that there is not enough room left on the site to manage rainwater. Remember, the smaller the total impervious area, the less expensive managing and treating runoff will be.
Techniques that you can use to minimize the ISA and EIA include:
- Design smaller buildings, build up rather than out (within local ordinance or bylaw requirements), and cluster buildings so that walls, services and construction space are shared.
- Minimize driveway length and width, install a “Hollywood” driveway (described later in this section), or share a driveway with neighboring properties. Reduce parking area where feasible.
- Maximize the areas of vegetation and absorbent landscapes. Create a rain garden to receive runoff from roofs, patios, and/or driveways.
- Convert areas of existing or planned impervious paving to permeable paving products, ensuring their proper installation.
- Install a green roof wherever practical and suitable (you will likely need advice from a qualified design professional).
Green roof (H. RUEGGEBERG)
Discharge of runoff must be designed to adapt to site conditions, particularly where slope stability is an issue. Infiltrating runoff to ground near the top of bluffs and steep slopes may decrease the stability of these slopes. On these sites, drain water discharge, including drainage from rain gardens, green roofs and other LID facilities, away from erodible slopes. Lining rain gardens and installing underdrains and overflow drains to a collection system may be necessary. In some situations, flows may need to be hard piped to the beach to avoid erosion and bank failure. Consult with a geotechnical engineer in these situations, or contact your local planning department to ensure that you are complying with any shoreline or hazard guidelines.
To achieve points for low ISA%:
- Determine the size of the lot based on a property survey or from your site plan.
- Measure the ISA including existing and proposed roof areas, driveways, parking areas, patios and paths.
- Calculate the percent of the site that is covered by ISA.
- From the Points Available table, see if you qualify for points based on your lot size and ISA%. If not, and you are planning a new project or renovation, determine if there are other design options that could be used to reduce the ISA on the site.
To achieve points for low EIA%:
- In your design plan, delineate all existing and proposed impervious areas and show how water is drained from these surfaces, indicating the water flow path. If you are installing one or more LID features (rainwater management facilities), indicate what they are and where they are located on the design plan. Calculate the area of the ISAs that are draining to these facilities and enter this area in the applicable “Treated Area” section in the EIA Calculation Table below. Also show emergency overflow paths.
- Working with a qualified professional or using stormwater design guidelines provided by your local government, calculate the size of suitably designed rainwater management facilities needed to manage each ISA and indicate this on your design plan. A sufficient amount of land needs to be available to manage the runoff from each impervious area.
- Use the EIA Calculation Table on the next page to calculate the Total Treated Area and the EIA% for the site.
- From the Points Available table above, see if you qualify for points based on your lot size and EIA%. If not, and you are designing a new project or renovation, determine if there are other rainwater management options that could be used to reduce the EIA on the site.
Note that achieving a low EIA does not change your ISA – that remains constant.
Marine: A site plan showing the low built footprint of this new development.
EIA Calculation Table
|Insert m2 or ft2||
|Lot area||From existing conditions or design plan|
|Impervious surface area (ISA)||ALL hard artificial surface areas|
|Green roof||An engineered roof that is partially or completely covered with vegetation and a growing medium (enhanced soil) over a waterproof membrane; may also include additional layers such as a root barrier and drainage and irrigation systems.|
|Rain garden||A concave landscape area vegetated with plants that can withstand periodic inundation, which receives drainage from a roof, downspout or other adjacent impervious surface. Runoff is allowed to pond temporarily while infiltrating into deeply constructed soils below.|
|Pervious paving||A surface layer that allows rainfall to percolate into an underlying base where rainfall is either infiltrated to underlying soils (if they are porous enough) or removed by a subsurface drain. The surface component can be: porous asphalt or porous concrete; concrete or plastic grid structures filled with gravel or vegetated soil; or concrete modular pavers with gapped joints that allow water to percolate through. Note that pervious paving requires a properly designed underlying base adapted to site conditions.|
|Absorbent landscape||Area with enhanced soils (at least 12” depth and at least 10% organic content) that receives drainage from a roof, downspout or other adjacent impervious surface|
|Other (state method)|
|Total treated area||= sum of above treated areas|
|Effective impervious area (%)||= (ISA – Total Treated Area)/Lot Area x 100%|
* This is the area of impervious surface treated by the LID feature, not the area of the feature itself (though for green roofs, the area treated and area of green roof are the same).
For more information
BC Ministry of Environment (2010). Stormwater Planning: A Guidebook for British Columbia. http://bc.waterbalance.ca/resources/guidance-documents/ or http://www.toolkit.bc.ca/resource/stormwater-planning-guidebook-british-columbia
BC Ministry of Environment, 2012. Develop with Care: Section Three – Site Development and Management. http://www.env.gov.bc.ca/wld/documents/bmp/devwithcare/#
Partnership for Water Sustainability in BC – Rainwater Management. http://waterbucket.ca/rm/?sid=18&id=236&type=single
Bakeman, S., D. Gariepy, D. Howie, J. Killelea, F. Labib, and E. Obrien, 2012. Stormwater Management Manual for Western Washington, Washington State Dept. of Ecology, Olympia, WA. 1039 p. http://www.ecy.wa.gov/programs/wq/stormwater/manual.html
Low Impact Development Centre (as of 2015). www.lowimpactdevelopment.org
Portland Sustainable Stormwater (as of 2015) http://www.portlandonline.com/bes/index.cfm?c=34598
Puget Sound Action Team and Washington State University. 2005. Low Impact Development Technical Guidance Manual for Puget Sound. http://www.psp.wa.gov/LID_manual.php
San Juan County Conservation District. Low Impact Development Best Management Practices http://sanjuanislandscd.org/water-2/lid-bmp/
Seattle Stormwater Manual http://www.seattle.gov/dpd/codesrules/codes/stormwater/default.htm
Soils for Salmon. www.soilsforsalmon.org
Washington State University. Rain Garden Handbook for Western WA Homeowners: http://raingarden.wsu.edu/
Washington State Department of Ecology Stormwater webpage: http://www.ecy.wa.gov/programs/wq/stormwater/index.html.
Water Environment Research Foundation LID BMP Cost Calculator www.werf.org/bmpcost