Created by The North Carolina School of Science and Math.The North Carolina School of Science and Math Copyright North Carolina Department of Public Instruction.North Carolina Department of Public Instruction Created by The North Carolina School of Science and Math.The North Carolina School of Science and Math Copyright North Carolina Department of Public Instruction.North Carolina Department of Public Instruction

 Effects of Urbanization ◦ Impermeable areas reduce infiltration ◦ Stormwater velocity in streams increases the erosion potential ◦ Nutrients are delivered to streams ◦ Increase of pollutants from roads, building, and managed landscapes ◦ Increase in metals and chemicals from vehicles ◦ Sediment is the number one water quality concern by volume in North Carolina

 Know your Watershed ◦ Contact your local Soil & Water Conservation District ◦ Learn more about your local streams and the water quality ◦ Identify the stormwater system at your site ◦ Learn the path of the stormwater from your site to the nearest stream

 Identify the Water Quality Concern ◦ Observe stormwater runoff from impervious areas  Observe roof top runoff and it’s path to the stormwater system  Observe parking lot or driveway stormwater runoff and it’s path to a stormwater system

Fig. 2 Heavy runoff from parking lot during rain event Fig.1 Current site: Royall Parking Lot

 Is the stormwater “Connected”? ◦ Not all stormwater creates a water quality concern ◦ Connected Imperviousness  Impervious areas that immediately drain to a stormwater pipe or system are considered connected  If impervious rooftop or sheet flow runoff flow across permeable vegetated area, it may be receiving treatment and considered non connected(not a water quality concern)  The area must be vegetated or have at least 80% coverage  The runoff must sheet flow across at least 30 feet before it re-concentrates

 Soils and designing stormwater BMP’s ◦ Contact your local Soil & Water Conservation District and obtain a copy of the soil survey with the detailed description of a soil series  Complete a site investigation  Does the site remain saturated 1.5 days after a rain event  Dig a hole 2-foot deep, or to the depth of the bottom of the proposed BMP, whichever is deeper  Check for wetland soils  If wetland soils are within 1’ of the surface, the site is likely to be poorly drained  Complete a simplified soil infiltration test

 Complete a simplified infiltration test ◦ Call 811 and have utilities located prior to digging ◦ This is a test to check the permeability of the soil ◦ Dig a hole 1’ x 1’ by 1.5’ or 2’ deep ( or one foot below the bottom of the BMP) ◦ Fill the hole with water ◦ Monitor the hole and record how long it takes to drain ◦ Once the hole has drained, fill the hole again and record how long it takes to drain a second time ◦ It is best to dig 2 or 3 test holes in the proposed BMP location

Digging Infiltration test holes Fill the holes with water and monitor the time to drain

 Siting a Rain Garden & Structural Considerations ◦ The rain garden should not be within 10’ of any building foundation ◦ The rain garden should be at least 25’ from a well head and down slope from the well head ◦ The rain garden should be at least 25’ from a septic field and placed lateral to the field( not upslope or down slope) ◦ The rain garden should not be placed in an area with underground utilities ( call 811 and locate before digging)

 Siting a rain garden ◦ Select a site that is part sun or full sun ◦ Avoid placing a rain garden in an area that stormwater drains to such as a ditch or constructed waterway ◦ Do not concentrate stormwater flow and add to an existing drainage system

Measure the distance from the building Use a tape measure

 The “First Flush” ◦ Research has indicated that pollutants that have collected on impervious areas will be washed off within the first half inch of runoff during a rain event. ◦ The standard for capturing the “First Flush” in North Carolina is based on the work completed by North Carolina State University, Biological and Agricultural and Engineering Department for the North Carolina Department Of Natural resources.

 A stormwater BMP will theoretically be able to treat 90% of the pollutants as long as it is designed to capture 90% of a rain event. ◦ In North Carolina the rain event of 1” will produce the runoff for most areas of the state. ◦ In the CAMA( Coastal Area Management Act) counties in North Carolina the rain event is considered to be 1.5”.  Visit the NCDENR map of CAMA Counties

 Determining watershed and area draining to the BMP ◦ Measure the impervious area (square feet) ◦ Measure the pervious area (square feet) ◦ Total the impervious and pervious areas to get the total watershed area (square feet) ◦ Tools  Use tape measure or a measuring wheel to measure driveways, sidewalks or parking lots  Rooftops can be measured by using GIS or by using a tape measure on the ground under the roof eaves

 Impervious Area = 6390 square ft  Pervious Area = 1278 square ft  Total Drainage Area = 7668 square ft  Imperv. Percentage in Watershed(%) = 83  Pervious Percentage in Watershed(%)=17  Rv (runoff coefficient) = 0.80  V(runoff volume) = 511cubic ft  Partial sun exposure  Appropriate site; not wet/saturated 1.5 days after rainfall  Soil Type: urban soil  Rain Garden Size: 1024 square ft  Peak storm water runoff ◦ C (runoff coefficient) = 0.83 ◦ I (peak storm water runoff) = 7.2 in/hr ◦ Q (peak flow) = cubic ft/second

 Calculate the percentage of the watershed area that is impervious ◦ Example Impervious sqft= 6390 ◦ Total area 7668 ◦ 6390 divided by 7668= ◦ % Impervious=83% ◦ The balance of the watershed is the pervious percentage ( =17, 17% pervious)

 Rv= (0.009*I)  Rv-Fraction of rainfall that will produce runoff  I- Percentage of Impervious runoff  Example Rv= (0.009*83)  Rv=.80

 V=Rv*A*(p/12)  Where: ◦ Rv=Runoff Coefficient ◦ I=Connected impervious percentage of watershed ◦ V=Volume of runoff ◦ A=Area that drains to the BMP ◦ P=Depth of storm to be captured (inches of rainfall)

 V=Rv*A*(p/12) ◦ Rv= Runoff Volume ◦ I=Connected Impervious ◦ V=Volume of Runoff ◦ A=Area that drains to BMP ◦ P=Depth of Storm to be captured(inches) ◦ Example ◦ V=.80*7668*(1/12) ◦ V= cubic feet

 C=(Impervious area) x0.95+(Pervious area) x 0.25/watershed area  C= (6390)x.95 + (1278) x.25/7668  C=.83333

 Rational Method  Q=C x I x A  Q=Peak Flow in cubic feet  C=Runoff Coefficient  A=Watershed area being treated in acres  I=Intensity of storm event to be captured(in/hr)Available from the National Weather Service  Q=.833 x 7.2 x  Q=

 Weir Equation  L=Q/(Cw x H 1.5)  Q=Flow (Peak flow 10 year storm event)  Cw=Weir Coefficient(use 3.0)  L= Length of Weir  H=Height of water over top of weir(use 0.5 feet)  L=1.05/(3.0x0.125)  L=2.8 linear feet

 Impervious Area = 6390 square ft  Pervious Area = 1278 square ft  Total Drainage Area = 7668 square ft  Imperv. Percentage in Watershed(%) = 83  Pervious Percentage in Watershed(%)=17  Rv (runoff coefficient) = 0.80  V(runoff volume) = 511cubic ft  Partial sun exposure  Appropriate site; not wet/saturated 1.5 days after rainfall  Soil Type: urban soil  Rain Garden Size: 1024 square ft  Peak storm water runoff ◦ C (runoff coefficient) = 0.83 ◦ I (peak storm water runoff) = 7.2 in/hr ◦ Q (peak flow) = cubic ft/second

 Evaluate the Site  Identify the water quality concern  Review the soil survey and complete an infiltration test  Measure the watershed area that the BMP will capture  Calculate the size of the rain garden  Check to see if the site is suitable for the size of the BMP required to provide treatment