b'STORMWATERCATALOGDESIGN CONSIDERATIONS Supply: The Rational Method is used to calculate the Filters: As rainwater comes into the system from the roof orpotential supply of rainwater runoff:collection area, there is a need for a first flush or pre-filtrationQ = CIA0.623treatment. This is done to remove as much debris as possibleWhere:from the rainwater before it enters the storage system. Q=Average Monthly Rainwater Runoff Rate from drainage area or Average Monthly Supply gal/monthWith first flush systems, a volume of the rainwater isC=Runoff Coefficient, the fraction of rainfall on the drainage diverted to eliminate contaminates associated with it.area that becomes stormwater runoff. Runoff coefficients There is no exact calculation to determine how much initialcan range from as high as 0.80-0.85 (for a well-constructed water needs to be diverted because there are a numbercorrugated-iron roof) to 0.10-0.20 (for a compactedsoil surface). of variables that would determine the effectiveness ofI=Average Intensity of Rainfall (in/hr)washing the contaminants off the collection surface, justA= Drainage Area (sqft)as there are variables determining the makeup of the contaminants themselves.the monthly demand for water usage. Deficiencies in For example, the slope and smoothness of the collectionmonthly precipitation are typically made up by piping surface, the intensity of the rain event, the length ofcostly city water to the system. No one can outguess the time between events (which adds to the amount ofweather month to month so make up water piping and accumulated contaminants), and the nature of theassociated valves are necessary in any system. The trick contaminants themselves add to the difficulty ofis to minimize the use of make-up water through good determining just how much rain should be diverted duringplanning during this phase of the sizing process. Under first flush. In order to effectively wash a collection surface,sizing a system defeats the purpose of this Green Building a rain intensity of one-tenth of an inch of rain per hour isBMP and ultimately the owner will realize limited savings needed to wash a sloped roof. A flat or near-flat collectionfrom the investment.surface requires 0.18 inches of rain per hour for an effective washing of the surface. The following table can be utilized for selecting appropriate Runoff Coefficient (C):Park recommends pre-treatment through filtration whichmay prove more efficient than diverting the first flush of AREA DESCRIPTION VS. RUNOFF COEFFICIENTa rainwater harvesting system. If using a roof for aAREA DESCRIPTION RUNOFF COEFFICIENT Ccollection area that drains into gutters, calculate theBusinessamount of rainfall area that will be drained into everyDowntown0.70-0.95gutter feeding your system. If a gutter receives an amount of runoff that requires multiple downspouts, filtrationNeighborhood0.50-0.70devices should be installed for each downspout. ResidentialSingle-Family0.30-0.50Mulitunites, detached0.40-0.60 SIZING Multiunites, attached0.60-0.75A best management practice (BMP) for sizing a rainwaterResidential (suburban) 0.25-0.40harvesting system is to determine the volume of water thatApartment0.50-0.70can be captured and stored (the supply) and this should equal or exceed the volume of water used (the demand). IndustrialLight 0.50-0.80Another factor to consider is the loss of rainwater to firstHeavy0.60-0.90flush, evaporation, splash-out, or overshoot from theParks, cemeteries0.10-0.25gutters in hard rains and leaks. Rough collection surfacesPlaygrounds0.20-0.35are less efficient at conveying water because waterRailroad yard0.20-0.35captured in pore spaces tends to be lost to evaporation.Unimproved0.10-0.30Also impacting achievable efficiency is the inability of theCharacters of Surface Runoff Coefficient Csystem to capture all water during intense rainfall events.PavementsFor instance, if the flow-through capacity of a filter-typeAsphatic and contrete0.70-0.95roof washer is exceeded, spillage may occur. Additionally,Brick0.70-0.85once storage tanks are full, rainwater is lost as overflow.Roofs0.75-0.95To solve for the average rainfall intensity I, find Lawns, sandy soilthe annual precipitation for the area then divide by Flat, 2 percent0.05-0.1012 months to determine monthly average and ultimatelyAverage, 2-7 percent0.10-0.15the monthly supply. Consider seasonal adjustmentsSteep, 7 percent0.15-0.20depending on the application. Lawns, heavy soilFlat, 2 percent0.13-0.17Monthly precipitation is the key calculation to theAverage, 2-7 percent0.18-0.22rainwater harvest supply as it must equal or exceedSteep, 7 percent0.25-0.35104 INFO@PARKUSA.COM|888-611-7275|WWW.PARKUSA.COM'