Calculation of drawdown rate Qr

Therese, The days is not a default value, but an example from the Maryland Stormwater Design Manual. Also in this manual is this definition, "The detention lag time (T) for the one-year storm is defined as the interval between the center of mass of the inflow hydrograph and the center of mass of the outflow hydrograph." (p. 48). Does that help?

Dear Amy and Therese, Considering that we are using the ACP to calculate the reduction of the volume of stormwater runoff, what should be the time interval we have to use? Bare in mind that we are in a tropical zone and the average 30% rainfall in the month with highest average rainfall is over 70 mm. Thanks a lot.

Time interval refers to the spacing (time) between points used to plot a hydrograph. But I do not think this is what you are referring to? Are you trying to develop a percentil storm using local rainfall data?

Hi Michael, Thanks for your prompt reply. Our understanding is that this credit requires a diminishing in the volume of water sent to the city rainwater drainage system (in this case this is achieved by a reduction on the flow). The sizing of this retention tank, based on the number above is becoming massive and therefore we are trying to understand if we should consider the volume of run for one hour our smaller interval. Also if we decide to discard the water, when and how long should I take to do that? Best

The time interval in this equation refers to the interval between design storms. So: If you are designing for 1-year storm 31,536,000 seconds. Divide the volume of your tank by that to determine the minimum drawdown rate. Then you need to design an orifice Q=C(2gh)^1/2 that meets that rate. See the SSc 6.1 forum for a more detailed discussion of this. http://www.leeduser.com/credit/NC-2009/SSc6.1