Runoff volumes

The Example 2 in SSc6.1 captures rainfall for the 90% average annual rainfall event. LEED gives watershed equivalent amounts to treat in SSc6.2 that can be used for credit compliance of SSc6.2 (see footnote at the bottom of p101 BD&C Userguide). Example 2 in SSc6.1 goes on to state that "In this example, the captured rain must be drained within 3 days, or at a minimum rate of 1.4 gpm, for the tank to be emptied before the next storm. If the drainage rate is slower, full capacity cannot be assumed to be available during the 2-year 24-hour design storm." Does this imply that a 2-year 24-hour design storm rain volume = rainfall for the 90% average annual rainfall event? I ask because I've been made aware of projects that hit SSc6 with a sledge hammer by simply arguing they have cisturn capture capacity for 1.5 inches of rain per 24hrs. Background is that in Germany (and some other countries), rainfall calculations are generally made using different statistical information (also based on IDF charts) given as say e.g. 5 min 2 year frequency event rates and volumes. The only way for our projects to get SSc6 credits is to pay the weather service lots of money for the adapted information.

Is there any correlation between 1 and 2 year 24hr design storm depths and watershed classification? This may dramatically simplify things.

Hi Jean, hi all, I can see you have been going through the same problem that mine. In Denmark, a 2-year 10 min rate (0,0168 l/s.m2) is used for rainfall calculations. Have you found any way to convert our European rainfall rates to 2-year 24-hour depths? Or to calculate it from rainfall statistical data? Do you have any 2-year 24-hour depth for Germany? Kind regards, Nicolas

"for Germany" will not cut it. One side of the mountain could be vastly different from the other side, and depending on local geography the rainfall could differ significantly. The national weather office in Germany has the actual data of many if not all the weather stations in Germany and from that data produce the statistical "2-year 5-min" and other rainfall rates statistics. For a price they would probably deliver the Frequency-Intensity-Duration, and Frequency-Depth-Duration graphs for the closest weather station from your location from which one can read-off the 2-year and 100-year recurrence frequency and respective rainfall intensities and depths that you'll require. Denmark may have a similar office, otherwise the German office is also worth contacting as they have international data as well (at least that's what a colleague has told me).

Also see this program used to simulate hydrological problems: http://www.hec.usace.army.mil/software/legacysoftware/hec1/hec1-download.htm