The key for volume calculation is knowing the 1 and 2-year, 24 hour storm depth. This is calculated fordifferent locations and it is a long process and it is not the same as calculating runoff rate using the Rational Method and multiplying by time. For this reason, most jurisdictions publish a series of 24-hour storm depths (1,2,10,50, and 100 year depths). LEED asks that we calculate volumes for the 1 and 2-year 24 hour storm to show that there is no net increase in the volume of runoff from the site after development..
For example, a 2-year 24-hour storm depth for Baltimore, Maryland is 3.2 inches.
When the 2-year 24 hour storm depth is known, it can be multiplied by the run-off coefficient (a dimensionless number) and the area of the site to calculate a total volume. The only variable in that calculation from predevelopment conditions to post development conditions is the runoff coefficient. Therefore, the purpose behind the credit is to develop a stormwater management program that does not change the combined runoff coefficient. Pavement will have a higher coefficient, bioswales will have a lower coefficient, detention ponds may have a zero coefficient for a 2-year, 24 hour storm, depending on your overall design. The runoff coefficients from all the various sub-basins, multiplied by the associated areas for the sub-basins can be combined to develop a combined run-off coefficient for the entire study are of the site..
Jean Marais
b.i.g. Bechtold DesignBuilder Expert832 thumbs up
May 5, 2010 - 9:24 am
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.
Jean Marais
b.i.g. Bechtold DesignBuilder Expert832 thumbs up
May 7, 2010 - 6:05 am
Is there any correlation between 1 and 2 year 24hr design storm depths and watershed classification? This may dramatically simplify things.
Nicolas Galiotto
OwnerBuildGreen.dk
27 thumbs up
September 27, 2010 - 10:13 am
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
Jean Marais
b.i.g. Bechtold DesignBuilder Expert832 thumbs up
September 28, 2010 - 2:40 am
"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).
Jean Marais
b.i.g. Bechtold DesignBuilder Expert832 thumbs up
September 28, 2010 - 2:53 am
Also see this program used to simulate hydrological problems:
http://www.hec.usace.army.mil/software/legacysoftware/hec1/hec1-download...