NC 2009 SSc6.1: Stormwater Design—Quantity Control

Runoff Volumes

This credit is actually extremely technical.

I can still use the Rational MethodA formula that can be used for calculating stormwater flow rates. Q = CIA, where C represents a coefficient for physical drainage area, I is the rainfall intensity, and A is area. The method is suitable for watersheds smaller than 300 acres in size. to deturmine the Runoff Rates, but as for the Runoff Quantities, I'm stuck. The US NRCS/SCS charts are obviously made from US data and therefore not suitable for projects outside the US.

I don't find anything helpful on the internet.

PS. Nice site.

Runoff volumes

The way that we have calculated run-off volume is to calculate a weighted average run-off coefficient, as you would for the Rational MethodA formula that can be used for calculating stormwater flow rates. Q = CIA, where C represents a coefficient for physical drainage area, I is the rainfall intensity, and A is area. The method is suitable for watersheds smaller than 300 acres in size.. You do this for pre and post development conditions, and essentially, the Run-off Coefficient needs to be the same. For instance, if the Run-off Coefficient is 0.35 for an undeveloped site, and the Run-off Coefficent for the developed site is 0.70, then the volume that must be mitigated is the difference in Run-off Coefficient times the rainfall for the one-and two-year 24 hour design storms. The approach to meeting the volume requirement can be with green-roofs, porous pavment, cisterns and reuse, infiltration basins, bioswales, etc.

Working through the rational

Working through the rational methodA formula that can be used for calculating stormwater flow rates. Q = CIA, where C represents a coefficient for physical drainage area, I is the rainfall intensity, and A is area. The method is suitable for watersheds smaller than 300 acres in size., I've calculated the Time of Concentration (Tc), then itterated using IDF (intensity-duration-frequency) charts until the duration = time of concentration (since the Tc is influenced by the corrisponding intensity at a given duration).

This really means to me that even though you can calculate this for a given statistical return period, i.e. 2 years, the duration of "24hrs" is badly worded and should only apply to run-off VOLUMES using the SCS methode after finding the run-off RATES with the rational method.

The weighted average Run-off Coefficent should then be used with the Intensity given at the corrisponding Tc as calculated using the Rational Method giving the required run-off RATES, but this still leaves me short for calculating the VOLUMES, which as previously stated, is usually done using American Charts based on American rainfall data.

I mean which CN number must I use if I'm building somewhere in Brazil? Does anyone have a concrete example of the complete set of calculations?
I know in America this is done by the Civil Engineer, but it does't alway fall into his camp in all parts of the world...even though maybe it should.

Runoff volumes

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 MethodA formula that can be used for calculating stormwater flow rates. Q = CIA, where C represents a coefficient for physical drainage area, I is the rainfall intensity, and A is area. The method is suitable for watersheds smaller than 300 acres in size. 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..

Runoff volumes

Thanks Mr. Hurst. That points me in the right direction. For a long time I was trying to find the correlation between the run-off rates and the rainfall depth with no success. Understandable, as it's a whole different set of data from the local weather service. We had big problems in Lybia trying to find proper rainfall data of any sort what so ever so I was hoping there were correlations to draw at least rough estimates from the data I had available.
It also seems I was wrong about the CN charts for the SCS method. They represent the soil attributes (and their part in holding or restraining the run-off) and would most likely still be usable internationally.

This also clarifies the use of the "24hr" return period used in the lead requirement language. It applies then to the run-off Volume calculation and not (if using the Rational MethodA formula that can be used for calculating stormwater flow rates. Q = CIA, where C represents a coefficient for physical drainage area, I is the rainfall intensity, and A is area. The method is suitable for watersheds smaller than 300 acres in size.) the run-off Rates.

Keep up the great work and let me know when I can help out with the site.

calculation of Cr 6.1 Stormwater design-Quantity Control

I would like to request help for the calculation of Cr 6.1 Stormwater design-Quantity Control. The details are as follows.

I have a site with an area of 28460 SQ.FT.

Pre-Development: The site was covered with flat vegetation (Surface Type) which has a Runoff Coefficient of 0.10. This will be complying for Option-1, since the existing site is with imperviousnessResistance to penetration by a liquid and is calculated as the percentage of area covered by a paving system that does not allow moisture to soak into the ground. less than 50%.

Post-Development: The site is designed with 23274 SQ.FT. of hardscape including the building and pavements with same Runoff Coefficient, 0.95. The rest 5286 SQ.FT. was left for landscape.

Perform the sample calculation using a data for rainfall.

The credit says if existing

The credit says if existing imperviousnessResistance to penetration by a liquid and is calculated as the percentage of area covered by a paving system that does not allow moisture to soak into the ground. is less than 50%, then
implement a stormwater management plan that prevents the
post-development 1.5 year, 24-hour peak discharge rate from exceeding
the pre-developemnt 1.5 year, 2 4-hour peak discharge rate.

Assume Q= Runoff Quantity

Then Q pre dev (for 1.5 year, 24-hour peak) <= Q post dev (for
1.5 year, 24-hour peak)

Q pre = Rainwater Runoff = Average rainfall X Surface Area X Runoff Coefficient

Pre development the site area, 28460 sq.ft(2643.9 sq.mt)., was flat vegetation.
Post Development the site was designed to have 23274 sq.ft(2162.1
sq.mt.) of hardscape including building and pavements with runoff
coefficient of 0.95 and the rest of 5286 sq.f.t(491.08 sq.mt.) of flat
landscape.

Calculation for 1.5 year, 24-hour peak discharge:

Predevlopment = 0.115 m X 2643.9 m2 X 0.1 = 30.40485 m3
= 30.40485 X 1000 litres

Whereas, 0.115=average rainfall
2643.9 m2 = Area
0.1= Runoff Coefficient

Post Development = (0.115m X 2612.1 m2 X 0.95) + (0.115m X 491.08 m2 X 0.1)
= 285.37 m3 + 5.6 m3
= 290.97 m3
= 290.97 X 1000 litres

Since the Post development runoff is higher than the predevelopment
run-off, we need to design for reducing the runoff from the site.

Let me know if there is any changes to the calculation

calculation of Cr 6.1 Stormwater design-Quantity Control

Your calculations are correct. You will have to develop strategies to infiltrate, evapotranspire, or reuse 260.57 m3 of stormwater to qualify for the credit..

However, depending on your location, please verify that 0.115 m is your 1.5 year, 24 hour storm. That seems high. You are trying to find the rainfall amount for the 1 and 2-year 24 hour storms.

Calculations for SSc6.1

Calculation of a 2-year, 24 hour storm is a lengthy process. See my comments above on this issue. Rainfall amounts, especially for one year, will not provide you with a 2-year, 24 hour storm event. This is information that you have to obtain from the local jursidiction. I have seen are some world atlas books that may provide a figure if there is no local jurisdiction, but they are not very accurate and should be used if you cannot obtain the information locally to your project.

I gave an example above for a 2-year, 24 hour storm for Baltimore. I found this value on the internet.

Hope this helps.