View Full Version : Best Design Storm 6-min vs 30-min or 1 hour

March 3, 2015, 01:48 PM
We are working on the first sanitary sewer model of our system. We have good physical system data - inverts, pipes, connectivity. We have a lot of metering data of decent quality and we have use that with local population and employment data to develop per capita flow rates and loaded into the model. That is all ok. The metering also captured a few storms with the maximum near a 1 year event.

We have loaded the model with a type ii distribution, at six minute intervals, for 2, 5 and 10 year storms assuming the rainfall occurs simultaneously throughout the system. Ignoring spatial distribution of the storm is a conservative feature we are ok with. However...ARE WE BEING OVERLY CONSERVATIVE BY ALSO USING A 6-MINUTE DISTRIBUTION?

I can add a lot of detail if you want.

Pascale Duteriez
March 4, 2015, 01:49 AM
I'd say the time distribution of your storm data should be smaller than the concentration time of your catchments. 30-min and 1 hour may be ok for large rural catchments but considering you're working on urban catchments, I'd rather go with 6 minute intervals. Using 30 minute or 1 hour intervals may result in lowering the rainfall pic intensity and therefore the maximum flow in the sewer system.

did you try to load a real metured storm with different distributions (6-min, 30-min and 1 hour) and check the results ? Are there différences ? which solution gives the more accurate results regarding metering data ?

Ann Pugh
March 4, 2015, 06:42 PM
Great question and one that comes up quite regularly. One of the benefits of hydraulic modeling software is that you don't have to guess which is the critical storm - you can run each of the scenarios and assess the results. Being able to utilize the system storage by running a dynamic simulation is a key component of the analysis - if you have a flat system then even if your pipes are undersized for the peak flows, the system may often retard the flows without spillage. Sometimes in these instances a longer duration storm might be more of a concern. In Australia I've seen 24 hour storms prove to be critical on some sewer models.