critical storm detention method

[wds1]

How do you calculate the critical Storm? I use Hydrocad and I like it's simplicity. I would like to know how other calculate their critical storms. In the Critical Storm Detention method you determin the %increase for 1-yr volume(V1) between Post and Predevelopment(post V1/Pre V1-1.0)*100, and using the table below you determine the critical storm:

0-10% increase 1 yr critical stm.
10-50% 5 yr
50-100% 10 yr
100-250% 25 yr
250-500% 50 yr
> 500% 100 yr

Then you must hold all your less frequent Post storms up to and including the critical storm to the 1-yr Pre flows. Ie. if my %increase = 75%, then it's a 10 yr critical storm, I must hold the Post Q_1,2,5, and 10yr flow to the Pre Q1 flow. The highier Post storms are held to their Pre levels. The critical storm accounts for the increase in volume due to development that adds up downstream to cause flooding.
I determine my critical storm for the whole site. Some plan reviewers want you to calculate a seperate critical storm for each drainage area that flows off the site.

 

[psmart]

It's a pretty common requirement to hold most (if not all) of the events to the predevelopment peak flow. This is usualy done in HydroCAD by constructing separate pre/post models and comparing the results.

You can use the pond sizing report to estimate the storage required for this purpose. For details see

http://www.hydrocad.net/pondsizing.htm

In your case, it sounds like only some of the events must be evaluated for this requirement. But the process is the same.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[RWF7437]

"How do you calculate the critical Storm? "

Depends on how you choose to define the Critical Storm. But it is almost certainly NOT the so called 100 year storm, which has nothing to do with 100 years.