Stormwater Detention Estimates

Last month, I discussed the couple of different ways AutoCAD Civil 3d, Hydraflow Hydrographs and Autodesk Storm and Sanitary Analysis presented stage-storage information.  Today – let’s take a step back and figure out how to estimate storage required. 

Many development regulations require engineers to not to exceed the pre-development release rate on new developments.  Current trends in stormwater detention design are beginning to focus more on matching the pre-development runoff volume.  [That is probably a better method – but it is not the focus of this post]

So -  let’s estimate the development on the 100-Acre Wood.  [Sorry – Winnie – Christopher Robin is building townhomes]  It has ‘C’ Class soils, and the approximate density will be 1/4 acre units. The curve number estimated is 83.


Add the SCS Hydrograph, the proposed development criteria (above) and review the results! (below)


We are using the SCS Type II hydrograph – let’s imagine it is the local regulations requirement for the area around the 100-Aacre Wood.  It’s very peaky, and probably a poor estimate of actual storm volumes. [and if I recall – the 100-acre Wood is on the wrong side of the pond. We, in Illinois, use the Circular 173 – Huff Distributions.]

If you open a Pond, and switch over to the Pond Tools tab, you will see this:


If you know your pre-development requirements – you would enter your target release rates at the Event’s Target column.  [something like these below – maybe a 0.04 cfs/acre release for the 2-year and 0.015 cfs/acre release for the 100-year event]


If you have contour areas, you can switch to the Storage Tab and enter some stage-area estimates to see if your pond can hold these volumes.  This provides you a good basis to manipulate your Civil 3d grading objects!


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