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!