|
|
Hydrology Design Problem 2 - Determine stormwater runoff using the NRCS method:
|
|
A 5 acre (0.0078125 square mile) site is proposed to be developed into a commercial industrial site. The site consists of woods in fair condition over type A soils.
The finished site will consist of open lawn space with parking areas, office and storage buildings with total impervious area of 65%. If the local land development regulations require that the developed site retain all post development runoff above the predevelopment condition for a 100 year storm, determine the volume that must be stored in a drainage retention pond onsite if the 100 year rainfall is 11 inches. |
1. From the hydrology submenu open the composite curve number for connected impervious area calculation, CNCompCon.aspx. Input the "Title" of the calculation, the "Designer/Checker" information.
Next open the reference tables and determine the CN number for the open lawn space in type A soils, assuming fair condition. Use a CN = 49. Now enter the variables and click on the "Calculate" button. Read the results. The post development composite CN is 80.85. Use 81.
Next go back to the reference tables and determine the CN number for the site in the predevelopment condition. Use CN = 36.
|
|
2.
From the hydrology submenu open up the NRCS runoff calculation, SCSRunoff.aspx. Input the "Title" of the calculation, the "Designer/Checker" information and the variables for the predevelopment condition of the site.
Click on the "Calculate" button and read the results. The predevelopment runoff is 2.20 inches.
|
|
3. Staying in the same NRCS runoff calculation, SCSRunoff.aspx. Input the "Title" of the calculation, the "Designer/Checker" information and the variables for the post development condition of the site.
Click on the "Calculate" button and read the results. The post development runoff is 8.61 inches. The difference in runoff is 8.61 in - 2.20 in = 6.41 inches.
|
|
4.
From the hydrology submenu open up the NRCS runoff volume calculation, RunoffVolume.aspx. Input the "Title" of the calculation, the "Designer/Checker" information and the variables.
Click on the "Calculate" button and read the results. The post minus pre development runoff volume is 2.67 Ac-ft.
|
|
5. Summary
|
The difference in runoff between the predevelopment condition and the post development condition of the site is 6.41 inches. The required storage volume of the onsite retention pond is 2.67 ac-ft = 116,341.5 cubic feet.
|
|
|