Storm sewer system draining into a detention basin

[BillsSis]

hello all,
I am reviewing a proposed construction site's drainage report. Included in it is a detention basin to capture the water after construction, slow it down and then take it to the ditch of the adjoining state highway. The parking lot area of the proposed construction site has a storm sewer system collectively taking the storm water into the detention basin. Would I analyze the storm sewer system according to HEC 22, thereby whatever end value runoff I get from the system analysis, I would put into the detention basin? Also, there is a small area of grass which will drain into the same detention basin.

My other question...will each area - the detained one using the storm sewer system and the undetained one (the small area of grass) - have their own hydrograph (peak flow and volume) going into the detention basin?

Thanks for any insight!

 

[gbam]

HEC22 is appropriate. The tail water on the storm drain system should be in accordance with HEC22, Chapter 7, Table 7-3. The tail water would be the routed water surface elevation within your detention basin based on the appropriate design storm.

 

[pinkpig]

if local government review agency allows, it seems it is just easier to model the whole thing in a dynamic model which consists of storm sewer, pond, street (as major a drainage system for spread analysis), inlets (model it as weir and/or orifice).

 

[beej67]

I have never seen a local jurisdiction that would support a true time-series dynamic model of a storm drain system in land development. They're way too fixated on their pipe charts.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[HydrologyStudio]

Sorry for the late reply but perhaps you can file this for future reviews where a storm sewer system outfalls into a detention pond. The way to model that is like this:

1. After the storm sewer analysis has been completed (assuming Rational method was used), note the total C x A value at the very end of the storm sewer system (CA).
2. Then note the total time of concentration value here as well (Tc).
3. In your hydrology software, create an equivalent rational method runoff hydrograph using the above CA and Tc (gets intensity from Tc).
4. Create another runoff hydrograph representing the grass area.
5. Combine the two hydrographs into one final runoff hydrograph.
6. Use this combined hydrograph as the inflow hydrograph into your detention pond.
7. Design/analyze the detention pond using traditional procedures.

We get this question frequently at Hydrology Studio.
Hope this helps.

Terry Stringer
Better Hydraulics & Hydrology Software

http://www.HydrologyStudio.com