flow rate computation with 484 vs. Delmarva UH

[superfuzz]

The Delmarva unit hydrograph gives lower flow rates for a given CN, A, and tc than does the 484 unit hydrograph. In NJ, I've been directed to use the 484 for runoff computation in the northern part of the state and Delmarva in the southern part. If I build a parking lot in the northern part of the state and use 484, I will get a specific flow rate. But if I build an identical parking lot in the southern part, I will get a lower flow because of the Delmarva hydrograph. This doesn't seem right. How can I get 2 different flow rates off the same surface? Shouldn't they be the same? Wouldn't it be appropriate to use 484 in South Jersey in this case?

 

[psmart]

The two unit hydrographs have a different peak factor (484 vs. 284), hence they produce a different runoff rate for the same precipitation.

Basically, the peak factor is a function of the average terrain. A lower peak factor is used for the Delmarva peninsula to account for the flatter terrain and greater storage in surface depressions. Other (low) values are used for some other coastal areas, such as Florida.

If the runoff is from a parking lot alone, it's arguable that the same UH would be used for any location. But standard practice is to use the same UH for all surfaces in the region where a given UH applies.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[psmart]

On the other hand, there is some rationale for using the same UH when comparing impervious surfaces. Here is one of a number of documents that google found on the subject:

http://www.state.nj.us/agriculture//pdf/delmarvabulletin.pdf

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[cvg]

if you use the rational method, you get the same Q for both parking lots, but it is different than either the 484 or delmarva method... For a small area such as a parking lot, why use a UH? Seems like overkill.