Calculating Baseflow from an Underdrain 1

[levelhead]

Hi all:

I am in the process of reviewing some hydraulic calculations for a residential subdivision. The site has high groundwater elevations throughout the area (within 1-foot from ground surface).

In reviewing the calculations, it seems the engineer used a typical rainfall distribution type curve to model the "base flow" from the underdrains. (2.5 ft wide by 1.5-ft deep with a perforated pipe near the bottom [3.5 ft below finished grade]).

First, I would think that the base flow would be a relatively consistent flow regardless of a rainfall event ... it may have seasonal fluctuations, but should be relatively consistent, overall.

Second, Any tips on methodology for approximating the base flow from this type of system would be helpful.

Thanks in advance.

 

[dicksewerrat]

I think the rainfall event will give peak flows, not base flow. Base flow will come from the ground water. do you have a geotech report?

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[JasonG]

I agree with you that the rainfall distribution is not appropriate if the drain is typically submerged.

If the drain can be assumed to be submerged then I would use an assumed head and calculate the baseflow by evaluating the maximum flow through the perforations and through the pipe itself. The smaller flow would control.

The ADS website has some helpful information for calculating the capacity of perforated pipes.