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Infiltration Basin Design Using HydroCAD With High Ksat Value

Infiltration Basin Design Using HydroCAD With High Ksat Value

Infiltration Basin Design Using HydroCAD With High Ksat Value

(OP)
I am working on an infiltration basin design in eastern North Carolina. We had a licensed soil scientist investigate the site and he found that SHWT was only 33" (we expected around 60" for this area) and that the hydraulic conductivity, Ksat value, was 2.68 in/hr (we were expecting around 0.50 in/hr). The problem now is that due to such a high Ksat value, the post-development peak flow rate due to exfiltration alone ends up being 0.27 cfs while the pre-development peak flow rate for the entire site is only 0.26 cfs. The Onslow soils at the project site belong to hydrologic soil group A so a very low curve number of 39 has to be used for mostly grassed surface at the project site.

The only way that I see out of this problem is having the client buy more land to accommodate a bigger and more shallower infiltration basin since seasonal high water table ended up being so shallow and putting some sand media that would artificially lower exfiltration rate from 2.68 in/hr to a smaller value that would result in the post-development peak flow rate lower than pre-development peak flow rate. Has anyone here ran into an issue like this before and if so how did you resolve it? Also, does anyone know of any suppliers/manufacturers that have a variety of sand media with a range of different Ksat values from which I could choose my desired sand media?

Another thing that I thought of and this is a very long shot is that HydroCAD considers peak flow rate due to exfiltration to be "discarded" flow. I've thought about reaching out to the state review agency and seeing if we could disregard the "discarded" flow due to exfiltration in peak flow comparison of pre-development vs post-development since the discarded flow will not affect any natural streams because the runoff volume is infiltrated directly through the ground. There will be some bypassed peak flow since runoff volume from about half of the project area will be bypassed and will not reach the infiltration basin (the 0.27 cfs post-development peak flow rate is due to runoff from about half of project area alone). So essentially, I was thinking of asking the state for variance by having them disregard post-development peak flow due to exfiltration (discarded peak flow).

Pre-development peak flow = 0.26 cfs

Post-development peak flow due to drainage area tributary to infiltration basin = 0.27 cfs (this is discarded flow so essentially treat it as 0 cfs)
Post-development peak flow due to bypassed drainage area that does not reach infiltration basin = 0.19 cfs

Post-dev = 0.19 cfs < Pre-dev = 0.26 cfs

Any thoughts whether I would have any luck in convincing them to disregard discarded flow due to exfiltration? The rule that they put in place for controlling peak flow rate during one year storm was due to the fact that they wanted to preserve natural hydrology of watershed and not to disturb natural streams and the like. On one hand, we are sort of accomplishing this by having the runoff volume in the infiltration basin exfiltrate through the ground below. However, on the other hand, we are putting more water in the ground (0.27 cfs vs 0.26 cfs) so we are not technically keeping peak flow rate below pre-development peak flow rate. Any thoughts?

RE: Infiltration Basin Design Using HydroCAD With High Ksat Value

you're discarded flow is just that, discarded. Your runoff rate is 0 cfs from your basin

RE: Infiltration Basin Design Using HydroCAD With High Ksat Value

I'm with Twinkie. I never count infiltrated flows as ruonff for the purposes of peak discharge matching, and I've never found any region that did require that. I have a lot of experience in coastal Florida, coastal Georgia, and inland NC.

In HydroCAD, set your infiltration outlet control line to a secondary discharge, and then route it to a dead end dummy node. Route everything else to a primary outfall node. Compare your peak discharges between the predevelopment site and the primary node only.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com

RE: Infiltration Basin Design Using HydroCAD With High Ksat Value

(OP)
Thank you very much for your input Twinkie and beej67. I was hoping that I could treat discarded flow as being effectively 0 for my peak flow comparison. This will make my life a lot easier.

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