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Exfiltration Area HELP

Exfiltration Area HELP

Exfiltration Area HELP

Good afternoon, I'm just curious if anyone has any workarounds for the following?

When modeling ponds (detention basins) with exfiltration outlets and constant velocity outfall type, we need a way to exclude a certain square footage of the bottom surface area from exfiltration.

Review agencies are now requiring that we exclude the forebay(pre-treatment) areas from the infiltrating area. Unfortunately, there are work arounds but no good way to accurately model the exfiltration area while still keeping the total bottom area available for volume.

One thought was to simply use constant flow, converted to cfs, instead of velocity (in/hr) because we can calculate the flow based on the reduced bottom footprint. The downfall to that is we don't take advantage of exfiltration on the side slopes. When we use velocity, the flow will continue to increase as the detention pond fills because we are taking advantage exfiltration in the side slopes.

The Discharge multiplier simply reduces the rate. In a perfect world, it would be nice if another multiplier option could be added to control the available exfiltration "area" as a percentage of the bottom surface area of the pond. That way, say 85% of the bottom of the pond could be used for exfiltration but 100% is still used for storage volume.

Some guys here have been modeling the forebays as separate ponds that then link to the main pond but i don't like that. Another option was to set the bottom of the pond 0.01' lower and make that area 85% smaller than the 0.0' elevation. The problem with that is I believe the exfiltration will start in the remaining 15% once the pond elevation raises that 0.1'

I'm open to suggestions but would really love more flexibility added to the software for exfiltration.


Jon Falkowski, P.E.

RE: Exfiltration Area HELP

To prevent exfiltration through the lower portions of the pond select "Allow exfiltration .. Only above invert" and set the Invert Elevation to the desired level. This will exclude the area at and below that elevation. The exact area that is excluded will be shown on the pond summary report in the outlet listing. This can be surface area, wetted area, or horizontal area, depending on your selection.

For further details click the Help button on the exfiltration screen.

Peter Smart
HydroCAD Software

RE: Exfiltration Area HELP

I understand the challenge. Peter's solution would work if the forebay was lower than the main basin:

forebay------remaining bottom
\.............. ________________________/

But it would not work if the forebay was at the same elevation:

forebay------remaining bottom
\ ......... __.........................................../

The latter example is common in the coastal environment where I practice with high ground water and regulatory requirements for the practice to be a measured distance above the ground water.

I have experienced similar from regulators. My recollection is that I did not include the area nor volume of the forebay. It is cumbersome and we are losing some side slope infiltration. It sounds minor, but many times we are squeezing everything we can to get something to work, especially on small sites.

Perhaps a wish list item?
a. define an area within a storage that is excluded from exfiltration calcs.
b. define an embedded storage that is excluded from exfiltration calcs (this would be ideal, but may be difficult to program).

RE: Exfiltration Area HELP

To exclude an individual storage definition from exfiltration, simply un-check "Allow Exfiltration" in that storage screen.

Peter Smart
HydroCAD Software

RE: Exfiltration Area HELP


This sounded promising; however, it does not appear available for the scenario I had in mind:

I created a prismatic storage for the basin. (#1)
The forebay (#2) is storage embedded in the basin
Outlet is Exfiltration

The "Allow Exfiltration" is greyed out for the forebay.

RE: Exfiltration Area HELP

Terry is correct, looking for a way to model when the forebay is part of the overall storage...only separated from the rest of the storage area by a gabion stone wall. We can't exclude the volume used by the forebay and area above the forebay in larger storm events or ponds would have to get much larger. We just need a way to tell hydrocad to only use XX % of the bottom surface area for exfiltration.


RE: Exfiltration Area HELP

TerryScan: The forebay should not be embedded storage. That doesn't apply to this situation, and will force the checkbox on but grayed-out.

To recap: Model the forebay and the main storage in a single pond, but use a separate line on the storage tab to define each volume. Then setup your exfiltration, using the velocity option. Now go back to the storage definition for the forebay and un-check the box for "Allow exfiltration". Or, if you prefer to restrict exfiltration to a given min/max elevation you can set these points on the exfiltration screen. If your cutoff is based on horizontal position or some other arbitrary zones, then you can setup a separate storage definition for each zone. All scenarios are easily modeled using these techniques.

Peter Smart
HydroCAD Software

RE: Exfiltration Area HELP

Thanks Peter, I will try that!

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