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Retention Basin Considerations

Retention Basin Considerations

(OP)
I have a retention basin that I would like to construct in a C/D type soil. The depth to water table is shown to be at a range of 12 to 30 inches below grade. The soils info a have also says that the Capacity of the most limiting layer to transmit water is Moderately low to Moderately high (0.06 to 0.20 in/hr). This info is from Web Soil Survey.

I'm being told that in order to put the basin in this type of soil that it needs to have a compacted clay liner with clay that is brought in from offsite. Does this sound correct?

RE: Retention Basin Considerations

Something's not right here. These are marginal conditions for a retention/infiltration basin. No idea why you would want to reduce the infiltration capacity even further by installing an (impervious) clay liner. Is this really a retention pond - or a detention pond? The liner would only make sense for a detention pond.

Peter Smart
HydroCAD Software
www.hydrocad.net

RE: Retention Basin Considerations

It depends on local codes. Some retention basins are built to slow the flow into a nearby drainage system with minimal local recharge.

Others encourage local recharge to minimize any off site flow. As such, no clay liner is used.

RE: Retention Basin Considerations

A basin designed to "slow the flow into a nearby drainage system with minimal recharge" is generally classified as a detention pond, not a retention pond.

Peter Smart
HydroCAD Software
www.hydrocad.net

RE: Retention Basin Considerations

neither stormwater detention or retention ponds here are designed to limit infiltration, which generally is encouraged. however, in some cases infiltration is not desired due to water quality issues or a desire to lower groundwater levels, so a lining might be useful

RE: Retention Basin Considerations

when bedrock or the water table is in proximity to the base grade of the stormwater retention/detention/infiltration pond a liner is reqired in some states. That's my take on the matter. . .

f-d

¡papá gordo ain’t no madre flaca!

RE: Retention Basin Considerations

(OP)
This basin will be used as a landscape feature in the front of the owner's building with a fountain. We did a test hole down to 11' and the material on site appears to be good clay. The water table was not as high as predicted. In fact after we reached the 11' mark no water had initially entered the hole. After a while, there was a couple inches of water in the bottom. Any more thoughts would be appreciated.

RE: Retention Basin Considerations

So you want to make a pond. Thus an impervious liner is needed in the soil you describe. Many possibilities exist. One alternative is the incorporation of a fraction of bentonite into the exiting soil. Lab tests will help determine the percentage needed for the result desired. There are a few different ways to do the job in the field. Bentonite is sold at driller supply places and plumbing shops in the form used by drillers in drilling wells, etc. One brand is called Volclay. Powedered is best. Granulated is more difficult to achieve same results.

RE: Retention Basin Considerations

ok - from now on in order to avoid unnecessary confusion, this is to be called a "landscaping water feature" or a "gold fish pond", not a "retention basin"...

RE: Retention Basin Considerations

(OP)
No it's both the retention basin and landscape feature. Just need to be sure it keeps a permanent pool.

RE: Retention Basin Considerations

Quote (cvg)

neither stormwater detention or retention ponds here are designed to limit infiltration, which generally is encouraged.

This isn't true for all regions of the country. I've been forced to put a clay liner in ponds with high infiltration rate soils because the reviewers thought we got more water quality from wetland plants and reuptake than we would out of raw infiltration. We all probably realize that's dumb, but sometimes it's easier not to argue.

Quote (fatdad)

when bedrock or the water table is in proximity to the base grade of the stormwater retention/detention/infiltration pond a liner is reqired in some states. That's my take on the matter. . .

If they're worried about contamination of the groundwater table, absolutely. Or subsurface karst features.

MES11:
If I were you, I would start by doing a water balance calculation to see whether the infiltration through the soil is going to draw the pond down significantly compared to the expected runoff into the pond. You may be able to justify the pond as-is with no additional soil amendments.

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

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