Detention Pond and Seasonal High Water Table Question

[zazapon]

I have designed a detention only pond that will attenuate the required storms (1 through 25-year 24-hour storms in my jurisdiction). In my design, I have accounted for tailwater conditions at the outlet. I obtained 10-year and 25-year tailwater elevations from digital flood maps. The city reviewer only had one review comment on this design and it was the following.

"Ensure that the bottom of the pond is higher than the SHWT."

The seasonal high water table is about one foot above basin bottom. However, I designed this as a detention only pond. It is not being used to get local credit for nitrogen and phosphorus treatment/reduction of 10% since the developer will pay a nutrient offset buydown fee. Since I am using the pond for detention only and since I have already accounted for tailwater conditions I do not think that the seasonal high water table should be an issue here at all since my headwater will be higher than taiwater during all storm events.

So my question is whether I am missing something that I'm unaware of and if so did anyone ever get a similar comment from a review agency? My hunch is that the reviewer might be thinking that it is a "dry pond" in a sense that it can be used for 10% reduction of nitrogen and phosphorus but then again I made it pretty clear in my narrative that this was a detention only pond. If anyone has any advice/argument on how to deal with this review comment I would appreciate it. I was going to tell him what I posted here and but wanted to see if anyone here ever ran into this issue before or whether his concern is valid. The worst thing that could happen I think that the first foot of the pond would have standing water for a few days (weeks at most) while seasonal high water table was at its highest point.

Thanks

 

[zelgar]

First, check the regulations regarding the pond. There may be regulations that the base must be above the SHWT.

Also, if you're constructing below the SHWT, then you won't have all of the capacity of the pond when the HWT is above the base of your pond.

 

[Twinkie]

Most jurisdictions want you to be 2' above SHWT, check to ensure that you are in compliance.

 

[beej67]

The seasonal high water table is about one foot above basin bottom.

So that is a problem.

It sounds from context clues like you might be in Florida. That or you're someplace similar. If your SHWT is a foot above your detention pond invert, then your pond will not be dry during the wet season, it will have a foot of water in it. From a routing standpoint, you'll need to set your tailwater and your starting elevation for your routing at the SHWT.

If for some reason you're sure that the pond is a foot lower than the SHWT but the tailwater is below the pond invert, then you may be in a situation where you're draining the water table down a foot as the groundwater drains out of your pond. This could lower the groundwater table on your neighbors property, impacting their ponds, adjacent wetlands, etc. There may be rules against that.

Make sense?


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