PLD Freeboard and Overflow Requirements

[Matt Simpson]

Hello,

I have a question regarding PLD freeboard and overflow
requirements.

The situation that I have in question is a sump location that has
about 3/4 of an acre draining to it. While it would be suffficent to
place and area-inlet in the low point and call it 'good', we are
placing a PLD in the low point. There is no flood control detention
above this PLD.

My question is now that we have changed the outlet from an area-inlet
to a water quality facility do we now have more requirements in regard
to freeboard and a spillway?

I have looked through the new Vol. 3 of the Manual have not seen
requirements or suggestions for either of these on a PLD.


Thanks,
Matt

[Ben-UWLLC]

Matt,

I do not see a need for freeboard on the WQCV in a PLD/Rain Garden. You do
need to consider the surcharge depth you will need in order to get the water
that exceeds the WQCV into the overflow inlet. This has to occur without
backing up water onto the streets or other facilities in a way that may
exceed drainage standards.

Also, the effects of major runoff event (i.e., 100-year) needs to be
accounted for so that properties other than those owned by your client are
not flooded to a greater degree than occur before these improvements are in
place. Regardless, if the facility in combination with the overall site
changes can cause flood damages to business or homes in the area, it is an
issue that needs to be addressed.

Ben Urbonas, PE, WRE
Urban Watersheds, LLC
845 S Cove Way
Denver, CO 80209
Phone: (M) 303-728-4449
(H&O) 303-765-0155

Hello,

Thanks,
Matt

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[Ken MacKenzie]

Well put, Ben. Matt, you need to look at the ponding depth necessary to
pass your required design flow through the overflow outlet, and map that
water surface elevation on the adjacent land to determine the hazard risk.
If it's just your client's parking, warning signs may be sufficient. If
ponding outside the limits of the rain garden is unacceptable then
depressing the rain garden to create the necessary overflow head within
the rain garden perimeter is the way to go. -Ken.