Credit

[Dizzle]

For several storm water ordinances I've seen they make you store the difference between the 2-yr 24-hour storm pre-developed condition, and the 25-yr 24-hour storm post developed condition. The problem I see is that they ding you twice...once for the post developed condition, and once for the increase from a 2-yr to a 25-yr storm. I understand the post developed condition vs. pre developed condition, but it seems like they both should be for 25-yr storms, instead of only giving you credit for the 2-yr storm. Does anybody know why they don't give you credit for a 25-yr 24-hour storm pre-existing condition instead of the 2-yr? Thanks

 

[cvg]

they are killing two birds with one stone here
a) making sure post development runoff does not increase
b) storing first flush runoff to improve water quality

it is not unusual to require storage of the 50- or 100-year storm, post development so count yourself lucky

 

[msquared48]

It's 100 here.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[beej67]

Never seen that. I did once see an ordinance that made you match the following conditions though:

100 post < 50 pre
50 post < 25 pre
25 post < 10 pre
10 post < 5 pre
5 post < 2 pre
2 post < 1 pre

The bottom one was one of the hardest ones to meet. They have since changed their ordinance.



Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[msquared48]

Our usual standards are not to exceed:

50% of the 2 year flow;
the 25 year; and
the 100 year.

So we usually have to use a three stage orifice - and a lot of storage volume with heavy siltation and water quality control. Most of the drainage basins here flow into Salmon streams, so it is VERY restricted.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[Dizzle]

Thanks for the information.

 

[LincolnPE]

Along with the attenuating the peak flows, they want to address the fact you are sending more volume downstream in the developed condition.

 

[LincolnPE]

Here, the regulator decrees maximum allowable release rates for each design storm on a unit flowrate basis (cfs/acre) per NRCS Hydrologic Soil Group Type (A, B, C&D).

This creates a 6 x 3 matrix of release rates since soil groups C&D are grouped together.

 

[beej67]

Sounds like a headache. Where are you at Lincoln?

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[LincolnPE]

Colorado Front Range.

We don't try to attenuate to all of the design storms (2,5,10,25,50,100) in every pond. Typically we have only two or three outlet controls on the site development ponds:

1) Water Quality Control (first flush event), 2) maybe the 10yr, 3) always the 100yr.

The other events are there if needed for perhaps large regional detention pond controls, various basin Master Plan requirements, etc.