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(OP)
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





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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
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Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
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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
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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
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This creates a 6 x 3 matrix of release rates since soil groups C&D are grouped together.
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Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com
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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.