20061161 Ver 2_Staff Comments_20070110

06-1 lblv2 Cobblestone Subdivision
Subject: 06-1161v2 Cobblestone Subdivision
From: Annette Lucas <[e-mail]>
Date: Wed, 10 Jan 2007 12:26:26 -0500
To: Ian McMillan <[e-mail]>
Ian,
I have reviewed the level spreaders associated with this project. I cannot find
ariy details of the level spreader - these would have to be included before I could
approve the design of the diffuse flow system.
The worksheets for level spreaders 2 and 4 look fine and could be approved with the
submittal of appropriate detailed plans. Level spreaders 1 and 3 are not acceptable
at this time. Level spreaders may not be placed in series in order to obtain the
required length for treatment. This project was submitted prior to Jan. 1 so level
spreaders may be designed according to the old guidance. As such, the project may
include have level spreaders up to 300 feet in length although we strongly
discourage that practice. Level spreader 1 would need to be 300 feet in length and
level spreader 3 would need to be 194 feet in length - each a continuous level
spreader set at absolutely level elevation. I think the chance of failure for these
devices would be high.
If this is unacceptable to the developer, the other two options I see are:
- Attenuate the 10-year storm with a small detention device and then release at a
slower rate to a properly designed level spreader of appropriate length (preferably
no greater than 130 feet, per the new level spreader guidance).
- Provide a properly designed stormwater BMP to treat the stormwater from drainage
areas 1 and 3, which may -then be released to a shorter level spreader or discharged
directly to the stream.
Annette
1 of 1 1/10/2007 2:13 PM