Thank you for the opportunity to provide comments on Guidance Memo No. 22-2012 - Stormwater Management and Erosion & Sediment Control Design Guide. I am a reviewer for Henrico County and have the following comments on this guidance memo.

• Section 3.302.1 C 3.a This section restricts post-development stormwater velocities in RCP to 10 ft/sec or less in order to be considered non-erosive. Please provide a source for this standard. What about other types of pipes? (HDPE, corrugated metal, etc).

• Section 3.303.1 C.3 “The stormwater conveyance system enters a mapped FEMA floodplain” . This contradicts VAC25-870-66 C3.c which provides an option for limits of analysis for flood protection to include mapped floodplains adopted by ordinance of a locality, not just FEMA mapped floodplains.

• Section 3.303.2 B.3 “The stormwater conveyance system enters a mapped FEMA floodplain” . This contradicts VAC25-870-66 C3.c which states an option for limits of analysis to include mapped floodplains adopted by ordinance of a locality, not just FEMA mapped floodplains.

• Section 3.305.2 D. For certain slopes in dense underbrush, the maximum length of sheet flow calculated by this equation is less than the minimum sheet flow length in DEQ Stormwater Design Specification No. 2 Sheet Flow to Filter or Open Space from the BMP clearinghouse. 

• Section 5.200 B. “Post-development drainage areas should deviate from the pre-development condition by no more than plus or minus 10%.” A “where practicable” clause would be beneficial here. For example, for some sites, there may be no existing downstream stormwater conveyance system and the more feasible option is to redirect flow to another discharge point on the site and detain to meet channel or flood protection. Or, in some cases, the downstream system is experiencing erosion or flooding and the more environmentally-friendly option would be to direct some of all of that drainage area to a different outfall that can still meet channel & flood protection.

• Section 5.303. This section provides guidance for curve numbers for a bare earth condition. However, sections 2.302.D. and Section 2.303 G both refer to runoff coefficients in Section 5.303 – can runoff coefficients for bare earth conditions (for use with the rational method) be included here as well?

Please feel free to contact me if there are any questions regarding the above comments.

Regards,

Alex Gruendl

Review Engineer

Henrico County

gru018@henrico.us