§ 25.142. First flush basins and sediment collection units.

When stand-alone BMPs such as permanent first flush basins and prefabricated sediment collection units are proposed or required for a specific site, the following design standards shall apply:

A.

A first flush basin or pre-fabricated sediment collection unit shall contain storage volume for the first one (1) inch of runoff from the on-site impervious tributary area. The storage volume of a first flush basin can be calculated as:

V _ff (cf) = A x C x 3,630 cf/ac-impervious

B.

The outlet of a first flush basin or sediment collection unit shall be designed to release the first flush volume over 24—36 hours.

C.

The outlet of a first flush basin or sediment collection unit shall not be submerged by the receiving watercourse at a 10-year design level.

D.

The first flush basin or sediment collection unit shall contain a bypass structure and/or berm to allow the 10-year peak flow to bypass without hydraulic interference.