Open Channel Credit

Open Channel Credit

The open channel credit provides an incentive to design new developments with open channel roads, rather than curb and gutter. The credit needs to explicitly define performance criteria for the site design to receive credit.

Example:

Credit may be given when open grass channels are used to reduce the volume of runoff and pollutants during smaller storms. The area draining to the open channel can be applied to water quality and recharge requirements. While the curve numbers for channel protection and peak flow control will not change, the time of concentration may increase.

Open Channel Credit: Summary
Goals	Encourage the use of open channels in site design to treat stormwater runoff and increase the time of concentration.
Stormwater Management Objectives	Recharge, Water Quality	Subtract area draining to the buffer from the recharge and water quality requirements.
	Channel Protection/ Flood Control	Possible adjustments to the time of concentration.

To receive credit, the following criteria must be met:

The credit is applied to moderate to low density residential land uses (maximum density of 3 du/ac).
The maximum flow velocity for runoff from the one-inch rainfall shall be less than or equal to 1.0 fps
The bottom width shall be 6 feet maximum.
The side slopes shall be 3:1 or flatter.
The channel slope shall be less than or equal to 3.0%.
Credit is documented at a concept plan stage and verified with the final grading plan and with an as-built certification.
The length of the grass channel shall be equal to the roadway length treated by it.
Specific Channel Design Criteria may vary, and channel protection may be more critical in arid regions.

The open channel credit can be expressed as a function of the area draining to open channels, such that:

C = (AOC/A)WQ_vWhere:
C = Open Channel Credit (Ac-ft)
A_oc = Area Draining to the Open Channels (Acres)
A = Total Site Area (Acres)
Wq_v = Original Water Quality Volume.

The water quality and recharge volumes can then be reduced by the volume of the credit (C). The example below illustrates how this credit would be applied.