Stream Buffer or Filter Strip Credit

This credit encourages the use of stream buffers and filter strips to treat stormwater runoff at the site level. Specific criteria for the buffer itself will vary between communities.

Example:

Designers can receive credit for treating stormwater runoff areas adjacent to a filter strip or designated stream buffer using site grading. Credits include:

• Area draining to the buffer is subtracted from the calculations for water quality and recharge volume.
• The curve number of areas draining to a buffer preserved in natural vegetation shall receive a curve number adjustment to reflect woods in good condition.

To receive the credit, the buffer must meet the following criteria:

• The minimum undisturbed buffer or filter strip width shall be 50 feet.
• The buffer shall meet the maintenance and design requirements described in the buffer ordinance.
• The maximum contributing length to the buffer or filter strip shall be 150 feet for residential developments, and 75 feet for commercial developments.
• If the overland flow path is greater than 50 feet, a level spreader shall be used to establish sheet flow.
• The average contributing overland slope shall be less than or equal to 3 percent.
• The buffer shall be preserved in a conservation easement or similar protective mechanism.

  Specific buffer requirements will depend on the stream order, and also vary regionally. More detail is provided in the Buffer Ordinance.

The credit for both recharge and water quality volumes can be determined based on the required water quality volume, and the fraction of the site draining to buffers, such that:

C = (A_B/A)WQ_v

Where:
C = Buffer Credit (ac-ft)
A_b = Area Draining to the Buffer (Acres)
A = Total Site Area (Acres)
Wq_v = Original Water Quality Volume (ac-ft)

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.