§ 35-29.6. Streambank protection.

Streambank protection is the second design focus area as shown in 35-29B. There are three options by which an owner may provide adequate streambank protection downstream of a proposed development. The first step is to perform the required downstream assessment as described in section 35-29.7. If it is determined that the proposed project does not exceed acceptable downstream velocities or the downstream conditions are improved to adequately handle the increased velocity, then no additional streambank protection is required. If on-site or downstream improvements are required for streambank protection, easements or right-of-entry agreements will need to be obtained by the owner. If the downstream assessment shows that the velocities are within acceptable limits as provided by this division, then no streambank protection is required.

(1)

Option 1: Reinforce/Stabilize Downstream Conditions. If the increased velocities are greater than the allowable velocity of the downstream receiving system, then the owner must reinforce/stabilize the downstream conveyance system. The proposed modifications must be designed so that the downstream system is protected from post-development velocities. The owner must provide supporting calculations and/or documentation that the downstream velocities do not exceed the allowable range once the downstream modifications are installed.

Allowable bank protection methods include riprap, gabions, and bio-engineered methods. Sections 3.2 and 4.0 of the NCTCOG-iSWM Hydraulics Technical Manual provide guidance for designing stone riprap for open channels, culvert, culvert outfall protection, riprap aprons for erosion protection at outfalls, and riprap basins for energy dissipation.

(2)

Option 2: Install stormwater controls to maintain existing downstream conditions. The owner must use on-site controls to keep downstream post-development dishcarges at or below allowable velocity limits. The owner must provide supporting calculations and/or documentation that the on-site controls will be designed such that downstream velocities for the three storm events (streambank protection, conveyance, and flood mitigation) are within an allowable range once the controls are installed.

(3)

Option 3: Control the release of the 1-yr, twenty-four (24) hour storm event. Twenty-four hours of extended detention shall be provided for on-site, post-developed runoff generated by the 1-year, 24-hour rainfall event to protect downstream channels. The required volume for extended detention is referred to as the Streambank Protection Volume (denoted SPv). The reduction in the frequency and duration of bankfull flows through the controlled release provided by extended detention of the SPv will reduce the bank scour rate and severity.

(Ord. No. 2017-10008, § II, 10-26-17)