Slide 40 of 49
Notes:
Establishing a criteria to provide channel protection storage is extremely challenging. On the one hand, stream geomorphology is a very complex process that is largely site specific. On the other hand, a good design criteria needs to be easily applied yet scientifically valid. Based on the research that has been conducted to date, there appears to be two options available for protecting stream channels from erosion.
The first is 24 hour extended detention of the one year, 24 hour storm event. The one year storm is controlled to release at the pre-development one year peak discharge. The rationale for this criteria is that runoff will be stored and released in such a gradual manner that critical erosive velocities will seldom be exceeded in downstream channels. In waters where cold water fisheries are a concern, only 12 hours of detention may be provided.
The second method is the distributed runoff control (DRC). The method states that channel erosion is minimized if the erosion potential of the channel boundary materials is maintained constant to pre-development conditions over the range of available flows, such that the channel is just able to move the dominant particle size of the bedload. This requires a geomorphic assessment to determine the bankfull channel characteristics and thresholds for channel stability and bedload movement.
Obviously, DRC requires a much more rigorous analysis by a qualified geomorphologist; however, in cases where the resource is deemed to be high quality, this more detailed approach may be desirable and warranted.
The channel protection requirement may not apply to: very small sites, direct discharges to 4th order or greater streams where the development area is less than 5% of the watershed area, lakes and reservoirs, or to tidal waters.