As noted above, the proposed peak flows meet the above reductions, thereby achieving off-site stability, therefore; this section is not applicable.
C. Modify the waterway or channel to a stable design condition.
As noted above, the proposed peak flows meet the above reductions, thereby achieving off-site stability, therefore; this section is not applicable.
In addition to ensuring erosion does not occur at the point of discharge, areas downstream and beyond the immediate area of site development may be damaged due to erosive forces resulting from extended duration of hydrograph peak flows. An unintended consequence of the practice of detaining and slowly releasing stormwater is the ability for peak flows to be sustained for longer periods of time, offering an opportunity for upstream discharges to coincide with project site discharges. The resulting combined discharge may be equal to or even exceed that of the pre-development condition.
To limit the potential for such an occurrence the designer may choose either of two approaches for downstream stability protection:
Examine hydraulic characteristics of the receiving stream channel considering upstream discharge in combination with site discharge to assess channel stability. The scope and scale of the analysis shall be appropriate to the scale of the project and the post development peak discharge rate and volume. Of particular concern are hydraulic control points, (culverts, bridges, etc.), bends in streams and sudden changes in channel cross sections downstream of the discharge point. The following may be utilized to assess stability:
The proposed design utilizes approach 2 as described below.
In lieu of performing a comprehensive watershed analysis, design a detention facility that reduces peak flows to the following levels. Infiltration may be used to meet these criteria:
2-year storm – 50% of the predevelopment peak
10-year storm – 75% of the predevelopment peak