All three sub-catchments failed to meet the erosional standards demonstrating the ability of the standards to predict unstable geomorphic processes in streams. Standards developed from urbanized reference streams could possibly better represent SCMs in urbanized watersheds. Depending on the long-term management goal of the unstable stream, this metric is not applicable for all urbanized watersheds due to the discrepancy between the field and calculated unit critical discharges. The unit critical discharge metric appeared to adequately represent urbanized stream geomorphic processes for the sub-catchment undergoing urbanization (4.5% difference) but not for the mature urbanized sub-catchments (47.5% and 68.8% difference).

The standards were applied to an urbanized watershed (one sub-catchment containing a structural SCM and two sub-catchments without) in Raleigh, North Carolina. Ninety-four percent of the unit critical discharges were less than the 2-year 24-h event, indicating the necessity of controlling higher frequency sub-bankfull flows. These standards represented benchmarks of stable, naturally eroding reference streams.

The unit critical discharge represents a threshold that, once exceeded, incipient motion of the d 65 particle can occur. The d 65 substrate size, pattern, profile, and dimension of 33 reference stream cross-sections in Piedmont North Carolina were modeled using the continuous simulations program, SWMM, to develop (1) a unit critical discharge metric in L/s/ha, Q c = 0.0035( d 65) 1.5048, (2) allowable annual erosional hour standard, Log(AAEH) = −1.26Log( d 65) + 1.21, and (3) allowable volume of eroded bedload standard, Log(AV) = −0.64( Q c) − 1.52, for watersheds containing SCMs discharging into surface waters. SCMs are possibly subjecting streams to longer and more frequent periods of erosion, increasing stream channel instability. Most SCMs target lower frequency storms, usually the 2-and/or 10-year discharge events, but leave peak flows resulting from higher frequency storms uncontrolled. In part, design standards for SCMs include peak flow attenuation to maintain pre-development flow conditions to those of undeveloped watersheds or return urbanized, developed watersheds back to the pre-developed state. This study evaluated the potential impacts of sub-bankfull flows produced by stormwater control measures (SCMs) on stream geomorphic stability.

Pcswmm orifice