Stormwater control measure (SCM) design standards to limit stream erosion for Piedmont North Carolina

► Erosion metrics for stormwater control measures (SCMs) that target stream geomorphology. ► Stormwater critical discharge standards can be established from the d 65 of streams in Piedmont region of North Carolina. ► Annual erosional hours and sediment transported in urbanized watersheds can predict uncontrolled erosion within streams. ► Catchment design and SCM sizing can reduce negative impact of development on stream geomorphologic health. ► Better critical discharge standards may be possible by analyzing stable and healthy urban stream segments. This study evaluated the potential impacts of sub-bankfull flows produced by stormwater control measures (SCMs) on stream geomorphic stability. 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. 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. SCMs are possibly subjecting streams to longer and more frequent periods of erosion, increasing stream channel instability. 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. The unit critical discharge represents a threshold that, once exceeded, incipient motion of the d 65 particle can occur. These standards represented benchmarks of stable, naturally eroding reference streams. 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. The standards were applied to an urbanized watershed (one sub-catchment containing a structural SCM and two sub-catchments without) in Raleigh, North Carolina. 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). Depending on