![]() ![]() In contrast, WVDNR (2012) suggests a runoff volume reduction equal to the design volume of the practice, based upon a 1-inch water storm event, which results in a 90% total suspended solids (TSS) credit. (2014) provides a method for estimating SPSC sediment and nutrient removal credits based upon retrofit curves. Several recommendations are available on how to estimate the amount of pollutants removed by SPSC systems. Pools that are wider than they are long (length: width 5%) or where there is a high water table may have limited runoff reduction and/or water quality benefits (Koryto, Hunt, and 7 Thompson, Schwartz, and Hathaway 2020). Pools are located between steps or cascades and should be at least 10 feet long and more than twice the length of the preceding rock weir or cascade to dissipate energy. ![]() This shape will direct flows towards the center of the SPSC channel. The rock weirs are constructed in a shallow parabola that opens downstream, such that the arms of the weir form 70 degree angles with the bank. Cascades are larger structures that can have drops up to 5 feet. Steps are constructed of large boulders and cobbles (see Figure 3) and have drops of no more than 1 foot, where a drop is the elevation difference between the low points on two consecutive weirs. Army Corps of Engineers will likely be required prior to construction.Īt its most basic level, an SPSC is a modified channel consisting of pools alternating with steps or cascades. ![]() Because SPSCs can involve construction in a waterway, a permit from local and state agencies or the U.S. It is also important that the SPSC be connected to the receiving stream in a way that minimizes the potential for a knickpoint to form on the floodplain at the SPSC outlet, which could potentially undermine the SPSC. Because the 100-year storm recurs relatively infrequently and most stormwater drainage systems are designed to only handle flows from 10-year or 25-year storms, this approach results in a conservative design. Designs should be modified on a site-by-site basis, depending on what is required to fit the SPSC within the existing channel, while ensuring the SPSC can convey the peak flow rate from a 100-year storm without causing scour of the weirs. ![]() By constructing the SPSC in an existing first- order stream or erosion gully, runoff can continue to be discharged into the receiving stream at the same location, but with less energy. How are Step Pool Stormwater Conveyances Designed and Built? General Design GuidelinesĪlthough the SPSC is technically a constructed or modified channel, it should be designed to minimize alterations to the receiving stream and surrounding habitat. Information on crediting will improve as more SPSCs are implemented and monitored. Water quality credits are based upon relatively few studies.It is a relatively new best management practice (BMP), so design and maintenance experience is limited.Fosters the creation of both aquatic and terrestrial habitat (MDNR 2018).Facilitates establishment of vegetation.Reconnects the stream with the floodplain and the water table.Retains a wide range of aggregates, which are needed to form stable channels.Retains woody debris, which facilitates pollutant removal processes such as denitrification.Prevents erosion and subsequent increased downstream sediment pollution.Can be used in a wide variety of locations if properly designed.While an SPSC may be designed for a site with a valley or channel slope greater than 10%, these applications should be used with caution, and the design should be modified (e.g., by reinforcing the rock weirs) to compensate for the increased hydraulic forces that will result. SPSCs with a slope greater than 5% will generally provide fewer water quality benefits than those on gentler slopes, so they should be avoided unless absolutely necessary, and should only be undertaken by an experienced designer. SPSCs are effective for either first-order streams or eroded gullies with intermittent flows with valley and channel slopes between 2% and 10%. Where Can Step Pool Stormwater Conveyance be Used? The step-pool channel morphology of steep SPSCs dissipates flow energy during storm events, and the pool beds filter and absorb water during low flow conditions. These rock weirs are designed to mimic the rocks and logs found in naturally occurring step-pool channels. On steep slopes, rock weirs are spaced across the channel, perpendicular to the flow direction at pre- determined distances downslope, creating a series of shallow pools that slow stormwater and reduce erosion. How Does Step Pool Stormwater Conveyance Work? ![]()
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