Common use of DETAILED PROJECT DESCRIPTION Clause in Contracts

DETAILED PROJECT DESCRIPTION. The project location is a deeply incised and actively eroding stormwater channel. The device will treat an 8.87-acre watershed and will improve two head cuts. The RSC will be installed in a section the ephemeral stormwater channel that is approximately 90 feet in length. There is an approximate seven to eight-foot headcut located at the upstream end of the project reach; the channel downstream from this point averages approximately 15 feet in width from top of bank to top of bank and is incised, on average, approximately 4-5 feet. At the downstream end of the reach there is another headcut, approximately 4-5 feet in elevation. The RSC will end just upstream of the location of this existing downstream headcut. In a RSC, water conveyance and processing is accomplished via a constructed network of sand seepage berms, pools, and boulder (or cobble, depending on the modeled flow) weirs. The arrangement of these features raises the incised invert of the incised channel back to pre-disturbance elevation and forms a new surface topography that controls the surface and subsurface hydrology. The modifications necessary to establish the sand seepage hydrology result in the creation of a series of vegetated stilling pools, sand seepage beds replete with above and below-ground biomass, and associated flow paths through the sand/mulch filter media. The physical effect of the pools and the vegetation planted on the lateral sides of the channel in the filtration media reduce water velocity and facilitate removal of suspended solid particles and associated nutrients and contaminants. Uptake of dissolved nutrients and adsorption of oils and greases by the many plant stems present in the pools yields additional benefits. The conceptual design/structure of the RSC at the proposed location currently involves filling the incised channel with a mixture of sand (80%) and hardwood mulch or chips (20%). A parabolic ▇▇▇▇ structure will be installed at the upper headcut, at existing grade, to control grade and direct water into the RSC downstream. Continuing downstream, the incised channel will be filled with the sand/mulch filtration media, and stabilized with a series of 3 (or more) additional parabolic ▇▇▇▇ structures, of varying length and slope, depending on the existing topography. Each ▇▇▇▇ will have a pool immediately upstream for high flow storage and to enhance infiltration of stormwater downward through the sand/mulch filtration media. At the downstream end of the RSC, material will be excavated down to or below the invert of the channel immediately downstream of the headcut there. A parabolic ▇▇▇▇ will be installed at the location of the headcut itself, which will control grade at that point in the channel, and will extend upstream as far as needed to stabilize the media in place and accommodate the site topography and channel continuity. The largest pool in the RSC will be just upstream of this last parabolic ▇▇▇▇. The RSC design will restore the basic channel invert elevations to their condition before development upstream caused excess runoff and erosion. The design of the RSC will be such that much of the stormwater flow will be treated by the sand/mulch filter media and directed downstream. The structure of the RSC encourages stormwater infiltration below the sand/mulch filtration media, which also helps to recreate pre-disturbance conditions and ▇▇▇▇▇ stormwater flows. After construction, monitoring will be undertaken for 12 months to determine the functionality and performance of RSC’s for nutrient load reduction in Durham’s Triassic Basin soil type. A Master of Science (MS) student will focus on this research, producing a final report and presentation. This information will be provided to the engineering and design community and may be used to increase the amount of nutrient and sediment load reduction credit for this recently approved practice. Currently, the North Carolina Division of Water Resources (DWR) Stormwater Best Management Practices Manual (NCBMP Manual) conservatively applies credits to these devices as if they are sand filters. Evidence from their use in the Chesapeake Bay region and other parts of North Carolina suggests they provide greater nutrient load reductions than sand filters. An additional benefit of these devices is that that they may be installed in steep, eroded areas not suitable for traditional stormwater control measures. NCSU continues to be involved in research with other organizations such as the Upper Neuse River Basin Association (UNRBA), North Carolina Department of Transportation (NCDOT), North Carolina Department of Environment and Natural Resources (NCDENR), and municipalities such as Durham to develop innovative stormwater control measures. Data from this project may be useful for future refinement of models and crediting to assist the regulated community in achieving stringent nutrient reduction management strategies such as the Falls and Jordan Lake Rules.