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Protocol for Outlet Analysis at Highway Sites

Regulations require NCDOT to ensure all post-construction stormwater discharges are conveyed without causing downstream erosion. Natural flow pathways may be altered to ensure construction is feasible; however, liability for any substantial damage is a concern. As of date, NCDOT’s Guidelines for Drainage Studies and Hydraulic Design lack specific methods to assess outlets for overland erosion, resulting in evaluations that rely on best professional judgement. Somewhat related, previous research in North Carolina has included culvert assessments and stream stability as well as erosion and discharge metrics for stormwater control measures (SCMs). However, this previous research did not consider overland erosion or the effects of watershed area, hydrologic soil groups, and other NCDOT approved outlet types. This project will reduce the gap in knowledge and consist of four primary objectives: (1) determine the current conditions of NCDOT-managed outlets and identify which characteristics affect downslope stability, (2) develop a replicable outlet analysis protocol that uses a ranking system rather than best professional judgement for assessments, (3) create design standards for outlets that minimize erosion and cost, and (4) produce a Microsoft Excel-based tool that synthesizes the project results. Prior to any work, North Carolina State University (NCSU) will collaborate with NCDOT to explicitly define stable and unstable downslope receiving areas, which will form the basis of this project. These objectives will be accomplished through a series of tasks including: 1. Conduct a literature review of (a) stormwater outlet performance and their design and (b) erosional thresholds for downslope land uses, 2. Establish the current conditions for NCDOT-managed outlets and downslope receiving areas, 3. Identify the outlet characteristics impacting the stability of downslope receiving areas, 4. Develop and calibrate an outlet analysis protocol to be used throughout the state, 5. Monitor a subset of field assessed sites for water quality, hydrology, and landscape changes, 6. Create design standards for outlets that minimize overland erosion, 7. Perform a cost analysis comparing the proposed and current design standards, and 8. Incorporate the project results into a Microsoft Excel-based tool for practicing engineers. Prior to any field assessments, an ArcGIS analysis will be conducted to ensure an appropriate range of conditions and outlets are reflected in the surveying and protocol. It is anticipated 40 to 50 sites will be surveyed in the Piedmont ecoregion and 20 sites each will be assessed in the Mountain ecoregion. Faculty and students at the University of North Carolina at Charlotte (UNCC) will assist NCSU with the Mountain and Piedmont site assessments within proximity of Charlotte, NC. NCSU will prepare a project specific QAPP following the guidance provided in NCDOT’s research program QAPP prior to monitoring any site for water quality, hydrology, and landscape changes. Monitoring stations will be set up at the outlet, intermediate, and outfall locations for each site. Intermediate locations will be determined through the literature review and field assessments. Faculty and staff at UNCC will collect and analyze data for two sites located in proximity to Charlotte, NC. The data will be used for model validation and calibration, and the model will incorporate the characteristics found to most affect receiving area stability. Resultant erosive velocities and temporal erosional exposure will be used to create design standards for outlets. These standards will then be simulated within the calibrated model to assess the reduction in erosion. Costs associated with the proposed and current outlet design standards will be compared in terms of material costs and pollutant reduction. Cost-benefit analyses in terms of material costs and pollutant reduction will be calculated to compare proposed and current outlet design standards. Unit prices provided by NCDOT will be included in the analyses. A Microsoft Excel based tool will be created to synthesize the outlet protocol, design standards, and associated costs developed from the project. This tool will enable designers to select outlet designs using basic parameters (i.e. USDA Web Soil Survey data) while estimating pollutant load reductions and construction costs. The project deliverables will be disseminated through training workshops, peer-reviewed journal publications, and engineering conferences.