Integrating 1D and 2D Riverine and Collection System Models

Many communities across the U.S. suffer from routine and often severe flooding due to a combination of riverine impacts and old, undersized stormwater collection systems. But how many communities have the opportunity to develop detailed hydrologic and hydraulic models of both systems on a watershed scale? The City of Fayetteville NC had the vision and financial commitment to analyze drainage issues across all 12 of its watersheds. With its 12.0 square mile area and a stream length of 9.5 river miles, the Blounts Creek Watershed is the largest and most developed of the City's watersheds. A Master Plan was prepared for this watershed that included developing detailed 1D and 2D hydrologic and hydraulic models using HEC-HMS/HEC-RAS for the primary (riverine) system and ICM for the secondary (collection) system; performing a field assessment of high priority stream reaches; and identifying issues and problem areas using the modeling and field assessment results to inform required mitigation with new drainage solutions. The primary collection system was modeled using HEC-HMS and HEC-RAS modeling platforms. Local county pre-preliminary HEC-RAS models developed under a Countywide Flood Insurance Study (FIS) report update were used. The base models were reviewed in detail and updated to include new survey. A combination of 1D and 2D modeling techniques were used to represent the primary system operation. The secondary system was modeled using InfoWorks ICM. To improve project efficiency and best use resources, hands-on practices were coupled with automated methods to streamline hydrologic parameter and hydraulic network development. The project team created multiple ESRI based innovations including tools that identified the longest flow path and calculated the time of concentration for the surface runoff within each subcatchment as well as a tool that used model status codes within the GIS data to build InfoWorks ICM compatible model networks. The primary and secondary system models were run under several design storms and the model results were analyzed to define current levels of service as defined by the City. The primary and secondary system model results were mapped together to enable the City to fully understand how the systems impact each other and identify areas of significant surface flooding and structure impacts (Concern Areas). The results indicated that 135 locations experience significant flooding impacts, where the stormwater infrastructure (pipe, inlet, channel) does not meet the City's defined level of service. Specifically, flooding impacts were shown to significantly affect 47.2 miles of roadway, 13 road crossings, and 471 structures, as well as prevent accessibility to 328 structures. From the erosion assessment, four areas were identified as erosion concerns. These 135 areas were assessed, scored, and ranked based on severity of the flood or erosion risk. From this prioritization exercise, 101 areas were selected for further analysis to include developing proposed solutions and evaluating their ability to mitigate the flooding and erosion impacts. The selection of areas for development of proposed solutions was primarily oriented around mitigation of flooding in the public right-of-way. Structural flooding mitigation was evaluated as an auxiliary benefit to right-of-way improvements. Potential downstream impacts were also evaluated. Proposed solution conditions were then incorporated into the watershed models and flood risks were re-evaluated under future rainfall conditions. These proposed solutions range in size and complexity, from simple culvert upgrades to bridge refurbishments and floodplain development. The planning level estimated cost for design and construction of all 100 proposed solutions is $277,552,400. The proposed solutions were prioritized and one of them, totaling just over $20M, was selected to move forward to the current Capital Improvement Plan (CIP). The remaining proposed solutions are considered backlog for future years. This presentation will step through the overall program and vision of the City in establishing and implementing the watershed master plan program and the process of developing, validating, and integrating the 1D and 2D riverine and collection system models with several project innovations to efficiently and cost effectively develop a CIP program to mitigate stormwater flooding.