Shifting Focus for the Future: How to Adapt a Current Plan to Create a Resilient Flood Protection System - Case Study Grand Forks, ND South End Drainway and Levee System

Following the catastrophic 1997 Red River of the North (RRN) flood, the City of Grand Forks, North Dakota and the US Army Corps of Engineers designed and constructed a massive flood protection system that involved an extensive levee and floodway system along the RRN and significant interior drainage improvements, including the South End Drainway (SED), Closure Structure, and Pump Station. At the time of the design and construction, much of the City's south end was undeveloped, so numerous assumptions had to be made about future land use and anticipated stormwater management. Over the past 20 years, both recent and anticipated future development density exceeds what was anticipated for design. Recently, a rapid snowmelt combined with heavy spring rainfall occurred in Spring 2022 that created flooding conditions throughout the City's south end that were more severe than expected, suggesting that the City had more vulnerabilities in their interior drainage system than previously thought. To update the master plan for the South End Drainway watershed, substantial work was needed to accurately reflect current hydrologic and hydraulic conditions. This work included: 1.Installing rainfall and water level monitoring equipment throughout the watershed since previous work around the City indicated that rainfall-runoff responses were more complex than typical watersheds. 2.Creating a new XP-SWMM two-dimensional (2D) hydrologic and hydraulic model that used a combination of rain-on-grid and subcatchment modeling. The model covered about 7.8 square miles (20 square kilometers) and included over 3,100 links to represent the underground storm drain system. 3.Creating an hourly, continuous (almost 75 years) hydrologic simulation simulating rainfall, snowpack accumulation, and snowpack depletion to determine the most severe historical events. 4.Reviewing both the timing and magnitude of RRN flood stages to determine the likelihood of a severe hydrologic event on the interior drainage system combined with a gate closure / pumping condition. 5.Assessing the presence and implications of trends in hydrologic severity for both high RRN stages and severe rainfall / snowmelt conditions within the SED watershed, and 6.Determining the 1% Annual Chance Event (ACE) for both a spring snowmelt / rainfall event with gates closed and summer thunderstorm with gates open conditions. Once the updated XP-SWMM model was created, it was discovered that both spring and summer conditions exceeded the City's maximum allowable elevations along the SED and that the gates closed with spring rainfall and snowmelt occurring was the worst-case scenario. Based on the flat hydraulic grade line during the worst-case scenario, it was evident that either the system did not have sufficient storage or that the existing pumps lacked sufficient capacity. Master planning work began with developing a conceptual full build-out land use plan for the entire planning area. The full build-out land use plan confirmed that anticipated land use densities and corresponding impervious area amounts would be much higher than originally assumed. The current conditions XP-SWMM models were updated to reflect full build-out land use and a variety of stormwater detention standards and routing changes were explored to determine the optimal approach to lowering SED flood elevations to below the maximum allowable elevations while also accommodating future development in the watershed. Key conclusions from the evaluation were that: 1.The 1% ACE summer thunderstorm became the controlling event indicating that development expansion in the City's south end would have a nuanced impact on the performance of the SED. 2.With the storage needed to control the 1% ACE summer thunderstorm to near the maximum allowable elevations in the SED, the maximum 1% ACE spring snowmelt / rainfall event flood elevations would lower substantially because the increase in storage more than offset the modest hydrologic change from frozen ground to impervious cover. 3.The needed storage for new development was two to five times greater than the City's current standards, but this increased storage provided for new development would avoid tens of millions of dollars in new City-funded infrastructure. 4.Feasible routing changes (including pumped diversions) had only modest benefits or impacts whereas storage provided a much more efficient solution to reducing flood risk in the SED. The findings of this study are useful for the stormwater professional community because the study demonstrates: 1.Using statistical probability and a variety of analysis approaches to determine the 1% annual chance event for a variety of conditions (summer thunderstorm and spring / snowmelt). 2.The need to revisit and update stormwater master plans since development densities are typically much denser than anticipated as recent as 10 to 20 years ago, 3.The benefits of 2D modeling in flat topography to accurately simulate runoff and flood peaks reaching critical drainage infrastructure, 4.The benefits of collecting monitoring data to validate models that are rarely checked against data, and 5.Balancing development and City costs for storm drainage and flood protection infrastructure to create an optimal solution that alleviates existing flood risk and provides a resilient plan to shifts in hydrology as land use and climate changes.