Cities in the Coachella Valley have a history of dealing with damaging summer flash floods. Intense thunderstorms can dump several inches of rain in a short time while runoff rushes from the nearby San Jacinto Mountains into the lowland flats where the cities are built.
The City of La Quinta suffered back-to-back damaging summer storms and flooding. In early September 2014, a storm producing rainfall depths of almost 3 inches in 1 hour flooded and damaged parts of the city. According to the National Oceanic and Atmospheric Administration, the event was close to a 500-year (1-hour) storm. In August 2013, a similar short-duration, high-intensity storm flooded and damaged many of the same areas.
Modeling Complex Stormwater Interactions
After the 2014 storm, the city wanted a high-level drainage analysis and undertook the most complex hydrologic/hydraulic evaluation ever completed in the Coachella Valley. As part of the high-profile project, Dudek engineers specializing in drainage were tasked with developing an advanced modeling technique to evaluate the urban rainfall-runoff characteristics for the site.
“Urban stormwater modeling has come a long way since the days of rational method and manning’s equation,” said Dudek’s Tom Ryan, PE, who led the modeling team. “Current models use a more advanced hydrodynamic calculation process than the traditional energy equation or pressure-and-momentum alone. Fully dynamic models utilize the shallow water equations or full Saint Venant equations that allow engineers to evaluate complex drainage systems to include the effects of surcharge and backwater effects more accurately.”
XP Solution’s XPSWMM model was used to develop a comprehensive two-dimensional (2D) surface model that included almost 3 square miles of developed urban area. The 2D surface model was linked to fully dynamic one-dimensional model (1D) of the city’s storm drain network. A surface, digital terrain model was created using detailed LIDAR topographic data.
Analyzing the Surface–Subsurface Links
A network of surface and subsurface storm drain facilities was added to the model based on available as-built plans and field reconnaissance. The surface model was “linked” to the subsurface model at catch basin inlets, culvert and pipe system inlets, and outfalls. The linked locations are where flows from the surface and subsurface (storm drains) communicate. For example, if a subsurface storm drain becomes pressurized, it will reverse out of the catchbasins and flow down the streets within the surface model, and vice-versa.
“When using these types of advanced models, hydrology must be calculated to produce rainfall runoff hydrographs, not just peak flows,” Ryan said.
Dudek engineers used the Riverside County hydrology methodology to calculate the hillside, or non-urban area, runoff while the direct rainfall method (DRM) was applied to urbanized areas. DRM allows modelers to add rainfall across the entire surface grid, allowing the program to calculate drainage areas, directional paths, and time-of-concentrations.
Several models were created in XPSWMM for various storm events. A 500-year (existing condition) storm event was prepared for reference, to correlate to the September 2014 storm event.
Storm photographs of known flooded areas were used to correlate the model parameters at these locations with the depths in the photographs. Once the model was correlated for the existing conditions, evaluations were prepared for flood mitigation measures. The proposed measures included increasing the existing storm drain facilities, adding flood attenuation basins, and relocating some of the facility outfalls to more suitable locations.
The proposed measures are currently under final review by the city.
Advanced Modeling Benefit
Complex analysis provides a realistic and cost-effective basis for project designs to alleviate municipal flood issues. The main benefits are the ability to acquire immediate feedback for each proposed alternative, and to identify the most cost-effective drainage solution.