Engineering Beyond Typical Computer Floodplain Modeling
The El Paso area has mountain terrain, desert alluvial fans, and flatlands that lead into the Rio Grande Valley – a hydrologist’s nightmare. Then, too, as the city has expanded into the mountainous areas, many natural arroyos have been transformed into concrete channels. Arroyo 1 is located in the rapidly developing West Side of the Franklin Mountains. This arroyo is a great concern to El Paso Water. FEMA issued a LOMR for Arroyo 1 in 2005 showing the 100‐year storm event within the concrete-lined channel banks. The current effective Hydrologic Engineering Center’s River Analysis System suggests the Bear Ridge Channel should contain a 4.3‐inch 24‐hour 100‐year storm event within the channel banks. The FEMA effective flow is 1580 CFS for the upper region of Arroyo 1.
However, on July 10, 2015, an intense storm event occurred, causing water to jump the banks of Arroyo 1 at a 90‐degree curve and damage adjacent rock wall structures and the concrete channel in a 35‐foot radius located in Arroyo 1. PSC analyzed NOAA radar images, estimating rainfall intensity at 2.5 in/hour ‐ equating to approximately 1.25 inches over the 30-minute storm event. A flow rate was estimated at 330 CFS with a velocity of 25 ft/s. Rain gauges reported only 1.29 inches of precipitation depth for this storm event over 24 hours.
PSC used hand calculations for superelevation, minimum curve radius, channel developed length and culvert inlet control equations in a detailed analysis, which revealed deficiencies in the geometric and hydraulic design of the existing constructed channel. PSC’s engineering calculations demonstrated how a 6‐ to 7 ‐foot superelevation of the water surface in the sharp bend exceeded the channel banks in the recorded rainfall. The superelevation in the 100‐year storm event was estimated at 15‐17 feet. Typical 1D hydraulic models do not account for superelevation.
Just downstream of the bend are two 6’x 4’drainage culverts underneath a roadway. PSC’s additional hydraulic analysis, which included sediment and debris loading, showed the FEMA 100‐year effective flow rate will not pass through the culverts without overtopping, and indicated wave interaction between the bend the roadway embankment leading to damage of the concrete lining. The analysis showed the usual 1D HEC‐RAS model under‐predicted the limits of the flood boundary.
PSC proposed two alternatives to reduce velocities of floodwaters approaching the 90‐degree bend and the culvert from 30 FPS to less than 3 FPS in the 100‐year event and recommended an upstream sediment/detention basin to reduce debris loads and peak flow in the concrete‐lined channel. Reduction of peak flow in the channel avoids the need to replace the Contessa Ridge culverts. El Paso Water selected a stilling basin alternate and requested PSC to add design of the suggested upstream sediment/detention basin to the project.
Sometimes when you deal with structures with challenging bends, careful analysis supersedes the previous computer modeling. Since reviewers, including FEMA, accepted the HEC‐RAS model of the channel at face value, the mapped floodplain did not reflect the actual flood risk. An in‐depth engineering assessment of all factors, going beyond typical computer modeling, revealed the need to reduce flow velocity and quantity as a solution to this situation.