Estimating Stage Discharge Uncertainty for Flood Damage Assessment

Abstract

A flood damage assessment study was recently completed for the proposed North Levee at the confluence of Salt and Gila Rivers in Phoenix, Arizona under the U.S. Army Corps of Engineers’ (Corps) Tres Rios environmental restoration project . This paper presents the methodology applied to estimate stage discharge uncertainty and levee freeboard under regulated flows. U.S. Army Corps of Engineers’ Flood Damage Reduction Analysis software HEC-FDA was used to obtain necessary freeboard for the proposed levee at a minimum 95% level, i.e., a non -exceedance probability of 0.95, fo r levee overtopping. The knowledge of high discharges (> 100 year) was important for setting the levee heights . A rating curve was generated in HEC-RAS using peak discharges of up to 500-year return period, and stage uncertainties were computed per Corps of Engineers’ Engineering Manual EM 11102-1619 at several locations along the study reach based on the 100-year water surface elevations. Model uncertainty was computed based on Manning’s n sensitivity analysis using HEC-RAS. As a second step in the le vee freeboard determination, graphical exceedance probability functions were introduced in HEC -FDA. The levee freeboard was then calculated to be sufficient to maintain the minimum 95% reliability for the 100 -year event along the Salt-Gila river being protected.