Flood Study of the Columbia River Basin

This paper presents a novel approach for assessing flood risks in the Columbia River Basin. Colder Associates Inc. (Golder) conducted a study to assist our client in making decisions regarding flood insurance for a site in St. Helens, OR, near River Kilometer 140.8 of the Columbia River. A hydrologic model was used to combine historical hydrographs from tributary sub-basins covering the entire Columbia River Basin upstream of Longview, WA, which is very near St. Helens. The effects of the 5 largest flood control dams were included in the model. Extensive calibration was performed to ensure appropriate statistical similarity to historical behavior. The magnitudes of the contributing hydrographs were modified using a Monte Carlo analysis with a sufficient number of realizations to estimate the 100- and 500-year events with 95% confidence. Results from the hydrologic model were input to a HEC-UNET (UNET) hydraulic model of the Lower Columbia River to predict river flood stages at St. Helens.