Physical Model Study of Marmot Dam Removal: Cofferdam Notch Location and Resulting Fluvial Responses

Abstract

This report summarizes observations made for a set of experiments conducted using the physical model of the Sandy River and Marmot Dam constructed for Portland General Electric (PGE). The experiments focused on the location of the cofferdam notch and its impact on the immediate sediment remobilization, knickpoint location and trajectory, volume of removal, and location of stranded sediment. The motivation for the study was to provide insights on how and if the position of a cofferdam notch will have an impact on how the site fails and how the reservoir sediments are remobilized. Based on early experiments with the model, PGE expressed concern that some failure scenarios resulted in abandonment of large terraces of sediment near the dam site, posing potential public safety issues. One goal of these experiments was to determine if cofferdam notch location could be positioned to minimize the volume of sediment stranded in terraces. Eight model scenarios were completed for this study. Seven of the scenarios examined a failure discharge of 2500 cfs (cubic feet per second), the minimum failure design discharge. Within these seven scenarios, we examined three notch positions; river right (north bank of river), center, and river left (south bank of river). In an eighth scenario we examined a river right notch location and failure at a high discharge of 5500 cfs. Sediment mixtures used in the model were scaled to sediment core data of the Sandy River reservoir sediment. The data and observations indicate that at the minimum design failure discharge of 2500 cfs, notch position does impact the location of cofferdam failure as well as the location of the first major knickpoint and its trajectory. The data suggest that a river left notch position minimizes the extent of stranded sediment terraces and a river right notch tends to result in larger terraces. A center notch position yielded similar results to the river right notch. At a discharge of 5500 cfs, results suggest that notch position is less important than at lower discharge rates, as the knickpoint is more or less bank to bank and is able to mobilize sediment more effectively.