Indirect Sediment Transport Measurement In The Upper White River On Mount Rainier

The White River is subject to sediment laden inflows from the steep upper basin tributaries and glaciers draining Mt Rainier in western Washington State. Mud Mountain Dam (MMD), the Corps’ flood risk management reservoir, is located at RM 28. A Fish Passage Barrier (FPB) is approximately 6 miles downstream of Mud Mountain Dam, at RM 24.8, where upstream migrating fish are collected and trucked to a release site upstream of MMD. Both MMD and the FPB collect sediment during high flow events and that sediment is then flushed downstream at lower flows. A critical component for understanding the White River is the bedload transport rate. Major data gaps are the sediment volume transported past MMD and depositing at the FPB, the volume passing the FPB during sediment flushing events, and the fate of the sediment flushed from the FPB. Estimates of the sediment supply to MMD from the Upper White River are limited to sampling from 1974-1976 using a 3-inch Helley-Smith. However, bed surface grain size distributions measured D90 size more than twice the size of the Helley-Smith in the Upper White River. Despite this incongruity, direct bedload sampling has not been repeated upstream of RM 8 due to the difficulties involved in the actual sampling and the large sediment sizes known to transport through the river. On-going work is focused on indirect sediment monitoring at the FPB. Fine sediment in suspension and washload sediment pass the FPB continuously and can be estimated from continuous turbidity records. The larger suspended sediment and gravels pass the FPB only when the flows exceed 4,000 cfs and the gates are lowered. We are taking advantage of the accumulation and release of sediment in the forebay of the FPB to do repeat bathymetric surveys. ADCP surveys are conducted over the reaches upstream and downstream of the FPB before and after each sediment release. By differencing the data, we have quantified the volume of sediment transported through the barrier during two flood seasons and applied a lower bound for the volume of sediment moving through the Upper White River. Hydrophones are being added to the monitoring in fall, 2022. A pair of hydrophones was installed 850 feet (approx. 8 channel widths) downstream of the FPB. Hydrophone records will be analyzed for sediment transport timing and volume. The ADCP measurements will continue during this year, and the recordings from the hydrophones will be calibrated to the volume being transported past the FPB over a flood season and, hopefully, different flow rates. By the end of the calibration period, we hope to be able to use only the hydrophone system to measure the total sediment volumes moving past the FPB.