USGS Washington Water Science Center
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WA294 - Approximation of Mudflow-Caused Flood Elevations at Trojan Nuclear Powerplant - Completed FY1983
Problem - The Water Resources Investigation (WRI) Report 82-4125, Mudflow Hazards Along the Toutle and Cowlitz Rivers from a Hypothetical Failure of Spirit Lake Blockage, has caused some concern in the Nuclear Regulatory Commission (NRC) that such a mudflow, it if were to occur, could possibly produce or introduce hazardous flood elevations in the Columbia River at the Trojan Nuclear Plant. As the Trojan Plant is scheduled to reopen on or about July 10, 1983, after several months of shutdown for service, and the results of this investigation could possibly impact that reopening, there is some urgency in the NRC request.
Objectives - The grade elevation at Trojan, 45 feet mean sea level, is the design flood elevation. Floods above that elevation could seriously impact Trojan facilities and operations. The purpose of this investigation is to determine if the approximate elevation of a flood at Trojan due to a mudflow as described in WRI 82-4125 might exceed 45 feet mean sea level.
Approach - In Phase I the same kinematic-wave model used for the WRI will be applied to the Columbia River, to simulate the effect of the Cowlitz River tributary flow on the Columbia River. In one case the Columbia will be assumed in steady state at peak flow or a coincident mean or median flood; in another case it will be assumed at slack high tide to detain maximum sediment deposit in its channel, followed with a steady-state peak of a mean or median Columbia River flood over the deposit. If results exceed 45 feet mean sea level we will proceed with Phase II which is the review and, if appropriate, revising of assumptions made in Phase I, using assumptions modified to reflect expected or more realistic conditions and occurrences.