13000000095 Continental Dam Inundation Report

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CONTINENTAL DAM - 2 - July 11, 2013 INUNDATION MAPPING REPORT Table 1. Continental Reservoir Dam Failure Summary of Elevation Data Description 1925 & 1990 Plans (feet, unknown datum) SEO Capacity Table (feet, gage height) Dam Crest 10290 86.5 Emergency Spillway Crest 10280 76.5 Outlet Pipe (Upstream Invert) 10203.5 0 Approx. Natural Ground under Embankment 10200 -3.5 The elevations cited in the remainder of this memo are in reference to the gage height. The elevation of the upstream invert of the outlet culvert defined on Revised Sheet 4 of 5 of the 1925 plan set closely matched gage height 0. The approximate surface area of Continental Reservoir was calculated using the SEO capacity table for each gage height. At a gage height of 1 foot the surface area was calculated as 3.7 acres. Based on this small surface area it was concluded that any water which may be stored below gage height 0 is negligible relative to the maximum storage volume of 22,678 acre-feet above gage height 0. Though no extra storage below gage height 0 was considered in this analysis, the breach bottom was assumed to extend to gage height -3.5 because this is the approximate natural ground elevation underneath the embankment. An elevation-area-capacity curve based on the SEO capacity table is provided in Appendix B. Two empirical methods for predicting the average breach width and failure time were compared. The methods, which are included as spreadsheets in Appendix C are the Froehlich (2008) equations and the MacDonald & Langridge – Monopolis/Washington State (MLM-WA, 2007) equations (collectively referred to as the “empirical equations”). The empirical equations predict the average breach width and failure time as a function of depth of water, dam height, and volume of water stored. The MLM-WA method also takes into consideration the estimated volume of embankment eroded. The Froehlich method resulted in predicted bottom and average breach widths of 181 and 244 feet, respectively, and a breach formation time of 1.08 hours. However, based on the supplemental “as- built” sheet in the 1925 plan set, the canyon will constrain the breach to a bottom width of approximately 90 feet and an average width of approximately 215 feet. The breach formation time of 1.08 hours remains unchanged because it is independent from the breach width in the Froehlich method. The MLM-WA method resulted in a predicted bottom breach width of 55 feet, an average breach width of 235 feet, and a formation time of 2.99 hours. The “Results Check” section of the MLM- WA spreadsheet also predicted that the erosion rate calculated by the equations was slower that what can be reasonably expected. For this reason, the breach parameters predicted by the MLM- WA method were not considered for use in the breach simulation of Continental Dam.