DWR_2726118 - Laserfiche WebLink

 Page 6 Upper and Lower Emerald Valley Dam Failure Report January 23, 2015 4.2 Lower Emerald Valley Dam Lower Emerald Valley dam was a larger structure than UEV. During the interview process, there were anecdotal reports that the dam actually failed previously in 1997, and was subsequently reconstructed. A review of aerial photographs from between 1997 and 2011 would support this. The photographs indicate the repair of the “1997 Breach” were located at the same location as the 2013 Breach. The aerial photos indicate the downstream slopes in the area of the 1997 Breach were very steep and did NOT match the remainder of the dam’s relatively gentle downstream slopes. This could have played a significant factor in the rapidity of the dam breach and concentration of overtopping flows on the left end of the dam. Additionally, the dam appears to have been constructed on a timber foundation to support embankment fill on organic foundation materials at time of original construction (timeframe unknown). Based on our field work, as part of the original construction, a timber “seepage cutoff” wall was constructed through the core of the dam. The timbers were well chinked and mortared together, as illustrated in photos in the appendices. It is likely this element of the dam was NOT repaired during 1997 Breach. 4.2.1 Outlet / “Spillway” Configuration Prior to breaching, it appears there was a 3-ft diameter CMP drop inlet principal spillway located near the left end of the dam. It is unknown if this pipe had a trashrack where it controlled the normal water surface elevation in the reservoir. Additionally, there is a 3-ft diameter CMP “emergency” spillway located on the right end of the dam. Evidence would suggest there was an 18- inch steel low level outlet near the left central portion of the dam, with a valve/manhole on the downstream slope. It is unknown if this valve was operational. Figure 7: LEV Plan View of Dam/Breach