Laserfiche WebLink
<br />STUDY RESULTS <br /> <br />Low water or fair weather antecedent conditions were adopted for <br />the dam failure. It is considered by the County that these <br />conditions would comprise the greatest risk for loss of life and <br />property damage. The reservoir was assumed to be at normal pool <br />(spillway) elevation 1881.5 feet msl. The breach was assumed to <br />take 10 minutes, have a rectangular shape 300 feet wide and <br />extending 138.5 feet below the spillway crest (to the streambed <br />elevation downstream). The downstream flow preceding the breach <br />was taken as 5,000 cubic feet per second (cfs). This was <br />necessary for the model to solve for initial downstream <br />conditions. This is a rather large flow for fair weather <br />(summer) conditions when, for example, a flow on the order of <br />200 cfs might be expected in the South Umpqua river. However, <br />when considering that the maximum computed dam breach flow at <br />the gaging station near Brockway was 53,000 cfs, the river stage <br />at this flow only needs to change about 1.4 feet to account for <br />5,000 cfs. <br /> <br />Breach simulations were also performed for PMF and December 1964 <br />flood conditions. For both of these conditions, it was assumed <br />that peak flows were occurring throughout the river system when <br />failure occurred. For the 1964 flood condition, the dam failure <br />flood wave resulted in a 4 foot rise in water level at the <br />gaging station near Brockway as the flood wave passed. This <br />rise occurred over a three hour period. For the PMF condition, <br />the water level rise was 3.6 feet over a three hour period. <br /> <br />The adopted fair weather breach indicates that water levels <br />would be expected to rise quickly as the flood wave passes <br />downstream. In the upper reaches of the study area, the water <br />level would drop quickly as the peak pas5ed. In the lower <br />reaches, the water level would tend to remain near the peak <br />stage for a time as a result of flood wave attenuation due in <br />part to channel and overbank storage. For example, at Glendale, <br />Cow Creek would rise on the order of 22 feet in about one half <br />hour, and then recede at a rate of about 4.5 feet per hour for <br />the next 2.5 hours. In the Winston area, the water level would <br />rise about 15 feet in three hours, then gradually rise about <br />another 8 feet over the next 6 hours. The water level would <br />then recede about 3 feet per hour. At the confluence of the <br />North and South Umpqua Rivers (end of study area), the flood <br />wave would arrive about 17 hours after dam failure. The river <br />would rise about 12 feet in two hours, then gradually rise about <br />another 2 feet and remain within bank for about 10 hours. The <br />river would then recede at about 1.5 feet per hour. It is <br />expected that real time information on river stages and travel <br />times would be available by then. <br /> <br />/ <br /> <br />B 5 of 11 <br />