Figure 25. --Summary of zero intensity lines for earthquakes of record in the vicinity of Trinity Dam. installations A, B, and C; and 55 surface settlement points. The installation and operation of these instruments are described in designations E-27 through E-35 of the Earth Manual, revised edition, 1963. The instruments were installed by contract under Government supervision as the dam was being built (see sec. 359). B. Spillway, Outlet Works, and Auxiliary Outlet Works 1. Design Criteria 46. Design Flood Studies. (a) Inflow Design Flood.--The inflow design flood for Trinity Dam has a peak of 160,000 second-feet and a 6-day volume of 489,000 acre-feet. This design event represents the floodfrom a midwinter rain falling on a moderate cover of new snow. (b) Drainage Area. --The drainage area includes 688 square miles of the Trinity Alps, some of the most rugged and precipitous terrain in California. Elevations range from about 2000 feet at the damsite to over 8000 feet on some of the mountain peaks. The thin soil cover is largely decomposed granite and shale. Vegetative cover on the lower elevations is manzanita and chapanal, while the higher elevations are covered with heavy conifer forest. (c) Design Storm.--The design storm was derived by maximizing the observed storms of December 7 through 11, 1939, and November 28 through December 4, 1941. This design storm has a 6-hour value of 4. 4 inches and a 74-hour total of 17.7 inches and is applicable to the period of November through March. An antecedent snow cover of 5.5 inches water equivalent was assumed to be on the basin when the rainstorm occurred. Retention loss rates ranging from 0.2 inch per hour at the start of the storm to a minimum of 0.13 inch per hour were subtracted from the storm rainfall. (d) Flood Hydrograph. --The basin lag time of 17 hours was selected after analyses of recorded floods. The unitgraph procedure was used to develop the inflow design flood hydrograph, utilizing the rainfall excesses and snowmelt runoffs from the design storm studies. For routing the inflow design flood through the reservoir, a water surface at the top of the conservation storage was assumed to prevail at the beginning of the hydrograph period. (e) Diversion During Construction. --Hydrographs for use in planning diversion requirements during construction were based on a frequency study of the streamflow records at the Lewiston gage. These hydrographs, shown in figure 29, have the following characteristics: Figure 29.--Diversion requirement hydrographs based on Trinity River flow at Lewiston gaging station. |