CHAPTER IV. Design—SPILLWAY AND OUTLET WORKS

A. Spillway and Outlet Works Design Criteria

23. Design Flood Studies, (a) Historical Flood.--The maximum flood of record at Navajo damsite is estimated to have been about 67, 000 second-feet on October 5, 1911. The peak discharge measured on October 6 at the nearest downstream gaging station at Bloomfield, N. Mex., was 80, 000 second-feet.

(b) Development of a Snountelt Flood. —The maximum probable snowmelt flood was computed based on the maximum probable spring temperature sequence, together with a seasonal precipitation and assumed snowmelt accumulation of 202 percent of normal. To determine the temperature-runoff relationship and to compute the flood discharge figures, the hydrothermogram procedure was used. Temperatures at Durango, Colo., with a

2-day lag and a base temperature of 45° F., were used in this determination. The volume of total seasonal snowmelt of 16.1 inches was determined by extraction from two U. S. Weather Eureau publications, Technical Paper No. 4 and Cooperative Studies Section Report No. 9. A runoff of 50 percent of this value was assumed, based on the ratios from the years 1941 to 1956. The final seasonal design volume of runoff thus computed is 2, 766, 000 acre-feet. Added to the maximum probable snowmelt flood was a maximum recorded spring rain flood which occurred on June 29, 1927. The combined flood had a peak flow of 56, 600 second-feet and a 30-day volume of 1, 426,100 acre-feet. This flood was compared with rain floods to determine the most critical flood for the spillway design.

(c) Development of the Maximm Probable Rain Flood. —Two design storms were prepared:

(1) A spring storm of 4.61 inches in 84 hours combined with the snow flood of record; and

(2) a fall- or tropical-type storm of 5. 57 inches in 36 hours. The second storm resulted in a flood having the highest peak flow, but which did not prove to be the most critical flood overall. For the fall-type storm, a retention loss rate of 0.15 inch per hour was used. For the spring storm, the basin was considered in two parts, above and below the 8, 000-foot contour. A retention loss rate of 0.05 inch per hour was used for the upper basin and 0.15 inch per hour for the lower basin. The spring-type inflow design flood when combined with a snow flood of record, which occurred in the spring of 1941, resulted in a peak flow of 107, 000 second-feet, a 15-day volume of 675, 500 acre-feet, and a 30-day volume of about 1, 038, 000 acre-feet. The maximum probable fall- or tropical-type storm, which could occur only in the fall, had a peak flow of 138, 000 second-feet and a 4-day volume of 396, 000 acre-feet.

(d) /touting Criteria. —The three design floods were routed with the reservoir full to top of joint-use storage at the beginning of the hydrographs. The inflow design spring rain flood, having a peak of 107, 000 second-feet and a 15-day volume of 675, 500 acre-feet, was the most critical of the three inflow design floods for the spillway design. The inflow design flood hydrograph is shown on figure 20.

(e) Frequency Studies. —To evaluate diversion requirements and reser.-oir operations during construction, the probable 5-, 10-, and 25-year flood hydrographs were prepared using the following values taken from the probability curves included in the flood studies:

1/Less than 5-year spring flood.

Hydrographs of the above floods are shown on figure 21.

24. Reservoir Operations, (a) Reservoir Storage Allocations. --The reservoir storage allocations were computed as follows (see sec. 14 for final as-built allocations):