Determination of channel capacity of the Merced River downstream from Merced Falls Dam, Merced County, California

Abstract

This study evaluates the adequacy of a reach of the Merced River between Merced Falls and the confluence with the San Joaquin River to carry flood releases from New Exchequer and McSwain Dams and Reservoirs. The flood release from these reservoirs is to be restricted so that flows will not exceed 6,000 cfs (cubic feet per second) in t he Merced River at the gaging station near Stevinson (about 5 miles upstream from the mouth). Computed floodwater profiles based on channel conditions in late 1967 and observed water-surface profiles for historic floods were used in the analysis. The conclusions reached are contingent on there being no levee failures during periods of high flow. Evaluation of historical flood records at gaging stations between Merced Falls and Stevinson indicates that a reduction in peak discharge occurs as the floods traverse the study reach. Reduction in peak discharge is dependent on river stage, prior river flows, magnitude of tributary inflow, bank storage and infiltration to the ground-water reservoir, and diversion for irrigation. For example, in late June 1967 a peak discharge of 9,860 cfs at Merced Falls was reduced by irrigation diversions to 6,850 cfs at a site 3 miles downstream and further reduced to 6,490 cfs at the gaging station near Stevinson. Backwater effect from high stages on t he San Joaquin River will increase stages on the Merced River and the effect may extend as far as 8 miles up stream from the junction of the two rivers. Backwater may cause some overbank flooding along the 8-mile reach when Merced River flows at Stevinson exceed 4,000 cfs . Most of the area subject to such inundation lies between levees that are a considerable distance--up to half a mile-- from the main channel . In the reach upstream from the backwater effect, a discharge of 4, 500 cfs near Stevinson will cause flooding at two locations, one of which is about 10 miles upstream an the other about 18.5 miles upstream from the confluence . When the discharge is 6,000 cfs , additional sites will be inundated, particularly the 20.5-mile reach of channel downstream from the Highway 99 crossing. A discharge of 6,000 cfs will pass through all bridge openings in the reach but overbank flooding may occur if debris should lodge against the bridge piers and significantly reduce the waterway area . Peak flows in Dry Creek, as measured 18.7 miles upstream from the mouth of Dry Creek, will be attenuated due to channel storage and will increase the discharge of the Merced River at Cressey by only about 50 per cent of t he Dry Creek peak discharge. Furthermore, Dry Creek seldom carries floodflows during periods of high water on the Merced River.