FLOOD03848 - Laserfiche WebLink

Home Browse Search

I I I I I I I I I I I I I I I I I I I 27 S~spanded Sediment Load Catchment Period Suspended Se~lment Stream location Area,lIll of Record load, t/mf /y Cherry Creek Melvin 360 1942-1948 722 Clear Creek IdahaSprlngs 264 1953-1955 125 South Platte Henderson 4,713 1942-1944 299 1946-1948 South Platte Ft. Morgan 14,810 1944.1948 124 28 Enginet!rshasmeasuredthesefluv1aldeposfts. Dorough (198J) reports that 3035 acre-feet of sediment accumulated in the lake in 17.4 years. A large portion of this came with the June 1965 flood. This extreme flood also contributed an immense quantity of trees and driftwood which was sub. sequently burned and burfed. The 3035 acre-feet is equivalent to a yield of approximately 600 tons per square mile per year. The measured dry unit weight of deposits in Cherry Creek is 60 pounds per cubic foot. ClearCreek. For the proposed damsite in Arvada the Corps of Engineers (1978) used a value of 0.10 acre.feet per square mile per year for allotting sediment storage in the reservoir. The catchment area is 520 square miles. That is approximately 165 tons per square mile per year. Sediment Balance lOO.year period. That is equivalent to an annual sediment yield of approxi- mately 0.07 acre-feet per square mile or 110 tons per square mile. The amount of sediment deposited in Chatfield Lake in the periOd from the completion of the dam in 1975 to September 19BO has been measured by the Corps of Engineers, ()l1aha District. Dorough (1983) reports that because materials were taken from the reservoir area to build the dam, the initial survey was modified an unknown amount. Therefore, the current esti. mate is unre1iable. Almost all of the sediment entering the lake is stored in the lake and in the reach of river immediately upstream. The sediment deposited 'in the reservoir behind the City of Denver's Cheesman Dam was measured in 1931. The depos1ts amounted to 1040 acre-feet in 30 years, which reduces to 0.02 acre-feet per square mile per year or approximately 30 tons per square mile per year. As discussed previously. the estimated annual suspended sediment load in the South Platte River at Henderson is 130 tons per square mile prior to the construction of t~,e three flood control dams in the Metro- politan Denver area. The sediment yield is greater than this value by an amount between 10 and 25 percent for bed loadandan unknown amount because the samples were taken at the water surface. According to the Corps of Engineers (1978) the annual total load is 299 tOns per square mile. After the closure of Chatfield, Cherry Creek and Bear Creek Cams, the main sources of sediment are Sand and Clear Creek and the erodible banks and bed of the South Platte River between Chatfield and Henderson. Bear Creek. The laO-year sediment storage allotment for the Bear Creek reservoir is 2400 acre-feet (Corps of Engineers, 1973). The catchment area is 260 square miles, giving a sediment yield of approximately 150 tons per square mile per year. Cherry Creek. Almost all the sediment derived from the Cherry Creek catchment is stored in the lake behind Cherry Creek dam. The Corps of From the information assembled above. one can formulate a tentative and approximate sediment yield balance for the South Platte River in the study reach prior to the closures of the flood control dams. Sediment Yield Balance Stream Location Annual Sediment Load,tons South Platte River Bear Creek Cherry Creek Sand Creek Clear Creek Other tributaries Chatfield Mouth Mouth Mouth Mouth South Platte River 330,000 40,000 300,000 150,000 100,000 BO,OOO 1,000,000 South Platte River Henderson With the flood control dams at Chatfield. Bear Creek and Cherry Creek, the sediment supply is estimated to have decreased by approximately BO percent. Need for Data There is a critical need for sediment data to aid in planning the development of the South Platte River in the Metropolitan area. The first priority is to obtain the particle sizes of sediment at single crossings between concurrent mcander bends. Studies of other rivers indicate that itis the size of material on the crossings which controls the rate of degradation. At the same time the sediment data are collected, the crossings should be surveyed accurately. Sequential surveys will be used to determine the degradation rate and the need for riverbed control.