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WWRC 89-07
Sediment Transport Relations and Channel Maintenance Implications for Big Sandstone Creek

Executive Summary

Study Objectives
The objective of this study has been to investigate sediment transport and storage processes in the Big Sandstone Creek drainage in the vicinity of proposed water diversion structures and relate these findings to the need for channel maintenance flow releases.

Study Approach
In July 1986, three reaches were selected for study in the Big Sandstone Creek drainage. Two reaches were located at proposed diversion points on the North and South Forks, while a third was downstream below the confluence of these tributaries. Multiple transects were established at each reach to investigate bedload and suspended load transport and sediment storage characteristics through the 1987 and 1988 spring runoff seasons. Based upon dimensionless flow duration analysis and the developed sediment transport relations, mean annual sediment budgets were determined for each reach. Also, simulated hydrographs and sediment budgets were developed for the lowermost study reach by applying a development scenario to the measured 1987 and 1988 natural hydrographs.

Study Findings
Under the natural flow regime, sediment storage in the Big Sandstone reaches is quite stable. High, short duration flows import more finer material (less than 2.0 mm) into the reaches than is exported, while lower, more frequent discharges tend to export this excess, thus maintaining a relative balance. Dominant discharges are less than 10 times the average annual flow and are less than bankfull.

Analysis of natural and simulated post-development average annual sediment budgets indicates that channel aggradation and encroachment should not occur in upper Big Sandstone Creek as a result of water development. This conclusion is supported by the findings of Wesche et al (1988), who found that the dimensions of steep, rough mountain stream channels could be maintained despite significant flow depletion in the forest snowpack zone. The key to maintaining such channels does not appear to be the release of relatively large channel maintenance flow regimes, but rather an effective erosion control program during and after construction.

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