Bed material routing and streambed composition in alluvial channels

Graduation date: 1981A descriptive model of bed material routing was developed for alluvial streams exhibiting sequences of pools and armored riffles. The model assumes that channel geometry, sediment transport competence and the availability of sediments for transport are all non-uniform in the downstream direction. Bedload transport is described as occurring in two relatively distinct phases. Phase I involves the transport of\ud predominantly sand-sized bed materials over stable riffles. Phase II occurs at flows which are greater than those required to entrain riffle armor and involves the transport of riffle sediments in addition to Phase I sediments. Phase I bedload transport was sampled during three high flow events at Flynn Creek, a 2 km², third order drainage in the Oregon Coast Range and at Huntington Creek in the Wasatch Plateau of central Utah during a controlled release of water from Electric Lake reservoir. A power relationship existed between Phase I bedload transport and discharge at Flynn Creek. The relationship was consistent between storms and between years. At Huntington Creek, Phase I bedload transport correlated with discharge on the rising limb of the hydrograph, but transport rates decreased over time at constant discharge, indicating a sediment supply control over Phase I transport. Phase II bedload transport was sampled at Flynn Creek during a 1.8 year return period streamflow event in February, 1979. Bedload transport rates were closely correlated with the storage and release\ud of bed material from the riffle at the bedload sampling cross section; transport peaks corresponded to periods of scour and large decreases in transport occurred during periods of deposition. Particle size analysis of the bed material in transport showed as much as a 12-fold increase in the transport of large (> 12.5 mm diameter) bed material during periods of riffle scour. Phase II transport, which involves rapid scour and redeposition of riffles, can be described as highly non-uniform in a downstream direction and highly unsteady at any point over time. At Flynn Creek, significant scour and redeposition of riffles can be expected to occur on an average of once every one to two years.\ud Corresponding to the two phases of bedload transport are two processes which result in changes in the particle size composition of\ud riffle sediments. First, the intrusion of fine sediments into the pore spaces of stable riffles dominates during Phase I transport. Second, the deposition of entirely new riffle features occurs during\ud Phase II transport. Deposition is often associated with one or more streambed scour and fill sequences and is the direct result of instantaneous differences in sediment transport rates between different\ud channel locations. The particle size gradation of riffles is determined primarily by bed material deposition. Sampling at Flynn Creek indicated that a riffle contains the range of sediment sizes found in transport at the time of deposition but, over all, is more coarse than the particle\ud size distribution of sediment in transport. In flume studies of the deposition process, riffle composition was shown to be sensitive to the rate of sediment transport at the time of deposition--becoming more coarse at lower transport rates. Small but significant (p = 0.95) decreases in the median particle diameter of riffle sediments resulted as the sediment mixture in transport increased from a 1:1 to a 5:1 sand-to-gravel ratio. Flume studies showed the intrusion process to be selective towards finer sediments which cannot be filtered by the riffle sediment\ud matrix. Intrusion may decrease or even cease if filterable sediments block surface pores in riffle substrate. Intrusion, if it occurs, is a function of two factors: (1) the pore size distribution of the stable riffle feature, and (2) the particle size distribution and concentration of suspended and Phase I bedload sediments in transport immediately above the streambed