Temperature effects in alluvial streams

Abstract

A laboratory investigation was conducted to determine the effects of water temperature on sediment discharge close to the bed (bed-load discharge), and on bed roughness and geometry in alluvial, open-channel flows. Three types of experiments were made: 1) Low-transport, flat-bed experiments in which all of the sediment discharged moved as bed load; 2) high-transport, flat-bed experiments with fine sands wherein there was considerable suspended sediment discharge; and 3) a series of experiments where the discharge was kept constant and the velocity varied to produce ripple, dune, and flat-bed configurations. The experiments were made in pairs. In each pair the velocity and depth were the same or nearly the same, but in one experiment the water temperature was from 15°C to 20°C higher than in the other. It was found that in low-transport, flat-bed flows where particle transport is by rolling and sliding along the bed, a 15°C to 20°C increase in water temperature can produce a relatively large change in sediment discharge. The nature of this change depends on the flow condition at the bed. With hydrodynamically smooth flow there is an increase in sediment discharge with increase in water temperature; whereas in transition from smooth to rough an increase in water temperature effects a reduction in sediment discharge. With fully-rough flow which obtains at boundary Reynolds numbers larger than approximately 200, sediment discharge does not depend on water temperature. A phenomenological explanation has been presented for these observed temperature effects on sediment discharge. In high-transport, flat-bed flows with suspended sediment transport, it was observed that the temperature effects on bed-load discharge are qualitatively the same as those which obtain in low-transport, flat-bed flows of approximately the same boundary Reynolds numbers. It was also found that under certain flow conditions a change in water temperature alone can cause a change in bed form. The nature of this change in bed form seems to be related to the boundary Reynolds numbers R'_(*b) of the flows. For R'_(*b) less than a value near 8 bed form transitions were accomplished at lower velocities in a warm water flow than in a cold water flow at the same discharge; whereas for larger values of R'_(*b) contrary temperature effects on bed form transitions have been observed.

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