Graduate Thesis Or Dissertation

 

Hydraulic influences on pool morphology : a laboratory investigation Public Deposited

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  • A flume study was conducted to investigate the influences of changing hydraulic conditions on the bed morphology of a scoured pool. During a simulated storm runoff event, responses of the pool were tested under different conditions: clearwater flow, sediment supply at steady rates, and two different sediment mixtures of the pool material between two fixed riffles. Bedload transport in this modelled riffle-pool-riffle sequence was measured to better understand the transport system in a gravel-bed stream. In addition, an attempt was made to verify the hypothesis of 'competence reversal' between the upstream riffle and subsequent pool. This hypothesis tries to explain areal sorting mechanisms in pools and riffles. Significant influences on the scour shape of a pool are the quantity and quality of the upstream sediment supply and the grain size distribution of the bed material itself. Higher sediment input into the riffle-pool-riffle system results in shallower pools. Vertical velocity profiles, approaching logarithmic profiles similar to those on the riffle, are then characteristic. Negative and zero bottom velocities are predominant for those pools shaped under clearwater conditions. The equilibrium scour depth in the case of a heterogeneous bed with a protecting surface layer of coarse particles (armour layer) might be a misleading indicator of the actual scour processes. Partial armouring in the pool can cause deeper local scour than in a non-armoured bed. The total volume of scoured material, however, might still be less than in the case of a non-armoured bed. Hence, even though the depth of a scoured pool can be larger in an armoured bed, less bed material might be transported out of the system. Under steady flow and steady sediment feed conditions, bedload transport occurs in an unsteady, pulse-wise pattern. Sediment piles up within the pool and moves downstream in waves. 'Competence reversal' could not be verified in the conducted experiments. Under the modelled flow conditions, bottom and mean velocities in the pool never exceeded those on the riffle -- one possible condition for competence reversal. The analysis of other hydraulic parameters also did not indicate any possible reversal. A more detailed investigation of increasing turbulence with increasing discharge in the pool is suggested. This could be a main driving force for large particle transport from the pool onto the downstream riffle during different flow conditions.
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