One-dimensional modeling of bed evolution in a gravel bed river subject to a cycled flood hydrograph

How does a mountain river adjust to accommodate repeated flood hydrographs? Do flood hydrographs cause major cycles of aggradation and degradation of the river bed? Here flume experiments are used to explore this problem. The response of a gravel bed river to repeated floods is modeled in the simplest possible way. The gravel is well sorted, the flume is operated in sediment feed mode, and the gravel feed rate is held constant. The flow discharge, on the other hand, is specified in terms of the repetition of the same hydrograph until mobile bed equilibrium (averaged over the hydrograph) is achieved. The results of the experiments demonstrate a remarkable trade-off. In a short inlet "boundary layer" (transition region) the bed elevation and bed slope fluctuate cyclically with the changing flow discharge, while the gravel transport rate remains nearly equal to the constant feed rate. Downstream of this short reach, however, the bed elevation and bed slope do not fluctuate in response to the hydrograph; all the fluctuation is transferred to the gravel transport rate. These results are verified in terms of one-dimensional analytical and numerical modeling. This modeling shows that the trade-off is inevitable as long as the morphologic response time of the reach in question is sufficiently long compared to the duration of a single hydrograph. The implication is that gravel bed rivers tend to adjust to hydrographs so as to minimize the response of the bed and maximize the response of the bed load transport rate to fluctuating flow discharge.