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. Gravel size is held uniform, 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 eight flood tests demonstrate a remarkable tradeoff. In a short “boundary layer” inlet reach, the bed elevation (measured with a sonar-transducer system, simultaneously at different streamwise positions of the flume and with a high temporal resolution) 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 1D analytical and numerical modeling. This modeling shows that the tradeoff is inevitable as long as the morphologic response time of the reach in question is sufficiently long compared to the time 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 bedload transport rate to fluctuating flow discharge.

Major accomplishments:

Papers
Wong, Miguel, and G. Parker (2006), One-dimensional modeling of bed evolution in a gravel-bed river subject to a cycled flood hydrograph, Journal of Geophysical Research – Earth Surface, 111, doi:10.1029/2006JF000478.