Lateral versus downstream transport of gravel in gravel-bed meandering rivers

Abstract:

The degree to which gravel is exchanged laterally from eroding banks to point bars rather than transported downstream is largely unknown in gravel-bed meanders. This is crucial for understanding the potential effects bank stabilization on channel form. We use a compilation of field data to calculate the number of bends required for the supply of gravel from bank erosion to equal transport rate of gravel through a reach (N_b). A low value of N_b suggests that most of the gravel transport by the river is derived from local bank erosion and that, in essence, gravel is being shifted from outer bank to downstream bars with little net downslope flux. We compared the migration rate in 18 gravel-bed meanders rivers with calculations of the gravel transport capacity. Using the average bend length measured for the reach, and assuming the fraction of gravel in the banks ranged from 0.1 to 0.8 of the bank height, N_b was < 1 bend for 12 of the 18 rivers and generally < 10 bends for the remainder of the rivers. The meanders with N_b<1 had Shields stresses less than 0.044, which is the median Shields stress of 115 gravel-bed meanders in the literature. We compared these results to 9 gravel-bed meanders where gravel transport rates were available but the migration rates were unknown. For these rivers, we assumed migration rates ranged from 0.005-0.1 widths/yr (the range observed for gravel-bed meanders) and the gravel fraction in the banks ranged from 0.1-0.8. N_b was ranged from <1 to 20 bends, but was generally higher than for the gravel-bed meanders where we calculated the gravel transport capacity. This is not surprising because 7 of the 9 rivers had Shields stresses > 0.045, and higher gravel transport rates would be expected for this dataset. Our calculations suggest that for many gravel-bed meanders, gravel is being exchanged between the bed and banks within one bend, and even gravel-bed meanders with higher Shields stresses are likely exchanging gravel within a given reach. Limiting channel migration may have a deleterious effect on gravel routing in many gravel-bed meanders.