Physically-based model of downstream fining in bedrock streams with side input and verification with field data

Bedload particles in bedrock streams receiving side input from hillslopes may or may not show a clear pattern of size reduction in the downstream direction. Both abrasion and selective sortingmay play important roles in mediating downstream fining. The objective of this study is to develop a physically-based model of downstream fining in bedrock streams based on both processes. A surfaced-based gravel transport relation for size mixture due to Parker (1990) is employed here to account for the effect of selective sorting (differential transport). While the model produces silt and sand by abrasion here it is also assumed to loosely capture particle splitting (as well as crushing, cracking, and grinding) via a lumped abrasion coefficient embodied in Sternberg's law. The model has been verified against field data from Vieux-Habitants River in Guadeloupe Island, which is located in the Caribbean Sea. The river shows clear downstream fining, and this pattern is captured reasonably well by the model. The verification of the model against the field data as well as field observations, however, suggest that the uses of a full flood hydrograph and an adapted sediment transport formula that captures the effect of drag forces associated with large, immobile boulders are essential for an improved model. In addition, rock splitting may play a vital role in downstrem fining (especially for weaker rock types). The order of magnitude of size reduction obtained from the model results suggest that abrasion (including splitting) and selective sorting can be equally important for downstream fining in bedrock streams only in certain cases, such as streams with relatively smaller grain sizes and lower slopes. In some cases, the results show that selective sorting (differential transport) in one extreme flooding period may not play a dominant role in downstream fining (such as inVieux- Habitants River). However, selective sorting due to a full flood hydrograph (or selective entrainment), such that smaller sizes predominate in the load during the lower flows but the largest sizes are only moved near the peak flow, appears to characterize Vieux-Habitants River. The results also suggest that future work may need to explicitly consider splitting mechanisms rather than lumping them in a Sternberg-type coefficient.