The Formation of Incisional Boundary Layers In Bedrock-Alluvial Rivers Subjected to Spatiotemporally Varying Alluvial Transport

Abstract:

The Capacity-based Saltation Abrasion (CSA) bedrock incision introduced by Sklar and Dietrich (2004) has opened new horizons in the study of the morphodynamics of mixed bedrock-alluvial rivers. The CSA formulation has a major limitation, however, in that it relates bedrock cover only to the sediment supply rate, without providing a mechanism to route this sediment downstream. As a result, the formulation cannot treat the morphodynamics of bedrock incision under the condition of waves of alluviation passing down the river. The Macro-Roughness-based Saltation-Abrasion-Alluviation (MRSAA) model introduced here a) relates areal fraction of alluvial cover to the thickness to which characteristic bedrock roughness elements are buried in alluvium (rather than sediment supply rate), and b) tracks alluvial thickness via an Exner equation of sediment conservation modified to capture both at-capacity and below-capacity transport. MRSAA thus has the ability to track the waves of alluviation and rarification that are likely to characterize the morphodyamics of bedrock-alluvial landscapes subject to dynamic relative base level change (e.g. rock uplift), where discrete landslides and debris flows may be common. Here we apply MRSAA to the case of a 1D channel subject to repeated “sedimentographs” of sediment supply at the upstream end. We show that under such conditions, an “incisional boundary layer” forms in the first ~ 20 km downstream of the sediment feed point, where the mean bedrock slope can differ substantially from that associated with constant sediment feed rate. Farther downstream, the sediment wave is damped and smeared out, and the long-term bedrock morphodynamics differ little from that associated with a constant feed rate. Here we consider both the cases of a repeated periodic sedimentograph and a randomized sedimentograph.