Bedforms in Sand Bed Rivers

Abstract

Bedforms are topographic features of river beds that are smaller than channel-scale bar forms and that have specific geometric properties. There are three broad classes of bedforms commonly observed in sand bed rivers: ripples, high angle dunes (HADs) and low angle dunes (LADs). There are also a variety of other bedforms (upper-stage plane beds, antidunes and cyclic steps) that develop under certain hydraulic conditions, that are rarer in sand bed rivers, but do occur. The equilibrium size of ripples is a function of grain size, fluid viscosity and transport stage. The equilibrium size of dunes in rivers is a function of transport stage, and not flow depth as is traditionally assumed, although dunes do get larger with the size of river systems. Bedforms are a key component of flow resistance in rivers, which can be partitioned between grain and form resistance. In sand-bedded rivers, form resistance is primarily generated by bedforms. The methods to partition grain and form resistance are based on empirical coefficients, which limit their use. Understanding flow separation dynamics and eddy generation over bedforms is leading to more rational, physically-based methods for predicting the form-related components of boundary shear stress and flow resistance. Bedform growth and decay occurs by some combination of local flow dynamics over topography, which create sediment flux divergences, whose phase relative to bed topography cause bedforms to get larger or smaller, and bedform kinematics that cause merging and splitting of bedforms to create larger or smaller features. Relations between transport stage and bedform height, length and migration rate allow calculation of bedform-related sediment fluxes in rivers. Inversion of those relations permit first-order paleoreconstructions of flows that created cross strata generated by bedforms in rivers.