Mapping 2‐D Bedload Rates Throughout a Sand‐Bed River Reach From High‐Resolution Acoustical Surveys of Migrating Bedforms

Abstract

Measuring bedload in sand-bed channels is important for a number of river and coastal management issues (e.g., sediment budget, channel stability, navigation, ecological habitat, coastal retreat) but it remains a challenging operation. Due to technical difficulties, cost, and field hazards, manual sampling techniques are often replaced by surrogate techniques such as dune-tracking from acoustical sounding of the bed topography. Bedform characteristics such as bedform height, wavelength, and velocity can be determined with reasonable accuracy, then used to determine bedload if two successive surveys of bed elevation are available. Based on the Exner equation for sediment continuity, the mean bedload rate q b over a bedform wave length can be expressed as (Simons et al., 1965):where p is the bedform porosity, V b is the bedform velocity, H b is the mean bedform height (thickness) and q b,0 is a constant bedload rate that may be interpreted as the transport rate below the bedform baseline (Simons et al., 1965) or as the fraction of bed material load that moves intermittently in near-bed suspension (McElroy & Mohrig, 2009). Traditionally, the mean bedform height is computed as H b = α b H b,max where H b,max is the crest-to-trough height of bedforms and α b is the bedform shape factor. Most often, dunes are assumed to be triangular in shape, hence α b = 0.5.A number of methods for determining bedform characteristics from a bed-elevation profile (or longitudinal transect) exist (see van der Mark & Blom, 2007 for a review). The detection of bedforms can be based on either manually selected crests and troughs, local extremes, or zero crossings in the detrended bed-elevation profile. These methods come with a variable degree of subjectivity, usually requiring some choices and thresholds to be set to avoid including smaller, superimposed bedforms or minor variations in the bed-elevation profile. Traditionally,