The characteristics and morphodynamics of sedimentary structure in gravel-bed rivers: implications for sediment entrainment

While Earth Scientists acknowledge that bed structure influences flow resistance and sediment transport, relatively little is known about the structural properties of water-worked gravel beds and how bed structure influences particle stability. To address this concern, this research first explored the structural characteristics of 11 representative patches from nine gravel bars from six humid temperate and two dryland rivers. Streambed microtopography was analysed using a suite of statistical metrics, including bed elevation moments, variograms and local inclination, slope and aspect. To investigate how bed structure influences bed stability, for two humid temperate and three dryland patches, measurements of force balance parameters (pivoting angle, projection and exposure) were used to estimate sediment entrainment thresholds. Finally, the temporal dynamics of bed structure was examined through repeat surveys of two humid temperate patches over a 28-month period. These patches were divided into three sections to investigate the natural temporal variability of bed structure and the effects of shorter and longer periods of flow in reworking an unstructured surface. The humid temperate patches were characterised by a water-worked grain- and bedform-scale structure which, while variable, was comparable to previous observations (e.g. near-normal bed elevation pdfs; grain size dependent σz). The dryland patches were, for the bed elevation pdfs, statistical moment Kuz* and variograms, structurally distinct from their humid temperate counterparts. Differences in bed structure between the humid temperate and dryland patches influenced grain pivoting angle and protrusion and, hence, particle stability. For the humid temperate patches, streambed structure was largely maintained over a range of competent flows. Following surface treatment, larger adjustments in bed structure occurred during initial water-working: subsequent changes were comparatively modest. However, since changes in bed structure were subtle, it was difficult to determine whether a natural bed structure had fully formed during the initial period of restructuring.