Morphology, flow hydraulics and sediment movement in rill networks

Rill networks act as conduits that transfer water and sediment rapidly across the surface of hillslope systems. Knickpoint migration and birfurcation are key factors in network expansion, but their effects on network sediment delivery have received little attention, thus inhibiting their inclusion in general concepts of hillslope sediment delivery. Understanding water and sediment movement through rill networks allows forecasts of sediment transport and erosion processes to be made, and is central to the study of landscape evolution and implementation of soil erosion control measures.; Three sets of laboratory flume experiments are undertaken to examine the evolution of a rill network and the area it drains, how this is organized during a storm period and resulting morphological response. The purpose of this study is to: (1) examine the evolution of soil surface roughness on different slope angles during the initial stages of a storm; (2) describe the mechanisms of rill network inception and expansion; (3) prepare sediment budgets for rill networks developed on a natural soil under simulated rainfall in a laboratory flume; (4) identify changes is the relationship between hydraulic indices and sediment transport as a network expands; and (5) identify the effect of knickpoint migration through confluences on rill hydraulics and sediment flux patterns.; Interrill surface roughness on planar slopes begins as a normally-distributed function of particle size and aggregate size distribution and develops according to the presence of water stable aggregates and large clasts to determine initial stages of flow concentration and network development. Multiple aspects of rill network evolution are identified: scour hole and knickpoint formation on an initial surface; flow concentration; scour and knickpoint migration; knickpoint bifurcation and tributary development. Water and sediment budget dynamics are adequately explained by consideration of temporal and spatial variations in discharge, stream power, shear velocity and sediment transport capacity. Detailed measurements of flow characteristics at junctions show decreased velocities, shear stress and stream power, along with increased turbulence and frictional energy loss as a knickpoint migrates through the confluence hydrodynamic zone.