Relationships between rill configuration, flow energy, tributary confluence angles and rill bank collapse

This thesis examines the impact of changes in configuration and confluence geometry on rill development processes under simulated rainfall or runon discharge. Experiments on rill configuration were carried out in a large flume (L = 22 m) under simulated rainfall, using loamy sand and sandy loam soils. Confluence experiments were performed in the same flume, with controlled runon delivered through symmetrical tributaries at varying confluence angles. An eroded rill was stabilized to allow analysis of the relationship between rill configuration and flow energy. The effect of changes in soil water content and rill flow conditions on bank collapse was analyzed in a moulded, unstabilized, intensely instrumented meander channel.; Temporal distribution of stream power in the rill incision experiments and spatial distribution of flow energy gradients along the stabilized loamy sand and sandy loam rill show a tendency towards minimization. Energy dissipation increased with the increase in the flow width/depth ratio, used as an index of cross-sectional channel shape. In the stabilized rill, flow acceleration reduced energy dissipation on the loamy sand but not on the sandy loam. The greater number of sections with knickpoints and chutes which developed in the sandy loam rill obscured a clear relationship between flow acceleration and energy gradient. In wide, shallow rectangular channels, wave resistance dominated with increasing bed roughness, while in narrow, deep, semi-circular channels surface and form resistance were dominant.; Results from confluence experiments showed that smaller confluence angles (<45°) produced braided flow and meandering in the zone further downstream from the junction point (>2 m).; The critical threshold for rill bank collapse coincided with knickpoint migration. Minimal soil shear strength in the bank toe and critical gravitational force generated bank instability while some matric potential in unsaturated parts of the bank and direction of hydraulic gradients away from bank face can be important stabilizing factors.; The complexity of rill configuration-energy pattern interactions in these controlled laboratory conditions, does raise significant questions about the potential accuracy of energy-based rill erosion models in field conditions, and therefore about the practical potential of physically-based soil erosion prediction.