Sedimentation and erosion patterns within anabranching channels in a lowland river restoration project

Phase 2A of the Salt River Ecosystem Restoration Project (SRERP) was implemented to increase transport efficiency of water and sediment through a low gradient river reach to alleviate flooding on adjacent properties. The SRERP utilizes a unique anabranching channel design that concentrates base flows within a single deep, narrow channel overflowing onto an alternating series of higher elevation active benches at flood stages. This paper investigated the performance of the project’s hydraulic conveyance and general utility of anabranching channels as a restoration alternative by assessing the distribution and magnitude of deposition and erosion response patterns. Aggradation was not observed within the main channel. High-resolution surveyed reaches experienced mean elevation decreases between 0.08m and 0.29m indicating effective discharge rates transported dominant grain sizes in suspension. Along some reaches, bed scour was sufficient to undercut banks, producing slumps which may affect long-term conveyance capacities. Lateral bank scour was limited to reaches exposed to daily tidal flows. Variable deposition patterns were observed within secondary channels, depending on cumulative precipitation, dominant hydrology, and channel entrance orientation. Isolated tidal flows resulted in deposition, while long duration flood flows produced intertidal floodplain scour. Within fluvially-dominated benches, uniform longitudinal deposition of fine-grained sediments was associated with low channel entrance flow rates. Higher entrance flow rates resulted in concentrated deposition of coarse-grained particles, up to 0.21m, and a longitudinal gradient of decreasing sediment sizes and magnitude. This project confirmed the suitability of anabranching channel systems for efficient hydraulic conveyance within fluvial reaches of lowland rivers and provides general recommendations for future designs