Suspended and bedload dynamics in a tidally influenced river: The river Dyfi, Wales, UK: The river Dyfi, Wales, UK

In fluvial-estuarine systems, three main physical processes zones can be identified: a) river-dominated; b) tide- dominated; and c) wave-dominated. In the river-dominated zone, water and sediment transport is directed seaward, whilst in the marine-forced zones, water and sediment fluxes may be bi-directional. Although the spatiotemporal dynamics of water and sediment fluxes in both river and marine-forced systems have been comprehensively studied, the controls, timing and magnitude of material transport through the tidally-influenced zone is only poorly understood. Here, initial results are presented from a field experiment that aims to elucidate the relative role of river and tidal flows on the net pattern and magnitude of sediment transport through a 10 km tidally influenced reach of the River Dyfi in west Wales, UK, which drains into Cardigan Bay and the Irish Sea. Specifically, we report material fluxes under a set of contrasting marine and fluvial forcing scenarios (spring/neap tides and low/flood river flows). The experimental design incorporated measurement of suspended and bedload material fluxes using a combination of time-integrated and stage-dependent sampling, turbidimetry and coarse particle tracing. These measurements were tied to the hydraulic regime through a network of bi-directional flow measurements in the study reach. Additional analysis of material provenance was established through mineralogical and particle size classification. Results from a 3 month field campaign demonstrated a net seaward sediment transport (suspended and bedload) through the tidally-influenced system. Whilst tidal forcing of suspended transport is evident, these fluxes are an order of magnitude lower than those observed in river flood conditions (concentrations vary between 1-100 mg/l in tidal cycles but exceed 3000 mg/l in river floods). Fine to coarse gravels were entrained and transported 1-10 m during bankfull river floods, however, there was little comparable transport in tidal cycles, although very fine gravels were moved during high spring tides. Mineralogical, geochemical and particle size analyses are revealing contrasting sediment provenance dynamics during river floods and tidal cycles. These data will be used to establish the relative frequencies, magnitudes and flux directions of components of the seasonal sediment budget due to tide and river currents.