Physical, hydrodynamic and biological controls on sediment delivery to shoreline in the tidal flats of Morecombe Bay, UK

Tidal flats moderate storm impacts and they enhance the stability of shoreline ecosystems and infrastructure. Morecambe Bay, which has the largest tidal flats in the United Kingdom, is presently a net accumulator of sediment with a flood-dominant tidal asymmetry. However, the rapidly changing morphology limits the ability of surveying to accurately quantify rates of sediment transport and deposition. Estimates of sediment delivery to the shoreline and surrounding salt marshes have large uncertainty, and one key factor is the uncertainty from seasonal controls on sediment transport. Here we report on two field deployments (summer 2018 and winter 2019) to explore seasonal controls on sediment fluxes.

Multiple instrument platforms were positioned on intertidal flats in the bay over a spring-neap cycle. The tidal range up to 3.5 m inundated these platforms for 1-4 h per semidiurnal tide. Instruments measured 3D ripple morphology and migration, mean bed level change, suspended sediment concentration and grain size from a combination of acoustic and optical sensors Acoustic Doppler Velocimetery provided hydrodynamic forcing (tides, waves, turbulence). Winter surveys also measured Chlorophyll-a, Fluorescent dissolved organic matter (FDOM), and extracellular polymeric substances (EPS). Bed samples were collected from both seasons for grain size and EPS in the substratum.

Results will be assessed from the perspective of the initiation of sediment motion, factoring in controls due to biology, sediment sorting, tidal range and wave conditions to examine seasonal variability. Initial results suggest that net transport toward the shoreline was higher in winter than summer. In winter, flood tides eroded the bed while ebb tides deposited sediment. The opposite was found in the summer. Ripple morphology varied from 3D to 2D during individual inundation periods.