The Effect of Varying Wave Skewness and Asymmetry on Sediment Transport across Intertidal Flats

Intertidal flats and salt marshes provide a natural coastal defence and are important ecological habitats for a wide range of flora and fauna. Due to their significance, they are often subject to conservation legislation. Salt marshes are eroded by strong wave action, yet waves can also transport sediment across intertidal flats to the marshes. Understanding the asymmetric sediment dynamics (supply and loss) is vital for understanding long-term system stability. Time-varying water level over intertidal flats modulates the wave activity and suspended sediment transport. Wave skewness (wave orbital velocity asymmetry) and asymmetry (wave acceleration asymmetry), due to friction from the seabed, affect the transport of sediment to and from the marshes.

The present investigation measures the effect of the varying water level across intertidal flats on wave skewness and asymmetry, and the resultant suspended sediment transport. An improved understanding of hydrodynamic conditions across intertidal flats will allow better assessment of the sediment transport that leads to change in intertidal flat morphology and marsh retreat and accretion.

A transect of five frames, each with an acoustic Doppler velocimeter to measure the hydrodynamics and an optical backscatter sensor to measure the suspended sediment, among other instruments, were deployed across the macro intertidal flats off Flookburgh, in Morecambe Bay, UK during winter 2019. Offshore wave climate was provided by a nearby buoy. These data give the water level, wave properties, currents and sediment fluxes for each site. Gradients and asymmetries of these parameters across the intertidal flats vary with the inundation and retreat of the tide, resulting in temporal and spatial variation in sediment erosion, transport and deposition. By investigating these gradients across the intertidal flats, the resulting asymmetries in sediment transport to and from the marshes can be studied and conclusions drawn about sediment transfer.