Wave and Tide Influence on Headland Bypassing and Shelf Scale Sand Transport
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Waves and tidal currents resuspend and transport littoral and shelf sediments, and it is important to understand the processes impacting the net transport of sand, with implications for various topics including benthic habitats, marine operations, marine spatial planning, and long-term coastal change. Continental shelf areas comprise ~8% of global sea floor, and embayed beaches separated by rocky headlands represent 50% of global shorelines. These areas host a large proportion of global marine economic activity and recreation. Sandy sediments are potentially mobilised by wave-induced currents across > 40% of the earth’s shelf. Mobilised sediments are susceptible to net transport by tidal residual currents and currents induced by wave asymmetry due to wave shoaling. Net sand transport pathways govern the transport and fate of littoral and shelf sand, influencing bedform morphology, contaminant dispersal, sediment distribution, and morphological evolution. Quantification of the potential for longshore bypassing of sand around headlands is necessary for evaluation of coastal sediment budgets and long-term coastal change. The processes driving net sand transport at shelf, regional and littoral scales, including headland bypassing, will form the focus of this thesis.
Shelf-scale assessments of the dominant drivers of sand transport often do not fully consider wave-tide interactions (WTI), due to the computationally intensive coupled numerical modelling required. WTI non-linearly enhance bed shear stress and apparent roughness due to interaction between the wave and current bottom boundary layers, change the current profile, and modulate wave forcing, The influence of WTI on headland bypassing is an area of ongoing research, and while bypassing rates have been shown to be predictable for idealised headlands, it remains to test the predictability of headland bypassing for realistic headland morphologies and sediment availability. In this thesis, it is shown that WTI can dominate net sand transport in mixed tide and wave energy conditions, the dominant forcing mode and potential magnitude of net sand transport are predictable from readily available data, and headland bypassing rates can be parameterised under wave-dominated conditions, depending upon headland cross-shore length, surf zone width, headland toe depth and spatial sediment coverage.
Sand transport in energetic macrotidal environments can be heavily influenced by waves. Median (50% exceedance) waves generally enhance net sand transport in the tidal direction, while extreme (1% exceedance) waves can dominate net sand transport, increasing it by an order of magnitude and potentially reversing transport direction. A novel continental shelf classification scheme is presented, based on wave, tide and WTI dominance of net sand transport. Here, “WTI” includes radiation stresses, stokes drift, enhanced bottom friction and bed shear stress, current and depth-induced wave refraction, Doppler shift and wave-blocking. Application of this scheme to the macrotidal South West UK shelf shows WTI are a dominant or sub-dominant contributor to net sand transport under extreme waves and spring tides.
To enable application of this classification scheme to wider shelf areas in a computationally efficient way, a k-Nearest Neighbour (kNN) algorithm was trained on model data using readily available uncoupled wave and tide parameters, median grain size and water depth. The dominant mode and magnitude of net sand transport were predicted with 81.9% and 90.8% accuracy, respectively. This kNN classifier was applied to the Northwest European shelf over a synthetic, statistically representative year. The relative influence of waves, tides and WTI varies across the shelf, dependent upon variability in wave exposure, tidal regime, grain size and local bathymetry. Net sand transport in meso-macrotidal areas is tide-dominated, while shallow regions with finer sediments such as Dogger Bank and the German/ Denmark Shelf areas are wave-dominated. WTI dominate on the Netherlands (NL) Shelf and in deeper areas of the North and Celtic Seas. Observed sand wave morphologies on the NL Shelf vary significantly between dominant sand transport modes under extreme waves and spring tides (95% confidence level). Sand waves increase in length and asymmetry, and decrease in height, for increased wave dominance.
To examine the controls on headland sand bypassing, numerically modelled bypassing rates are quantified for 29 headlands along the macrotidal, wave-exposed North Coast of Cornwall under varied forcing conditions. Bypassing is wave-dominated under energetic (>5% exceedance) waves, with tides acting as a secondary control. WTI dominate bypassing for median waves. Non-uniform sediment coverage can reduce headland bypassing by several orders of magnitude when sand is unavailable at the headland toe. A primary control on bypassing is headland cross-shore length relative to surf zone width, whilst toe depth is an important secondary control. By adapting and developing an existing parameterisation, bypassing rates are predictable with a Mean Absolute Error of a factor of 4.6. Only two of the 29 headlands block headland bypassing along this coastline under all forcing conditions, indicating the potential ubiquity of headland bypassing on embayed coasts.
The findings of this thesis emphasise the critical need to consider wave-tide interactions when considering net sand transport in energetic environments globally, where previously tides alone or uncoupled waves may have been considered. The kNN approach applied here can efficiently indicate the dominant processes driving sand transport under a variety of conditions and across large spatial domains, of value to modellers in similar environments. This method will allow efficient inter-regional comparison and sensitivity testing to changing climate conditions. This work demonstrates headland bypassing is amenable to parameterisation in wave-dominated conditions. Such parametrisations of bypassing in realistic settings are entirely novel and the ability to predict bypassing within an order of magnitude is highly useful. This thesis highlights the extent to which headland bypassing occurs with implications for embayed coasts worldwide.