Headland-associated Sandbanks Research Articles

Impact of a headland-associated sandbank on shoreline dynamics

Sandbanks can occur downdrift of headlands across embayments as a result of tidal currents and wave forcing, and may present a potentially valuable resource of marine aggregate material. Large sandbanks can alter tidal flows and, through refraction and dissipation, increase or decrease wave exposure along the shoreline of an embayment. The impact of offshore sandbanks on shoreline morphodynamics at varying timescales has not been rigorously investigated. Here we show, for the first time, that removing, lowering or raising a headland-associated sandbank can have a significant impact on longshore sediment transport. A Delft3D model of Start Bay and Skerries Bank, UK, validated on hydrodynamic observations, was used to conduct the numerical experiments. Results indicate that removing or lowering the bank generally reduces dissipation and increases wave heights, increasing longshore flux in the lee of the bank. Raising the bank generally has the opposite effect, increasing dissipation and lowering flux. Under oblique waves, the bank may protect the adjacent shoreline by refracting waves away from the coast. Shoreline flux for the region directly onshore of the bank acts as a continuous function of bank height (R2 = 0.7), but correlations for distal regions are increasingly non-linear or absent. A raised bank would reduce transport rates to near-zero across extensive sections of the bay, reversing the long-term net flux at points distal (>5 km from bank) from the bank location, causing currently eroding regions to accrete. Our results demonstrate that moderate (≤5 m) elevation changes to a sandbank will likely produce significant variations in wave power at the shoreline, which will: (i) modify the shoreline response for individual storms; and (ii) substantially change shoreline morphology at decadal timescales. From a broader perspective, this study can be used to inform planning decisions involving naturally varying banks and/or potential mining of sandbanks, aiding shoreline protection strategies for locations with similar embayment-sandbank configurations.

Russia: Stroyspetsmontazha – carbon black from scrap tyres

Sandbanks can occur downdrift of headlands across embayments as a result of tidal currents and wave forcing, and may present a potentially valuable resource of marine aggregate material. Large sandbanks can alter tidal flows and, through refraction and dissipation, increase or decrease wave exposure along the shoreline of an embayment. The impact of offshore sandbanks on shoreline morphodynamics at varying timescales has not been rigorously investigated. Here we show, for the first time, that removing, lowering or raising a headland-associated sandbank can have a significant impact on longshore sediment transport. A Delft3D model of Start Bay and Skerries Bank, UK, validated on hydrodynamic observations, was used to conduct the numerical experiments. Results indicate that removing or lowering the bank generally reduces dissipation and increases wave heights, increasing longshore flux in the lee of the bank. Raising the bank generally has the opposite effect, increasing dissipation and lowering flux. Under oblique waves, the bank may protect the adjacent shoreline by refracting waves away from the coast. Shoreline flux for the region directly onshore of the bank acts as a continuous function of bank height (R2 = 0.7), but correlations for distal regions are increasingly non-linear or absent. A raised bank would reduce transport rates to near-zero across extensive sections of the bay, reversing the long-term net flux at points distal (>5 km from bank) from the bank location, causing currently eroding regions to accrete. Our results demonstrate that moderate (≤5 m) elevation changes to a sandbank will likely produce significant variations in wave power at the shoreline, which will: (i) modify the shoreline response for individual storms; and (ii) substantially change shoreline morphology at decadal timescales. From a broader perspective, this study can be used to inform planning decisions involving naturally varying banks and/or potential mining of sandbanks, aiding shoreline protection strategies for locations with similar embayment-sandbank configurations.

Effects of waves on the initiation of headland-associated sandbanks

Linear sandbanks appear in the lee of coastal headlands where the hydrodynamics are dominated by strong tidal currents and the seabed is characterized by an abundance of sands. They may develop as symmetrical sandbanks on either sides of the headland or as an unique banner bank. The present study numerically investigates the combined effects of waves and tide on the initial development of headland-associated sandbanks. A morphological model based on the coupling of the wave propagation module SWAN (Simulating WAves Nearshore) with the three-dimensional circulation module COHERENS (COupled Hydrodynamical-Ecological model for REgioNal and Shelf seas) is applied to an idealized Gaussian shaped headland for waves conditions varying in heights and directions at the offshore boundary. The coupling considers the effects of the interactions between the wave and current bottom boundary layers, namely the enhanced levels of turbulence near the bottom and the increase of the total bottom shear stress. Waves substantially modify the initial development of sandbanks formed by suspension narrowing their width and reorienting them along the side of the headland. They weakly impact the morphogenesis of sandbanks by bedload favoring on a short-time scale the growth of symmetric circular-shaped features and a central depositional spit prolonging the headland tip. Waves of transverse directions toward the tip of the headland contribute to the initiation by suspension of a well-developed feature in the headland side of low energy limiting the seabed evolution in the exposed area.

Short-term dynamics and maintenance processes of headland-associated sandbanks: Shambles Bank, English Channel, UK

The short-term (over a spring tidal cycle) dynamics of a headland-associated sandbank (Shambles Bank, English Channel) are investigated, by means of field measurements (synchronous data, using an acoustic Doppler current profiler (ADCP) and repeated side-scan sonar imagery) and two-dimensional (2D) hydrodynamic and sediment transport models. The dynamics of the bank are described in terms of along- and cross-bank velocity components, sand transport pathways and bedform asymmetries. The results showed the occurrence of a net bedload convergent zone along the crest of the bank as a result of anti-clockwise veering of sediment transport towards the crest. This transport pattern is suggested to be the result of the dynamic interplay between two distinct tidally related processes acting over the sandbank, during each phase of the tidal cycle: (a) the formation and different stages of a transient tidal eddy that drives bedload movement during the flood phase of the tide and (b) the bottom-friction induced by the presence of the sandbank that governs the bedload transport dynamics during the ebb phase of the tidal cycle.

Water and sediment movement around a coastal headland: Portland Bill, southern UK

Numerical simulations of tidal flow and sand transport around a coastal headland (Portland Bill, southern UK) were undertaken to investigate patterns of sand transport during the development of tidally induced transient eddies. Results obtained from a 2-D finite-element hydrodynamic model (TELEMAC-2D) were combined with a sediment transport model (SEDTRANS), to simulate the sand transport processes around the headland. Simulation of the tidal flow around Portland Bill has shown the formation and evolution of tidally induced transient eddies, around the headland. During the evolution of these transient eddies, no current-induced bedload (transport) eddy is formed for either side of the headland. Net bedload sand transport direction, around a coastal headland, is the result of instantaneous gradients in bedload transport rates, during flood and ebb flows, rather than the average (residual) flow. Thus, the use of residual (water) circulation to describe patterns of sediment movement as bedload is not an appropriatedapproach. In the case study presented here, the distinct characteristics of the coastal and seabed morphology around the Isle of Portland (i.e. headland shape and the bathymetry) indicate that these parameters can be influencing tidal (flow) and sediment dispersion around the headland. Such an interpretation has broader implications and applications to headland-associated sandbanks elsewhere.

Sedimentary processes, bedforms and facies, associated with a coastal headland: Portland Bill, Southern UK

This investigation presents new findings on the sedimentary processes and deposits associated with a coastal headland, in a mixed tide–wave setting, using data compilation (bathymetric and sediment distribution maps), collected field data (side-scan sonar and seabed sampling) and sand transport modelling. Sand transport pathways are described: (a) on the basis of coupled-system seabed morphology/sand distribution; and (b) from bed shear stress and (medium-grained) sand transport rate simulations. The presence of a sequence of sedimentary deposits, associated with a complex suite of bedforms and sedimentary facies within an overall framework of limited sediment supply, represents a gradient in shear stress and sand transport towards the headland. These sequences are observed on both sides of Portland Bill, tending towards a symmetrical distribution. The sedimentary facies distribution is combined with sand transport rates and maximum bed shear stress distribution, to suggest a conceptual model for sand dispersal and deposit formation around headlands. Sand dispersal can be explained in terms of a headland-associated eddy/bedload convergent zone concept. Maximum bed shear stress is observed at the tip of the headland and is associated with bedrock exposed on the seabed. The sequence of sedimentary deposits, away from the headland, is: sandbanks (Shambles and Portland Banks); sand/gravel flats; sand shoals (Adamant and West Shoals); and rippled sand sheets. The banks lie in an area of very high bed shear stress ( u * =0.08 m s −1); this is greater than that for other tidal sandbanks, not associated with headlands. Bank formation is the result of the strong convergent (transport) component, enhanced by the development of transient headland eddies. The sandbanks are to some extent also morphologically controlled, i.e. the Portland Bank is affected by the deep bathymetry to the west side of Portland Bill. The principal characteristic of headland-associated eddy/bedload convergent zones is the development of bed shear stress convergent zones, on both sides of the headland. Two conceptually distinct regions of sedimentary processes, associated with the coastal headlands, are recognised: (a) an inner zone, with increasing gradients in sand transport and bed shear stress towards the headland; and (b) an outer zone, in which sand transport is away from the headland, associated with a decrease in the bed shear stress. These zones merge into a bed shear stress (sand bedload transport) convergent zone, which enhances the formation of sandbanks around headlands. Thus, headland-associated sedimentary deposits are a complex system, compared with the occurrence of isolated sandbanks described elsewhere. The identification of a sequence of deposits related to and extending much farther from the headland than previously supposed, demonstrates that the ‘tidal stirring concept’ of headland-associated sandbanks may explain the formation of headland-associated sandbanks; however, it ignores the presence of a suite of sedimentary deposits around such headlands. The conceptual model proposed here provides a sedimentological perspective on the formation and occurrence of headland-associated deposits, including sandbanks.

Large-scale bedforms as indicators of mutually evasive sand transport and the sequential infilling of wide-mouthed estuaries

Large-scale bedforms (2–10 m in vertical and 10 2–10 3 m in horizontal dimensions) found in wide-mouthed estuaries are described. Different bedform types occur depending upon the local availability of sand. With an increasing sand supply, sand ribbons grade into elongate trains of sand waves and then form sandwave fields. Inshore, headland-associated sand banks are formed which multiply into en-echelon sand banks. Based upon a review of data on directions of sand transport from the Bristol Channel and Thames Estuary, U.K., and from Moreton Bay, Australia, charts of ebb- and flood-dominant transport zones are constructed for lower estuarine environments which have undergone different degrees of infilling. Linear sand banks are seen to delimit partially the boundaries between opposing sand transport zones. Transport paths demonstrate how sediments derived from outside of the estuary are dispersed through ebb and flood transport zones, to supply other areas of net deposition. A comparison between different estuaries reveals that variations in the compexity of ebb- and flood-dominant transport zones and the morphologies of large-scale bedforms are coupled with apparent changes in the relative amounts of sand available to each system. A model for the sequential infilling of estuaries and the evolution of large-scale bedforms is presented and applied to the interpretation of present day examples. Vertical sequences predicted to be generated by such bedform evolution are described and discussed, in terms of their preservation in the geological record.