Aperiodic embayed sandy beach rotation and erosion-risk exposure on a hyper-muddy wave-exposed coast

<p><span>The 1500 km-long wave-exposed coast of the Guianas, South America, is characterized at any time by up to 20+ large distinct mud banks with suspended mud concentrations of up to 1000 g/l migrating from the Amazon delta to the Orinoco delta under the influence of wave-driven longshore transport. Banks can be up to 60 km-long, strongly dissipate waves, and are separated alongshore by ‘inter-bank’ sectors of similar length. The latter are affected by shoreward propagation of much less dissipated waves that can generate rapid muddy shoreline erosion and reworking of beaches and cheniers formed from sand supplied by rivers draining the crystalline rocks of the Guiana Shield.</span></p><p><span>About 500 km northwest of the mouths of the Amazon, the pervasive mud and its effects on the nearshore wave regime determine, for the embayed, headland-bound beaches in French Guiana, outcomes that are important  from a long-term management perspective. These beaches have come under urban pressures and assure recreational and ecological functions such as provision of nesting sites for marine turtles. The sand-mud interactions, processes of sand segregation from mud, sediment transport modes, and morphodynamics associated with these beaches over timescales ranging from weeks to several decades, were analyzed from aerial photographs, satellite images, aerial photogrammetry, and field experiments. The longer bay beaches are exposed to longshore transport when mud is temporarily scarce (inter-bank phases), and subject in parts to overwash. During inter-bank phases, ‘normal’ westward sand transport along these beaches is generated by waves from E to NE, but is counter-balanced during bank phases by eastward drift at the leading edge of a bank as waves are refracted over the bank. This counter-drift prevails at a ‘mobile’ rotation front that moves with the bank’s leading edge migrating at rates of 1 to 2.5 km a year. As the bank passes, it further shelters beaches from wave reworking, with eventual re-exposure to waves and ‘normal’ drift following complete mud-bank passage. In the context of the ‘closed’ sand budget of these beaches, headlands spatially constrain sand mobility, and the unique mode of rotation induced by mud-bank refraction of waves plays an important role by counter-balancing unidirectional longshore transport that could otherwise result in permanent deprivation of updrift beach sectors of sand. Due to variability in bank-migration rates and spacing, normal drift and counter-drift may prevail, respectively, over periods exceeding two years but of unknown duration. The variability of this time frame of rotation poses a challenge to the implementation of set-back lines necessary to avoid the impingement of urbanization and sea-front activities on the long-term (>decadal) bandwidth of beach affected by rotation, which involves aperiodic and variable erosion and accretion in different parts of the beach. In this context of aperiodic beach rotation, prediction of mud-bank migration rates downdrift of the Amazon and of the imminent arrival of a mud bank, coupled with the firm implementation of shoreline development setback lines, are necessary to mitigate risks from erosion and overwash events.    </span></p>

Regionally-Coherent Embayment Rotation: Behavioural Response to Bi-Directional Waves and Atmospheric Forcing

Bi-directional wave climates often drive beach rotation, increasing erosional risk at semi-sheltered locations. Identification of rotation and forcing mechanisms is vital to future coastal defence. In this study, regional investigation of modelled wave data revealed strong bi-directionality between dominant south-westerly and sub-dominant easterly waves for 14 offshore locations along the length of the south coast of England, U.K. South-westerly wave power was well correlated to positive phases of the West Europe Pressure Anomaly (WEPA), whilst easterly wave power was well correlated with negative phases of the North Atlantic Oscillation (NAO). Additionally, decadal records of beach morphological change and associated wave forcing, were investigated for 22 coastal sites across the same region. Significant rotational behaviour was identified at 11 sites, leading to the creation of a rotation index. Beach rotation was attributed to shoreline angle, with the strongest rotation occurring at south-east-facing beaches, with high obliquity to dominant south-westerly waves. The beach rotation index was well correlated with the normalized balance of wave power from opposing south-westerly and easterly directions. Direct correlations between beach rotation and WEPA at two sites showed that future forecasts of atmospheric indices may allow prediction of rotational beach state, at seasonal scales.