Aeolian dune development and evolution on a macro-tidal coast with a complex wind regime, Lincolnshire coast, UK

Coastal foredunes are natural aeolian bedforms located landward of the backshore and which interact continuously with the beach. Traditionally, coastal dunes have been associated with onshore winds, however they can be found under more complex wind regimes where offshore winds are common such as the UK East coast, Northern Ireland and New Zealand. This research investigates the ways in which foredune-beach interactions occur under a complex wind regime at a range of overlapping temporal and spatial scales and is innovative in that it explicitly links small-scale processes and morphodynamic behaviour to large scale and long-term dynamics. The study area is the north Lincolnshire coast, East England. Detailed observations of airflow at three locations under varying wind regimes revealed considerable spatial variations in wind velocity and direction, however it was possible to determine a general model of how foredune topography deflected and modified airflow and the resultant geomorphological implications (i.e. erosion and deposition). During direct offshore and onshore winds, airflow remained attached and undeflected; and distinct zones of flow deceleration and acceleration could be identified. During oblique winds airflow was deflected to become more parallel to the dune crest. The field sites used are characterized by a seasonal erosion/accretion cycle and a series of increasingly complex models was developed and tested to determine whether it was possible to predict sand volume changes in the foredune-beach system based on a limited number of variables. The model predictions were tested against detailed digital terrain models at a seasonal timescale. The model prediction that best matched the observed (surveyed) sand volume changes included wind speed, direction, grain size, fetch effect controlled by beach inundation and angle of wind approach was accurate to within ±10% for 18 out of 48 tests at the seasonal scale and 6 out of 12 tests over periods of >5 years. A key variable influencing foredune-beach sand volume is the magnitude and frequency of storm surge events and this was not factored in to the model, but may explain the model-observation mismatch over the medium-term on two occasions. Over the past 120 years historical maps and aerial photographs indicate long-term foredune accretion of approximately 2 m year-1 at the three study sites (1891-2010). At this timescale, rates of coastal foredune accretion reflect the low occurrence of severe storm surges and suggest rapid post-storm recovery. The morphological response of the foredune-beach morphology is considered to be a combination of controlling and forcing factors. Process-responses within the system, associated with nearshore interactions and sediment transfer from the littoral drift, are compiled into a multi-scale morphodynamic model. Important to match appropriate dataset to scale of research question or management plan being explored. In the case of management, long-term records of past activity are necessary to predict the future but also to understand natural responses of system to short-term impact such as storm surge

A 150-year record of coastline dynamics within a sediment cell: Eastern England

Coastal sediment cells reflect processes operating at a range of scales, but it is the medium spatial and temporal scales (decades to centuries) that are of greatest interest for coastal management. This paper focuses on coastline position change within a single sediment cell over 150 years where the geomorphology includes cliffs, beaches and saltmarshes. The focus is the east coast of England from Flamborough Head to Gibraltar Point. Although the updrift sector of this sediment cell has been studied for well over a century, the downdrift sector has attracted significantly less attention. Using topographic profiles, bathymetric profiles, aerial photographs and historical maps we mapped coastline erosion and accretion using the Digital Shoreline Analysis System (DSAS) and calculated volumetric changes for different morphometric units. Rapid erosion of the updrift Holderness cliffs has been counterbalanced with accretion on beaches along the downdrift Lincolnshire coast. The amount of accretion in Lincolnshire corresponds to around 29% of the volume of sediment eroded from Holderness. Much of the eroded cliff material is likely to be deposited temporarily into nearshore and offshore sand banks before being redistributed by cross-shore currents. An exploration of storm surge impact on long-term erosion and accretion rates showed no clear relationship between storm surge frequency and change in coastline position, however this may be in part due to the relative timing of storm occurrence and data acquisition. The Jenkinson daily weather type classification was found to be a reasonable proxy for the occurrence of strong onshore winds which may offer scope for further investigation of the role of forcing factors over time periods beyond the length of the meteorological and tidal station records. Winter North Atlantic Oscillation phase was not a good indicator of storminess on the east coast of England but may be a useful proxy for quiescence

Post-storm geomorphic recovery and resilience of a prograding coastal dune system

Geomorphic resilience is the capacity of a system to recover to pre-disturbance conditions following a perturbation. The 2013/14 Atlantic winter storm period had extensive geomorphological impacts and provides an opportunity to assess coastline resilience. This paper uses high spatio-temporal resolution data to quantify the beach-dune response and subsequent recovery of a prograding coastline following the 5 December 2013 North Sea storm surge. It demonstrates that despite the high water levels and destructive nature of the storm, the beach-dune system recovered sediment rapidly over the first post-storm year. Within four years the dune advance had exceeded the seawards position expected based on long-term coastal trends but had not yet recovered the pre storm foredune profile. Cumulative evidence from numerous European locations suggests one of the stormiest periods on record triggered only a minor disturbance to what appear to be highly resilient beach-dune systems

Measuring surface moisture on a sandy beach based on corrected intensity data of a mobile terrestrial LiDAR

Surface moisture plays a key role in limiting the aeolian transport on sandy beaches. However, the existing measurement techniques cannot adequately characterize the spatial and temporal distribution of the beach surface moisture. In this study, a mobile terrestrial LiDAR (MTL) is demonstrated as a promising method to detect the beach surface moisture using a phase-based Z&F/Leica HDS6100 laser scanner mounted on an all-terrain vehicle. Firstly, two sets of indoor calibration experiments were conducted so as to comprehensively investigate the effect of distance, incidence angle and sand moisture contents on the backscattered intensity by means of sand samples with an average grain diameter of 0.12 mm. A moisture estimation model was developed which eliminated the effects of the incidence angle and distance (it only relates to the target surface reflectance). The experimental results reveal both the distance and incidence angle influencing the backscattered intensity of the sand samples. The standard error of the moisture model amounts to 2.0% moisture, which is considerably lower than the results of the photographic method. Moreover, a field measurement was conducted using the MTL system on a sandy beach in Belgium. The accuracy and robustness of the beach surface moisture derived from the MTL data was evaluated. The results show that the MTL is a highly suitable technique to accurately and robustly measure the surface moisture variations on a sandy beach with an ultra-high spatial resolution (centimeter-level) in a short time span (12 x 200 m per minute)

Predicting Morphodynamics for Beach Intertidal Systems in the North Sea: A Space-Time Stochastic Approach

The ability to accurately predict beach morphodynamics is of primary interest for coastal scientists and managers. With this goal in mind, a stochastic model of a sandy macrotidal barred beach is developed that is based on cross-shore elevation profiles. Intertidal elevation was monitored from monthly to annually for 19 years through Real Time Kinematics-GPS (RTK-GPS) and LiDAR surveys, and monthly during two years with an RTK-GPS. In addition, during two campaigns of about two weeks, intensive surveys on a daily basis were performed with an RTK-GPS on a different set of profiles. Based on the measurements, space and time variograms are constructed in order to assess the spatial and temporal dependencies of these elevations. A separable space-time covariance model is then built from them in order to generate a large number of plausible future profiles at arbitrary time instants t + t, starting from observed profiles at time instants t. For each simulation, the total displaced sand volume is computed and a distribution is obtained. The mean of this distribution is in good agreement with the total displaced sand volume measured on the profiles, provided that they are lower than 45 m3/m. The time variogram also shows that 90% of maximum variability is reached for a time interval t of three years. These results demonstrate how the temporal evolution of an integrated property, like the total displaced sand volume, can be estimated over time. This suggests that a similar stochastic approach could be useful for estimating other properties as long as one is able to capture the stochastic space-time variability of the underlying processes

35 years of monitoring the Belgian sandy coastline

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Morphological diversity and complex sediment recirculation on the ebb delta of a macrotidal inlet (Normandy, France): A multiple LiDAR dataset approach

International audienceThe shoreline in the vicinity of inlets can exhibit considerable variability in morphology in both space and time.Most studies on inlets and their adjacent shores have focused on the morphodynamics of sediment by-passingmechanisms generated by longshore transport. For the first time, the morphology, sedimentary features, sedimentbudgets and patterns of evolution of the shoreline and ebb delta in a macrotidal inlet system have been investigatedusing seven LiDAR topographic surveys in Normandy, France, over a period of 3.7 years fromFebruary2009 to October 2012. The ebb delta shows strong development on the northern flank of the inlet, expressed by alarge sand spit and two types of superimposed dynamic sandy features: eight long-crested and highly mobiletransverse bars and a large swash bar. Sand transport fromN–S on the updrift beach feeds the growth of the distalpart of the spit. This sand supply is further augmented by the onshore movement of a large swash barwelding tothe upper foreshore. However, the main topographic changes were induced by the northward migration of thetransverse bars on the ebb platform. This is driven by strong northward-directed tidal currents parallel to theshore. The bars exhibit a more complex morphology and dynamics along the seaward margin of the ebb deltawhere their mobility is controlled by wave action. Topographic measurements suggest a clear sand recirculationpattern. In this morphodynamic model, sand coming from the updrift upper beach is transported southwardand deposited at the distal end of the spit, where it serves to construct transverse bars close to the tidal inlet.Transverse bar migration ends in the wave-exposed northern margin of the ebb delta,where they are integratedinto the shallow dissipative shoreface sand sink. This sink nourishes the southward longshore transport to feedgrowth of the large swash bar and southward spit elongation. This semi-circular recirculation cell model involvesan inversion of sand movement close to the inlet and emphasizes the combined role of tidal currents and wavesin the large-scale 3D ebb–delta sediment dynamics in this macrotidal setting, in contrast to the much morecommonly reported alongshore sediment by-passing mode of microtidal inlets

Relating Hydrodynamic Forcing and Topographic Response for Tide-Dominated Sandy Beaches

To relate hydrodynamic forcing and topographic response for a tide-dominated sandy beach, extensive field measurements were carried out in the intertidal zone. Hydrodynamics and beach topography were monitored during a total of 12 weeks at two different study sites: one with a featureless intertidal zone and one with intertidal bars. The results of both study sites indicate that the intertidal beach grows when wave steepness is small, whereas it erodes when wave steepness is large. Spring-neap variations in tidal current direction heavily distort this trend: strong spring tidal currents transport sediment away from the beach, resulting in enhanced erosion. Tide-induced beach volume changes are on the same order of magnitude as wave-induced changes. Besides waves and tides, the effect of variations in the amount of sediment supply is substantial, with enhanced accretion when the sediment supply is large. The effect of variations in sediment supply on the intertidal beach topography is subordinate to the effect of waves and tide, though. From this study, it is concluded that larger waves are primarily erosive, but they can also enhance the natural sediment supply. Furthermore, it is found that tidal currents can be equally important as waves in shaping the beach topography, especially during spring tide on macrotidal beaches

Understanding Coastal Resilience of the Belgian West Coast

Topobathymetric monitoring carried out in the past 30 years revealed that the amount of sand in the active zone of the Belgian West Coast increased substantially. Correcting for sand works carried out, the rate of natural feeding of the area was estimated to be 10 mm/year, which is significantly more than the local sea level rise rate of 2 to 3 mm/year. One concludes that this coastal zone, with a length of ca. 16 km, has shown a natural resilience against sea level rise. The question remains which processes govern this behavior and where natural input of sand to the system occurs. Using available coastal monitoring data for the Belgian coast, as well as a state-of-the-art sand transport model, revealed that different processes drive a cross-shore natural feeding from offshore to the coastline. The spatial distribution of this cross-shore natural feeding is determined by the existence of a gully-sand bank system. The outcome of this research was a conceptual model for the large-scale sand exchange in the study area which is implemented in an 1D coastline model. The most important element in these models was the cross-shore natural feeding of the active zone via a shoreface connected ridge amounting to 95,000 m3/year in the period 2000–2020