Two new conceptual models for the formation and degradation of baymouth spits by longshore drift and fluvial discharge (Iguape,SE Brazil)

This study describes the formation of two successive baymouth spits systems on the south‐eastern Brazilian coast and the degradation of the first system. The study area includes the Jureia Beach spit, the deflected Ribeira de Iguape River mouth, the central Iguape sandy headland, the Icapara Inlet of the Mar Pequeno Lagoon and the northern end of the Comprida Island barrier spit. The wave and river flow patterns were combined with the coastline evolution and the alongshore migration rates deduced from satellite images. Initially, both spits showed convergent alongshore migration rates equal to or less than 83 m/yr. However, the extreme river flow due to high rainfall during a very strong El Niño event in 1983 eroded the inland side of the Jureia Beach spit, which finally retreated due to wave erosion. In 1989, a sand bank emerged in the river mouth, which attached to the central headland forming a recurved northeastward spit. In 1994, the high fluvial discharge associated with another very strong El Niño event caused the landward migration of the new spit and emersion of a second sand bank. This second sand bank merged with the Jureia Beach spit in 1997 at an alongshore migration rate of 1795.6 m/yr. Wave erosion of the central headland continued and the attached spit disappeared in 2000. In 2009, the headland erosion merged the river mouth and the Icapara Inlet, which resulted in flanking baymouth spits in a configuration that remains today. Therefore, two models for the formation of baymouth spits have been documented for wave‐dominated microtidal coasts in humid tropical regions with intense fluvial discharge. The convergent longshore migration of the spits is controlled by both the bidirectional longshore drift and the fluvial discharge, the latter eroding the fronting spit, supplying sediments and acting as a hydraulic blockage for longshore drift. Copyright © 2017 John Wiley & Sons, Ltd.     

The geomorphology and evolution of a large barrier spit: Farewell Spit,New Zealand

Farewell Spit is a 25 km long barrier spit that marks the end of a littoral drift system, almost 1000 km in length that runs along South Island, New Zealand. The spit is composed of barchan dunes over 20 m high, sand sheets over 1 km wide and vegetated linear dunes. Analysis of aerial photography indicates a rapid colonization of the spit by vegetation which has expanded in area by 75% since 1950. Vegetation colonization preferentially occurs on the southern side of the spit, with its northern margin characterized by barchan dunes which migrate at rates of up to 64 m/yr. Sand sourced from longshore drift appears to be the primary source of beach sediment, which is then transported into the dune field by the persistent westerly winds of the Roaring 40s. While there has been significant dune roll‐over on the surface of the spit, its overall area has remained much the same for the past 54 years. Occasional cyclone events cause erosion, but this is balanced by aeolian sediment transport. It would appear that extension of the subaerial portion of the spit is related to the development of shells banks at its downdrift end which are periodically welded to the main spit by dune extension. Farewell Spit therefore provides an ideal example of a barrier environment where longshore sediment supply and aeolian transport dominates geomorphic evolution. This differentiates the study site from other barrier environments where overwash or tidal inlet development often characterizes recent landform evolution. Copyright © 2010 John Wiley & Sons, Ltd.     

Wave energy and clast transport in eastern Tasman Bay,New Zealand

Coarse‐gravel beaches are common features along the eastern margin of Tasman Bay, at the north end of South Island, New Zealand. Although these features have traditionally been interpreted as spits, contemporary incident wave energy appears too small to transport boulders and cobbles persistently along the beaches and platforms by longshore drift. An alternative explanation suggests that boulder beaches are essentially derived in situ from resistant bedrock, which lies seaward and was buried by gravel during the Holocene sea level rise. Wind, wave and clast size data from Cable Bay and the Nelson Boulder Bank were used to resolve this problem. Wave and wind data indicate that waves reaching these areas are derived locally in Tasman Bay, and are limited in size and energy. Hindcasting predicts a 4·7 m wave could propagate from Tasman Bay. However, during Cyclone Yalli, the most intense storm in nearly 40 years of wind records, the largest wave measured in the nearby area of Cable Bay was only 2·7 m high. Maximum orbital velocity on the seabed beneath a 4·7 m is calculated to be 2·9 m s^?1, which cannot initiate transport of clasts greater than 0·15 m in diameter. Clasts on the gravel platforms have average diameters greater than this, but some clasts may be as large as 1·0 m in diameter. By comparison, a swash run‐up method predicts that a wave 4·7 m high can transport clasts no larger than 0·3 m in diameter. These data and approximate calculations strongly suggest that the present wave environment in eastern Tasman Bay is not capable of consistently transporting clasts on the boulder platforms by longshore drift. Reduced sea levels in the pre‐Holocene period would further reduce wave energies available in Tasman Bay. Copyright © 2006 John Wiley & Sons, Ltd.     

Beach erosion as a function of variations in the sediment budget,Sandy Hook,New Jersey,U.S.A.

Many of the world's beaches have recently been eroding, even on progradational landforms. This study uses the sediment budget approach to identify and rank the causes of the hazard along Sandy Hook spit where the primary recreational beach has been eroding at about 10 m/yr since 1953 and 23 m/yr in the 1970s. Large spatial variations in longshore sediment transport are found to result from differences in refracted wave energies and intersegmental sediment transport. Erosion results from a 60 per cent deficit (-270,000 m³/yr) in the sediment budget that is primarily caused by (1) refraction induced locally high waves that increase the transport rate by 100,000 m³, and (2) shore protection structures that have lessened the longshore sediment inputs by an additional 100,000 m³/yr. A storm index is presented to analyse secular climatic variation. It suggests that the annual sediment transport rate may vary by as much as ±50 per cent about the mean and that recently, above normal storm wave energies are responsible for about 60,000 m³/yr of the budget deficit. Rising sea levels and storm overwash each account for only about one per cent of the sediment loss. Pulses of sediment, induced by accelerated erosion at the feeder beach locale of spit segments, are found to move downdrift. They alter the geomorphology of the spit through episodic extensions of the spit segments with lag times exceeding one year per segment.     

A comparison of reef‐protected environments in ­Western Australia: the central west and Ningaloo coasts

Variability in the regional setting and morphology of cuspate forelands on the west coast of Western Australia is examined in this paper. In accordance with this aim, principal differences in the geologic and geomorphologic setting of three prominent sites on the west coast were established and their association with historical changes and contemporary oceanographic processes was examined. The cuspate forelands investigated are Jurien Bay, Winderabandi Point and Turquoise Bay. The most significant differences in geologic setting are associated with the structure and location of an extensive offshore reef system. Morphologically, the reef alters from south to north, changing from a discontinuous ridge parallel to the shore along the central west coast, to a nearly continuous fringing reef at Ningaloo. The reefs vary in distance from the shore, being farthest in the south and closest in the north and they impound a series of inshore basins, or lagoons. The deeper southern basins are dominated by locally generated wind waves and wind‐generated currents. The shallower northern basins are most markedly affected by tidal currents and wave pumping across the reef flats. The large cuspate foreland at Jurien on the central west coast has undergone shoreline configuration change in response to changing phases of storminess as well as in response to a change in focus for sediment deposition as a result of offshore reef erosion. At Winderabandi Point on the Ningaloo coast, relict Pleistocene limestone has provided the focus for sedimentation and morphology has been controlled by a balance in refracted wave energy and nearshore currents driven by tidal and wave set‐up variability. At Turquoise Bay, where the lagoonal basin is most shallow and narrow, the morphology of the foreland suggests that it may at some stage have been migratory, but its present asymmetrical shape is maintained by strong northerly longshore drift and strong currents exiting the lagoon through a nearby gap in the reef crest. Fundamental differences between the two coastal regions include the structure of the offshore reef, processes driving flow of water within the lagoons and the role of storminess in evolution of coastal landforms. Although many questions regarding storm surge dynamics and landform change remain unanswered, this research provides a significant contribution to the understanding of the evolution of morphological systems in low‐wave‐energy protected environments. Copyright © 2000 John Wiley & Sons, Ltd.     

The fetch effect on aeolian sediment transport on a sandy beach: a case study from Magilligan Strand,Northern Ireland

Experiments were conducted on Magilligan Strand, Northern Ireland, to assess the influence of the fetch effect on aeolian sediment transport. During each experiment surface sediments were uniformly dry and unhindered by vegetation or debris. The leading edge of erodible material was well defined, with the limit of wave up‐rush demarcating the wet–dry boundary; the work was conducted during low tides. A number of electronic and integrating traps were utilised, with two ultrasonic anemometers used to measure wind direction and velocity at 1 Hz. The combination of 1^o direction data and trap locations resulted in a range of fetch distances, from 2 to 26 m. Data integrated over 15‐minute intervals (corresponding to the integrating trap data) revealed a distinct trend for all the experiments. An initial rapid increase in the transport rate occurred over a short distance (4–9 m). This maximum transport rate was maintained for a further 5–6 m before a steady decay in the flux followed, as fetch distance increased. A measured reduction in wind speed (6–8%) across the beach suggests a negative feedback mechanism may be responsible for the diminishing transport rate: the saltating grains induce energy dissipation, thus reducing the capability of the wind to maintain transport. For one experiment, the presence of compact sediment patches may also have contributed to the reduction of the transport rate. The decay trend calls into question the utility of the fetch effect as an important parameter in aeolian studies that seek to understand sediment budgets of the foredune‐beach zone. Copyright © 2016 John Wiley & Sons, Ltd.     

The formation and closure of the Big Breach of Sacalin spit associated with extreme shoreline retreat and shoreface erosion

We document a case of exceptionally large natural breaching of a sandy spit (Sacalin spit, Danube Delta) using multiannual to seasonal surveys of topography and bathymetry on successive cross-barrier and shoreface profiles, LiDAR data, satellite imagery, and wind and wave data. The large breach, which quickly reached 3.4 km in May 2014, is attributed to morphological preconditioning of the narrow (50–150 m) barrier, which was susceptible to breaching even during moderate storm conditions. The event switched the barrier's decadal evolution from low cross-shore transport to high cross-shore transport over the barrier, which is an order of magnitude larger than during the non-breach period. Upper shoreface erosion, as indicated by the extensive erosion down to −4 m, indicates that this zone is a significant source for the rebuilding of the barrier. Barrier recovery and widening trigger a negative feedback which limits the back-barrier sediment transfer. As a result, back-barrier deposition decreases whilst the barrier aggradation through overwash becomes more frequent. The Big Breach (TBB) closed naturally in three years. The very high deposition rate of sediment in the breach is a testimony of the high sediment volumes supplied by the longshore transport and the high sediment released through shoreface retreat, and resulted in widening the barrier to a maximum of 1 km. Since the newly-formed barrier shoreline retreated 500 m, this reveals that barrier breaching is an important mechanism which significantly accelerates the landward migration of the barrier system and is a proof of the highly non-linear morphodynamics involved in the barrier island translation. © 2019 John Wiley & Sons, Ltd.     

Lithological controls on soft cliff planshape evolution under high and low sediment availability

This paper addresses a series of geomorphic questions relating to large‐scale (> 1 km), long‐term (100 – 1,000 years) coastal planshape evolution. Previous research on soft‐cliff coasts has recognised the role of protective fronting beach volumes on reducing rates of cliff toe retreat. However, it is the maintenance of this critical threshold that ultimately determines two contrasting modes of shoreline behaviour: Mode A, in which there is little beach sediment and shoreline evolution is controlled by material strength; and, Mode B, when ample beach sediment means that shoreline evolution is controlled by longshore sediment transport. Here we use a numerical model (SCAPE) to investigate temporal and spatial changes in beach volume on a broader range of feedbacks than considered in previous models. The transition between Mode A and Mode B coasts is defined by relative sediment inputs to outputs and used to explore how these contrasting modes control the evolution of an initial linear frontage exhibiting longshore changes in cliff lithology (material resistance and the proportion of beach grade material in the eroded bedrock). Under Mode A, relative changes in material resistance result in long term heterogeneous rates of retreat, which result in the development of persistent headland and embayment features. However, under Mode B, feedbacks between coastal planshape, longshore sediment transport, beach volume and wave energy result in steady state retreat rates regardless of longshore variations in resistance. Results are compared and contrasted to previous simulations and site specific examples and a conceptual model of Mode A and Mode B interactions presented. Copyright © 2015 John Wiley & Sons, Ltd.     

Regional wind fields and dunefield migration,southern Brazil

This paper examines a seemingly anomalous situation in southern Brazil where the dunefields on Santa Catarina Island (e.g. Joaquina Beach) migrate to the NNW, almost completely the opposite direction (c. 160) to the dunefields immediately to the south (e.g. Pinheira Beach), and some much further to the north (e.g. Cabo Frio) which migrate to the SSW. A variety of mechanisms are examined to explain the differences in dunefield migration including grain size variations, topographic effects on local winds, shoreline orientation, and regional wind field changes. The mean grain sizes of the two beaches, Pinheira and Joaquina, are not sufficiently different to restrict aeolian sediment transport in either place, nor to account for a lack of transport from the NNE to the SSW in the case of Joaquina. Some topographic steering of the wind is likely but could not account for the long‐term average difference in migration trends of the island dunefields compared to the mainland dunefields. While the orientation of the shoreline to prevailing winds is an important control on beach and dune sediment transport, it is not the dominant controlling mechanism. An analysis of the regional wind patterns demonstrates that there is a major shift in the regional wind field near the island such that the dominant island winds blow from the SW/SSW while those further south blow from the NE. It is concluded that this is the predominant reason for the divergence in the direction of migration of the dunefields. Copyright © 2006 John Wiley & Sons, Ltd.     

Hindcast of breaking waves and its impact at an island sheltered coast,Karwar

Variability in the characteristics of depth-induced wave breakers along a non-uniform coastal topography and its impact on the morpho-sedimentary processes is examined at the island sheltered wave-dominated micro-tidal coast, Karwar, west coast of India. Waves are simulated using the coupled wind wave model, SWAN nested in WAVEWATCH III, forced by the reanalysis winds from different sources (NCEP/NCAR, ECMWF, and NCEP/CFSR). Impact of the wave breakers is evaluated through mean longshore current and sediment transport for various wave energy conditions across different coastal morphology. Study revealed that the NCEP/CFSR wind is comparatively reasonable in simulation of nearshore waves using the SWAN model nested by 2D wave spectra generated from WAVEWATCH III. The Galvin formula for estimating mean longshore current using the crest wave period and the Kamphuis approximation for longshore sediment transport is observed realistically at the sheltered coastal environment while the coast interacts with spilling and plunging breakers.     

Aeolian sediment transport on a recovering storm‐eroded foredune with sand fences

Studies of sediment transport on developed coasts provide perspective on how human adjustments alter natural processes. Deployment of sand‐trapping fences is a common adjustment that changes the characteristics of the dune ramp and its role in linking sediment transfers from the backshore to the foredune. Fence effects were evaluated in the field using anemometer arrays and vertical sediment traps placed across a beach and dune at Seaside Park, New Jersey, USA during onshore and longshore winds. The foredune is 18 m wide and 4.5 m above the backshore. The mean speed of onshore winds at 0.5 m elevation decreased by 17% from the berm crest to the upper ramp and 36% in the lee of a fence there. Sediment transport during mean wind speeds up to 8.0 m s^?1 at 0.5 m elevation was < 0.06 kg m^?1 h^?1 on the berm crest and backshore where fetch distances were < 45 m and surface sediment was relatively coarse (0.74–0.85 mm) but increased to 5.63 kg m^?1 h^?1 on the upper ramp aided by the longer fetch distances (up to 82 m) and finer grain size of the source sediment there (0.52 mm). Sediment transport along the berm crest and backshore during longshore winds, where fetch distances were > 200 m, was up to 58.69 kg m^?1 h^?1, about three orders of magnitude greater than during the onshore winds. Fences can displace the toe of the ramp farther seaward than would occur under natural conditions. They can create a gentler slope and change the shape of the ramp to a more convex form. A fence on the ramp can cut off a portion of sediment supply to the upper slope. Decisions about fence placement thus should consider these morphologic changes in addition to the effects on dune volume. Copyright © 2017 John Wiley & Sons, Ltd.     

The extreme 2013/2014 winter storms: hydrodynamic forcing and coastal response along the southwest coast of England

The southwest coast of England was subjected to an unusually energetic sequence of Atlantic storms during the 2013/2014 winter, with the 8‐week period from mid‐December to mid‐February representing the most energetic period since at least 1953. A regional analysis of the hydrodynamic forcing and morphological response of these storms along the SW coast of England highlighted the importance of both storm‐ and site‐specific conditions. The key factor that controls the Atlantic storm wave conditions along the south coast of southwest England is the storm track. Energetic inshore wave conditions along this coast require a relatively southward storm track which enables offshore waves to propagate up the English Channel relatively unimpeded. The timing of the storm in relation to the tidal stage is also important, and coastal impacts along the macro‐tidal southwest coast of England are maximised when the peak storm waves coincide with spring high tide. The role of storm surge is limited and rarely exceeds 1 m. The geomorphic storm response along the southwest coast of England displayed considerable spatial variability; this is mainly attributed to the embayed nature of the coastline and the associated variability in coastal orientation. On west‐facing beaches typical of the north coast, the westerly Atlantic storm waves approached the coastline shore‐parallel, and the prevailing storm response was offshore sediment transport. Many of these north coast beaches experienced extensive beach and dune erosion, and some of the beaches were completely stripped of sediment, exposing a rocky shore platform. On the south coast, the westerly Atlantic storm waves refract and diffract to become southerly inshore storm waves and for the southeast‐facing beaches this results in large incident wave angles and strong eastward littoral drift. Many south coast beaches exhibited rotation, with the western part of the beaches eroding and the eastern part accreting. © 2015 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.     

Sand spit dynamics in a large tidal-range environment: Insight from multiple LiDAR,UAV and hydrodynamic measurements on multiple spit hook development,breaching, reconstruction,and shoreline changes

Sand spits with distal hooks have been well documented from coasts with low to moderate tidal ranges, unlike high tidal-range environments. Datasets from 15 LiDAR and 3 UAV surveys between 2009 and 2019 on the Agon spit in Normandy (France), a setting with one of the largest tidal ranges in the world (mean spring tidal range: 11 m), combined with in-situ hydrodynamic records between 2013 and 2017, highlight a three-stage pattern of spit hook evolution. Stage 1 (2009–2013) commenced with the onshore migration and attachment of a swash bar, followed by persistent spit accretion updrift of the bar and erosion downdrift because of the slow speed of bar migration in this large tidal-range environment. In stage 2 (2013–2016), three overwash events and a 220 m-wide breach culminating in the total destruction of the spit during winter 2015–2016 involved the landward mobilization of thousands of cubic metres of sand. These events occurred during short durations (a few hours) when spring high tides coincided with relatively energetic waves, underscoring the importance of storms in rapid spit morphological change. Strong spring tidal currents maintained the breach. Stage 3 (2016–2019) has involved new hook construction through welding of a swash bar and spit longshore extension, highlighting the resilience of the spit over the 10-year period, and involving a positive sediment balance of 244 000 m³. The three stages bring out, by virtue of the temporal density of LiDAR and UAV data used, a high detail of spit evolution relative to earlier studies in this macrotidal setting. The large tidal range strongly modulates the role of waves and wave-generated longshore currents, the main process drivers of spit evolution, by favouring long periods of inertia in the course of the spring–neap tidal cycle, but also brief episodes of significant morphological change when storm waves coincide with spring high tides. © 2020 John Wiley & Sons, Ltd.     

The effects of surface moisture on aeolian sediment transport threshold and mass flux on a beach

This paper presents results from a study designed to explore the effects of beach surface moisture and fetch effects on the threshold of movement, intensity of sand transport by wind and mass flux. The experiment was carried out over a period of five weeks at Greenwich Dunes, Prince Edward Island, Canada in May and June 2002. Moisture content was measured with a Delta‐T moisture probe over a 50 m by 25 m grid established on the beach. Measurements of wind speed and direction were made with arrays of cup anemometers and a two‐dimensional sonic anemometer. Transport intensity was measured at a height of 2–4 cm above the bed using omnidirectional saltation probes which count the impact of saltating grains on a piezoelectric crystal. Anemometers and saltation probes were sampled at 1 Hz. Sand transport was measured with vertical integrating sand traps over periods of 10–20 minutes. Results show that where there is a considerable supply of dry sand the saltation system responds very rapidly (1–2 s) to fluctuations in wind speed, i.e. to wind gusts. Where sand supply from the surface is limited by moisture, mean transport rates are much lower and this reflects in both a reduction in the instantaneous transport rate and in a transport system that becomes increasingly intermittent. Threshold wind speed is significantly correlated with an increase in surface moisture content near the upwind end of the beach fetch, but the relationship is not significant at the downwind end where sediment transport is initiated primarily by saltation impact from upwind. Mass flux increases with increasing fetch length and the relationship is described best by a power function. Further work is necessary to develop a theoretical function to predict the increase in transport with fetch distance as well as the critical fetch distance. Copyright © 2007 John Wiley & Sons, Ltd.     

Aeolian fetch distance and secondary airflow effects: the influence of micro‐scale variables on meso‐scale foredune development

Unsuccessful attempts to use process‐scale models to predict long‐term aeolian sediment transport patterns have long been a feature of aeolian research. It has been proposed that one approach to overcome these problems is to identify micro‐scale variables that are important at longer timescales. This paper assesses the contribution of two system variables (secondary airflow patterns and fetch distance) to medium‐term (months to years) dune development. The micro‐scale importance of these variables had been established during previous work at the site (Magilligan Strand, Northern Ireland). Three methods were employed. First, sand drift potentials were calculated using 2 years of regional wind data and a sediment transport model. Second, wind data and large trench traps (2 m length × 1 m width × 1·5 m depth) were used to assess the actual sediment transport patterns over a 2‐month period. Third, a remote‐sensing technique for the identification of fetch distance, a saltation impact sensor (Safire) and wind data were utilized to gauge, qualitatively, sediment transport patterns over a 1‐month period. Secondary airflow effects were found to play a major role in the sediment flux patterns at these timescales, with measured and predicted rates matching closely during the trench trap study. The results suggest that fetch distance is an unimportant variable at this site. Copyright © 2007 John Wiley & Sons, Ltd.     

Characteristics and dynamics of multiple intertidal bars,north Lincolnshire,England

Multiple intertidal bars and troughs, often referred to as ‘ridges and runnels’, are significant features on many macrotidal sandy beaches. Along the coastline of England and Wales, they are particularly prevalent in the vicinity of estuaries, where the nearshore gradient is gentle and a large surplus of sediment is generally present. This paper examines the dynamics of such bar systems along the north Lincolnshire coast. A digital elevation model of the intertidal morphology obtained using LIDAR demonstrates that three to five intertidal bars are consistently present with a spacing of approximately 100 m. The largest and most pronounced bars (height = 0·5–0·8 m) are found around mean sea level, whereas the least developed bars (height = 0·2–0·5 m) occur in the lower intertidal zone. Annual aerial photographs of the intertidal bar morphology were inspected to try to track individual bars from year to year to derive bar migration rates; however, there is little resemblance between concurrent photographs, and ‘resetting’ of the intertidal profile occurs on an annual basis. Three‐dimensional beach surveys were conducted monthly at three locations along the north Lincolnshire coast over a one‐year period. The intertidal bar morphology responds strongly to the seasonal variation in the forcing conditions, and bars are least numerous and flattest during the more energetic winter months. Morphological changes over the monthly time scale are strongly affected by longshore sediment transport processes and the intertidal bar morphology can migrate along the beach at rates of up to 30 m per month. The behaviour of intertidal bars is complex and varies over a range of spatial and temporal scales in response to a combination of forcing factors (e.g. incident wave energy, different types of wave processes, longshore and cross‐shore sediment transport), relaxation time and morphodynamic feedback. Copyright © 2005 John Wiley & Sons, Ltd.     

Conceptualizing delta forms and processes in Arctic coastal environments

Climate warming in the Arctic directly causes two opposite changes in Arctic coastal systems: increased melt‐water discharge through rivers induces extra influx of sediments and extended open water season increases wave impact which reworks and erodes the shores. A shoreline change analysis along the southern coast of Disko Island in western Greenland was conducted with aerial photographs and satellite images from 1964, 1985, and 2012. The decadal morphologic evolution of this 85 km section showed that large parts of the coast had undergone very limited changes. However, two deltas were highly dynamic and popped up as hotspots. The Tuapaat delta and Skansen delta showed large progradation rates (1.5 and 7 m/yr) and migration of the adjacent barriers and spits. The dynamic behavior at the delta mouths was mainly caused by classic delta channel lobe switching at one delta (Tuapaat), and by a breach of the fringing spit at the other delta (Skansen). The longshore and cross‐shore transports are responsible for reworking the sediment with a result of migrating delta mouths and adjacent subaqueous mouth bars. Seaward progradation of the deltas is limited due to the steep nature of the bathymetry in Disko Bay. Finally, a schematic conceptual overview of processes and associated morphological responses for deltas in Arctic environments is presented, including the climate drivers affecting delta evolution. Copyright © 2016 John Wiley & Sons, Ltd.     

Subsurface erosion,scarp retreat,and sedimentation at Mountain Lake,Virginia, USA: groundwater geomorphology in a flow‐through lake with subsurface drainage

The complete natural drainage in 2008, 2011, and 2012 of Mountain Lake in Giles County, Virginia, allowed detailed observations of the only natural lake basin in the southern Appalachian Mountains. Here we use these observations to support geomorphic analysis and develop a model of basin evolution, which may advance the understanding of rare flow‐through lakes with subsurface drainage elsewhere. Key features included (a) an angle‐of‐repose slope with a smoothly concave planform across the entire 260 m width of the north end of the basin, (b) an arc of steep‐sided depressions along the deep northern margin of the basin floor, and (c) an abrupt transition between colluvial and finer‐grained sedimentary deposits on the floor. Our geomorphic analysis suggests that subsurface erosion has enabled long‐term northward scarp retreat in the basin by removing water and sediment. Mountain Lake formed on the northern limb of a breached anticline along the Eastern Continental Divide, where strong‐over‐weak stratigraphy and a small watershed have enabled the basin to evolve generally as follows. (1) Pond Drain, a first‐order tributary of the New River, incised north‐dipping sandstones and underlying shales on the northern limb of the anticline. The valley floor subsequently accumulated meters to tens of meters of mostly late Pleistocene colluvial fill. (2) Subsurface drainage developed likely along the contact between the sandstones and shales, facilitated by pre‐existing fractures. (3) Ongoing subsurface erosion has progressively undermined the sandstone, causing scarp retreat along the northern margin of the basin while a surface stream intermittently incised the shallow southern end. Sedimentary deposits indicate that only the deeper northern portion of the basin is usually flooded under Holocene conditions. Our basin evolution model suggests slow development of the basin over hundreds of thousands of years rather than sudden damming by a catastrophic landslide. Copyright © 2018 John Wiley & Sons, Ltd.     

Beach Morphodynamics influenced by an ebb‐tidal delta on the north Florida Atlantic coast

Tidal inlets interrupt longshore sediment transport, thereby exerting an influence on adjacent beach morphology. To investigate the details and spatial extent of an inlet's influence, we examine beach topographic change along a 1.5 km coastal reach adjacent to Matanzas Inlet, on the Florida Atlantic coast. Analyses of beach morphology reveal a behavioral change between 0.64 and 0.86 km from the inlet channel centerline, interpreted to represent the spatial extent of inlet influence. Beyond this boundary, the beach is narrow, exhibits a statistically significant inverse correlation of shoreline position with offshore wave conditions, and has a uniform alongshore pattern in temporal behavior, as determined from empirical orthogonal function (EOF) analysis. On the inlet side of the boundary, the beach experiences monotonic widening (with proximity to the inlet), lacks spatial consistency in correlation between shoreline position and wave conditions, and exhibits an irregular pattern in spatial EOF modes. We augment the field observations with numerical modeling that provides calculations of wave setup and nearshore current patterns near the inlet, highlighting the effects of the ebb‐tidal delta on the assailing waves. The modeling results are verified by a natural experiment that occurred during May 2009, when a storm‐produced sedimentary mass accreted to the lower beach, then subsequently split into two oppositely directed waves of sediment that migrated away from the initial accretion site in the subsequent months. Our results suggest that the ebb‐tidal delta produces a pattern of wave setup that creates a pressure gradient driving an alongshore flow that opposes the longshore currents derived from breaking of obliquely oriented incident waves. The resulting recirculation pattern on the margin of the ebb‐tidal delta provides a mechanism through which the inlet influences adjacent barrier island beach morphology. Copyright © 2016 John Wiley & Sons, Ltd.     

Modelling wind‐blown sediment transport around single vegetation elements

Wind erosion is an important soil erosion and hence a soil degradation problem in the Sahelian zone of West Africa. Potentially, the characteristic dryland vegetation with scattered trees and shrubs can provide for soil erosion protection from wind erosion, but so far adequate quantification of vegetation impacts is lacking. The aim of this study was to develop a model of wind‐blown soil erosion and sediment transport around a single shrub‐type vegetation element. Starting with the selection of a suitable transport equation from four possible sediment transport equations, the effects of a single vegetation element on wind speed were parameterized. The modified wind speed was then applied to a sediment transport equation to model the change in sediment mass flux around a shrub. The model was tested with field data on wind speed and sediment transport measured around isolated shrubs in a farmer's field in the north of Burkina Faso. The simple empirical equation of Radok (Journal of Glaciology 19 : 123–129, 1977) performed best in modelling soil erosion and sediment transport, both for the entire event duration and for each minute within an event. Universal values for the empirical constants in the sediment transport equation could not be obtained because of the large variability in soil and roughness characteristics. The pattern of wind speed, soil erosion and sediment transport behind a shrub and on either side of it was modelled. The wind speed changed in the lee of the vegetation element depending on its porosity, height and downwind position. Wind speed was recovered to the upstream speed at a downwind distance of 7·5 times the height of the shrub. The variability in wind direction created a ‘rotating’ area of influence around the shrub. Compared to field measurements the model predicted an 8% larger reduction in sediment transport in the lee of the vegetation element, and a 22% larger increase beside the vegetation element. Copyright © 2011 John Wiley & Sons, Ltd.