Patrick A. Hesp

Foredunes and blowouts: initiation, geomorphology and dynamics

This paper reviews the initiation, dynamics, geomorphology and evolution of incipient foredunes, established foredunes, and blowouts. Incipient foredunes may be initiated in a variety of ways leading to the formation of ramps, terraces or ridges depending on progradation rates, vegetation type and cover, sediment transport rates and scale of erosional processes. Vegetation cover, morphology, growth rates, and types combine with aerodynamic processes to determine dune and swale morphology. Established foredunes are classified into five morpho-ecological types, and the flow behaviour over two types is detailed. A new model of foredune evolutionary paths at various possible time scales is presented. Blowout initiation, dynamics and sediment transport, and evolution is examined. Throughout the review, the gaps in present understanding of dune processes and a range of ideas for new research possibilities are provided.

Width, height and slope relationships and aerodynamic maintenance of barchans

The allometric relationship between the height of barchan dunes and horn to horn widths is re-examined. The addition of new data from barchans in the Namib Desert and elsewhere confirms the relationship that the height of a barchan is approximately one-tenth of the width of the dune. Height and width of the dune are closely linked to the three-dimensional morphology of the dune, in particular, the relatively constant mean angles of around 11° at which the side slopes of the dune are maintained. We suggest that the 3-D barchan form is directly controlled by aerodynamic processes. Constant feedback between three-dimensional stoss face (windward) shape of the dune, slipface height and horn to horn width, and shear stress produces an equilibrium dune form.

Seagrass loss associated with boat moorings at Rottnest Island, Western Australia

Loss of seagrasses caused by mooring damage at Rottnest Island, Western Australia, was studied using aerial photographs taken between 1941 and 1992. The temporal decline of seagrass beds damaged by moorings was studied by comparing areal coverages of seagrasses and sand patches, and increases in the length of the exposed seagrass edge within seagrass beds. The study concentrated on Rocky Bay and Thomson Bay where the bulk of permanent moorings are located. Rocky Bay is more exposed to the prevailing westerly swell direction and Thomson Bay is a protected east facing bay. The loss of seagrasses associated with moorings in Rocky Bay has been dramatic, with 18% of seagrass area lost between 1941 and 1992, and 13% between 1981 and 1992. The exposed edge of the beds increased by 230% between 1981 and 1992. Sand patches associated with moorings have coalesced in the shallow central west part of the bay. The change from single weighted swing moorings to three chained cyclone moorings has resulted in greater loss of seagrasses. Single cyclone moorings produce three circular holes in the seagrass bed. The areal loss recorded from Thomson Bay was less than 5%, yet the exposed edge of the seagrass beds doubled between 1941 and 1992. Fragmentation of the bed was visible near heavy use areas. Some regrowth was recorded to the north of the main ferry jetty where the sedimentary environment is depositional. The effects of moorings on seagrasses at Rottnest Island varied from devastating in Rocky Bay to small in Thomson Bay. The differences seem to be related to exposure to prevailing swell and whether the environments are erosional or depositional. Thus a single management protocol, such as cyclone moorings versus swing moorings, is inappropriate. Physical environments need to be assessed before determining the number and types of moorings allowable in bays at Rottnest Island.

Beach ridges, foredunes or transgressive dunefields? Definitions and an examination of the Torres to Tramandaí barrier system, Southern Brazil

Many prograded barriers and some dunefields in the world have been termed ‘beach ridge’ plains, but the actual genesis of the ‘ridges’ is often unknown. Use of the terms, berms, beach ridges and foredunes is also confusing in the literature because their definitions are highly variable and are commonly used interchangeably. Thus, the formation and definition of sand berms, beach ridges and foredunes is briefly reviewed. Beach ridges are re-defined as entirely wave formed deposits which are most commonly formed during high wave conditions and/or elevated water levels (e.g. storm surges). Foredunes are formed by aeolian sand deposition in vegetation on the backshore. Some dunefields in Brazil have been called beach ridge plains when they are, in fact, foredune plains, transgressive dunefields, or complex barriers (i.e. barriers comprising two types of dunes). The Holocene barrier extending from Torres to Tramandai in southern Brazil has been regarded as a beach ridge plain. The landforms of this Holocene barrier comprise wide, relatively linear, widely spaced (400–600m), shore parallel ridges on the landward half, and more closely spaced (80–400m), lobate and crescentic, discrete ridges on the seaward half. Low, rolling dunefields, sand sheets, nebkha fields and deflation plains occur between the ridges. The barrier is re-interpreted as a prograded, transgressive dunefield

Topographic Steering of Alongshore Airflow over a Vegetated Foredune: Greenwich Dunes, Prince Edward Island, Canada

Abstract High-frequency measurements of airflow from ultrasonic anemometers and time-averaged cup anemometer profiles were taken during an oblique alongshore sand-transporting event (6.7 m/s) over a vegetated foredune in May 2002 as part of a larger study on the sedimentary dynamics of a beach-dune complex in Greenwich Dunes, Prince Edward Island National Park, Canada. Local flow and sand transport pathways deviated significantly from the regional wind because of topographic steering of oblique alongshore airflow on the beach back toward the foredune in the backshore. Flow decelerated on the lower seaward slope of the foredune because of flow stagnation and vegetation-induced roughness effects. On the upper seaward slope, flow veered crest-parallel because of secondary flow effects, including potential flow reversal, acceleration, and interaction with faster regional flow. An inflection point in velocity profiles indicates a momentum sink at plant canopy height, and above this height exists a distinct shear layer. Flow acceleration occurs only on the upper slope above the plant canopy as the shear layer intensifies. Flow steadiness, as indicated by the coefficient of variation in horizontal (U) and vertical (W) velocity, also declines up the seaward slope, in contrast to measurements of flow over unvegetated dunes, perhaps because of increasing vegetation-induced drag and turbulence. Gusting and vertical bursting was evident as honami motion in the beachgrass and was responsible for maintaining intermittent saltation into the foredune. Temporal correlations exist between U, W, and flow angle. As flow shifted onshore, both U and W increased because of enhanced topographic forcing and increased vertical lift on the lower seaward slope, whereas when flow shifted alongshore, U and W declined because of reduced forcing and acceleration effects because the dune is effectively less steep to flow. Although these dunes align well with the vector of the regional resultant sediment drift potential, sand transport pathways followed local flow vectors of varying magnitude and direction. Offshore to oblique alongshore winds are typical of the summer wind regime, and, although frequently incompetent, they contribute to dune maintenance by cycling sand to the backshore for incipient foredune growth, scarp infilling, or both. To date, the role of secondary flows under alongshore winds in foredune morphodynamics has not been well documented.

7 – Disturbance Processes and Dynamics in Coastal Dunes

Coastal dunes occur throughout the world and thus in a diversity of climatic regimes. However, independent of the geographical location, they all share a set of environmental characteristics that greatly affect seed germination, seedling establishment, and adult performance. Repeatedly, and for a wide variety of locations ranging from tropical to subarctic latitudes, it has been demonstrated that the predominant factors that have a significant impact on the dynamics of coastal dune vegetation can be divided into two groups—environmental gradients, and recurring disturbances. Typically, salinity, substrate mobility, radiation, and nutrient contents vary following a gradient. Salinity, near surface wind speed, radiation, and substrate mobility decrease inland, while nutrient and biotic pressures increase. In turn, disturbance events occur mostly through wave scarping, water intrusion, substrate erosion, and burial by sand (or snow) and may be gradual or abrupt, and are commonly spatially and temporally variable. The intensity of these events will depend on the orientation of the coast, wind speed, time of year (or season), storm frequency and intensity, tide regime, surf zone-beach type, and perhaps mean sediment size. However, recent perspectives state that disturbances overlay environmental gradients, and their impact is, therefore, influenced by such gradients. In this sense, the relative importance of each depends on the spatial and temporal scale at which each occurs. In particular, transgressive dune fields are largely ignored in current geological, geomorphological, and ecological research and literature, yet they provide many opportunities for fundamental and exciting research.

Present and Future Challenges of Coastal Erosion in Latin America

ABSTRACT Silva, R.; Martínez, M.L.; Hesp, P.; Catalan, P.; Osorio, A. F.; Martell, R.; Fossati, M.; Miot da Silva, G.; Mariño-Tapia, I.; Pereira, P.; Cienfuegos, R.; Klein, A., and Govaere, G., 2014. Present and future challenges of coastal erosion in Latin America. The coastal zones of Latin America have many landforms and environments, including sedimentary cliffs, deeply incised estuaries, headlands, barrier coasts and low lying, muddy coastal plains. These forms will respond differently to the expected changes in climate and associated sea level rise, which may produce coastal erosion in the future. Considering the coasts of Latin America overall, erosion is not yet a serious threat, although it is widespread and it is severe in some parts. Major erosion problems are frequently associated with human intervention in sediment supply, with poor planning or with the morphodynamic nature of the coast. Permanent erosional processes, locally or regionally, are caused by tectonic subsidence, deforestation and the fragmentation of coastal ecosystems, land use changes and sediment deficits because of infrastructure built along the coast. In this article we analyse coastal erosion in Latin America and the challenges it presents to the region. We first highlight the relevance of Latin America in terms of its biodiversity; then we describe the population at risk, demographic trends and economic growth throughout the low lying coastal zones. We also examine the vulnerability of the region by analyzing the resilience of key coastal ecosystems after exposure to the most frequent hazards that affect coastal zones in Latin America, namely tropical cyclones, sea level rise, ocean acidification, earthquakes and tsunamis. Finally, we discuss seven case studies of coastal erosion across Latin America. We close the study by pinpointing the main areas of concern in Latin America and explore possible strategies to overcome erosion and thus sustain economic growth, minimize population risk and maintain biodiversity.

Offshore Aeolian Transport across a Low Foredune on a Developed Barrier Island

Abstract Sediment transport across a foredune and beach at Ocean City, New Jersey, was examined to identify the effect of houses, dune topography, sand fences, vegetation, and wrack lines during an offshore wind. Houses are as close as 18 m from the crest of the 2- to 3-m-high foredune and are up to 9.0 m high and 12.8 m wide and spaced 4.0 to 5.0 m apart. Data were gathered during a 1-day study on 3 March 2003 with the use of 15 vertical sand traps, 13 erosion pins, and eight sets of anemometers placed 0.3, 0.6, and 0.9 m above the ground surface. Wind speeds 0.9 m above the ground ranged from 1.61 to 3.75 m/s. Trapping rates were negligible on the vegetated dune crest and up to 2.03 kg m−1 h−1 near mid foreshore. The highest rate of trapping was 2.4 kg m−1 h−1 at a <10-m-wide unvegetated area in the dune. Houses shelter the dune from offshore winds and might contribute to dune stability. Increases in wind speed in the offshore direction cause greater rates of transport on the seaward side of the dune where vegetation has not had time to become established. Dune vegetation, remnant sand fences, wrack lines, and the sheltered area in the lee of the seaward dune ridge can trap sediment moving offshore during relatively low wind speeds. Sediment deposited at these obstacles could contribute to deflation of the backshore, which is the primary source of sediment moved to the inter-tidal foreshore. Losses of sediment from unvegetated portions of the dune to the beach can be overcome by replacing sand fences and suspending beach raking until the calmer summer months.

Flow dynamics in a trough blowout

The dynamics and geomorphological development of a trough blowout located at Fiona Beach in the Myall Lakes National Park in NSW, Australia are examined. Wind velocities and flow structure were measured utilising an array of miniature Rimco cup anemometers, Gill bi-vane and UVW instruments, and wind vanes. Flow measurements indicate that when the wind approaches the trough blowout parallel to the throat orientation, jets occur both in the deflation basin and along the erosional walls, relative flow deceleration and expansion occur up the depositional lobe, jets are formed over the depositional lobe crest accompanied by downwind flow separation on the leeward side of the lobe, and flow separation and the formation of corkscrew vortices occur over the crests of the erosional walls. Maximum erosion and transport occur up the deflation basin and onto the depositional lobe. Trough blowout morphologies are explained as a function of these flow patterns.When the wind approaches the blowout obliquely, the flow is steered considerably within the blowout. The degree and complexity of topographic steering is dependent on the blowout topography. The flow is usually extremely turbulent and large corkscrew vortices are common. The local topography of a blowout can be very important in determining flow patterns, overall sand transport and blowout evolutionary conditions and paths.Estimates of potential sand transport within the blowout may be up to two orders of magnitude lower than actual rates if remote wind data are used.

Coastal Dunes: Human Impact and Need for Restoration

A large proportion of the world population lives at or near the coast, which results in high impact and degradation of coastal ecosystems, such as the beach and coastal dunes. These ecosystems are particularly threatened by the impact of human activities because of the increasing demand of sun and sand tourism throughout the world. Because of this increase, conservation activities and restoration efforts will become more and more relevant. In spite of the urgent need for restoration, there are very few books focused on coastal dune restoration that cover a wide variety of coastal dune systems from different latitudes, including tropical and temperate dunes. In this book we have put together a set of restoration activities performed in different parts of the world. The goal was to gather information on state-of-the-art studies on the restoration of coastal sand dunes, covering a wide variety of landforms and approaches that can be of assistance in future coastal dune restoration projects. Different coastal land forms are included in this set of studies, such as foredunes, slacks/swales/deflation basins and plains, blowouts, parabolics, and transgressive dunefields. We follow a broad definition of “restoration,” which includes any effort to restore the original functionality of the previously existing ecosystem, but also efforts at rehabilitation, reclamation, and even designing ecosystems in drastically altered sites (such as after mining activities). The diverse array of contributions will hopefully stimulate further research that will lead to a better understanding of the actions required for improving restoration strategies.