Seaward Slope Research Articles

Formula for the maximum reference pressure at the interface of the breakwater core and filter layer

ABSTRACT A formula is proposed to determine the maximum reference pressure at the interface of the breakwater core and filter layer, based on small-scale physical model tests. Regular and irregular waves have been considered. It is found that the maximum reference pressure at the interface of the breakwater core and filter layer depends on the level of the water surface along the breakwater seaward slope. The proposed formula is verified using large-scale model results and real-scale field observations in non-overtopping and non-breaking wave conditions; it contains a nondimensional parameter that is a function of the wavelength and width of the core at a given water depth.

Caulerpa Bikinensis (Chlorophyta) Preference for the Mesophotic Depths of Pacific Atolls1

The marine green alga Caulerpa bikinensis W. R. Taylor was described as a new species in 1950 from dredged specimens (37–57 m depth) from Bikini Atoll and Rongelap Atoll in the Marshall Islands. Its relative rareness in collections from waters less than 30 m deep from these atolls indicated that it favored the mesophotic coral ecosystem (MCE) at depths of 30–150 m. Published and unpublished records of this species over the past 70 years show its habitat preference on the seaward slopes of atolls and in lagoons. Observations were based on collections from Pacific atolls of the Marshall Islands, Tuamotu Islands, Chuuk State in the Federated States of Micronesia, Cook Islands, and Johnston Atoll, and on the deep lagoonal reefs of Palau. Caulerpa bikinensis has also been recorded in shallow waters 1 m deep in dark microcavities on lagoonal pinnacles at Takapoto Atoll (Tuamotu Islands) and in Chuuk Lagoon, and 2 m deep at Raroia Atoll (Tuamotu Islands). Caulerpa bikinensis favors the deeper depths of the MCE for its low light intensity rather than its cooler temperature, and definitely shows a strong affinity to the high energy seaward slopes of calcareous Pacific atolls.

Factors controlling longshore variations of beach changes induced by Tropical Storm Eta (2020) along Pinellas County beaches, west-central Florida

Tropical Storm Eta impacted the coast of west-central Florida from 11 November to 12 November 2020 and generated high waves over elevated water levels for over 20 hours. A total of 148 beach and nearshore profiles, spaced about 300 m (984 ft) apart, were surveyed one to two weeks before and one to eight days after the storm to examine the beach changes along four barrier islands, including Sand Key, Treasure Island, Long Key, and Mullet Key. The high storm waves superimposed on elevated water level reached the toe of dunes or seawalls and caused dune erosion and overwash at various places. Throughout most of the coast, the dune, dry beach, and nearshore area was eroded and most of the sediment was deposited on the seaward slope of the nearshore bar, resulting in a roughly conserved sand volume above closure depth. The longshore variation of beach-profile volume loss demonstrates an overall southward decreasing trend, mainly due to a southward decreasing nearshore wave height as controlled by offshore bathymetry and shoreline configurations. The Storm Erosion Index (SEI) developed by Miller and Livermont (2008) captured the longshore variation of beach-profile volume loss reasonably well. The longshore variation of breaking wave height is the dominant factor controlling the longshore changes of SEI and beach erosion. Temporal variation of water level also played a significant role, while beach berm elevation was a minor factor. Although wider beaches tended to experience more volume loss from TS Eta due to the availability of sediment, they were effective in protecting the back beach and dune area from erosion. On the other hand, smaller profile-volume loss from narrow beach did not necessarily relate to less dune/ structure damage. The opposite is often true. Accurate evaluation of a storm’s severity in terms of erosion potential would benefit beach management especially under the circumstance of increasing storm activities due to climate change.

Impact of Management Regime and Regime Change on Gravel Barrier Response to a Major Storm Surge

Gravel barriers represent physiographic, hydrographic, sedimentary, and ecological boundaries between inshore and open marine offshore environments, where they provide numerous important functions. The morphosedimentary features of gravel barriers (e.g., steep, energy reflective form) have led to their characterization as effective coastal defense features during extreme hydrodynamic conditions. Consequently, gravel barriers have often been intensively managed to enhance coastal defense functions. The Blakeney Point Barrier System (BPBS), U.K., is one such example, which offers the opportunity to investigate the impact of alternative management regimes under extreme hydrodynamic conditions. The BPBS was actively re-profiled along its eastern section from the 1950s to the winter of 2005, whilst undergoing no active intervention along its western section. Combining an analysis of remotely sensed elevation datasets with numerical storm surge modeling, this paper finds that interventionist management introduces systemic differences in barrier morphological characteristics. Overly steepened barrier sections experience greater wave run-up extents during storm surge conditions, leading to more extreme morphological changes and landward barrier retreat. Furthermore, while high, steep barriers can be highly effective at preventing landward flooding, in cases where overwashing does occur, the resultant landward overtopping volume is typically higher than would be the case for a relatively lower crested barrier with a lower angled seaward slope. There is a growing preference within coastal risk management for less interventionist management regimes, incorporating natural processes. However, restoring natural processes does not immediately or inevitably result in a reduction in coastal risk. This paper contributes practical insights regarding the time taken for a previously managed barrier to relax to a more natural state, intermediary morphological states, and associated landward water flows during extreme events, all of which should be considered if gravel barriers are to be usefully integrated into broader risk management strategies.

High-Accuracy Remote Sensing Water Depth Retrieval for Coral Islands and Reefs Based on LSTM Neural Network

Li, X.M.; Ma, Y.; Leng, Z.H.; Zhang, J., and Lu, X.X., 2020. High-accuracy remote sensing water depth retrieval for coral islands and reefs based on LSTM neural network. In: Jung, H.-S.; Lee, S.; Ryu, J.-H., and Cui, T. (eds.), Advances in Geospatial Research of Coastal Environments. Journal of Coastal Research, Special Issue No. 102, pp. 21-32. Coconut Creek (Florida), ISSN 0749-0208.Accurate water depth data are essential to ensure navigational safety of ships operating in regions with coral islands and reefs; however, it is often difficult or impossible to conduct in situ bathymetric surveys in such areas. In this study, a Long-Short Term Memory (LSTM) neural network in deep learning was introduced for multispectral remote sensing detection of water depth, and an LSTM neural network model suitable for remote sensing water depth retrieval for coral islands and reefs was developed. The LSTM model retrieval result was optimal when using an Adam optimizer, batch size of 10 %, 2000 epochs and a 50/100 network structure. Compared with the classical Log-linear, Stumpf and improved Stumpf models, the LSTM model demonstrated better water depth retrieval capability with minimum mean absolute errors (MAEs) and mean relative errors (MREs) irrespective of whether using 300 or 2000 training points. The retrieval accuracy of the LSTM model with only 300 training points was better than the other three models with 2000 training points. Compared with SVR model, MRE of LSTM model reduced 19.03 % and 4.14 % respectively when using 300 and 2000 training points. Analysis of the Dong Island case showed that the water depth retrieval results based on the LSTM model clearly reflected the geomorphic units of the entire reef such as the reef flat, front slope (seaward slope), and patch reef. It also revealed subtle geomorphologic features of the reef flat surface and the valley system. The LSTM model demonstrated satisfactory performance for water depths of 5–25 m, with average MREs of 7 % and 9 % (minimum: 4.02 %, maximum: 14.62 %). These findings verify the application performance of the LSTM remote sensing water depth retrieval model both qualitatively and quantitatively.

Operational monitoring and forecasting of wave run-up on seawalls

In this work, a wave run-up monitoring system and a model for forecasting the wave run-up height on a seawall were developed. Electrical conductivity sensors were installed on the seaward slopes of seawalls to measure the wave run-up heights. The general packet radio service protocol was used to transmit the measured data in real time to the desired remote location. The Princeton Ocean Model and WAVEWATCH III were used to predict the water levels and ocean waves, respectively, by using the available wind fields. The empirical formulas recommended in the Coastal Engineering Manual (2011) and EurOtop (2018) were adopted to estimate the run-up height. The wave run-up heights were forecasted 72 h in advance and were renewed at 6-h intervals as new wind fields became available. The developed monitoring system and forecasting model were combined for operational monitoring and forecasting of wave run-up on seawalls. The wave run-up monitoring system was set up at three seawalls along the southwestern coast of Taiwan from 2013 to 2016. Consistency between the forecasted and measured wave run-up heights during typhoon periods demonstrated the feasibility of using the proposed method for monitoring and forecasting wave run-up heights. Furthermore, the multi-model ensemble approach was adopted to improve the unsatisfactory run-up forecasting performance during typhoon periods, and the forecasted run-up heights were eventually presented as a band with upper and lower limits as opposed to single values. The forecast results can be used to provide advance warning of possible wave overtopping and associated coastal flooding during typhoon periods.

Influence of oblique wave attack on wave overtopping at smooth and rough dikes with a berm

Oblique wave attack can significantly reduce the amount of wave overtopping at coastal structures compared to perpendicular wave attack. Guidelines exist to estimate the influence of oblique waves on wave overtopping. Guidelines to estimate the influence of a berm in the seaward slope are also available. These guidelines for the reducing effects of oblique waves and the reducing effects of a berm are frequently applied in combination, although limited data is available for this combination of reducing effects. To develop a method that can account for the reducing effects of oblique waves and a berm in the seaward slope of a dike, physical model tests have been performed in a wave basin on structures with smooth and rough slopes. Based on these data, wave overtopping guidelines for breaking waves on dikes have been validated and one of the guidelines has been extended to take the influence of oblique waves, berms and roughness into account.

Seismic stratigraphy and deep-water sedimentary evolution of the southern Mozambique margin: Central Terrace and Mozambique Fracture Zone

Based upon new seismic reflection data, this study reveals the development of a complex sedimentary system on the lower-slope terrace off Maputo on the southern Mozambique margin. The deep-water sedimentary system clearly shows the interaction of downslope and alongslope processes. Downslope features include: a pronounced canyon, slope creep, submarine slides, and multi-stage mass-transport deposits. Alongslope features include: contourite drifts and sediment waves, erosional moats, valleys, erosive surfaces and a distinctive contourite terrace. We show an evolution through time as follows: (1) restricted marine sedimentation during the Early Cretaceous after the initial break-up, with slope progradational clinoforms and hemipelagite drape developed; (2) open marine sedimentation characterized by the onset of alongslope sedimentary processes during the Late Cretaceous and development of a contourite depositional system; (3) downslope gravity processes dominated sedimentation during the Paleocene-Eocene, including development of a canyon and submarine fan; and (4) an alongslope contourite depositional system was re-established on the lower slope in the Middle Miocene, while turbidites (from a northern source) covered the Mozambique Basin. The Mozambique Fracture Zone marks a distinct and steep slope seaward of the contourite terrace, and has significantly influenced both contourite and turbidite deposition. Our study provides new insights into the onset and evolution of bottom current control on margin sedimentation, the interaction of downslope and alongslope processes and how these processes have jointly sculpted this part of the southern Mozambique margin.

The impact of internal waves on upper continental slopes: insights from the Mozambican margin (southwest Indian Ocean)

AbstractEvidences of sedimentation affected by oceanic circulation, such as nepheloid layers and contourites are often observed along continental slopes. However, the oceanographic processes controlling sedimentation along continental margins remain poorly understood. Multibeam bathymetry and high‐resolution seismic reflection data revealed a contourite depositional system in the Mozambican upper continental slope composed of a contourite terrace (a surface with a gentle seaward slope dominated by erosion) and a plastered drift (a convex‐shape sedimentary deposit). A continuous alongslope channel and a field of sand dunes (mainly migrating upslope), formed during Holocene, were identified in the contourite terrace at the present seafloor. Seismic reflection data of the water column show internal waves and boluses propagating in the pycnocline near the upper slope. The channel and the dunes are probably the result of the interaction of the observed internal waves with the seafloor under two different conditions. The alongslope channel is located in a zone where intense barotropic tidal currents may arrest internal solitary waves, generating a hydraulic jump and focused erosion. However, upslope migrating dunes may be formed by bottom currents induced by internal solitary waves of elevation propagating landwards in the pycnocline. These small‐scale sedimentary features generated by internal waves are superimposed on large‐scale contouritic deposits, such as plastered drifts and contourite terraces, which are related to geostrophic currents. These findings provide new insights into the oceanographic processes that control sedimentation along continental margins that will help interpretation of palaeoceanographic conditions from the sedimentary record. © 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd

Deepest known gastropod fauna: Species composition and distribution in the Kuril–Kamchatka Trench

The joint German–Russian expedition KuramBio II in 2016 provided a significant opportunity to reinvestigate the species composition and distribution of the abyssal and hadal gastropod fauna in the Kuril–Kamchatka Trench, which was sampled previously only by the Russian research vessel (R/V) Vityaz in the 1940–1960s. During the cruise of the German R/V Sonne, benthic animals were collected using an Agassiz trawl, an epibentic sledge, a multiple corer and a giant box corer. A total catch of 592 live specimens of non-heterobranch gastropods were classified into 86 species and 22 families of the subclasses Neomphaliones, Vetigastropoda, and Caenogastropoda. The abundance, richness and phylogenetic diversity were lower in the hadal zone than in the abyssal zone. Seven hadal stations (6,442–9,584 m) yielded a total of 101 specimens belonging to 22 species in 11 families, whereas catches from four abyssal stations on the landward and seaward slopes (5,102–6,221 m) included 491 specimens and 70 species in 21 families. The abundance, richness and diversity for the hadal zone are, however, particularly high in comparison with other oceanic trenches, due, presumably, to high primary production in the surface layer of the subarctic Northwest Pacific. The uniqueness of species composition increased with depth; sixteen out of the 22 species from the hadal stations (72.7%) were found exclusively in this depth zone. Two unnamed species of Neomphalidae and Sahlingia from 9,577 to 9,584 m represent the world’s deepest records for Neomphaliones and Vetigastropoda and suggest the as yet unidentified presence of the world-deepest cold-seep habitats in the Kuril–Kamchatka Trench.

Successive bleaching events cause mass coral mortality in Guam, Micronesia

The reefs of Guam, a high island in the Western Pacific, were impacted by an unprecedented succession of extreme environmental events beginning in 2013. Elevated SSTs induced severe island-wide bleaching in 2013, 2014, 2016, and 2017. Additionally, a major ENSO event triggered extreme low tides beginning in 2014 and extending through 2015, causing additional coral mortality from subaerial exposure on shallow reef flat platforms. Here, we present the results of preliminary analyses of environmental and biological data collected during each of these events. Accumulated heat stress in 2013 was the highest since satellite measurements began, but this record was exceeded in 2017. Overall, live coral cover declined by 37% at shallow reef flat sites along the western coast, and by 34% at shallow seaward slope sites around the island. Staghorn Acropora communities lost an estimated 36% live coral cover by 2017. Shallow seaward slope communities along the eastern windward coast were particularly devastated, with an estimated 60% of live coral cover lost between 2013 and 2017. Preliminary evidence suggests that some coral species are at high risk of extirpation from Guam’s waters. In light of predictions of the near-future onset of severe annual bleaching, and the possibility that the events of 2013–2017 may signal the early arrival of these conditions, the persistence of Guam’s current reef assemblages is in question. Here, we present detailed documentation of ongoing changes to community structure and the status of vulnerable reef taxa, as well as a critical assessment of our response protocol, which evolved annually as bleaching events unfolded. Such documentation and analysis are critical to formulating effective management strategies for the conservation of remaining reef diversity and function.

Tectonic discrimination of siliciclastic sedimentary record of the northern Tethyan margin at the end of the Triassic

Tectonic setting of both the uppermost Triassic Tomanova and the lowermost Jurassic Dudziniec formations can be characterized by a series of diagnostic discrimination diagrams. High-silica and low-silica multi-dimensional diagrams indicate that the sediments could have been deposited in the continental collision/rift setting. Diagrams of K 2 O/Na 2 O versus SiO 2 , SiO 2 /Al 2 O 3 versus K 2 O/Na 2 O and Th-Co-Zr/10 as well as Th-Sc-Zr/10 determine an origin from the tectonically active and/or passive continental margin setting. REE parameters indicate predominantly a passive margin tectonic setting of the sediments. The most probable source of clastic admixture in the Scythian to Domerian sediments in the Tatric Unit was situated in a remnant of the Variscan collisional orogen – the Vindelician Highlands. Occasional monsoonal rains occurring in orbitally controlled cycles transported weathered material from the source area down river valleys on the seaward slopes of the Variscan Vindelician Mountains towards the Tethyan Sea. The input of this material influenced also the marginal parts of the Fatricum (e.g., Vysoka or Havran units).

Statistical Analysis of the Stability of Rock Slopes

Physical model tests were performed in a wave flume at Deltares with rock armoured slopes. A shallow foreshore was present. At deep water, the same wave conditions were used, but by applying different water levels, the wave loading on the rock armoured slopes increased considerably with increasing water levels. This allowed an assessment of the effects of sea level rise. Damage was measured by using digital stereo photography (DSP), which provides information on each individual stone that is displaced. Two test series were performed five times. This allowed for a statistical analysis of the damage to rock armoured slopes, which is uncommon due to the absence of statistical information based on a systematic repetition of test series. The statistical analysis demonstrates the need for taking the mean damage into account in the design of rock armoured slopes. This is important in addition to characterising the damage itself by erosion areas and erosion depths. The relation between damage parameters, such as the erosion area and erosion depth, was obtained from the tests. Besides tests with a straight slope, tests with a berm in the seaward slopes were also performed. A new method to take the so-called length effect into account is proposed to extrapolate results from physical model tests to real structures. This length effect is important, but is normally overlooked in the design of rubble mound structures. Standard deviations based on the presented model tests were used.

STABILITY EFFECTS OF CUBE ARMOR UNIT PLACEMENT CONFIGURATIONS IN THE BERM OF A BREAKWATER

For the sake of overtopping, stability and economy, rubble mound breakwaters have been built with a berm in the seaward slope. Here, a breakwater model with cube armour units in the lower slope and a horizontal berm, and rock in the upper slope was tested experimentally. The purpose of the study was to investigate the stability of berms for two different configurations of the transition of armor units from the lower slope to the berm. Based on the test results, an effective configuration of cube units at the transition has been obtained.

Zooplankton in the Huangyan Atoll, South China Sea: A comparison of community structure between the lagoon and seaward reef slope

Relatively few studies have investigated the community structure of zooplankton in the atolls of the central South China Sea. Zooplankton were sampled at nine stations within the lagoon and four facing the seaward reef-slope in the Huangyan Atoll during May 2015. Lagoonal samples contained 48 zooplankton taxa ( H' =2.25±0.95; J =0.70±0.23) with planktonic larvae, copepods, and euphausids contributing 95.63% of total abundance. Seaward oceanic samples contained 114 zooplankton ( H' =5.27±0.27; J =0.90±0.03) with copepods, siphonophores, chaetognaths, appendicularians, and planktonic larvae accounting for 93.66% of abundance. Lagoonal zooplankton abundance was lower than that on the seaward reef-slope. Zooplankton community differed significantly between the lagoon and reef-slope groups ( R =0.708, P =0.001). The former was dominated by Macruran larvae, Brachyuran larvae, Stylocheiron carinatum, and Acartia spp., while the latter was predominately represented by Oikopleura longicauda, Chelophyes contorta and Copilia mirabilis. Chlorophyll a concentration was significantly higher in the lagoon than the reef slope regions ( P <0.01), which may be due to the nutrient input from fishing vessels near the lagoon mouth. Zooplankton abundance was negatively correlated with chlorophyll a concentration ( P <0.01). A clear difference was observed statistically in the species composition and abundance of zooplankton between the lagoon and the seaward slope at Huangyan Atoll, which suggests that it may be resulted from the influence by natural and anthropogenic interactions.

Sediment budget controls on foredune height: Comparing simulation model results with field data

AbstractThe form, height and volume of coastal foredunes reflects the long‐term interaction of a suite of nearshore and aeolian processes that control the amount of sand delivered to the foredune from the beach versus the amount removed or carried inland. In this paper, the morphological evolution of more than six decades is used to inform the development of a simple computer model that simulates foredune growth. The suggestion by others that increased steepness of the seaward slope will retard sediment supply from the beach to the foredune due to development of a flow stagnation zone in front of the foredune, hence limiting foredune growth, was examined. Our long‐term data demonstrate that sediment can be transferred from the beach to the foredune, even with a steep foredune stoss slope, primarily because much of the sediment transfer takes place under oblique rather than onshore winds. During such conditions, the apparent aspect ratio of the dune to the oncoming flow is less steep and conditions are not as favourable for the formation of a stagnation zone. The model shows that the rate of growth in foredune height varies as a function of sediment input from the beach and erosion due to storm events, as expected, but it also demonstrates that the rate of growth in foredune height per unit volume increase will decrease over time, which gives the perception of an equilibrium height having been reached asymptotically. As the foredune grows in size, an increasing volume of sediment is needed to yield a unit increase in height, therefore the apparent growth rate appears to slow. Copyright © 2018 John Wiley &amp; Sons, Ltd.

Semi-isolated, flat-topped carbonate platform (Oligo-Miocene, Sardinia, Italy): Sedimentary architecture and processes

The Chattian-Aquitanian carbonate system of Isili sub-basin (SE Sardinia, Italy) were studied to better understand the hydrodynamic processes controlling the formation of landward-downlapping, rhodolith-rich, giant clinoforms.The studied flat-topped platform was attached to an island (semi-isolated) and migrated onshore (landward) over a shallow marine, protected embayment. The depositional profile is characterized by four, sea to land, zones: seaward slope, flat-topped platform (submerged flat), landward slope and mixed siliciclastic-carbonate embayment. In particular, these zones record different levels, from high to moderate, of hydrodynamic energy and persistency. The paleodepth of the studied succession, based on T/D test on Amphistegina specimens, red-algal genera and geometric reconstruction of the overall system, ranges from −10 to −30m for the platform up to around −40m for the landward slope base. Thus, the flat-topped platform was permanently submerged and an extensive carbonate (sea grass, algal and bryozoan-algal) factory along with oyster framestones developed. The widespread presence in the barren zones of rhodolith-to-shell rich small to very small compound dunes (high-energy zone) suggests that the flat-topped platform was periodically swept by unidirectional, landward-directed currents. These currents allow the formation of large-scale, rhodolith-rich clinobeds along the landward slope (washover fan-like system). The presence of rodolith pavements developed along the flat-topped platform at or slightly below the fairweather wave base suggests that clinobeds were formed in a nearshore, shallow subtidal setting. Finally, the shallow marine, protected embayment (moderate-energy zone) represents the widening distal reaches of the currents flowing or along the clinoforms or from the coast basinward (river floods and/or ebb tidal currents).Despite few tidal-related sedimentary structures such as bi-directional ripples and small compound dunes separated by finer-grained “drapes” or reactivation surfaces were observed, the prevailing processes acting over the platform are unidirectional, landward-directed currents possibly associated with longshore currents and/or wave actions. However, the resultant migration of the whole system onshore (landward) cannot be easily explained with storm or wind-related processes. Thus the studied flat-topped platform seems to be controlled by long term tidal regulation within a meso to macro tidal regime. In particular, such meso/macro tidal environments experience multiannual to multidecennial phases of stronger/weaker tidal range fluctuations resembling periods of relatively sea highs and lows with respect to the mean sea level (0m). All the (wind, storm, wave and tidal) currents sweeping the flat-topped platform were maxima during phases of strong tidal fluctuations generating erosion and sediment transportation over the flat and accumulation on the landward slope (clinobeds). Conversely, during phases of weaker tidal range fluctuations overall currents were minima, clinoforms did not develop and factories widespread re-colonized the submerged flat. Therefore, the studied platform developed in a current-dominated and tidal modulated setting.Finally, the studied carbonates of Sardinia suggest that the Sardinian seaway and the incipient Provençal basin during the Chattian-Aquitanian were, locally, capable to generate meso to macro tidal conditions.

Offshore aeolian sediment transport across a human‐modified foredune

AbstractConcepts derived from previous studies of offshore winds on natural dunes are evaluated on a dune maintained for shore protection during three offshore wind events. The potential for offshore winds to form a lee‐side eddy on the backshore or transfer sediment from the dune and berm crest to the water are evaluated, as are differences in wind speed and sediment transport on the dune crest, berm crest and a pedestrian access gap.The dune is 18–20 m wide near the base and has a crest 4.5 m above backshore elevation. Two sand‐trapping fences facilitate accretion. Data were obtained from wind vanes on the crest and lee of the dune and anemometers and sand traps placed across the dune, on the beach berm crest and in the access gap. Mean wind direction above the dune crest varied from 11 to 3 deg from shore normal. No persistent recirculation eddy occurred on the 12 deg seaward slope. Wind speed on the berm crest was 85–89% of speed at the dune crest, but rates of sediment transport were 2.27 times greater during the strongest winds, indicating that a wide beach overcomes the transport limitation of a dune barrier. Limited transport on the seaward dune ramp indicates that losses to the water are mostly from the backshore, not the dune. The seaward slope gains sand from the landward slope and dune crest. Sand fences causing accretion on the dune ramp during onshore winds lower the seaward slope and reduce the likelihood of detached flows during offshore winds. Transport rates are higher in access gaps than on the dune crest despite lower wind speeds because of flatter slopes and absence of vegetation. Transport rates across dunes and through gaps can be reduced using vegetation and raised walkover structures. Copyright © 2017 John Wiley &amp; Sons, Ltd.

Are Sri Lankan Reefs Losing the Resilience? An Example from Coral Reefs in Sallitive Island

Coral reefs are known to undergo phase shift in to macro-algae and reefs have lost the capacity to remain in or return to a coral-dominated state. This shift will not only affect corals, but the others heavily depend on heterogeneous habitat afforded by corals. Sallitivu in Panichchankerni, Eastern province, is a small island (41328 m2). The Island is surrounded by a ring of coral reef with an elevated breaking reef crest, seaward slope and centred shallow lagoon (&lt;1.2m at high tide). The shore is entirely a thick layer of washed coral rubbles. The archived aerial photographs confirmed these coral rubbles present only after 2004 tsunami.Underwater visual census were carried out within the reef lagoon and three sites in reef slope using 30 m long belt transect. In addition to diversity of corals and algae, percentages of live, dead and bleached coral cover were recorded. The reef lagoon is shallow (10-60 cm) and much of the reef crest is exposed at low tide. Within the reef lagoon, the live coral cover was &lt;5%. Around 15% was observed recently bleached, 12% were overgrown by algae; Padina sp, Halimeda sp, Sargassum sp, Caulerpa recemosa and Dictyota sp, and the rest was dead corals smothered by sediments. The seaward slope with high wave action was mostly smothered dead corals with live corals &lt;3%. Coral colonies were represented by Acroporidae (branching and table corals)-Acropora cytherea, A. divaricata, A. formosa, A. gemmifera, A. grandis, A. hemprichii, A. hyacinthus, A. latistella, Acropora sp., A. nobilis, and A. robusta; (Foliaceous)-Montipora aequituberculata and M. hispida; Faviidae (sub massive and encrusting)-Favites halicora, Favites spinosa, Leptastrea purpurea and Platygyra daedalea; Pocilloporidae (Lace/cauliflower) -Pocillopora damicornis; Poritidae (Massive/submassive)-Porites evermanni, P. paliformis, and P. rus. Live corals observed were immature. Southern end of the reef slope had emerging corals among unstable coral rubbles. It is assumed that the degradation was started post-tsunami and continued due to natural stresses. The recovery of corals may hindered by macro-algal growth and resulting accumulation of sediments, smothering due to less wave action within reef lagoon and resulting recruitment and settlement failure. Recent bleaching would be due to exposure to direct sunlight during the change of tides and low sea level. Increasing oxygen level due to algal blooms would also prone for bleaching. Avoiding such undesirable phase shifts from coral dominance to algae and reverse them when occur, requires an urgent reform of scientific approaches to understand the processes causing the degradation. A better understanding on why some reefs rapidly degrade and others do not is critical. Most reef conservation efforts are directed toward reserve implementation, but new approaches are needed to sustain ecosystem function since demarcation of a marine reserve alone would not benefit in improving reef resilience.Keywords: Phase-shift, Coral reefs, Macro-algae, Bleaching, Sallitive Island

Wide Green Dikes: A sustainable adaptation option with benefits for both nature and landscape values?

The dual task of reinforcing the dikes along the Dutch coast and preparing for the effects of climate change have stimulated a search for innovative dike designs. Triggered by the presence of gently sloped grass-covered dikes that merge smoothly into the adjacent salt marshes along the German Wadden Sea coast, we assessed the potential costs and benefits of just such a ‘Wide Green Dike’ along the Dutch Wadden Sea coast, compared to the ‘Traditional Dike’, which is now used. The Traditional Dike has a steeper seaward slope and a stone or asphalt revetment along the dike toe. The assessment criteria were identified by local experts. We investigated, particularly, the spatial impact of a Wide Green Dike on the adjacent salt marshes under various climate scenarios. We also looked at the initial costs and clay demand for both dike designs. Furthermore, we conducted a quick-scan of the vegetation present halfway up the seaward slope of a Traditional Dike in the Netherlands and a Wide Green Dike nearby in Germany. Our analysis revealed four primary advantages of use of a Wide Green Dike along the Dutch part of the Dollard estuary: lower initial costs (based on standard unit prices), greater ease of doing repairs, increased adaptability and enhanced spatial quality compared to a Traditional Dike. Both types of dike were designed to withstand an extreme storm surge with an occurrence probability of 1/4000 years. Both fully met legally mandated engineering standards as well. Though the required crest height is comparable for both designs, the Wide Green Dike had a larger areal footprint, and would overlap more Natura 2000 area than a Traditional Dike. There would thus be a trade-off concerning salt-marsh area. Moreover, the vegetation examined halfway up the seaward slope of the German Wide Green Dike offered little ecological value. Nonetheless, cyclic harvesting of sediment for periodic dike reinforcement appeared to be a sustainable option for adapting to future sea level rise.