Wave Energy Regime Research Articles

TAPHONOMIC SIGNATURES IN DEAD SHELLS OF THE INVASIVE GASTROPOD RAPANA VENOSA (VALENCIENNES, 1846) AFTER TWO DECADES IN THE RÍO DE LA PLATA, URUGUAY

ABSTRACT Rapana venosa (Valenciennes, 1846) is an invasive gastropod, the arrival of which in the Río de La Plata estuary 22 years ago is well-documented. Rapana venosa shells were collected during two sampling events from four beaches with different substrate types and wave energy regimes to compare the taphonomic attributes under different environmental conditions. We analyzed the samples by comparing frequencies of taphonomic attributes. Our results show that intermediate-reflective beaches with rocky substrates were dominated by intermediate- to highly fragmented specimens, with high corrasion, intermediate to high bioerosion, low bioencrustation, and medium to large sizes. In contrast, intermediate-dissipative beaches with sandy substrate, mobile stones, and occasional consolidated sediments were dominated by less fragmented shells, high to intermediate corrasion, scarcer bioerosion, low bioencrustation, and small- to medium-sized specimens. Results suggest that significant taphonomic differences arise within two decades under natural conditions. These findings imply that paleoenvironmental signals derived from the taphonomic attributes of fossil assemblages emerge much faster than the potential duration of time averaging of shelly fossils in shallow marine settings.

Modelling and simulation of extreme wave heights around agatti island of lakshadweep, west coast of India

Small islands experience variation in wave energy regime along shorter shoreline due to refraction, diffraction and variability in direction of wave travel. Nearshore wave climate around these islands is quite complex due to wave transformation associated with configuration of islands, and high steepness in the bathymetry. Understanding wave energy will help in identifying suitable locations for coastal structures, wave energy devices and sediment budgeting. Assessment of extreme wave climate around the islands play a crucial role in the design and survival of coastal infrastructure for any developmental activities in islands. This study investigates the extreme wave climate around the Agatti island of Union Territory of Lakshadweep located in west coast of India. The spatial variability of wave power distribution around the Agatti island is analyzed based on 10 years of wave hindcast data from spectral wave modelling for the period between 2011 and 2020 using spectral wave model. In this study, third-generation spectral wave model was used to simulate the wave climate over the Arabian Sea. Regional model was established using ECMWF model winds and the offshore wave parameters were validated with deep water ocean observations AD07 and AD09 deployed in the Arabian Sea. To understand the nearshore wave transformation along the Agatti island, local wave model of fine resolution was established using high resolution bathymetry and is validated using the coastal buoy (CB02) deployed near Agatti. Nearshore wave parameters such as Significant wave height, Peak wave period (Tp) and Mean wave direction (θm) were analyzed. The wave power distribution around the island was quantified and demarcated for siting of offshore structures for developmental activities. The study reveals that the south zone of the island is more energetic with annual mean wave power of 8–12 kW/m and during monsoon, wave power more than 28 kW/m is observed. The extreme value theory based on Generalized Extreme Value distribution is applied to the extreme wave height estimate based on modelled wave data around the Agatti island. The extreme wave heights for different return periods were estimated around the Agatti island for design and analysis of coastal infrastructure.

Reef design and site hydrodynamics mediate oyster restoration and marsh stabilization outcomes.

The detrimental ecological impacts of engineered shoreline protection methods (e.g., seawalls) and the need to protect the coastal zone have prompted calls for greater use of natural and nature-based infrastructure (NNBI). To balance competing needs of structural stability and ecological functioning, managers require assessments of NNBI designs and materials for differing environmental settings (e.g., among wave-energy regimes). To examine the effects of setting and oyster-based NNBI design on the provision of shoreline protection, we constructed reefs from two substrates: a novel, biodegradable material (Oyster Catcher, OC) and traditional oyster shell bags (SB) on low- and high-energy eroding salt marsh shorelines, designated based on fetch and boat wake exposure. Both reef types buffered marsh elevation change on the high-energy shoreline relative to unaltered controls, but only SB reefs were able to do so on the low-energy shoreline. Additionally, both shorelines experienced high ambient rates of retreat and declines in marsh vegetation shoot density. Although constructed reefs did not mitigate marsh retreat on the low-energy shoreline, novel OC reefs significantly reduced retreat relative to SB reefs and control sites (no reefs) on the high-energy shoreline. Those SB reefs were severely damaged by storm events, increasing their areal footprints at the expense of vertical relief. Conversely, OC reefs on both shorelines exhibited steady oyster recruitment and growth and hosted higher densities of larger oysters. To successfully provide shoreline stabilization benefits, oyster-based NNBI must be structurally stable and able to promote sustained oyster recruitment and growth. Our results indicate that deliberate decisions regarding NNBI substrate, siting, and configuration can produce resilient reefs, which reduce rates of erosion and, in some cases, enhance vertical accretion along salt marsh edges. The growth trajectory, structural stability, and co-benefit provisioning of OC reefs demonstrate the potential of alternative restoration substrates to provide valuable oyster habitat along threatened marsh shorelines.

Reef response to sea-level and environmental changes in the Central South Pacific over the past 6000 years

Geological records of coastal system evolution during past higher and/or rising sea levels provide an important baseline for developing projections regarding the response of modern coastal systems to future sea-level rise. The mid-late Holocene corresponds to the most recent window into natural variability prior to the Anthropocene and involves slow-rate and low-amplitude sea-level changes that were mostly governed by a limited glacio-eustatic contribution, most likely sourced from Antarctica, and ‘glacial isostatic adjustment’ processes. This paper documents in unprecedented detail the response of coral reefs and coastal systems to changing accommodation space in relation to mid-late Holocene sea-level changes in French Polynesia. The sea-level curve that underpins this study has a global significance and documents a single short-lived sea-level highstand between 4.10 and 3.40 kyr BP. The amplitude of the highstand is less than one metre, within the range of the predicted sea level at the end of the current century. The reported relative sea-level changes are characterized by slow rates ranging from a few tens of millimetres per year to up to 2.5 mm/yr and by significant sea-level stability (stillstands) lasting more than a century and up to 250 years, defining a step-like pattern. Sea-level variability probably driven by climatic oscillations on interannual to millennial time scales is evidenced during the entire time window. The detailed reconstruction of reef development over the last 6000 years brings valuable information regarding coral reef dynamics and coastal processes during periods of higher sea level and wave energy regimes. The persistence of stable and optimal depositional environments over the last 6000 years is demonstrated by the constant overall composition and diversity of reef communities and the almost continuous development of coral microatolls. The facies distribution as well as the lateral extension and shift of facies belts have been governed by variations in accommodation space, which are controlled by relative sea-level changes and antecedent topography. The widespread development of mid-late Holocene reef deposits in coastal areas suggests that they have played a prominent role in global processes related to the formation and shaping of modern islands. • Coral reef responses to slow-rate, low-amplitude, high-frequency sea-level changes. • Continuous reef development over the last 6000 years in optimal conditions. • Mid-late Holocene coral assemblages similar to their modern counterparts. • Facies distribution governed by variations in accommodation space. • Prominent role played by reef deposits in the formation and shaping of modern islands.

Comparison of coastal hydrodynamics in different energy regime coasts along southwest coast of India

Study of the complex physical processes of beach-nearshore system helps to understand the coastal processes and facilitate coastal planning and management. Since the SW coast of India has very dynamic and contrasting hydrodynamic features, the Veli–Varkala stretch of southern Kerala (SK) and Munambam–Chettuwa sector of central Kerala (CK) were selected for a comparative study of their hydrodynamics. SK is a high wave energy coast compared to CK. The contrasting wave energy regimes result from the differences in the inner shelf slope and shoreline orientation. However, just as in the case of SK, waves and wave induced littoral processes are the dominant driving forces in the coastal processes of CK too. Due to the favorable hydrodynamic and sedimentological characteristics, mudbanks set in at different locations of CK affecting the coastal processes of the locations of its occurrence. Mudbanks do not occur along the SK due to the unfavorable sedimentological features. Artificial morphologies such as seawalls, groins, harbour breakwaters have a great role for coastal processes along both the sectors. The long term shoreline change along the SK and CK can be linked to the contrasting characteristics of longshore sediment transport along the coasts.

Modelling the response of soft cliffs to climate change: A statistical, process-response model using accumulated excess energy

Cliff retreat poses a threat to coastal communities worldwide. In soft cliff environments, coastal retreat may occur at rapid rates. The environmental significance of soft cliff environments, coupled with the societal impacts of cliff retreat, means that it is important to understand the processes and timings of soft cliff erosion. Here, a new statistical, process-response model of soft cliff erosion is proposed, based on the premise that wave energy delivered to the cliff toe is the key parameter forcing erosion. More specifically, and in contrast to prior studies that have sought to link erosion rates to indices of peak wave energy, we postulate that the Accumulated Excess wave Energy (AEE; the time integrated total wave energy above an erosion threshold), is the key hydraulic control on soft cliff erosion. The new model is calibrated to two UK sites with contrasting (Atlantic versus North Sea) wave energy regimes, before downscaled HadCM3 Global Climate Model outputs are used to assess how future (next ~100year) changes in sea-level and wave height affect retreat rates at the two study sites. We show that the AEE model accounts for between 46% and 89% of the variance in observed coastal erosion; representing an improvement over prior models that use measures only of peak wave energy. The AEE model suggests increases of between 0.5m.a–1 (92%) and 0.3m.a–1 (33%) in mean erosion rate by 2100 for sites in the Atlantic and North Sea, respectively. Furthermore, it is shown that at both sites extreme events, exceeding the 95th percentile value, are projected to become three to four times more likely under scenarios of future climate change, highlighting a need to understand and predict the extreme response of coastal cliff systems to changes in climate.

A boulder-strewn tidal flat, north shore of the Gulf of St. Lawrence, Québec

A clay flat strewn with ice-drifted boulders occurs in a sheltered embayment near Harrington Harbour, North Shore of the Gulf of St. Lawrence. The embayment, approx. 7 km2 in area with a slope gradient less than 0.2°, is under the influence of tides ranging from 1.4 to 2.2 m and of a low wave energy regime. Ice entirely covers the flat during 3 to 4 months per year but may be present up to 5 months. Ice rafting, ice pushing, and ice gauging are moderately important processes in the embayment.

Tidal influence on the intertidal bar morphology of two contrasting macrotidal beaches

Analyses of morphological and hydrodynamic data from two macrotidal beaches (mean spring ranges respectively 5.6 and 6.4 m) with intertidal bars and troughs were carried out in order to detect and explain the relationship between tides and bar morphological attributes (bar crest height and location) and morphological change. The following four aspects are treated in the paper: (1) tidal levels, vertical tidal translation rates and bar morphology, (2) tidal range and morphological relaxation, (3) the neap–spring tidal variation, and (4) tide-generated longshore currents. Bar distribution and bar heights across the profile are not related in a straightforward manner to tidal levels and expected vertical tidal translation rates at these levels, because the wave process domains involved in bar formation and bar morphological change are strongly modulated by wave energy, by tide-modulated water levels which vary with the neap–spring cycle, and by the beach morphology. The variability of these conditions may explain the large variability of bar–trough beach morphology. The relatively large tidal ranges and rapidly shifting wave domains across the profile imply long periods of non-exposure of the bars to wave processes during each semi-diurnal tidal cycle, and this tide-generated effect reduces the scope for prolonged wave reworking while contributing to enhancing the long relaxation times and morphological feedback which minimise change on these beaches. The effects of the neap–spring tidal range variation on wave activity on these beaches are subordinate to those due to the random wave energy regime, but this neap–spring variation appears to influence the morphologically important intertidal drainage channels associated with bar–trough topography. Although longshore tidal currents are relatively strong on these beaches, digital elevation models show no significant longshore bar and tidal channel migration over time, due to the absence of sustained strong wind conditions, which considerably reinforce such longshore tidal currents, and which therefore are the dominant control on longshore processes on these beaches. It is also tentatively suggested that there is no established relationship between tidal range and the number of intertidal bars on these beaches.

Influence of wave energy on Holocene coral reef development: an example from Ishigaki Island, Ryukyu Islands, Japan

Wave energy induced by ocean swell and local winds is an important factor controlling the coral reef development. Here we compare and contrast facies, ages, and accretion rates of reefs that developed independently along windward and leeward margins of the same island. As both reefs face the open ocean, swells provide a background of low- to medium-energy condition with additional energy along windward margins due to prevailing winds. This similarity of the wave-energy regimes (medium to high energy) produced a similar framework facies that was constructed by robust-branching Acropora. However, much greater lateral accretion rates were observed in the high-energy reef than in the medium-energy reef. Also, coral cobble facies were only found in the medium-energy reef, indicating that the reef framework was more susceptible to storm breakage than that in the high-energy reef. These differences are attributable to the differing biological and ecological responses of corals to wave energy and water motion.

Influence of Spacing on Mechanically Transplanted Seagrass Survival in a High Wave Energy Regime

Abstract The wave‐exposed nature of much of the southwestern Australian coastline considerably reduces the protective influence of seagrasses, and sediment movement appears to be relatively unaffected by their presence. Present seagrass restoration efforts focus on the deployment of large mechanically transplanted “sods” of seagrass as a means of combating the negative effects of water motion on transplant survival. The aim of this study was to investigate the combined role of wave energy and transplant spacing on sediment movement and transplant survival to provide guidance for seagrass transplantation in areas of high wave energy. One hundred sixty sods (0.25 m2) of seagrass were mechanically extracted from a mixed meadow consisting of Amphibolis griffithii (Cymodoceaceae) and Posidonia coriacea (Posidoniaceae) and planted in a high wave energy site with the treatments configured as three replicates of 16 sods placed in 4 × 4–meter squares at distances of 0.5, 1.0, and 2.0 meters apart. An additional 16 single sods were planted randomly throughout the site. Monitoring was conducted at two monthly intervals and consisted of counting the number of sods surviving and measuring the shoot density of seagrass species within each surviving sod. Sediment height was monitored using a series of sediment plates and an electronic sediment level sensor. Sod spacing had no significant effect upon transplant survival, which remained above 90% for 4 months after transplantation and then declined with the onset of winter (June to August). After 14 months individual sod survival was between 9% and 40%. Initial shoot densities were 200 to 500 shoots/m2 and declined to less than 50 shoots/m2. Sediment fluctuations up to 35 cm were noted, occasionally taking place over a matter of hours, and storms during winter caused significantly increased sediment movement. This probably curtailed rhizome extension and prevented the expansion of the transplants. This study indicates that the ability of seagrasses to influence sediment would appear to vary with the prevailing hydrodynamic regime and that a reappraisal of the notion that all seagrass communities trap sediment is necessary.

A Holocene progradation record from Okains Bay, Banks Peninsula, Canterbury, New Zealand

Fifty‐eight distinct ridges are preserved on the Holocene progradation plain in Okains Bay, Banks Peninsula, Canterbury. Of these, 48 represent beach berm and foredune complexes and the remaining 10 are transverse dune ridges. Periods of rapid coastal progradation are marked by multiple beach berm preservation, whereas intervening periods of lower sediment accumulation result in a stable coastline and transverse dune formation. Infilling of the bay began following sea‐level stabilisation in the mid Holocene. The fill is dominantly fine sand, which is derived from sediment carried around Banks Peninsula in the Southland Current and washed into Okains Bay by wave action. Variations in the progradation rate are therefore proxy indicators of coastal erosion in the Canterbury Bight. We demonstrate that there is little progradational fill preserved between c. 6500 and 2000 yr BP. This implies significant changes in sediment delivery to the Southland Current within the last 2000 yr, which we attribute to increased coastal erosion in South Canterbury. We speculate that this increasing erosion resulted from increased wave energy regimes, which in turn may relate to increasing Southern Hemisphere seasonality following the precessional cycle.

Vertical profiles of suspended sand concentration and size from multifrequency acoustic backscatter

Vertical profiles of suspended sand concentration and size are obtained from multifrequency acoustic profiling data collected in 1989 during a nearshore experiment at Stanhope Lane Beach, Prince Edward Island. The data were acquired with acoustic sounders operating at 1, 2.25, and 5 MHz. Independent estimates of concentration were made using optical backscatter sensors (OBSs). The algorithm for inversion of the three‐frequency backscatter data to particle size and concentration, based on the ratios of the different signals, was tested in laboratory experiments with an unconfined high Reynolds number suspended sediment jet. Results from the Stanhope Beach experiment are presented for three different surface wave energy regimes, with significant wave orbital velocities ranging from 0.3 to 0.8 m/s and peak wave periods of 4–6 s. The acoustic estimates of mean concentration are shown to be within 10% on average of those determined with the OBS nearest the bottom at 5‐ to 10‐cm height, over time scales ranging from 6.5 min to 4–6 s (one wave period). The acoustic estimates of suspended sediment size near the bottom are within 5–20% of the bottom sediment mean size. The statistical variability of the size estimates is high, with standard deviations in the estimates ranging between 30 and 50% of the mean. The time‐averaged concentration profiles exhibit an exponential decrease with height above an O(10)‐cm‐thick near‐bottom region of nonexpo‐nential decrease. In contrast, the time‐averaged mean size profiles decrease approximately linearly with height, and rather slowly, about 25% in 0.5 m.

Discriminant Analysis of Bermuda Carbonate Strand-Line Sediment

Research Article| January 01, 1972 Discriminant Analysis of Bermuda Carbonate Strand-Line Sediment SAM B UPCHURCH SAM B UPCHURCH Department of Geology, Michigan State University, East Lansing, Michigan 48823 Search for other works by this author on: GSW Google Scholar Author and Article Information SAM B UPCHURCH Department of Geology, Michigan State University, East Lansing, Michigan 48823 Publisher: Geological Society of America Received: 12 Mar 1970 Revision Received: 23 Jul 1971 First Online: 02 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Copyright © 1972, The Geological Society of America, Inc. Copyright is not claimed on any material prepared by U.S. government employees within the scope of their employment. GSA Bulletin (1972) 83 (1): 87–94. https://doi.org/10.1130/0016-7606(1972)83[87:DAOBCS]2.0.CO;2 Article history Received: 12 Mar 1970 Revision Received: 23 Jul 1971 First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation SAM B UPCHURCH; Discriminant Analysis of Bermuda Carbonate Strand-Line Sediment. GSA Bulletin 1972;; 83 (1): 87–94. doi: https://doi.org/10.1130/0016-7606(1972)83[87:DAOBCS]2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Discriminant function analysis is an effective tool for differentiation of strand-line environments, represented by calcareous sediment from the Bermuda Islands. Sediment samples from the beaches of Bermuda were separated on the basis of wave energy regime and tidal position: subtidal, intertidal, and supratidal (dune and eolian flat). Sediment from the beaches that face the lagoon is composed of fragments of (1) the green alga, Halimeda, (2) bivalves, and (3) Pleistocene, skeletal eolianite. Sediment from beaches that face the open ocean is composed of fragments of (1) the Foraminifera, Homotrema, (2) coral, (3) coralline algae, and (4) Pleistocene, skeletal eolianite. Differences in sediment texture and ability to discriminate depositional environments are related to skeletal composition of the sediment and to wave-energy regime. The combination of phi mean, phi standard deviation, skewness, and kurtosis is most efficient for separating all categories of strand sediment by discriminant analysis. Phi mean and standard deviation combined are also efficient variables for discrimination. Skewness and kurtosis are significantly lower in overall discriminant efficiency, and neither can be utilized successfully when paired with only one other textural variable. Different discriminant functions must be used on the samples from each shore to obtain optimum discrimination, which suggests that specific textural discrimination criteria may be locally, but not generally, valid. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Preliminary study of transverse bars

Transverse bars (finger bars) are extremely common along sandy beaches in low-to-moderate wave energy regimes. They are typically oriented normal or nearly normal to the beach toe. Each transverse bar acts as a “focusing lens” for advancing waves, controlling wave refraction in such a way that: ( 1) the transverse bar is maintained; ( 2) sediment is transported along the long axis of the bar; and ( 3) a subtle but nevertheless effective circulation pattern is established, whereby water is transported seaward from the surf, without rip currents. In areas where transverse bars are numerous and well developed, the onshore or offshore transport of sand along the bar axes may be more important than littoral drift. Field experiments delineated a subtle nearshore current pattern, caused by unequal distribution of wave energy over the foreshore. Currents of this type were studied also in model wave tanks. These studies indicate that a transverse bar causes shallow waves to be refracted so that wave energy increases over the bar and decreases between the bars. Bottom friction reduces the wave energy more rapidly over the crest than in the adjacent deeper water. If the bars are relatively short this loss of energy through bottom friction has little effect. The wave energy concentrated over the crest results in an onshore current along the axis. The onshore current divides near the breaker zone and returns seaward slowly between the bars. The currents form two meshed gyres of horizontal circulation, one on each side of each relatively short transverse bar. Over relatively long transverse bars, the greater loss of wave energy through bottom friction over the crests, compared with the deeper wave between the bars, results in a reduced amount of wave energy over the shoreward portions. The areas between these bars provide a greater capacity for onshore movement of water caused by the mass transport associated with shallow water waves. The seaward return of this water takes place over the crests of relatively long transverse bars. These bars produce four meshed horizontally circulating gyres with an offshore current over the shoreward portions and an onshore current at their seaward ends. Preliminary data indicate that current strength is a direct function of the depth of water over the bars and an exponential function of the initial wave height. Additional work is in progress.

Studies on Sedimentology of Shell Beds on Bermudian Patch Reefs: ABSTRACT

The upper surfaces of two patch reefs on the northern edge of the Bermuda platform were observed to consist of coral-coralline algal rock mounds amid a complex system of low-lying channels containing coarse- to fine-grained calcareous detritus. The rock mounds on the larger of the two reefs are noticeably concentrated toward the periphery. Detailed mapping of a part of one of these reefs indicates that the distribution of Spondylus valves and other coarse detritus bears a strong relation to the shape and width of the intermound channels. Oddly, 79% of the shells lay with the concave side up. The apparent preference for this generally unstable orientation is attributed to the lofting of shells during transport and the preferential burial of shells lying concave-side-downwa d. The mode of the fine fractions of samples taken from shelly areas consistently is toward the right of the mode of samples taken from adjacent nonshelly areas. It thus appears that the shells act as baffles to permit the local deposition of fine sediment. It is concluded that shell-bed sedimentology reflects the existence of two wave-energy regimes throughout the year. In winter, powerful storm waves determine the distribution of shell beds; calmer seas prevailing in summer months promote the preferential deposition of fines among the shelly material. Further, the rock-sand configuration on larger patch reefs suggests that the reefs may concentrate unimodal sand toward the center and bimodal sediments in the more narrow, circuitous, shell-rich channels toward the periphery. End_of_Article - Last_Page 736------------