Dissipative Beach Research Articles

Linear and nonlinear interactions of surface waves with bodies by a three-dimensional Rankine panel method

Linear and second-order surface wave interactions with floating and bottom-mounted bodies of realistic geometry are simulated in the time domain by a three-dimensional Rankine panel method. The fundamental stability analysis governing the propagation of transient wave disturbances on a panel mesh distributed on the free surface is carried out from first principles. The radiation condition is enforced by a dissipative beach selected to coincide with an outer annulus of panels. The fundamental physics governing the wave energy absorption is presented and the beach attributes are selected and validated for the linear and second-order problems. Computations are presented of the linear and sum-frequency second-order forces on a single and multiple truncated circular cylinders, and very good agreement is found with benchmark computations. The accuracy and efficiency of this method render it a promising candidate for the study of complex nonlinear wave induced phenomena upon offshore platforms, like springing and ringing.

Field observations of sand-mixing depths on steep beaches

A series of field studies were carried out on three Portuguese beaches (Garra˜o, Faro and Culatra) to assess sand-mixing depths under a range of wave heights (0.34–0.80 m), wave periods (5.1–7.0 s) and mean grain sizes (0.26 and 0.38 mm). All the studied beaches had a reflective profile with a steep upper foreshore (tanβ of 0.10–0.14) and a more gentle low-tide terrace. In all experiments, plunging waves were breaking on the beach face. The study has identified a linear correlation between significant wave height at breaking ( H b), and average sand-mixing depth ( Z m), whereby Z m = 0.27H b. The empirical relationship is ten times larger than a previous one proposed by other authors working on gentle-slope dissipative beaches, and confirms previous findings on similar reflective beaches carried out in the USA. It was also confirmed that Z m is related to wave period, but does not seem to be a function of mean sand size.

Variability of zonation patterns in temperate microtidal Uruguayan beaches with different morphodynamic types

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 160:197-207 (1997) - doi:10.3354/meps160197 Variability of zonation patterns in temperate microtidal Uruguayan beaches with different morphodynamic types Luis Giménez*, Beatriz Yannicelli Facultad de Ciencias, Sección Oceanografía, Tristán Narvaja 1674, 11200 Montevideo, Uruguay *Present address: Biologische Anstalt Helgoland, Meeresstation Helgoland, Postfach 180, D-27483 Helgoland, Germany Early studies on sandy beach zonation patterns have shown rather rigid schemes. Recent work has suggested that zonation changes with morphodynamic type and, in dissipative beaches, it also changes through time. This last finding leads to the conclusion that at least 1 yr of study is necessary to understand zonation patterns in dissipative sandy beaches. Here we report a 1 yr study (from March 1994 to March 1995) of 4 microtidal beaches with different morphodynamic types situated on the Atlantic coast of Uruguay. We show that zonation patterns (number of zones or belts) can change through time regardless of beach type. However, the morphodynamic characteristics of the beach seem to affect how frequently the zonation pattern can change by fusion or subdivision of zones, and which zones are involved in those processes. In beaches with flat slopes (toward the dissipative extremity), the lower zones were frequently fused and divided. The same occurred with the medium and upper zones in the beaches with steeper slopes (towards the reflective extremity). Our results suggest that spatial variability of the macrofauna is related to variability in the position of the swash zone, which in turn depends on beach slope and width. Variability of species distribution would also depend on morphological adaptations of organisms to move in such environments. We suggest 2 mechanisms of species movement to explain the variability in species distribution observed in this study: Swash Mediated Active Movement (SMAM) and Autonomous Active Movement (AAM). The first mechanism involves both an active and a passive component. AAM is independent of swash movements and affects species distribution on the upper levels of the beach. Sandy beaches · Seasonal variability · Zonation pattern · Morphodynamic types · Uruguay Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 160. Publication date: December 15, 1997 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 1997 Inter-Research.

Macroinfauna Zonation in Microtidal Sandy Beaches: is it Possible to Identify Patterns in Such Variable Environments?

Prior studies on sandy beach macroinfauna have been directed to elucidate generalizable zonation patterns. These studies, usually based on a reduced temporal scale (i.e. one sampling date), have often highlighted the discrepancies between their results and the classical, static zonation schemes. This paper demonstrates that snapshot studies might fail to obtain conclusive results about zonation patterns in microtidal sandy beaches, because these constitute dynamic environments subjected to many sources of uncertainty. For this purpose, a yearly study (February 1988–January 1989) of the macroinfauna was carried out on a dissipative beach on the Atlantic coast of Uruguay. An average zonation pattern with three main belts was recognized between the sand dunes and the lower levels of the swash zone when considering annual mean abundances. This pattern roughly matched traditional zonation schemes, whereas monthly patterns did not always fit them—with the exception of some species. Important spatial variability of the macroinfauna was observed, with aperiodic and seasonal components. The former was associated with unpredictable movements of tidal levels, with the macroinfauna changing its position, tracking the high water level. Concerning seasonal periodicity, the species tended to occupy the upper levels of their respective distribution areas during spring and summer, whereas in autumn and winter they inhabited the lower levels. The fact that faunal zones are dynamic, and track sea levels and seasons, diminishes the applicability of static traditional patterns. It was concluded that a yearly study is needed to give a more representative picture of the zonation patterns in microtidal sandy beaches.

Un segment proximal de rampe carbonatée d'âge protérozoïque supérieur au Nord du craton d'Afrique centrale (sud-est de la République centrafricaine)

Near Bakouma (southwest Central African Republic), the ante Pan African carbonated formations are deeply karstified and overlaid by uranium and phosphorus bearing sediments, probably Eocene in age. The sedimentological study, of drilling cores allows the proposal of a carbonated ramp model for the lithological pile. This ramp was backed on to a large argilaceous continental rise with feldspathic sandy deposits. In the marine domain, pelitic sediments of the coastal plain progressively change into dolomites deposited on a dissipative beach limited by a discontinuous and partially emerged stromatolic bioherm. In the south, the stromatolic bioherms of Kassa-Limassa are interbedded in lagoonal silicified dolomites bearing evidence of evaporitic processes. Native copper is locally present in shore deposits. The progradation of the facies is southwards. This model is compared to the beach of the carbonated ramp evidenced for the Schisto-calcaire Group in Congo. The carbonated ramps of Central Africa, established before the Pan-African orogeny, can be linked to the same tectono-eustatic regressive period as the Schisto-calcaire ramp (West-Congolian Supergroup). By comparaison with the results obtained in south Congo, the age of the carbonated ramp in the north of Central Africa could be near the Precambrian-Cambrian boundary.

Suspended sediment transport and morphological response on a dissipative beach

This paper reports on some results from a field experiment conducted under highly dissipative surf zone conditions, occurring during a storm in Lake Huron, Ontario, Canada. Measurements of sediment resuspension and sediment flux were conducted at a number of stations across a nearshore bar. This bar moved approximately 25 m offshore during the storm, at a rate of ≈2.5 m h −1. The maximum erosion depth over the former bar crest was 0.92 m, with simultaneous accretion on the lakeward slope of the bar reaching 0.63 m. During the early hours of the storm, the accretion rate over the lakeward slope was approximately 0.05 m h −1. Mean sediment concentrations were up to 6.4 g 1 −1, time series of sediment concentration were characterized by low-frequency fluctuations and the sediment resuspension was strongly constrained by infragravity waves. The major mechanism responsible for the offshore sediment flux was an offshore directed mean current, which reached a velocity of −0.34 m s −1. However, the reason for the erosion and the offshore migration of the bar was the strong spatial gradient in the cross-shore suspended sediment flux. This gradient was induced mainly by the infragravity waves, which transported sediment onshore at the former bar crest where the oscillatory flux balanced the mean flux, and offshore over the lakeward slope, reinforcing the mean flux at this location.

Adaptations of bivalves to different beach types

Burrowing ability, shape and density of 12 bivalve species from a wide range of beach types were compared as part of a general investigation of the adaptations of bivalves to the swash climates experienced on exposed sandy beaches. The genera used were Donax, Mesodesma, Tivela, Siliqua, Atactodea, Paphies and Donacilla. Burrowing rates varied widely and burrowing ability showed no relation to beach type along the reflective/dissipative beach gradient, i.e. from beaches with low wave energy and coarse sand to beaches with high wave energy and fine sand. Burrowing rate indices ( BRI's) ranged from 2 to 17 (rapid to very rapid) and tended to be higher for juveniles of most species. Bivalve species from dissipative beaches varied in shape from almost blade-shaped to almost spherical, whereas those from reflective beaches were more uniform generally wedge-shaped. Species with the most flattened shapes and (greatest height/width ratios) tended to burrow fastest. Striking interspecific differences were found in densities of whole intact bivalves, these ranging from 1.04 g · cm −3 to 2.10 g · cm −3. The highest densities were recorded in bivalves from reflective beaches and the lowest in bivalves from dissipative beaches. Bivalve species typical of intermediate and reflective beaches were successfully separated from those typical of dissipative beaches on the basis of their density, morphology and BRI, particularly their size and density, using discriminant analysis. It is concluded that small species with high density and streamlined shape are best adapted to the dynamic swash conditions that characterise reflective beaches.

The Interstitial Environment of Sandy Beaches

Abstract. The interstitial system of sandy beaches is lacunar and has its dimensions defined by the sand granulometry. It can be described by features such as pore size, porosity, permeability, and water content. The most important process occurring in this system, water filtration, is driven by inputs of freshwater from groundwater discharge, and inputs of seawater by tides, wave run‐up, and subtidal wave pumping. Reflective beaches have seawater input effected mainly by waves; they filter large water volumes with short residence times. Dissipative beaches display the opposite patterns, slowly filtering small volumes input by tides. Flow patterns and their effects on interstitial climate are described. The water table of the beach moves in response to groundwater discharge, tides, and waves and influences erosion/accretion processes on the beach face: a high water table promotes erosion. A series of moisture zones can be recognised from the dry surface sand at upper tide levels, to permanently saturated sand below the low tide water table, namely: a stratum of dry sand, a stratum of retention, a stratum of resurgence, and stratum of saturation. Interstitial chemistry is briefly described in terms of salinity changes, organic loads, oxygen content, and nutrient cycling. It is concluded that the interstitial environment of sandy beaches spans a continuum between physically and chemically controlled extremes: the former condition occurs on coarse sand reflective beaches, which experience low organic inputs and high filtration rates of large water volumes — resulting in powerful hydrodynamic forces; the latter occurs on dissipative beaches of fine sand, which are subject to high organic inputs and low filtration volumes — resulting in stagnation and steep vertical chemical gradients. Many intermediate situations occur and these are more favourable to interstitial life than either of the extremes.

BEACH-STATE CONTROLS ON AEOLIAN SAND DELIVERY TO COASTAL DUNES

Characteristics of morphodynamically reflective, intermediate, and dissipative beach systems, from the data of Short (1984), are used with an iterative aeolian sediment transport model to predict beach-state controls on transport rates. The predictions are based on the transport model of White (1979), the moisture effect model of Belly (1964), and the slope-effect model of Hardisty and Whitehouse (1988). The model adjusts profile geometry continuously as sand is preferentially eroded and deposited across the model beaches. For an assumed shear velocity of 0.50 ms-1, sediment transport rates were about 20% larger for the dissipative system compared to the reflective beach. For a shear velocity of 0.75 ms-1, about 80% more sand is removed from the dissipative beach. These results suggest that the Short and Hesp (1982) conceptual model of beach state-dune form relationships is valid and can be quantified. [Key words: coastal dunes, coastal geomorphology, beach state, beach modeling.]

Patterns of species richness in sandy beaches of South America

Species richness of the intertidal macroinfauna of exposed sandy beaches around South America is reviewed in relation to geographic location. This macroinfauna is dominated by cirolanid isopods ( Excirolana ), bivalves (Mesodesma and Donax) and opheliid and spionid polychaetes. In general, the upper shore of tropical and subtropical beaches is characterized by crabs (Ocypodidae), whereas on temperate beaches it is dominated by talitrid amphipods and cirolanid isopods. The middle shore is primarily occupied by cirolanids and bivalves, and hippid crabs, bivalves and amphipods dominate the lower beach. Generally, species richness increases from upper to lower beach levels. Studies carried out on exposed sandy beaches of south-central Chile (ca . 40°S) show that different beach states harbour differences in species richness, with the greatest species richness on dissipative beaches, and the least on beaches with reflective characteristics, a pattern also observed in Uruguay.

Community and Population Responses of the Macroinfauna to Physical Factors over a Range of Exposed Sandy Beaches in South-central Chile

Ten exposed sandy sites covering a range from reflective to dissipative beaches were sampled in south-central Chile to evaluate: (1) spatial changes in species richness, abundance and biomass of the intertidal macroinfauna in response to changes in mean grain size, beach face slope and beach type, and (2) spatial changes in abundance, biomass and body sizes of the most abundant species in response to changes in the physical factors. The number of species, abundance and biomass per beach in general decreased with increasing particle size and beach face slope (steeper beaches) and increased from reflective to dissipative conditions. The best fit for number of species was with Dean's parameter, a measure of beach type, whereas for abundance and biomass the best fits were found with particle size. The isopod Excirolana braziliensis and the anomuran Emerita analoga increased in abundance and biomass towards dissipative conditions, whereas Excirolana hirsuticauda showed the opposite trend in biomass and was significantly larger in beaches with steeper profiles. It is concluded that responses to changes in beach type are more pronounced at community level than within species populations.

Mechanisms for beach erosion during storms

Simultaneous time-series measurements of waves, currents, and suspended sediment concentrations (SSCs) were obtained from the surf zone of a high energy, macrotidal, dissipative beach (Llangennith, Gower, South Wales, U.K.) during “storm” and “calm” conditions. A collocated pressure transducer (PT), electromagnetic current meter (EMCM) and optical backscatter sensor (OBS) were used to measure waves, bi-directional currents and SSCs respectively. Incident wave heights were found to be saturated in the inner surf zone. As the incident waves decayed shorewards, low frequency (infragravity) oscillations in water level and current velocity grew. During the storm, the inner surf zone was dominated by strong (± 1m s −1), low frequency (≈0.01Hz), cross-shore fluid motions which accounted for up to 80% of the total spectral energy. Suspension events associated with the infragravity motions reached peak concentrations of over 70 g l −1, 0.04 m above the bed, and persisted for periods of 30–40 s. Co-spectra between the SSC and cross-shore velocity time-series were computed and used to examine the frequency dependence of the near-bed cross-shore (suspended) sediment transport rate, which was seen to be composed of mainly onshore transport due to asymmetric flows at incident wave frequencies, and predominantly offshore transport coupled with infragravity oscillations in the cross-shore current velocity. A mean (steady) transport component was also measured in association with the undertow (directed offshore). The combined effect of the infragravity band and mean offshore transport components was responsible for the erosion of the beach during the storm

Intertidal zonation patterns of macroinfauna over a range of exposed sandy beaches in south-central Chile

Ten sandy beaches, covering a range from reflective to dissipative conditions, were sampled in south-central Chile to determine whether different beach morphodynamic states support different macroinfaunal zonation patterns. Three patterns were distinguished through examination of kite diagrams and analysis using multivariate techniques: (1) in reflective beaches with coarse sand, a single zone of air-breathing crustaceans was located above the drift line; (2) a reflective beach with finer sands exhibited 2 faunal zones due to the addition of a second zone of cirolan~d isopods below the dnft line; and (3) in intermediate and dissipative beaches 3 faunal zones were delineated, the above 2 plus a broad zone covering the lower shore. Species representative of these zones are Orchestoidea tuberculata and Excirolana braziliensis in the upper shore, E. hirsuticauda and E. monodi in the middle shore and Ernerita analoga and Bathyporeiapus magellanicus in the lower shore. This study also showed that zones were ordinated along an intertidal gradient of sediment water content. The 3 zone pattern agrees with the worldwide scheme proposed by Dahl (1952; Oikos 4: 1-27). Body-size comparisons showed that individual sizes in some species differed across their intertidal distribution.

Suspended sediment transport in the surf zone: Response to incident wave and longshore current interaction

This paper reports the results of a field experiment carried out in the inner surf zone of a dissipative beach at San Marine, Oregon, in 1984. Observations were made at a location 70 m seaward of the mean high water line (mean depth 1.3 m) during a period when significant offshore wave heights were 3–5 m and the surf zone was 500 m wide. Instruments were deployed in a closely spaced array and measurements included cross-shore and longshore velocity, sea surface fluctuations, and sediment concentration profiles. The results reported in this paper are from a particular 44-min data run characterized by high waves and strong longshore currents. In the first section of this paper the field measurements are summarized to provide insight into the resuspension process and observed sediment concentrations and fluxes on time scales including incident and infragravity bands, and mean values for the entire data set. In the second section a numerical model is presented that incorporates the nonlinear interaction of waves and currents and calculates velocity profiles, boundary shear stress, suspended sediment profiles, and longshore and cross-shore particle flux over 0.5 s intervals throughout the time series. Finally, model calculations of mean suspended load and net longshore and cross-shore flux of sediment are compared to observations. The results show that the model calculates mean suspended load and net longshore and cross-shore flux of sediment within a factor of two of the observations. The model suggests that, for this data set, incident waves and longshore currents contribute equally to the suspended load. For the longshore particle flux, the current contribution is approximately 1.7 times the wave contribution (63% vs 37%), primarily because currents carry suspended sediment higher in the water column where it is advected by stronger longshore currents. Additionally, model results suggest that wave/current interactions enhance suspended sediment load and longshore sand transport by approximately 50–60% over the transport forced by waves alone.

Surf zone transformation of wave height to water depth ratios

A solid body of data exists on which to base engineering decisions regarding the maximum wave heights that can be achieved in shallow water at the point of incipient breaking. Effort is now being directed towards the study of how wave height varies as waves propagate through the surf zone and what parameters control this process. Early surf zone models assumed a constant value of C while later models incorporate a decay function for H h due to turbulence in the breaking wave. A few more recent studies have recognised a second process by which H h can decay through the surf zone where there is a strong underlying infragravity wave motion. The work presented here investigates the characteristics of this latter decay process on a natural exposed dissipative beach, including its magnitude and dual dependence on local mean water depth and distance from the breaker line. Some comparisons are then made with the decay characteristics of the former process.

A field study of wave reflections from an exposed dissipative beach, by R.C. Nelson and J. Gonsalves

A field study of wave reflections from an exposed dissipative beach, by R.C. Nelson and J. Gonsalves

A fiber optic sensor for monitoring suspended sediment

A new instrument that can be used to continuously monitor concentration of suspended particulates in close proximity to the sea bed ( z< 3.5 cm) is described. The instrument is composed of a narrow band source light, a photodiode detector, and glass fibers that transmit and receive light. Individual sensors can be sampled rapidly (8 Hz) and respond linearly to increasing concentrations of sediment over the range 0–100 gl −1. An array of sensors can be deployed from z=0.5 to 3.5 cm above the seabed providing simultaneous time series of suspended sediment concentration at five discrete elevations. Results of a field deployment in the swash zone of a dissipative beach indicate that the performance of the instrument is comparable to that of the standard optical backscatter sensor (OBS). The data show that se the standard optical backscatter sensor (OBS). The data show that sediment resuspension in the swash zone is strongly correlated with low frequency cross-shore flows with peak concentrations exceeding 200 gl −1.

A field study of wave reflections from an exposed dissipative beach

This paper describes a field study of wave reflections from an exposed dissipative beach, using only water-surface elevation data from five wave height sensors. Many insights were gained into beach reflections and their detection. A technique normally reserved for laboratory use was successfully applied to surf zone data to separate incident and reflected infragravity wave-energy spectra. The reflected wave energy was found to be significant for all wave periods greater than 20 seconds, and often total for wave periods greater than 30 seconds. The evidence was heavily weighted towards the observed reflected wave activity being essentially two dimensional with the influence of three-dimensional edge waves being minimal. Other findings relate to the value of wave energy spectra computed from a single surf-zone sensor, the relevance of single slope approximations of beach profiles and the relationship between incident infragravity wave-activity and the longer-period swash excursions on a beach face.

Beach cusps and beach dynamics: A quantitative field appraisal

The physical expression of beach cusps during the process of beach dynamics defines the objective of this report. A total of 93, largely fortnightly, volumetric beach change observations under cusp existence and extinction phases have been employed for this purpose. The above data were assessed alongside existing information on linear beach changes during both cusp phases from the study area. The study area is a 3 km long, moderate-high energy sandy beach fringing the southeast Atlantic coast of Nigeria. In the main, cusps are prone to develop during milder coastal conditions. In relation to the three principal beach states (dissipative, intermediate and reflective) cusp formation and persistence were most significant on beach sectors exhibiting reflective process signatures. However, in contrast to previous postulates, cusp development and disappearance phases did not reveal any unique relationship with the direction of volumetric beach change. The cusp phases correspond better with linear beach changes. Between 90–95% of the volumetric accretionary events during both cusp phases progressed through beach response routes characterized by beach scarp stability. Erosional events indicated a wider spread of response routes. It is asserted that on dissipative beaches, cusps play a passive role in the overall process of volumetric profile adjustment. The converse may be the case on highly reflective beaches.

Water filtration by dissipative beaches

Dissipative or low‐gradient beaches arc expected to filter low volumes of seawater. This idea was tested on the Oregon coast on two high‐energy dissipative beaches with medium and fine sand and found to be correct, filtered volumes being 0.1–7 m3 m−1 d−1. Input was mainly due to tidal effects as most wave energy was dissipated in the surf zone. During a very high‐energy event on a high tide, however, filtered volume increased by an order of magnitude as a consequence of increased wave input.