Measurements and modelling of beach groundwater flow in the swash-zone: a review

This paper reviews research on beach groundwater dynamics and identifies research questions which will need to be answered before swash zone sediment transport and beach profile evolution can be successfully modelled. Beach groundwater hydrodynamics are a result of combined forcing from the tide and waves at a range of frequencies, and a large number of observations exist which describe the shape and elevation of the beach watertable in response to tidal forcing at diurnal, semi-diurnal and spring-neap tidal frequencies. Models of beach watertable response to tidal forcing have been successfully validated; however, models of watertable response to wave forcing are less well developed and require verification. Improved predictions of swash zone sediment transport and beach profile evolution cannot be achieved unless the complex fluid and sediment interactions between the surface flow and the beach groundwater are better understood, particularly the sensitivity of sediment transport processes to flow perpendicular to the permeable bed.The presence of a capillary fringe, particularly when it lies just below the sand surface, has influences on beach groundwater dynamics. The presence of a capillary fringe can have a significant effect on the exchange of water between the ocean and the coastal aquifer, particularly in terms of the storage capacity of the aquifer. Field and laboratory observations have also shown that natural groundwater waves usually propagate faster and decay more slowly in aquifers with a capillary fringe, and observations which suggest that horizontal flows may also occur in the capillary zone have been reported. The effects of infiltration and exfiltration are generally invoked to explain why beaches with a low watertable tend to accrete and beaches with a high watertable tend to erode. However, the relative importance of processes such as infiltration losses in the swash, changes in the effective weight of the sediment, and modified shear stress due to boundary layer thinning, are not yet clear. Experimental work on the influence of seepage flows within sediment beds provides conflicting results concerning the effect on bed stability. Both modelling and experimental work indicates that the hydraulic conductivity of the beach is a critical parameter. However, hydraulic conductivity varies both spatially and temporally on beaches, particularly on gravel and mixed sand and gravel beaches. Another important, but poorly understood, consideration in beach groundwater studies is the role of air encapsulation during the wetting of beach sand.     

Beach-fill equilibration and dune growth at two large-scale nourishment sites

Natural dune growth after nourishment is often observed, and such growth plays an important role in beach management for coastal communities. Nourishment sand equilibration after construction is another important topic for project planning and design. Large-scale nourishment projects at Nags Head (NC, USA) (completed in 2011) and Bridgehampton–Sagaponack (NY, USA) (completed in 2014) are under comparatively high-wave energy conditions and offer new insight regarding these topics. After nourishment, a natural beach and inshore morphology were produced with high rates of dune growth by eolian transport. At Nags Head, volumetric dune growth averaged 8 m³/m/yr over the first 5 years following project completion, while Bridgehampton–Sagaponack averaged 9 m³/m/yr over the first 3 post-project years. Results are compared with the Bagnold (1941) analytical model predictions of dune growth and are shown to correlate and decay closely with dry-sand beach width as the nourished profile equilibrates. The extra volume and elevation in the dunes have provided a higher level of storm protection and have helped the sites avoid any major damage to oceanfront properties during hurricanes or numerous severe winter storms.     

Analysis of key parameters in a diffusion type beach profile evolution model

Diffusion type formulations are commonly used in beach profile evolution models. The practical idea behind that is to map the behaviour of the beach profile onto a simple mathematical model that exhibits the same behaviour under defined operating conditions. The success of this approach is based on the accurate determination of key parameters in the diffusion model that govern its behaviour, using observed beach behaviour in the field. In order to determine these parameters, i.e. diffusion coefficient and a time and space varying source function, we used observations of historic beach profiles at Milford-on-Sea beach in Christchurch Bay, Dorset, United Kingdom. The relationship between the diffusion coefficient and Dean's equilibrium profile was investigated, leading to a new interpretation of the diffusion coefficient in terms of the sediment characteristics. The analysis also shows the significance of the diffusion process in the medium to long term evolution of the beach profile. A canonical correlation analysis (CCA) was undertaken in order to identify patterns of behaviour between wave conditions and source terms, and the possible correlations between them. The analysis provides strong evidence of a useful link between the source term in the simple dynamical equation and the distribution of wave steepness.     

Litter burial and exhumation: spatial and temporal distribution on a cobble pocket beach.

A cobble beach (-6 diameter to -8 diameter) located on the South Wales coastline, UK, was studied over a three-month winter period to assess litter input levels. After total beach litter clearance, six surveys were conducted at consecutive spring tides which involved marking of previously unrecorded litter. The beach was soon inundated with debris, predominantly plastic beverage containers. Some marked litter was found to disappear from the beach surface, re-emerging weeks later which suggests that the potential for litter burial has been underestimated in litter research. Higher wave energies between surveys coincided with higher levels of previously unseen litter. These new inputs consisted of sea borne and exhumed litter. Items larger than the surrounding cobbles were found to work their way back to the surface of the beach after burial, smaller items remained buried. Pits dug into the cobble ridge confirmed the burial of mainly small items.     

A Markov model for beach changes on the holderness coast of England

Markov models offer an objective and quantitative method of assessing beach changes. For a stretch of the Holderness coast a beach classification scheme was devised and a probabilistic first order matrix model based on surveyed profile data was produced. This could describe and predict transitions between beach types and between different time periods. Different profile types dominated different coastal locations and seasonal variations were seen. In order to improve the accuracy of prediction throughout the year a second, ‘winter’, model was added to the original ‘summer’ one. Although the models had been prepared independently of wave conditions, a comparison of the wave record and beach transitions revealed that waves under 0·3–0·5 m high produced fairly static beaches; when waves were between 0·5 and 1·0 m the beach was more dynamic and variable, while waves over 1·0 m led to the depletion of the upper beach. This was broadly in accordance with published theory. Markov models have the advantage that they can be adjusted periodically if conditions change, and are thus useful for prediction on coasts for which no wave records exist.     

A model of inter-annual variability in beach levels

Beach profile data, collected twice per year at 19 stations over a 25 km length of coastline in Tremadoc Bay, have been analysed to quantify the inter-annual variability in beach levels over a 7 year period and the results compared against the output of a numerical model. Using hourly wind data as forcing, the morphological development of northern Tremadoc Bay was simulated by wave, tidal, longshore transport, total transport and bed level change models. The modelling methodology was efficient and innovative, allowing realistic simulations of long duration with a time step of 1 h, hence capturing the high frequency nature of wind events. The model was run for each of the 7 autumn/winter periods (generally November–April) and the modelled net change in beach levels compared with the data from all 19 stations. The model results had reasonable agreement with the beach profile surveys. However, the observed magnitude of bed level change in the bay lagged the model output by 1 year, indicating that sediment processes acting over a larger area are important in a relatively localised study of inter-annual variability.     

Apparent dynamic stability of the southeast African coast despite sea level rise

The coast of southeast Africa is dominated by sandy beaches that tend to be confined within log‐spiral or headland‐bound embayments. Investigations using serendipitous air imagery data set have been previously undertaken and conclusions drawn about the stability of the coast. We show that conclusions drawn from this data, with respect to the high water mark (HWM) position are fraught with errors, which include tidal state, pressure regime, beach slope, high‐swell erosion, seasonal and multi‐annual changes. We highlight and discuss these sources of error, together with their magnitudes. The most significant of these are the high‐swell, seasonal and multi‐annual variations. From case studies we show that the seasonal beach rotation and long‐term beach width variation are responsible for tens of metres of unaccounted HWM variation, 30 to 50 m is common, with maximums reaching 60 to 100 m. Overall the southeast African coastline appears to be in a state of long‐term dynamic equilibrium. There is no evidence of any sea‐level rise‐forced transgression in the coastal sediment budget, despite sea‐level rise (SLR). If such a signal is, in fact present, it is lost within the beach width variation. Some southeast African coastal reaches are suffering chronic erosion, but these are related to anthropogenic impacts. The extreme difficulty of placing a HWM, with any temporal validity on this coast precludes the routine use of the Bruun Rule. Although no transgressive signature is found, there is evidence of a decreasing coastal sand budget as a result of anthropogenic or natural climate change, or both. This decrease in the coastal sand volume is likely to result in increased future erosion. Copyright © 2016 John Wiley & Sons, Ltd.     

Quantitative estimation of sediment erosion and accretion processes in a micro-tidal coast

Spatio-temporal cross-shore profiles and textural characteristics are the key parameters for understanding dynamics of the inter-tidal sedimentary environment.This study describes short-term dynamics of the inter-tidal sedimentary environment at beaches along the micro-tidal coast.Further a correlation is estimated in cross-shore morphodynamics and textural characteristics of surface sediments.The sedimentary environment is examined for a complete annual cycle using monthly collected cross-shore profiles and sediment samples.The Devbag beach（northern side） and Ravindranath Tagore beach（southern side） at the Kali river mouth,Karwar,west coast of India are characterized from extremely gentle to average slope,and broadly composed of unimodal sands.The sedimentary environment is significantly composed of textures having fine to medium sand,well to moderately sorted,fine to coarse skewed,and platykurtic to leptokurtic in nature.During the annual cycle a reversal pattern is observed between the two adjacent beaches,where a slower rate of sediment accretion is observed at Devbag beach while Ravindranath Tagore beach exhibited erosion.The beach dynamics along with the propagation of south-west（SW） and south-west-west（SWW） waves towards the coast significantly exhibit a dominance of northward sediment transport with the existence of a northerly alongshore current.In addition,the study reveals that an eroded beach may not be significantly identified composed of coarse grains.The poor correlation in morpho-sedimentary characteristics reveals the prediction of grain characteristics based on beach profile and vice-versa is unrealistic.     

Analysis of the effects of different storm events on shoreline dynamics of an artificially embayed beach

A five‐year dataset of Argus‐derived mean intertidal positions has been analysed to characterize the shoreface variability in a beach protected by a system of groynes and a parallel low crested structure (Lido di Dante Ravenna, Italy). For the period 2004–2009, 84 intertidal beach bathymetries and shorelines at the zero sea level were used as indicators to assess beach changes in between a number of selected surveys and to determine characteristic patterns of the beach response to storm events from different directions. Variations in the shoreline at the zero sea levels have been quantified and analysed in conjunction with nearshore wave conditions and provenance linked to storm events. These fall into two categories: (1) storm events occurring during Bora (north‐eastern) wind conditions and (2) storm events occurring during Scirocco (south‐eastern) wind conditions. The results show that, apart from main beach advances of the whole protected beach due to nourishments periodically carried out, a marked variability is observed among the four sub‐cells into which the shoreface behaviour has been separately analysed. In particular, a dependence of beach rotation in the ‘artificially embayed’ area on the substantially bi‐directional wave climate has been shown: Bora and Scirocco storm events produce shoreline rotation in counterclockwise and clockwise directions, respectively, due to the occurrence of longshore currents in the opposite direction in the nearshore. An attempt was made to correlate the shoreface dynamics for the main rotation events (14 selected ones) to the wave attack intensity (as the total energy flux due to storm events). A relationship seems to occur (for each storm category) between the shoreline displacements estimated for each sub‐cell and the total energy flux computed for inter‐survey periods, supporting the occurrence of a link between the observed morphological changes and the hydrodynamic forcing associated with storm events in the five‐year monitoring period. Copyright © 2011 John Wiley & Sons, Ltd.     

Shoreline management plan for a protected but eroding coast along the southwest coast of India

Coastal erosion is a serious problem of concern along the southwest(SW) coast of India. Various coastal protection measures have been applied for the recovery of the coast, but the devastating effect of erosion still continues. The present study focuses on a coastal stretch situated on the southern sector of the SW coast of India, where Sundar and Sannasiraj(2006) proposed a groyne field along with an existing seawall to control severe erosion. In order to confirm the net littoral drift of this region and for a preliminary assessment of the performance of the groynes prior to construction of the proposed groyne field, two groynes were initially constructed as a pilot program in 2008-09. Periodic monitoring of shoreline position with the two groynes in place was carried out during 2009-14. A shoreline evolution model for the study region was setup, calibrated,and validated using field observations during 2010-11. In addition to traditional shoreline evolution modelling procedures, a profile simulation model was applied for simulating the shoreline behaviour during extreme monsoon seasons. The validated LITPACK model has been used to evaluate the performance of the proposed groyne field in controlling erosion, and the study also considered testing a modified transitional groyne field proposed as an alternative solution to the existing problem, and the modified transitional groyne field was found to be more effective than the prior design. A beach is expected to develop about 30-50 m within the groyne cells during the fair season which enhances the possibility of retaining a minimum beach width of 10 m during monsoon periods.     

A numerical model for shore-normal sediment size variation on a macrotidal beach

This paper describes a computer simulation model which is designed to predict the selective shore-normal sorting of grain sizes in the nearshore environment. The model simulates wave shoaling, wave height attenuation due to frictional losses and breaking, using linear theory up to the break point and a breaker decay model in the surf zone. Peak horizontal orbital velocities at the bed are calculated from Stokes second-order wave theory. The peak onshore and offshore velocities are used with the threshold expression of Komar and Miller (1975) to generate a spatial pattern of size variation of threshold grain diameter along a profile normal to the shore from deep water to the swash zone. The predicted grain size is used in an hydraulic interpretation of grain size distribution on the intertidal profile, based on the hydrodynamic variations over a tidal cycle on a macrotidal beach. The model is successful in predicting the broad pattern of increasing grain size in the onshore direction which has been observed in nature. Comparisons between measured and predicted grain size distributions indicate that the predictions of the model are better than those of previous models, but the model is more successful at predicting sediment size distributions than at predicting mean sizes on a beach profile.     

A proposed mechanism for storm beach sedimentation

One of the major problems of shingle beach dynamics is the method by which coarse gravel is transferred from beach face to storm beach, the latter often lying several metres above high spring tidal levels. The mechanism which is usually proposed, cites the action of plunging breakers as being central to this problem of sediment transfer. However, the nature of net residual fluid force of plunging breakers is deemed unsatisfactory for any substantial upbeach (onshore) sediment transport during storm conditions on gravel beaches. A mechanism is proposed by which high still water levels due to high astronomical tides, onshore storm force winds and allied wave surge, promote shoaling characteristics and beach profile changes which are conducive to spilling breaker development at tidal extremities. It is the net onshore fluid force vector of the spilling breaker overtopping the beach crest which may be the cause of extreme sedimentation events on the storm beach. An example of such sediment transfer is given for a known storm beach sedimentation event at Llanrhystyd gravel beach, West Wales, during February 1974. Process variables were monitored on several days allowing the use of an inshore breaker steepness criterion, to indicate positions in the tidal regime where plunging breakers give way to spilling forms. This example serves to suggest that more attention should be given to the nature and characteristics of shoaling waves, especially in respect of breaker type, when examining problems of shingle beach dynamics and sedimentation.     

Consequences of short‐term changes in coastal processes: a case study

In 1997–98, unique critical beach erosion led to structural failure along the Penarth, South Wales, UK coastline and anthropogenic activities, such as the construction of the Cardiff Bay Barrage and offshore marine aggregate dredging, were suggested as causes. The time‐frame of significant erosion was between 1995 and 1997 and forcing agents (extreme sea level and wind direction) and shoreline indicators (mean beach level and MHW) were analysed in order to assess change. Water level analysis showed that although there was no significant difference between actual and predicted mean sea levels, extreme sea levels at that time were significantly higher (t = 3·305; d.f. = 8; p < 0·05). Three wind direction analyses (annual mean, mean annual maximum gust and mean annual maximum gust ≥28 kn) between 1995 and 1997 also showed significant differences (p < 0·05). All comprised more easterly components which meant they approached the beach from the sea. Furthermore, gusts ≥28 kn from the northeast quadrant, that is, 0° to 90° true, were significantly more frequent during these years (t = 3·674; d.f. = 8; p < 0·01). Justification of statistical significances was established and there was supporting evidence of unusual meteorological conditions at that time. Relationships showed correlation between forcing agents (extreme sea level and wind direction) and shoreline indicators (mean beach level and Mean High Water). Furthermore, regression analysis showed winds from the northeast quadrant resulted in steeper longshore gradients, as a consequence of beach material loss. Therefore, it was concluded that the critical erosion of Penarth beach between 1995 and 1997 was caused by increased wave attack from the northeast and southeast quadrants, generated by unique significant changes in wind direction and extreme sea levels. Copyright © 2008 John Wiley & Sons, Ltd.     

Prevalence and distribution of fecal indicator organisms in South Florida beach sand and preliminary assessment of health effects associated with beach sand exposure

Fecal indicator levels in nearshore waters of South Florida are routinely monitored to assess microbial contamination at recreational beaches. However, samples of sand from the surf zone and upper beach are not monitored which is surprising since sand may accumulate and harbor fecal-derived organisms. This study examined the prevalence of fecal indicator organisms in tidally-affected beach sand and in upper beach sand and compared these counts to levels in the water. Since indicator organisms were statistically elevated in sand relative to water, the study also considered the potential health risks associated with beach use and exposure to sand. Fecal coliforms, Escherichia coli, enterococci, somatic coliphages, and F(+)-specific coliphages were enumerated from sand and water at three South Florida beaches (Ft. Lauderdale Beach, Hollywood Beach, and Hobie Beach) over a 2-year period. Bacteria were consistently more concentrated in 100g samples of beach sand (2-23 fold in wet sand and 30-460 fold in dry sand) compared to 100ml samples of water. Somatic coliphages were commonly recovered from both sand and water while F(+)-specific coliphages were less commonly detected. Seeding experiments revealed that a single specimen of gull feces significantly influenced enterococci levels in some 3.1m(2) of beach sand. Examination of beach sand on a micro-spatial scale demonstrated that the variation in enterococci density over short distances was considerable. Results of multiple linear regression analysis showed that the physical and chemical parameters monitored in this study could only minimally account for the variation observed in indicator densities. A pilot epidemiological study was conducted to examine whether the length of exposure to beach water and sand could be correlated with health risk. Logistic regression analysis results provided preliminary evidence that time spent in the wet sand and time spent in the water were associated with a dose-dependent increase in gastrointestinal illness.     

Beach profile evolution as an inverse problem

Beach evolution models are normally applied in a prognostic fashion, with parameters and boundary conditions estimated from previous experience or other forecasts. Here, we use observations of beach profiles to solve a beach profile evolution equation in an inverse manner to determine model parameters and source function. The data used to demonstrate the method are from Christchurch Bay in Dorset, UK. It was found that there is a significant contribution from diffusive processes to the morphodynamic evolution of the beach profiles and that the development and disappearance of near-shore coastal features such as upper beach berms and inter- and sub-tidal bars are well captured by the source function in the governing equation.     

Daily morphological changes determined by high‐energy events on an embayed beach: a qualitative model

This paper examines the daily morphological responses of Sununga Beach, an embayed beach located on the south‐eastern Brazilian coast, to storms in the South Atlantic Ocean. The main mechanisms and timing of beach erosion and accretion, the relationship between wave height and direction, and beach volume changes are considered, to establish a qualitative model for short‐term embayed beach morphological changes. The methodology consisted of daily topographic surveys during the month of May in 2001, 2002, and 2003, using an RTK‐GPS (real‐time kinematics – global positioning system). Weather and wave model results were used to correlate hydrodynamics and beach morphology. The results indicate that the morphodynamics of Sununga Beach are characterized by a process of beach rotation, which occurred more or less clearly during all three surveys. Unlike what has been commonly described in the literature for longer time intervals and alternations of fair and stormy weather, the beach rotation processes on Sununga Beach occurred under conditions of moderate‐to‐high wave energy change (wave heights greater than 2 m). An integrated evaluation of the behaviour of the meteorological aspects, together with beach morphology, enabled us to recognize that extra‐tropical cyclones were the most important agent in remobilizing the beach planform, whether in beach rotation or in cross‐shore erosion. Copyright © 2010 John Wiley & Sons, Ltd.     

A simple life-cycle method for predicting extreme shoreline erosion

The dynamical responses of a shoreline over long-term (years or decades) is a non-linear and time-dependent random process. It is affected by both longshore and cross-shore sediment transports. The former tends to cause cumulative changes in the mean shoreline position while the latter usually only leads to beach profile fluctuations relative to the moving mean beach profile. Due to the time-dependency of the process the life-cycle approach is ideally suited to formulate the probability distribution of extreme shoreline erosion. A model based on such approach and using standard Monte Carlo simulation techniques has been reported by Dong and Chen (1999). In this paper a simplified procedure is developed by introducing the assumption that the longshore and cross-shore processes are statistically independent. This then allows the probability distribution of the extreme erosion to be calculated in terms of the marginal probability distributions of the maximum recessions due to purely longshore and purely cross-shore transport. This method was applied to two idealised shoreline configurations and its usefulness for engineering applications is evaluated by comparison with the full Monte Carlo method.     

闽粤沿海全新世垂直构造运动的速率

基于华南地区新构造运动具有断块垂直差异运动的特征,本文主要根据目前已测定的,分布于闽粤沿海地区的若干个全新世海相沉积物的放射性年代,海拔高度等数据,初步估算和对比闽粤沿海不同断块构造区的垂直构造运动的速率,并探讨它与地震活动的关系     

Embryo dune development drivers: beach morphology,growing season precipitation,and storms

For development of embryo dunes on the highly dynamic land–sea boundary, summer growth and the absence of winter erosion are essential. Other than that, however, we know little about the specific conditions that favour embryo dune development. This study explores the boundary conditions for early dune development to enable better predictions of natural dune expansion. Using a 30 year time series of aerial photographs of 33 sites along the Dutch coast, we assessed the influence of beach morphology (beach width and tidal range), meteorological conditions (storm characteristics, wind speed, growing season precipitation, and temperature), and sand nourishment on early dune development. We examined the presence and area of embryo dunes in relation to beach width and tidal range, and compared changes in embryo dune area to meteorological conditions and whether sand nourishment had been applied. We found that the presence and area of embryo dunes increased with increasing beach width. Over time, embryo dune area was negatively correlated with storm intensity and frequency. Embryo dune area was positively correlated with precipitation in the growing season and sand nourishment. Embryo dune area increased in periods of low storm frequency and in wet summers, and decreased in periods of high storm frequency or intensity. We conclude that beach morphology is highly influential in determining the potential for new dune development, and wide beaches enable development of larger embryo dune fields. Sand nourishment stimulates dune development by increasing beach width. Finally, weather conditions and non‐interrupted sequences of years without high‐intensity storms determine whether progressive dune development will take place. Copyright © 2017 John Wiley & Sons, Ltd.     

Berm formation and dynamics on a gently sloping beach; the effect of water level and swash overtopping

Berm formation and morphological development of the beach face have been observed during a neap–neap tidal cycle on the gently sloping and accreting beach at Vejers, Denmark. During the field campaign, an intertidal bar migrated onshore and stabilized as a berm on the foreshore. A new intertidal bar occurred on the lower beach face, migrated onshore on the rising tide and finally merged with the pre‐existing berm. As the tide continued to rise, the new berm translated further onshore as an intertidal bar to the uppermost part of the foreshore. The sediment transport during the berm transition was onshore directed in the upper swash and offshore directed in the lower swash. This berm development can be described through both the neap‐berm, ridge‐and‐runnel and berm‐ridge development concepts proposed by Hine (Sedimentology 1979; 26: 333–351), and all three stages were observed during only three tidal cycles. The main factors controlling this fast transformation were the gentle slope of the cross‐shore profile, rapid water level translation rates, substantial swash overtopping of the berm, and low infiltration rates. Despite the onshore migration of intertidal bars and berm formation, no net foreshore accretion took place during the field campaign. This was largely due to the formation of rip channels with strong rip currents cutting through the intertidal bars and the berm, which acted as a sediment drain in the profile. Copyright © 2009 John Wiley & Sons, Ltd.