Dune Erosion Research Articles

AN ANALYTICAL TOOL FOR LONG-TERM DUNE MANAGEMENT – CASE STUDY: YSTAD SANDSKOG BEACH, SWEDEN

Climate change calls for coastal management to consider longer time scales. The main objective of this study is to develop an analytical tool for long-term dune management to help evaluate the risk associated with high water levels, high waves, and their joint probability, as well as the probability of dune erosion, overwash, and breaching to occur. The tool also aims to identify vulnerable dune sections. A case study was made in Ystad Sandskog beach located on the south coast of Sweden. Data covering 20 years were used to investigate probabilities and consequences resulting from extreme waves and water levels. The data were also manipulated in order to indicate future implications in the context of climate change. The analytical tool turns out to be easy to implement and to interpret. The case study shows that Ystad Sandskog beach is subject to dune erosion and that many houses and infrastructure located behind the dunes may be exposed to more frequent flooding in the future.

COMPARATIVE MORPHODYNAMICS BETWEEN EXPOSED AND REEF PROTECTED BEACHES UNDER HURRRICANE CONDITIONS

It has been widely documented that coral reefs play a very important role in the dissipation of wave energy. Observations of beach morphology, made before and after a major hurricane that hit SE Mexico in October 2005 (Wilma), show widespread erosion at an exposed beach (Cancun), but unexpected beach accretion 25 km south at a beach fronted by a fringing reef (Puerto Morelos). The DELFT 3D morphodynamic model is used at a regional scale to explain the divergent morphological behavior of both beaches and explore the processes involved at both sites. A combination of offshore transport at the exposed beach, large scale southward sediment transport and onshore transport along the reef protected beaches seems to be the main explanation for the observed morphological behavior. Dune erosion and beach face sand relocation are also partially responsible for the beach progradation at the reef protected site.

LAND COVER EFFECT ON DUNE EROSION AND OVERWASH

In this study, a numerical morphodynamic model eXtreme Beach behaviour (XBeach) was used to simulate the effects of land cover on the shape of the post storm profile s at two test sites on barrier islands at Outer Banks, North Carolina. The simulations included dune erosion and overwash regimes and were carried out with and without the inclusion of land cover effects. The model results were assessed by comparing post storm profiles extracted from XBeach and Light Detection And Ranging (LiDAR) data. The simulations showed that the morphological response of the profiles was sensitive to land cover and inclusion of these effects improved the model results.

ANALYSIS AND MODELLING OF EXTREME LOCALISED DUNE EROSION EVENTS ALONG DENMARK'S NORTH SEA HIGH WATER DUNE BARRIER

This paper presents the results of an investigation into cases of extreme localised dune erosion along Denmark’s west coast. Cases of extreme dune erosion of over 20m are identified between 1977 and 2012. These are compared to the closest storm events. However, no correlation is found between the occurrence of these extreme erosion events and storm intensity or return period. A general increasing prevalence of sand bars is identified as well as an absence of extreme dune erosion in several of the more recent intense storms. Results of XBeach numerical modelling on a local example of extreme dune erosion showed that the offshore bathymetry and sand bar size has a significant effect on the amount of erosion that occurs at the dune face during a storm.

EROSION OF VEGETATED COASTAL DUNES

Coastal dunes can protect communities and infrastructure against storm surge and wave attack. They provide a natural barrier of granular material that erodes during storm conditions and dissipates wave energy in the process. Vegetation is abundantly present in most coastal dunes and may enhance the capability of the dune to withstand erosion. However, reliable data allowing quantification of the erosion process of dunes are sparse, in particular, data involving live plants. In this study, physical model experiments in a moveable-bed wave flume were carried out to investigate the effect of live plants on the wave-induced dune erosion process. The dune grass Sporobolus virginicus was grown in pots with dune sediment, transplanted into test dunes and subjected to both regular and irregular wave attack. Measurements of hydrodynamics and profile evolution show that the vegetation does affect the eroded dune volume and the dune scarp retreat rate. During wave flume testing vegetation density ranged from 0 to 80 plants per square meter and plant maturity (i.e. age) was varied between 0 and 12 weeks but further tests are necessary to substantiate the observed trends. The maturity of the plants’ root system yielded the most significant impact on dune erosion as plants with more developed roots were able to reduce the eroded dune volume more than plants with less mature root systems.

EROSION PROGRESSION OF WOODED COASTAL DUNE

Active plantation of woody plants over coastal sand dunes has been known to reduce the wave overtopping and overwash. In this study interactions among waves, sand dune and woody plants were investigated experimentally. Wooden plants were represented by cylindrical wooden dowels. Two tests were conducted to examine the placement density of the dowels. Additional two tests were conducted to examine the toppling effect of the dowels covering the dune. The measured data indicate that toppling of the dowels on the foreslope of the dune diminishes the effectiveness of the dowels in reducing dune erosion and overwash because the dowels on the foreslope reduced wave uprush and overtopping. The data collected in these experimental study were used to calibrate and verify the numerical model CSHORE.

FULLY PROBABILISTIC DUNE SAFETY ASSESSMENT USING AN ADVANCED PROBABILISTIC METHOD

In this paper, an advanced probabilistic method - Adaptive Directional Importance Sampling (ADIS) - is used in concert with a process-based dune erosion model - XBeach - to perform a dune safety assessment. The efficiency of the probabilistic method removes the restriction of only using simple empirical dune erosion models for probabilistic calculations. The proposed combination of ADIS and XBeach is able to handle complex cross-shore profiles that empirical dune erosion models cannot, as shown in several case studies.

MODELLING DUNE EROSION OF THE SEFTON COAST, LIVERPOOL BAY, UK

Dune erosion at sandy coastal systems poses significant threat to the hinterland areas due to flooding and subsequent damage to life and property. The Sefton Coast (Fig.1), bounded by Mersey (South) and Ribble (North) estuaries in the Liverpool Bay, represents about 20% of the UK’s dune system, has a high recreational value, form an effective coastal flood defense and consists of a significant number of conservation areas of national and international interest. This area is however susceptible to erosion during storm events where landward retreat occurs leading to dune breach and coastal flooding. Present study focused on an area around the Formby Point of the Sefton Coast (dashed-line polygon in Fig.1), which is a highly dynamic sedimentary system. The dune system at Formby Point beach showed continuous accretion (max ~4.7 m/year) during the last century but has become an eroding beach since early twentieth century (max. ~6.2 m/year). However, beach accretion can still be seen in far south and north of Fomby Point. The Sefton Coast experiences a semi-diurnal macrotidal regime with a mean spring tidal range reaching 8.22 m (Esteves et al., 2012). Mean annual significant wave height (Hs) is about 0.53 m while extremes exceed 5 m (Brown et al., 2010). The aim of this study is to investigate the response of the Formby Point dune system to extreme storm conditions based on the March 2010 storm event (max. Hs>3.5 m, see Brown, 2010). We use the 2DH process-based morphological model XBeach, which has been developed to investigate dune erosion under extreme conditions (Roelvink et al., 2009), in combination with the SWAN spectral wave model (Booij et al., 1999). The model set-up consists of a coarse-grid model (Sefton) and a fine-grid model (Formby Point) which encloses the area of interest. The Sefton model is driven by the Liverpool Bay WaveNet buoy wave data (Esteves et al., 2012). The outputs of the Sefton model provide wave and water level boundary conditions to the Formby Point morphodynamic model to investigate the beach response to the selected storm event. The model setup described above is then used to simulate nearshore hydrodynamics and morphodynamics. Fig. 1shows a snapshot of simulated Hs corresponding to the storm event occurred in the Liverpool Bay during 30 March and 02 April 2010, at the mean tidal level. Offshore waves are significantly attenuated when travelled from offshore to nearshore. Fig. 2 shows the model simulated exposure of the dune system and the beach during Low Water (LW) (a) and High Water (HW) (b). At LW, the beach exposes up to about 3 km while at HW, the entire beach drowns, exposing the dune system to incoming waves. Therefore, a storm event combined with HW may lead to damage the dune system and hence flooding of the hinterland of the River Alte area, due to the low dune height in this area. Relatively high morphological changes are observed around the Crosby channel area (see location in Fig. 1). The storm driven dune erosion occurred during the HW. Large storm waves propagate along the Crosby channel towards the River Alte area. The River Alte area is susceptible to coastal flooding during storms, as a result of the low dune crest in this area. Work is currently in progress to predict the dune evolution and flooding under the future climate change and sea level rise scenarios.

PILE FENCE TO ENHANCE DUNE RESILIENCY

A beach with a berm and dune provides storm protection but wave overtopping and overwash of the dune may lead to dune breaching and inundation. Dune erosion and overwash may be reduced by piles placed linearly along the toe of the dune. This study will explore whether piles can be used to make dunes more resilient against storm surge and waves. Six laboratory tests were conducted to observe the effect of pile fence porosity, location and pile toppling. It is recommended that a pile fence porosity of 0.5 or less be used to obtain significant increase in dune resiliency. Presence of the pile fence may increase offshore sediment transport resulting in more deposition in the surf zone. The pile fence should be placed near the dune toe to reduce wave uprush on the dune foreslope and to minimize pile toppling due to wave action.

Application of empirical wave run-up formulas to the Polish Baltic Sea coast.

Advanced, multidimensional models are typically applied when researching processes occurring in the nearshore. Relatively simple, empirical equations are commonly used in coastal engineering practice in order to estimate extreme wave run-up on beaches and coastal structures. However, they were mostly calibrated to the characteristics of oceanic coasts, which have different wave regime than a semi-enclosed basin like the Baltic Sea. In this paper we apply the formulas to the Polish Baltic Sea coast. The equations were adjusted to match local conditions in two test sites in Międzyzdroje and Dziwnówek, where beaches are under continuous video surveillance. Data from WAM wave model and coastal gauge stations were used, as well as precise measurements of the beaches' cross-sections. More than 600 run-up events spanning from June to December 2013 were analysed, including surges causing dune erosion. Extreme wave run-up R2% was calculated and presented as a percentage value indicating what part of the beach was inundated. The method had a root-mean-square error of 6.1 and 6.5 percentage points depending on the test site. We consider it is a fast and computationally undemanding alternative to morphodynamic models. It will constitute a part of the SatBałtyk Operating System-Shores, delivering forecasts of wave run-up on the beaches for the entire Polish coastline.

Evaluation of wave runup predictions from numerical and parametric models

Wave runup during storms is a primary driver of coastal evolution, including shoreline and dune erosion and barrier island overwash. Runup and its components, setup and swash, can be predicted from a parameterized model that was developed by comparing runup observations to offshore wave height, wave period, and local beach slope. Because observations during extreme storms are often unavailable, a numerical model is used to simulate the storm-driven runup to compare to the parameterized model and then develop an approach to improve the accuracy of the parameterization. Numerically simulated and parameterized runup were compared to observations to evaluate model accuracies. The analysis demonstrated that setup was accurately predicted by both the parameterized model and numerical simulations. Infragravity swash heights were most accurately predicted by the parameterized model. The numerical model suffered from bias and gain errors that depended on whether a one-dimensional or two-dimensional spatial domain was used. Nonetheless, all of the predictions were significantly correlated to the observations, implying that the systematic errors can be corrected. The numerical simulations did not resolve the incident-band swash motions, as expected, and the parameterized model performed best at predicting incident-band swash heights. An assimilated prediction using a weighted average of the parameterized model and the numerical simulations resulted in a reduction in prediction error variance. Finally, the numerical simulations were extended to include storm conditions that have not been previously observed. These results indicated that the parameterized predictions of setup may need modification for extreme conditions; numerical simulations can be used to extend the validity of the parameterized predictions of infragravity swash; and numerical simulations systematically underpredict incident swash, which is relatively unimportant under extreme conditions.

Vertical Cylinder Density and Toppling Effects on Dune Erosion and Overwash

AbstractInteractions among waves, sand dunes, and vertical cylinders are investigated in a laboratory experiment. A cross-shore numerical model is expanded to include the drag force acting on cylinders. The hydrodynamic variables modified by the cylinders are used to predict bed load and suspended sand transport rates. The expanded model is compared with five previous tests. The drag coefficient is calibrated to obtain similar agreement for the dunes with and without the cylinders. Four tests were conducted to examine the placement density and toppling of the cylinders. The effectiveness of the cylinders in reducing dune erosion and overwash decreased significantly with the density decrease and cylinder toppling. The reduced effectiveness is reproduced by the expanded model with the calibrated drag coefficient. However, a simple toppling criterion needs to be adjusted for each test for the reproduction of the observed landward progression of cylinder toppling during the dune profile evolution. Field data ...

Recording of selected effects and hazards caused by current and expected storm events in the Baltic Sea coastal zone

Furmańczyk, K., Andrzejewski, P., Benedyczak, R., Bugajny, N., Cieszyński, Ł., Dudzińska-Nowak, J., Giza, A., Paprotny, D., Terefenko, P., Zawiślak, T., 2014. Recording of selected effects and hazards caused by current and expected storm events in the Baltic Sea coastal zone. In: Green, A.N. and Cooper, J.A.G. (eds.), Proceedings 13th International Coastal Symposium (Durban, South Africa), Journal of Coastal Research, Special Issue No. 70, pp. 338–342, ISSN 0749-0208.Routine monitoring has already become a legal obligation and apparent necessity for EU members, especially of such dynamic and important habitat as the Baltic Sea and its shores. One of the subsystems of SatBałtyk, a Polish research project for maritime observation, is dedicated to coastal monitoring. It focuses on recording and forecasting the consequences of storm events along the Baltic's Polish shoreline. Three sites (7–12 km-long sections of dune coast) were selected for detailed research: the Dziwnow Spit, Jamno and Bukowo lakes' spits and part of the Hel Peninsula. Work started in 2010 and will continue until 2015. A numerical model, XBeach, was implemented in order to provide forecasts of several parameters, i.e. beach flooding, dune erosion, suspension of matter in the water after storm and rip current occurrence. It combines external data with in situ measurements by the researchers. Many pieces of equipment were installed for this research: a satellite receiving station, underwater probe ADCP (Acoustic Doppler Current Profiler), a tide gauge, a hyperspectral radiometer and two video cameras for constant shoreline observation. The system is currently being calibrated and verified; an online visualisation available to the public is due to be launched as a long-lasting outcome of the project.

Probabilistic estimation of coastal dune erosion and recession by statistical simulation of storm events

Knowledge about future oceanographic events will assist governments to better manage risk in coastal zones, a crucial task in the light of projected sea level rise, population growth and economic development. In this study, a 31-year data set of deep water wave climate parameters and bathymetry measurements (yearly cross-shore transect surveys) at Noordwijk, the Netherlands, were analysed to jointly estimate storm events variates of deep water wave conditions, and to probabilistically compute dune erosion volume and the resulting dune retreat distance with the simulated wave climate and possible local sea level rise scenarios by 2100. The probabilistic dune retreat models were applied and adjusted to the study site. Based on the outcomes of this application, a modelling technique can be established to propose a framework for probabilistically describing the coastal erosion and recession along the coast protected by dunes.

Spatio-temporal variability in accretion and erosion of coastal foredunes in the Netherlands: regional climate and local topography.

Depending on the amount of aeolian sediment input and dune erosion, dune size and morphology change over time. Since coastal foredunes play an important role in the Dutch coastal defence, it is important to have good insight in the main factors that control these changes. In this paper the temporal variations in foredune erosion and accretion were studied in relation to proxies for aeolian transport potential and storminess using yearly elevation measurements from 1965 to 2012 for six sections of the Dutch coast. Longshore differences in the relative impacts of erosion and accretion were examined in relation to local beach width. The results show that temporal variability in foredune accretion and erosion is highest in narrow beach sections. Here, dune erosion alternates with accretion, with variability displaying strong correlations with yearly values of storminess (maximum sea levels). In wider beach sections, dune erosion is less frequent, with lower temporal variability and stronger correlations with time series of transport potential. In erosion dominated years, eroded volumes decrease from narrow to wider beaches. When accretion dominates, dune-volume changes are relatively constant alongshore. Dune erosion is therefore suggested to control spatial variability in dune-volume changes. On a scale of decades, the volume of foredunes tends to increase more on wider beaches. However, where widths exceed 200 to 300 m, this trend is no longer observed.

Erosion and its rate on an accumulative Polish dune coast: the effects of the January 2012 storm surge

The Polish coast is a non-tidal area; its shores are affected mainly by autumn-winter storm surges. Those of 6 and 14 January 2012 are representative of the forces driving the erosion of normally accumulative sections of coastal dunes, monitored by the author since 1997. The sea level maximum during these two storm surges reached 1.2 to 1.5m amsl along the Polish coast. Land forms up to 3m amsl were inundated. Beaches and low parts of the coast up to this height were rebuilt by sea waves attacking the coast for almost 12days. Quantitative analyses of the morphological dynamics of the coastal dunes are presented for 57 profiles located along the coast. Only those accumulative sections of the Polish coast are analysed where sand accumulation did occur and led to new foredune development. The mean rate of dune erosion was 2.5m3 per square metre with an average toe retreat of 1.4m. Erosion understood as dune retreat was greater when a beach was lower (correlation coefficient 0.8). Dune erosion did not occur on coasts with beaches higher than 3.2m or on lower ones covered by embryo dunes.

Erosional Impact on Caspian Sea Coasts Stability Capacity

Stability capacity determination of beach against marine erosion forces in the southern coasts of the Caspian Sea (Babolsar to Noor) is the main target in this paper. By dividing study areas to 14 zones, shoreline displacement rate, land covering deformation and sand dune erosion vulnerability tendency have been measured with remote sensing and the geographical information system. In field monitoring, the geometric structure of the beach was mapped and 42 sediment samples have been obtained from the beach zone (Berm, shoreline, 0.5 meter depth). Then the beach stability capacity levels that due to several criterions such as: (sediment texture, geometry structure, sand dune accumulation, and land use development and land cover changes) have been calculated by GIS in universal ranking system model (URSM). Results showed that the highest stability capacity rates belong to Talar beach (zone 1) and Arabkheil beach (zone 2) and the eastern part of Mahmoodabad beach (zone 9). By contrast, Mahmoodabad (zone 10) and Noor (zone 14) were the most unstable regions against sea-levels and hydrodynamic forces.

A relationship to describe the cumulative impact of storm clusters on beach erosion

Estimation of erosion volumes for adequate dry beach buffer zones is commonly estimated on the basis of a single extreme event, such as the 1 in 100year storm. However, the cumulative impact of several smaller, closely spaced storms can lead to equal, if not more, dry beach loss, but this is often not quantified. Here we use a calibrated model for dune erosion, XBeach, to hindcast the cumulative erosion impact of a series of historical storms that impacted the Gold Coast, Queensland region in 1967. Over a 6-month period, four named cyclones (Dinah, Barbara, Elaine, and Glenda) and three East Coast Lows caused a cumulative erosion volume greater than the predicted 1 in 100year event. Results presented here show that XBeach was capable of reproducing the measured dry beach erosion volume to within 21% and shoreline retreat to within 10%. The storms were then run in 17 different sequences to determine if sequencing influenced final modeled erosion volumes. It is shown that storm sequencing did not significantly affect the total eroded volumes. However, individual storm volumes were influenced by the antecedent state of the beach (i.e. prior cumulative erosion). Power-law relationships between cumulative energy density (∑E) and eroded volume (∆V) as well as cumulative wave power ((∑P)) and eroded volume (∆V) both explained more than 94% of the modeled dry beach erosion for the 1967 storm sequences. When the relationship was compared with observed and modeled erosion volumes for similar beaches but different storm forcing, the inclusion of pre-storm beach swash slope (βswash) in the parameterization was found to increase the applicability of the power-law relationship over a broader range of conditions.

Potential Late-Holocene Disjunction ofSequoia sempervirenson the Central Oregon Coast

A large (> 200-cm diameter) coast redwood (Sequoia sempervirens) stump occurring upright on an Oregon beach, 257 km from the northern limit of its native distribution, may be a remnant of an extinct disjunct population or it may be the result of vertical emplacement of a drift log. Holocene tree stumps in situ in paleosols commonly emerge on the shore platform as a result of a complex history of dune activity, subsidence, and erosion. An historical account reported organic soils associated with the stump during the late 1800s; such soils are normally removed quickly by wave action after exposure. We used several methods to corroborate this account. The stump’s in situ origin is supported by 1) a radiocarbon age indicating a death date between 1820 and 1720 years ago, coeval with other paleosols and in situ stumps; 2) its height and upright position, which are only matched by other in situ stumps; and 3) a photograph from 1912 showing uneroded wood inconsistent with a sea-drift history. Other evidence failed to support the in situ origin: 1) ground-penetrating radar did not reveal an associated paleosol, 2) the age of a nearby paleosols and stumps within 3 km were younger than the stump, and 3) three paleosols did not yield redwood pollen or wood. The only support for a sea-drift origin is that its age slightly predates a known tsunami that may have emplaced the stump. The balance of evidence suggests that the redwood stump is a remnant of an extinct late-Holocene disjunct population.

Effects of Woody Plants on Dune Erosion and Overwash

AbstractA laboratory experiment consisting of five tests was conducted to examine the effects of woody plants on erosion and overwash of high and low dunes. Scarping occurred on the foreslope for three high dune tests and no scarping occurred for two low dune tests. A narrow vegetation zone on a steep backslope of a high dune did not reduce wave overtopping and overwash in comparison with the corresponding bare dune. A wide vegetation covering the high dune reduced foreslope scarping, prevented wave overtopping initially, and reduced sand overwash after the initiation of wave overtopping. A wide vegetation zone covering an entire low dune reduced dune erosion by retarding wave uprush and reducing wave overtopping and overwash.