Journal of Marine Science and Engineering

Article Dune Volume Changes at Decadal Timescales and Its Relation with Potential Aeolian Transport

Glenn Strypsteen 1,* , Rik Houthuys 2 and Pieter Rauwoens 1

1 Construction TC, Dept. of Civil Engineering, Bruges Campus, KU Leuven, Spoorwegstraat 12, B-8200 Bruges, Belgium; [email protected] 2 Geoconsultant, Suikerkaai 8, B-1500 Halle, Belgium; [email protected] * Correspondence: [email protected]



Received: 11 September 2019; Accepted: 7 October 2019; Published: 8 October 2019


Abstract: Long-term changes in dune volume at the Belgian coast are analyzed based on measured data by airborne surveys available from 1979. For most of the 65 km long coastal stretch, dune volume increases linearly in time at a constant rate. Dune growth varies between 0–12.3 m3/m/year with an average dune growth of 6.2 m3/m/year, featuring large variations in longshore directions. Based on a wind data set from 2000–2017, it is found that potential aeolian sediment transport has its main drift from the west to southwest direction (onshore to oblique onshore). Based on a modified Bagnold model, onshore potential aeolian sediment transport ranges to a maximum of 9 m3/m/year, while longshore potential aeolian sediment transport could reach up to 20 m3/m/year. We found an important correlation between observed and predicted dune development at decadal timescales when zones with dune management activities are excluded. Most of the predicted data are within a factor of two of the measured values. The variability in potential transport is well related to the variability in dune volume changes at the considered spatial–temporal scale, suggesting that natural dune growth is primarily caused by aeolian sediment transport from the beach. It also suggests that annual differences in forcing and transport limiting conditions (wind and moisture) only have a modest effect on the overall variability of dune volume trends.

Keywords: aeolian sediment transport; dune development; Belgian coast; decadal timescales; annual timescales

1. Introduction Coastal dunes provide safety against flooding during storm events. They also have functions for recreation and nature conservation. Compared to hard engineering structures like sea dykes, coastal dunes are in favor because they are primarily being built by natural aeolian processes [1–3]. However, natural coastal dunes are dynamic features and the disadvantage, therefore, is that the provided safety level is variable in time [4]. Long-term coastal dune development is a net result of combined storm wave erosive and aeolian processes. It is the net result which determines if the dunes are eroding or growing. Predicting and evaluating these processes is a prerequisite for many management activities. In the past few decades, a lot of research has been carried out on dune erosion which can be simulated with good accuracy. Yet, predictions on aeolian sediment transport from the beach towards the dunes, leading to dune recovery, are still difficult to make [5–8]. This study examines how annual to decadal variations in wind climate (indirect potential aeolian transport) correlate with annual to decadal variations in dune volume along the Belgian coast. Decadal variations are of interest because they describe the general coastline development. Coastal dune development studies generally focus on the measurement of short-term transport processes in the timescale of hours to days [9–13]. Sometimes, at these timescales, a one to one relation

J. Mar. Sci. Eng. 2019, 7, 357; doi:10.3390/jmse7100357 www.mdpi.com/journal/jmse J. Mar. Sci. Eng. 2019, 7, 357 2 of 24 is found between predicted and observed values [14]. Coastal dune development is also frequently studied by the measurement of long-term topographical elevation variations in the timescale of months to years [8,10,15–17]. Primarily, these topographical elevation changes are related to calculated potential transport rates [18]. Long-term aeolian sediment transport from the beach towards coastal dunes is generally predicted by integrating hourly meteorological data, such as wind speed and direction from standard meteorological stations, into sediment transport equations (e.g., [19–23]). Unfortunately, at these timescales, the calculated potential sediment input in the dunes from the beach frequently do not agree with volume observations in the dunes (e.g., [10,15]). Keijsers et al. [18] and de Vries et al. [4] studied aeolian transport and dune behavior on annual to decadal timescales and found no significant correlation. Both found greater correlation between dune behavior and erosive events by storms. Though, for their study at the Dutch coast, Keijsers et al. [18] found stronger correlations between a time series of potential sediment transport and dune volume on wider beaches (>200 m). This suggests potentially stronger correlations at the Belgian coast, where beach widths are generally wider (between 150 m and 400 m). The wider beaches at the Belgian coast are primarily created by massive sand nourishments to cope with future flooding risks and hazards. Especially since the 1990s, nourishments are used to keep the sediment budgets along the coast positive. The nourished sand is naturally distributed in the coastal zone. When coastal dune development and its relation with potential aeolian sediment transport is studied on decadal timescales, the effects of erosive and accretive years should eventually average out. Annual differences would then be observed as small perturbations on the trend at decadal timescales. Potential transport should then, hypothetically, be well related to dune behavior at the considered spatial–temporal scale. This would suggest that dune growth (at locations not suffering too much from dune erosion) is primarily caused by aeolian sediment transport from the beach. The purpose of this paper is:

1. To gain insight into long-term dune development at the Belgian coast. This study is based on the analysis of airborne photogrammetric and airborne laser scanner (LiDAR) data of the dunes from 1979–2018. 2. To gain insight into annual potential aeolian sediment transport quantities and how it behaves on decadal timescales (long-term). Annual potential aeolian sediment transport along the Belgian coastline is estimated by the use of a modified Bagnold model, which has been validated by short-term field data of aeolian sediment transport rates [14], applied to a wind data set from 2000–2017. 3. To gain insight into the correlation between observed and predicted dune volume on an annual timescale. Year-to-year variations (between elevation measurements) in potential transport and dune volume changes are compared. 4. To gain insight into the correlation between observed and predicted dune volume on decadal timescales. Trend analysis on predicted and measured dune volume is conducted for comparisons on a decadal timescale. 5. To explain longshore variations of the correlations by distinguishing between ‘natural’ and ‘managed’ beach sections of the Belgian coast featuring dunes.

2. Regional Setting Located between France and the Netherlands (Figure1), the modest 65 km long Belgian coast was historically characterized by sandy beaches, backed by wide sand dunes, mudflats and tidal marshes [24]. However, over the last 50 years, these marshes and mudflats were modified by draining and diking them to develop agriculture, habitation and major economical recreation [25]. Nowadays, like many coastal areas worldwide, the Belgian coast faces increasing challenges due to coastal erosion, sea-level rise, and an increased risk of flooding. J. Mar. Sci. Eng. 2019, 7, 357 3 of 25

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Figure 1. The 65 km long Belgian coast is located at the North Sea, between the French and the Dutch Figureborder 1. andThe 65is kmdivided long Belgianinto 254 coast coastal is located sections. at the The North river Sea, ‘IJzer’ between flows the into French the and North the DutchSea at Figure 1. The 65 km long Belgian coast is located at the North Sea, between the French and the Dutch borderNieuwpoort. and is divided The green into areas 254 coastal indicate sections. the locations The river with ‘IJzer’ vegetated flows intocoastal the dunes. North Sea at Nieuwpoort. border and is divided into 254 coastal sections. The river ‘IJzer’ flows into the North Sea at The green areas indicate the locations with vegetated coastal dunes. Nieuwpoort. The green areas indicate the locations with vegetated coastal dunes. The Belgian coast is southwest–northeast orientated with a mean of 62° to the north (Figure 2). The Belgian coast is southwest–northeast orientated with a mean of 62◦ to the north (Figure2). LocalThe variations Belgian fromcoast theis southwest–no mean coastalrtheast orientation orientated are found, with aespecially mean of 62°around to the the north harbor (Figure mouths 2). Local variations from the mean coastal orientation are found, especially around the harbor mouths Local(Ostend variations (30 km) fromand Zeebruggethe mean coastal (50 km)), orientation and the natuare found,re reserve especially ‘het Zwin’, around on thethe borderharbor withmouths the (Ostend (30 km) and Zeebrugge (50 km)), and the nature reserve ‘het Zwin’, on the border with (OstendNetherlands. (30 km) and Zeebrugge (50 km)), and the nature reserve ‘het Zwin’, on the border with the the Netherlands. Netherlands.

Figure 2. Coastal orientation of the Belgian coast. It represents the orientation of dunes or dykes.

TheFigure mean 2. coastalCoastal orientationorientation is of 62 the◦ with Belgian respect coast. to It the repr northesents (red the line). orientation of dunes or dykes. The mean coastal orientation is 62° with respect to the north (red line). TheFigure Belgian 2. Coastal coast orientation consists of of sandy the Belgian beaches, coast. which It repr areesents generally the orientation up to 400 of m dunes wide or in dykes. the southwest The mean coastal orientation is 62° with respect to the north (red line). and onlyThe 150Belgian m in coast the northeast consists (Figureof sandy3). beaches, Beach width which is definedare generally here as up the to horizontal 400 m wide distance in the southwestThe Belgian and only coast 150 consists m in the of northeast sandy beaches, (Figure 3)which. Beach are width generally is defined up to here 400 as m the wide horizontal in the southwest and only 150 m in the northeast (Figure 3). Beach width is defined here as the horizontal

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1 betweendistance mean between low-water mean level low-water (+1.39 level m TAW (+1.391) and m duneTAW foot) and (+ 6.89dune m foot TAW). (+6.89 During m TAW). springtide, During beachspringtide, width can beach range width from can 100 range m to from 600 m 100 [26 m]. to The 600 upper m [26]. beach Thewidth, upper definedbeach width, as the defined horizontal as the distancehorizontal between distance mean between high-water mean level high-water (+4.39 m level TAW) (+4.39 and dunem TAW) foot (and+6.89 dune m TAW), foot (+6.89 ranges m fromTAW), 30ranges m to 100 from m. 30 The m beachesto 100 m. along The beaches the entire along coastline the en aretire verycoastline gently are sloping, very gently although sloping, the although slope increasesthe slope from increases west to from east west (Figure to east3). Due(Figure to the3). Du constructione to the construction of the jetties of the (approximately jetties (approximately 3 km in length)3 km in oflength) Zeebrugge of Zeebrugge harbor (between harbor (between 52–55 km 52 alongshore),–55 km alongshore), the beach the width beach is width relatively is relatively large, causinglarge, acausing very mild a very beach mild slope beach in thatslope region. in that region.

Figure 3. (A) Average beach width along the Belgian coast based on airborne surveys from 2000–2017. Figure 3. A) Average beach width along the Belgian coast based on airborne surveys from 2000–2017. Beach width is the horizontal distance between the average low-water level and the dune foot. Upper Beach width is the horizontal distance between the average low-water level and the dune foot. Upper beach width is the horizontal distance between the average high-water level and the dune foot or a beach width is the horizontal distance between the average high-water level and the dune foot or a hard defense structure. (B) Average beach slope along the Belgian coast based on airborne surveys hard defense structure. B) Average beach slope along the Belgian coast based on airborne surveys from 2000–2017. Error bars indicate the standard deviation. from 2000–2017. Error bars indicate the standard deviation. Due to a natural gradient and an increase in nourishments towards the northeast, the sand becomesDue gradually to a natural coarser gradient along the and coast, an increase from 150 inµ mnourishments in the west totowards up to 400 theµ mnortheast, in the east the [24 sand]. Figurebecomes4 shows gradually the mean coarser grain along size variation the coast, along from the 150 Belgian µm in coast.the west It is to from up to a unique400 µm data in the set east from [24]. theFigure year 2000 4 shows that coversthe mean the grain entire size Belgian variation coast along [27]. Threethe Belgian to four coast. samples It is from were a taken unique at thedata surface set from onthe the year upper 2000 beach, that and covers were the analyzed entire Belgian for grain coast size [27]. after Three removal to four of organic samples material. were taken On theat the basis surface of theon available the upper measurement beach, and data,were theanalyzed coast is for divided grain size into after zones, removal in which of theorganic nature material. of the beach On the sand basis is assumedof the available to be homogeneous. measurement Indata, total, the 16 coast zones is aredivi distinguished.ded into zones, The in which black linethe representsnature of the the beach 16 sand is assumed to be homogeneous. In total, 16 zones are distinguished. The black line represents

1 TAW1is TAW the Belgian is the reference Belgian level reference (Tweede level Algemene (Tweede Waterpassing) Algemene and isWaterpassing) located around meanand wateris located level ataround low tide springs. mean water level at low tide springs.

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the 16 homogeneous zones with each having a different grain size. The corresponding standard the 16 homogeneous zones with each having a different grain size. The corresponding standard homogeneousdeviation is also zones shown with (black each bars). having a different grain size. The corresponding standard deviation is alsodeviation shown is (black also shown bars). (black bars).

Figure 4. The grain size variation along the Belgian coast. There is an increase from France (0 km) to FigureFigure 4. 4.The The grain grain size size variation variation along along the the Belgian Belgian coast. coast. ThereThere isis anan increase increase from from France France (0 (0 km) km) to to the Netherlands (65 km). thethe Netherlands Netherlands (65 (65 km). km). Approximately half half of of the the Belgian Belgian coast coast consists consists of of vegetated vegetated coastal coastal dune duness (Figure (Figures 1 and1 and Figure5), Approximately half of the Belgian coast consists of vegetated coastal dunes (Figure 1 and Figure which5), which vary vary in heightin height between between+5 +5 m TAWm TAW and and+30 +30 m TAW,m TAW, with with most most ranging ranging between between+7 and+7 and+15 +15 m 5), which vary in height between +5 m TAW and +30 m TAW, with most ranging between +7 and +15 TAWm TAW [28 [28].]. According According to to Lebbe Lebbe et et al. al. [26 [26],], the the width width of of the the Belgian Belgian dunes dunes narrowsnarrows downdown fromfrom 2 km m TAW [28]. According to Lebbe et al. [26], the width of the Belgian dunes narrows down from 2 km west of the IJzer estuary to a few hundred meters eastwardseastwards of the IJzer. Landward of the dunes, an west of the IJzer estuary to a few hundred meters eastwards of the IJzer. Landward of the dunes, an outstretched and extensive coastal lowland is pres presentent at elevations between +1+1 and and +4+4 m m TAW TAW [28]. [28]. outstretched and extensive coastal lowland is present at elevations between +1 and +4 m TAW [28]. The remainingremaining part part of of the the Belgian Belgian coast, coast, especially especially around around coastal coastal cities, cities, is protected is protected by harbors, by harbors, groins, The remaining part of the Belgian coast, especially around coastal cities, is protected by harbors, seawalls,groins, seawalls, and sea and dykes sea (Figure dykes 5(Figure). The height5). The of heig theht sea of dykesthe sea is dykes approximately is approximately+9 m TAW. +9 m TAW. groins, seawalls, and sea dykes (Figure 5). The height of the sea dykes is approximately +9 m TAW.

Figure 5. Typical sights at the Belgian coast. Left: natura naturall beach-dune beach-dune system at Koksijde (11 km from Figure 5. Typical sights at the Belgian coast. Left: natural beach-dune system at Koksijde (11 km from the FrenchFrench border). border). Right: Right: managed managed beach-dyke beach-dyke system system at Mariakerke at Mariakerke (26 km (2 from6 km the from French the border).French the French border). Right: managed beach-dyke system at Mariakerke (26 km from the French border). 3. Methodsborder). 3. Methods 3.1.3. Methods Dune Volume Changes Along the Belgian Coast 3.1. DuneSince Volume 1979, the Changes Belgian Along government the Belgian has Coast been monitoring the eastern part of the coastline, and since 1983,3.1. Dune the entire Volume coastline Changes by Along either the annually Belgian orCoast bi-annually surveying cross-shore bathymetric profiles Since 1979, the Belgian government has been monitoring the eastern part of the coastline, and and collectingSince 1979, airborne the Belgian photogrammetric government and, has sincebeen 1999,moni airbornetoring the laser eastern scanner part (LiDAR) of the coastline, data [29, 30and]. since 1983, the entire coastline by either annually or bi-annually surveying cross-shore bathymetric Thesince surveys 1983, the took entire place coastline at different bytimes either per annually year. Determining or bi-annually and surveyin managingg cross-shore the seawall bathymetric safety level profiles and collecting airborne photogrammetric and, since 1999, airborne laser scanner (LiDAR) wasprofiles the initial and collecting idea to monitor airborne and photogrammetric survey the coastal and, morphology since 1999, in Belgiumairborne [ 31laser]. The scanner available (LiDAR) data data [29,30]. The surveys took place at different times per year. Determining and managing the allowsdata [29,30]. for specific The morphologicalsurveys took analyses.place at different Demonstrated times byper numerous year. Determining studies, LiDAR and datamanaging is able the to

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seawall safety level was the initial idea to monitor and survey the coastal morphology in Belgium [31]. The available data allows for specific morphological analyses. Demonstrated by numerous studies, LiDAR data is able to accurately represent beach topography over large stretches of coastline [32–34]. Sequentially, LiDAR surveys allow to very accurately monitor temporal shoreline changes [35]. Concerning the Belgian coast, LiDAR data covers the intertidal beach up to the sea-fronting dunes. For this study, the focus lies on the dunes. The Belgian coast is represented by a system of coastal sections adopted by the Flemish government, defined by fixed boundaries, which are also

J. Mar.used Sci. Eng.in this2019 ,study7, 357 [24]. There are 277 coastal sections, each approximately 250 m wide,6 ofwhere 24 morphology is monitored and surveyed. Section 1 is located, however, in France and sections 256 to 277 are located in the Netherlands. Hence, this study focusses on sections 2–255 (Figure 1). accuratelyIn the represent 1980s, beachon the topography basis of elevation over large boundaries, stretches the of coastline Belgian [coast32–34 was]. Sequentially, also divided LiDAR into five surveyscross-shore allow tohorizontal very accurately elevation monitor slices temporal[29,30]. Following shoreline Figure changes 6, [35slice]. Concerning1 comprises the the Belgian intertidal coast,beach LiDAR with data elevations covers the between intertidal +1.39 beach and up +4.39 to the m sea-fronting TAW, slice dunes. 2 is the For dry this beach study, thewith focus elevations lies on thebetween dunes. +4.39 The and Belgian +6.89 coast m TAW, is represented slice 3 represents by a system the ofdune coastal area sections above +6.89 adopted m TAW, by the slice Flemish 4 is the government,shoreface definedwith elevations by fixed boundaries,between −4.11 which m and are also+1.39 used m TAW, in this and study slice [24 5]. is There the sea are bottom 277 coastal below sections,−4.11 eachm TAW. approximately Initially, these 250 m elevations wide, where were morphology defined by is the monitored Department and surveyed. of Mobility Section and 1Public is located,Works however, (MOW) inrelative France to and the sections Z vertical 256 datum to 277 are(−4 locatedm, 1.5 m, in the4.5 Netherlands.m, and 7 m Z). Hence, Later, this these study were focussesconverted on sections to the 2–255 TAW (Figure vertical1). datum by subtracting 0.11 m. The elevations are conventional boundariesIn the 1980s, and on have the not basis been of elevationderived on boundaries, the basis of the tidal Belgian water coastlevel wasvariations also divided [24]. into five cross-shoreThe horizontal dune volume elevation is defined slices as [29 the,30 volume]. Following of sa Figurend above6, slice the 1dune comprises foot level the intertidaland is bounded beach by withfixed elevations vertical betweenplanes constituting+1.39 and +a 4.39fixed m landward TAW, slice limit. 2 is The the drydune beach foot level with along elevations the Belgian between coast +4.39was and defined+6.89 mat TAW,+6.89 m slice TAW. 3 represents The term the dune dune volu areame above has been+6.89 defined m TAW, purely slice 4based is the on shoreface bounding withplanes. elevations It results between in a dune4.11 mvolume and + 1.39that mcan TAW, also andbe calculated slice 5 is the in seaareas bottom with belowa seawall4.11 or mwith − − TAW.artificially Initially, raised these elevationstouristic berms, were definedwhere no by vegetate the Departmentd dunes are of Mobility found. It and is noticed Public that Works the (MOW) landward relativelimit tois theeither Z vertical positioned datum at (the4 foot m, 1.5 ofm, the 4.5 seawall, m, and or 7 mon Z). the Later, top, theseor even were at the converted landward to the side TAW of the − verticalfirst datumdune ridge. by subtracting However, 0.11 the m. varying The elevations landward are reference conventional is not boundaries of relevance, and havegiven not that been most derivedmorphological on the basis changes of tidal occur water in level the area variations included [24 ].in the section boundaries.

FigureFigure 6. The 6. The definition definition of the of the dune dune volume volume (number (number 3), where3), where the dunethe dune foot foot level level starts starts at + at6.89 +6.89 m m TAWTAW (Tweede (Tweede Algemene Algemene Waterpassing). Waterpassing). Figure Figure adapted adapted from from [24]. [24].

TheFigure dune volume7 gives an is definedoverview as of the all volume the available of sand du abovene volume the dune data foot with level respect and to is boundedthe first survey by fixedat verticalthe Belgian planes coast. constituting Blank bars a fixedindicate landward no performed limit. The LiDAR dune flights foot level and along thus theno available Belgian coast survey wasdata. defined Some at regions+6.89 m (especially TAW. The the term dune dune regions) volume ex hasperience been a defined growth purely in time based (yellow on boundingto red color). planes.Other It resultsregions in (in a duneparticular volume the that regions can alsothat beare calculated diked) remain in areas stable with in a seawalltime. or with artificially raised touristic berms, where no vegetated dunes are found. It is noticed that the landward limit is either positioned at the foot of the seawall, or on the top, or even at the landward side of the first dune ridge. However, the varying landward reference is not of relevance, given that most morphological changes occur in the area included in the section boundaries. Figure7 gives an overview of all the available dune volume data with respect to the first survey at the Belgian coast. Blank bars indicate no performed LiDAR flights and thus no available survey data. Some regions (especially the dune regions) experience a growth in time (yellow to red color). Other regions (in particular the regions that are diked) remain stable in time. J. Mar. Sci. Eng. 2019, 7, 357 7 of 25 J. Mar. Sci. Eng. 2019, 7, 357 7 of 24

Figure 7. Overview of available data on dune volume with respect to the first survey along the Belgian Figure 7. Overview of available data on dune volume with respect to the first survey along the Belgian coast, based on the annual surveys (positive values = deposition; negative values = erosion; 0 = stable). coast, based on the annual surveys (positive values = deposition; negative values = erosion; 0 = stable). The term dune volume has been defined purely based on bounding planes. Dune volume can also be The term dune volume has been defined purely based on bounding planes. Dune volume can also be calculated in areas with a seawall or with artificially raised touristic berms, where no vegetated dunes calculated in areas with a seawall or with artificially raised touristic berms, where no vegetated dunes are found. are found. 3.2. Calculation Procedure of Potential Transport 3.2. Calculation Procedure of Potential Transport Dune growth could be best explained by potential aeolian sediment input from the beach into the dunes.Dune By applyinggrowth could a time be series best ofexplained regional by wind potent dataial to aeolian transportsediment equations,input from potential the beach aeolian into thetransport dunes. can By beapplying calculated a time [23 series,36]. It of should regional be mentionedwind data to that aeolian potential transport transport equations, is the maximum potential aeoliantransport transport that can becan achieved. be calculated All winds [23,36]. above It should the threshold be mentioned velocity canthat transport potential sand, transport disregarding is the maximumall factors whichtransport negatively that can influencebe achieved. aeolian All winds sediment above transport, the threshold such asveloci precipitationty can transport and surficial sand, disregardingmoisture [37]. all Validated factors which by short-term negatively field influence campaigns aeolian [14], sediment where wind transport, speeds andsuch saturated as precipitation aeolian andtransport surficial rates moisture were measured, [37]. Validated a modified by short-term Bagnold modelfield campaigns [38] is used [14], to calculate where wind annual speeds potential and saturatedaeolian sand aeolian transport transport and rates is formulated were measured, by: a modified Bagnold model [38] is used to calculate annual potential aeolian sand transport and is formulated by:  s  d ρρ h 3 i 3600 α ∙ α ∙ 50 ∙ airair ∙(uu)33-u(u )3,,u u >u> u q =  B B * *,cr,cr * *,cr,cr q = · · dd50,ref · gg · ∗ − ∗ ∗ ∗ (1)(1)    0,0,u u*sediments.αB = Bagnold A value factor of (1.5–3.5). αB = 2 represents The coeffi cientnaturallyαB ranges graded from sands, 1.5–3.5 which and is is used dependent in the calculationon the surface of potential sediments. transport. A value Apart of αB from= 2 representsthe wind speed, naturally Equation graded (1) sands,assumes which conditions is used where in the allcalculation parameters of potentialare considered transport. cons Aparttant in from time, the making wind speed,the transpor Equationt variability (1) assumes solely conditions dependent where on theall parametersvariability in are wind considered speed [4]. constant in time, making the transport variability solely dependent on the variabilityThe critical in shear wind velocity speed [ 4of]. dry sand is defined as:

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The critical shear velocity of dry sand is defined as:

s" ! # ρs u ,cr = 0.10 1 g d50 (2) ∗ · ρair − · ·

3 3 2 where ρair = density of air (1.2 kg/m ); ρs = absolute density of sand grains (2650 kg/m ); g = 9.81 m/s ; d50 = median sand grain size (m). The shear velocity, u*, is the main driving factor in most transport formulae [39,40]. Determining the shear velocity, u*, requires either measurement of the velocity profile or knowledge of the aerodynamic roughness length [41]. A time series of hourly mean wind speed and wind direction were obtained from a meteorological station maintained by Ostend Airport. The station is about 1 km inland from the coastline and measured at a height of 4 m (Figure1). The wind data is available since the year 2000. It is likely that the roughness length varies significantly spatial–temporally [9]. This makes it inappropriate to use any of the frequently used transport formulae [41]. Hsu [21] suggested a straightforward equation for the prediction of aeolian sediment transport from hourly routine wind observations at meteorological stations. The equation is based on the relationship between the bed shear velocity, u*, and wind speed at a certain height above the surface, taken from field data from numerous study areas. This relationship for the dry beach area is given by u* = 0.040.U2m–10m, where U2m and U10m is the wind speed at 2 m and 10 m above the surface, respectively. This relationship is also the average value found by Strypsteen et al. [9]. In the potential aeolian sand transport calculations, the wind shear velocity, u*, is constant throughout the hour, in spite of the fact that this is practically not the case. Normally distributed fluctuations around the mean wind speed would cause, in theory, a larger amount of sediment to be transported by wind than would be given here [10]. According to Sarre [10], these inaccuracies and underestimations of potential aeolian transport are not important. Other transport limiting factors in coastal areas are of more importance, such as surface moisture, vegetation and beach dimensions [4]. Potential sediment transport will also be influenced by the fetch effect [42,43]. Delgado-Fernandez [43] described the effect of fetch length as an increase in sediment transport rate in the downwind direction until an equilibrium is reached. Longer fetch lengths often lead to higher transport rates under certain wind conditions. The limit where an equilibrium is reached is defined as the critical fetch. Critical fetch distance is not considered in this paper, since it is in the order of 10–50 m for wind speeds up to 15 m/s [44] and it is smaller than the average upper beach width at the Belgian coast. Applying a measured wind time series produces the total transport by summation of all calculated transport rates, qi (Equation (2)). It is of great importance that potential aeolian sediment transport is being modified according to the angle of wind approach along the shoreline in order to compare predicted with observed dune volume changes [1]. The predicted net sediment input by wind in the dunes (in m3/m) becomes:

n n 1 X 1 X h i Qcross shore = qi sin[α O] = qi sin ddj (3) − ρ − ρ b i=1 b i=1

Parallel or longshore winds should be modified by the cosine of the transport direction:

n 1 X h i Q = q cos dd (4) longshore ρ i j b i=1 where the angle, ddj, is the difference between the wind direction α and the coastal orientation, O (Figure8); n equals the total amount of hours in the measured time series, while assuming a bulk density 3 of sand of 1600 kg/m (ρb) (e.g., [38,45]). Hence, predicted sediment transport can be compared to J. Mar. Sci. Eng. 2019, 7, 357 9 of 24

J. Mar. Sci. Eng. 2019, 7, 357 9 of 25 observed dune volume changes in consecutive periods where (oblique) onshore winds were exceeding the threshold velocity.

Figure 8. Orientation (O) of the Belgian coastline and calculation procedure of potential transport. TheFigure orange 8. Orientation arrow shows (O) the of onshorethe Belgian direction coastline of sediment and calculation transport procedure towards the ofdunes potential and transport. is used to

explainThe orange dune arrow behavior. shows The the angle onshorα ise thedirection wind directionof sediment (with transport respect towards to the north) the dunes and the and angle is used ddj isto theexplain difference dune betweenbehavior. the The wind angle direction α is theand wind coastal direction orientation (with respect O. to the north) and the angle ddj is the difference between the wind direction and coastal orientation O. 4. Results and Discussion 4. Results and Discussion 4.1. Spatial–Temporal Variability in Dune Volume Changes: Linear Trends 4.1. Spatial–TemporalAs an illustrative Variability example of in an Dune analysis Volume at a Changes: natural duneLinear system, Trends Figure 9 shows the dune volume at section 50 of the Belgian coast. It is found that dune volume increases to a good approximation at a As an illustrative example of an analysis at a natural dune system, Figure 9 shows the dune constant rate in time (8.14 m3/m/year with a correlation of 0.99). To assess if this trend is valid for all volume at section 50 of the Belgian coast. It is found that dune volume increases to a good coastal dunes, the decadal trend in the calculation of dune volume changes is based on linear regression approximation at a constant rate in time (8.14 m³/m/year with a correlation of 0.99). To assess if this analysis. Fitting linear trendlines for all the coastal sections can show to what extent this linearity in trend is valid for all coastal dunes, the decadal trend in the calculation of dune volume changes is time is valid for the entire Belgian coastline. Linear trends are calculated for the entire period between based on linear regression analysis. Fitting linear trendlines for all the coastal sections can show to 1979–2018 and/or for the most recent trend. The correlation coefficient was calculated for all dune what extent this linearity in time is valid for the entire Belgian coastline. Linear trends are calculated sections experiencing dune growth or dune erosion. Figure 10 shows that higher correlation coefficients for the entire period between 1979–2018 and/or for the most recent trend. The correlation coefficient occur more for dune growth than for dune erosion. It is found that 80% of all coastal dunes with dune was calculated for all dune sections experiencing dune growth or dune erosion. Figure 10 shows that growth (total of 93%) have correlation coefficients higher than or equal to 0.9, which yields an overall higher correlation coefficients occur more for dune growth than for dune erosion. It is found that 80% average dune growth of 6.20 m3/m/year. This percentage decreases for smaller correlation coefficients. of all coastal dunes with dune growth (total of 93%) have correlation coefficients higher than or equal Similar observations are found by de Vries et al. [4]. For the negative rates of dune volume change (7% to 0.9, which yields an overall average dune growth of 6.20 m³/m/year. This percentage decreases for of all coastal dunes), the occurrence of a lower correlation coefficient increases. These calculations show smaller correlation coefficients. Similar observations are found by de Vries et al. [4]. For the negative that a substantial part of Belgian coastal dunes is excellently represented using a linear dune growth rates of dune volume change (7% of all coastal dunes), the occurrence of a lower correlation coefficient model in time. Figure 11 indicates that the locations of the coastal dunes, with correlation coefficients increases. These calculations show that a substantial part of Belgian coastal dunes is excellently larger than 0.9, are distributed over the total stretch of the Belgian coast. Figure 11 also shows the represented using a linear dune growth model in time. Figure 11 indicates that the locations of the annual dune development per dune section for the entire Belgian coastline. It is observed that in coastal dunes, with correlation coefficients larger than 0.9, are distributed over the total stretch of the most areas, dunes are growing. The annual dune growth ranges between 0–12.3 m3/m/year, based on Belgian coast. Figure 11 also shows the annual dune development per dune section for the entire observations between 1979–2018. Dune growth rates at the Holland coast have been calculated in the Belgian coastline. It is observed that in most areas, dunes are growing. The annual dune growth order of 0–40 m3/m/year, which is in the same order of magnitude [4]. ranges between 0–12.3 m³/m/year, based on observations between 1979–2018. Dune growth rates at the Holland coast have been calculated in the order of 0–40 m³/m/year, which is in the same order of magnitude [4].

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J. Mar.In Sci. some Eng. beach 2019, 7 sections, 357 along the Belgian coast, management activities are carried out which10 have of 25 J. Mar. Sci. Eng. 2019, 7, 357 10 of 25 a direct influence on dune development. In the following paragraphs, some typical management strategies and their effect on dune behavior are described in more detail.

Figure 9. Example of dune volume in time combined with a linear fit (correlation of determination R² FigureFigure 9. 9.Example Example of of dune dune volume volume in in time time combinedcombined withwith aa linear fitfit (correlation of of determination determination R² =2 0.99) for coastal section 50 (natural dunes). R == 0.99)0.99) for for coastal coastal section section 50 50 (natural (natural dunes). dunes).

FigureFigure 10. 10.Density Density of of occurrence occurrence of of linear linear dune dune behavior. behavior. Of Of the the dune dune sections, sections, 80% 80% show show correlation correlation Figure 10. Density of occurrence of linear dune behavior. Of the dune sections, 80% show correlation coecoefficientsfficients larger larger than than or or equal equal to to 0.9. 0.9. coefficients larger than or equal to 0.9.

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J. Mar. Sci. Eng. 2019, 7, 357 11 of 24

Figure 11. Dune behavior along the Belgian coast for all sections with vegetated dunes (approximately Figure 11. Dune behavior along the Belgian coast for all sections with vegetated dunes (approximately half of the coast). Red bars indicate the places where the correlation coefficient of the linear trend half of the coast). Red bars indicate the places where the correlation coefficient of the linear trend analysis is higher than or equal to 0.9. The red dashed line represents the average annual dune growth analysis is higher than or equal to 0.9. The red dashed line represents the average annual dune growth for correlations higher than or equal to 0.9. for correlations higher than or equal to 0.9. 4.2. Typical Management Strategies and Their Effect on Dune Behavior In some beach sections along the Belgian coast, management activities are carried out which have4.2.1. a Influence direct influence of Dune on Foot dune Reinforcement development. In the following paragraphs, some typical management strategies and their effect on dune behavior are described in more detail. Figure 12 shows dune volume changes between 1983 and 2018 for section 8. In the past, section 8, close to the French border, experienced intense dune erosion during storm events. For coastal safety, 4.2. Typical Management Strategies and Their Effect on Dune Behavior a dune foot reinforcement was constructed (concrete dyke) in the period 1976–1979. In 1990, this 4.2.1.concrete Influence dyke was of Dune extended Foot onReinforcement the east side after a severe storm surge. Massive erosion in the coastal zone was caused by heavy storms at the beginning of 1990 and storms in the period 1993–1994. In a partFigure of section 12 shows 8, a concrete dune volume wall was changes erected between in 1994 1983 above and the 2018 dune for foot section reinforcement 8. In the past, (Figure section 13). 8,Sand close was to alsothe appliedFrench border, above the experienced dune foot fromintense the dune low-water erosion line. during Sand storm was again events. supplied For coastal after safety,the storms a dune of 1–2foot January reinforcement 1995, 19 was February constructed 1996, (concrete 29 August dyke) 1996 in and the 29 period October 1976–1979. 1996. Due In to1990, the thisinfluence concrete of dune dyke foot was reinforcement, extended on the the east dune side volume after ofa severe section storm 8 does surge. not experience Massive erosion a linear in trend the coastalin time. zone The naturalwas caused behavior by heavy of the storms dune has at beenthe beginning completely of blocked. 1990 and As storms such, dunein the reinforcements period 1993– 1994.are not In suitable a part of for section climate-resilient 8, a concrete flood wall protection. was erected in 1994 above the dune foot reinforcement (Figure 13). Sand was also applied above the dune foot from the low-water line. Sand was again supplied after the storms of 1–2 January 1995, 19 February 1996, 29 August 1996 and 29 October 1996. Due to the influence of dune foot reinforcement, the dune volume of section 8 does not experience a linear trend in time. The natural behavior of the dune has been completely blocked. As such, dune reinforcements are not suitable for climate-resilient flood protection.

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Figure 12. Decadal dune evolution of the coastal dunes at section 8. FigureFigure 12. 12.Decadal Decadal dune dune evolution evolution of of the the coastal coastal dunes dunes at sectionat section 8. 8.

Figure 13. Concrete revetment and wall, built to reinforce the dune foot in section 8. Figure 13. Concrete revetment and wall, built to reinforce the dune foot in section 8. Figure 13. Concrete revetment and wall, built to reinforce the dune foot in section 8. 4.2.2. Effect of Dune Blowouts 4.2.2. Effect of Dune Blowouts 4.2.2.Since Effect the of airborne Dune Blowouts surveys started in 1983, the dunes of section 36 experienced a gradual linear Since the airborne surveys started in 1983, the dunes of section 36 experienced a gradual linear growthSince in time the due airborne to aeolian surveys sand started input in from 1983, the the beach dunes (Figure of section 14). In36 theexperienced beginning a ofgradual the 1990s, linear growth in time due to aeolian sand input from the beach (Figure 14). In the beginning of the 1990s, dunegrowth erosion in time started due to to occur aeolian due sand to the input formation from th ofe beach a dune (Figure blowout 14). (Figure In the 15beginning). The formation of the 1990s, of dune erosion started to occur due to the formation of a dune blowout (Figure 15). The formation of dunedune blowouts erosion tendsstarted to to start occur at patchesdue to the of bareformation sand, whereof a dune vegetation blowout does (Figure not grow.15). The Wind formation speeds of dune blowouts tends to start at patches of bare sand, where vegetation does not grow. Wind speeds abovedune the blowouts threshold tends erodes to start into at those patches patches, of bare blowing sand, the where sand vegetation out of the system.does not Note grow. that Wind the sandspeeds above the threshold erodes into those patches, blowing the sand out of the system. Note that the sand isabove feeding the the threshold dunes further erodes inland. into those Because patches, they blowin are notg the covered sand out by theof the LiDAR system. measurement, Note that the this sand is feeding the dunes further inland. Because they are not covered by the LiDAR measurement, this effisect feeding is not captured.the dunes These further blowouts inland. oftenBecause occur they on are stabilized not covered vegetated by the dunes. LiDAR Dune measurement, erosion was this effect is not captured. These blowouts often occur on stabilized vegetated dunes. Dune erosion3 was highesteffect betweenis not captured. 2000–2005. These Since blowouts 2005, the often intensity occur ofon dune stabilized erosion vegetated has decreased dunes. to Dune 7.26 merosion/m/year. was highest between 2000–2005. Since 2005, the intensity of dune erosion has decreased to 7.26 m³/m/year. highest between 2000–2005. Since 2005, the intensity of dune erosion has decreased to 7.26 m³/m/year.

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Figure 14. Decadal dune evolution of the coastal dunes at section 36. The solid magenta line indicates Figure 14. Decadal dune evolution of the coastal dunes at section 36. The solid magenta line indicates theFigure recent 14. dune Decadal trend. dune evolution of the coastal dunes at section 36. The solid magenta line indicates the recent dune trend. the recent dune trend.

Figure 15. Dune blowouts at sections 36 to 38. Figure 15. Dune blowouts at sections 36 to 38. 4.2.3. Influence of Dune Foot ProtectionFigure 15. Dune Measures blowouts at sections 36 to 38. 4.2.3. Influence of Dune Foot Protection Measures Figure 16 shows dune volume changes between 1983 and 2018 for section 72. In section 72 and a 4.2.3. Influence of Dune Foot Protection Measures part ofFigure section 16 shows 73, a dune dune foot volume protection changes measure between was 198 constructed3 and 2018 infor 1944 section in the 72. form In section of a concrete 72 and walla part (FigureFigure of section 1617 shows), 73, because a dune dune this foot volume area protection experienced changes measure between severe was 198 dune constructed3 and erosion. 2018 infor As1944 section an in indication, the 72. form In section of the a concrete location 72 and wallofa part the (Figure duneof section foot17), of73,because the a dune adjacent this foot area sectionprotection experienced (section measure severe 71) iswas approximatelydune constructed erosion. Asin 25 1944an m furtherindication, in the inlandform the of oflocation a theconcrete wall. of theSectionwall dune (Figure 72 foot experienced 17), of thebecause adjacent fairly this linear areasection experienced dune (section growth 71) severe until is approximately 1993.dune Majorerosion. dune 25 As m erosionan further indication, happened inland the of location due the towall. the of Sectionheavythe dune storms 72 footexperienced inof thethe 1990s.adjacent fairly Afterwards, linear section dune (section thegrowth dunes 71) until is experienced approximately 1993. Major slow dune 25 dune merosion further erosion happened inland or were of due basicallythe to wall. the heavystableSection asstorms 72 the experienced dune in the foot 1990s. couldfairly Afterwards, notlinear grow dune further the growth dunes seawards. until experienced 1993. Primary Major slow embryodune dune erosion duneerosion growthhappened or were is di due basicallyfficult to the to stabledevelopheavy as storms atthe the dune in dune the foot foot1990s. could and Afterwards, not most grow volume further the variations dunes seaw experiencedards. are Primary generally slow embryo found dune indune erosion the growth dune or footwere is difficult region. basically to developstable as at the the dune dune foot foot could and mostnot grow volume further variatio seawnsards. are Primarygenerally embryo found indune the growthdune foot is difficultregion. to develop at the dune foot and most volume variations are generally found in the dune foot region.

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Figure 16. DecadalDecadal dune dune evolution of the coastal dunes at section 72. The The solid solid magenta magenta line indicates Figure 16. Decadal dune evolution of the coastal dunes at section 72. The solid magenta line indicates the recent dune trend. the recent dune trend.

Figure 17. ConcreteConcrete wall wall as a dune foot protection measure in section 72. Figure 17. Concrete wall as a dune foot protection measure in section 72. 4.2.4. Combined Combined Influence Influence of Nourishments and Brushwood Fences 4.2.4. Combined Influence of Nourishments and Brushwood Fences Section 160160 (Figure (Figure 18 ),18), located located 41 km 41 from km the from French the border, French was border, fairly stablewas sincefairly measurements stable since measurementsstarted.Section However, 160 started. (Figure the heavyHowever, 18), spring located the storms heavy 41 spring ofkm 1990 from st hadorms the particularly of French 1990 had border, a ffparticularlyected thewas coastal affectedfairly dunesstable the coastal insince this dunespartmeasurements of in the this Belgian part started. of coast. the However, Belgian Due to thosethecoast. heavy storms, Due spring to large-scalethose storms storms, of beach 1990 large-scale had nourishments particularly beach werenourishments affected needed. the coastal Thesewere worksneeded.dunes werein These this carried partworks of out werethe during Belgian carried the coast. periodout during Due of 1992–2000 to the those period storms, and of attributed1992–2000 large-scale to and the beach attributed creation nourishments of to wide the creation beaches were ofandneeded. wide by thebeachesThese subsequent works and by were the plantation carriedsubsequent out of brushwood duringplantation the peof fences riodbrushwood (Figureof 1992–2000 fenc19).es Theand (Figure wideningattributed 19). The ofto widening thethe beachescreation of 3 theenhancedof wide beaches beaches aeolian enhanced and sand by beingaeolian the subseq blown sanduent to being the plantation dunes, blown causing ofto brushwood the a dunes, linear growthfenc causinges (Figure of a 12.18 linear 19). m /growthThem/year, widening whichof 12.18 of is wellm³/m/year,the beaches above thewhich enhanced average is well aeolian dune above growth sand the rate.beingaverage The blown dune sand to trappinggrowth the dunes, rate. efficiency causingThe sand of brushwooda lineartrapping growth fencesefficiency ofvaries 12.18 of brushwoodfromm³/m/year, site to sitefenceswhich and variesis is well particularly from above site the highto siteaverage if theand availability isdune particularly growth of sand highrate. is if largeThe the sandavailability [46]. trapping of sandefficiency is large of [46].brushwood fences varies from site to site and is particularly high if the availability of sand is large [46].

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Figure 18. Decadal dune evolution of the coastal dunes at section 160. The solid magenta line indicates Figure 18. Decadal dune evolution of the coastal dunes at section 160. The solid magenta line indicates theFigure recent 18. dune Decadal trend. dune evolution of the coastal dunes at section 160. The solid magenta line indicates the recent dune trend. the recent dune trend.

Figure 19. TheThe dunes at section 160, showing the plantation of brushwood fences. 4.2.5. InfluenceFigure of Excavation 19. The dunes Works at section 160, showing the plantation of brushwood fences. 4.2.5. Influence of Excavation Works 4.2.5.At Influence the Dutch of border, Excavation section Works 254 is the last section of the Belgian coast. In section 254, the tidal At the Dutch border, section 254 is the last section of the Belgian coast. In section 254, the tidal channel ‘the Zwingeul’ is present (Figure 20). Because of its free movement, this section has large channelAt ‘the the Zwingeul’Dutch border, is present section (Figure 254 is the 20). last Because section of of its the free Belgian movement, coast. Inthis section section 254, has the large tidal annual fluctuations in sand beach volume. The dunes grow fairly linear in time until the year 2016. annualchannel fluctuations ‘the Zwingeul’ in sand is beachpresent volume. (Figure The 20). dunes Because grow of itsfairly free linear movement, in time thisuntil section the year has 2016. large In the year 2016, large dune erosion occurred in this section because of large excavation works. Since Inannual the year fluctuations 2016, large in dune sand erosio beachn occurredvolume. Thein this dunes section grow because fairly oflinear large in excavation time until works.the year Since 2016. then, the dunes are continuously eroding at a rate of –12.84 m3/m/year (Figure 21). then,In the the year dunes 2016, are large continuously dune erosio erodingn occurred at a rate in this of –12.84 section m³/m/year because of (Figure large excavation 21). works. Since then, the dunes are continuously eroding at a rate of –12.84 m³/m/year (Figure 21).

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Figure 20. Tidal channel ‘the Zwingeul’ (sections 254 and 255). Figure 20. Tidal channel ‘the Zwingeul’ (sections 254 and 255).

Figure 21. Decadal dune evolution of the coastal dunes at section 254. The solid magenta line indicates Figure 21. Decadal dune evolution of the coastal dunes at section 254. The solid magenta line indicates the recent dune trend. Figurethe recent 21. Decadal dune trend. dune evolution of the coastal dunes at section 254. The solid magenta line indicates 4.3. Potentialthe recent Aeolian dune trend. Sediment Transport 4.3. Potential Aeolian Sediment Transport 4.3. PotentialIt is well Aeolian known Sediment that dune Transport growth is primarily governed by aeolian sediment processes. Therefore, expectationsIt is well are known that dune that volumedune growth variability is prim doesarily correlate governed with by variability aeolian insediment wind conditions. processes. It is well known that dune growth is primarily governed by aeolian sediment processes. PotentialTherefore, aeolian expectations sediment are transport that dune is calculated volume variability per coastal does dune correlate section between with variability the dates wherein wind a Therefore, expectations are that dune volume variability does correlate with variability in wind LiDARconditions. flight Potential is conducted aeolian using sediment the representative transport is median calculated grain per size coastal and coastal dune orientation.section between Hourly the conditions. Potential aeolian sediment transport is calculated per coastal dune section between the winddates data where is available a LiDAR since flight the is year conducted 2000 at Ostend using Airportthe representative (middle of themedian coastline grain and size approximately and coastal dates where a LiDAR flight is conducted using the representative median grain size and coastal 1orientation. km inland) andHourly is used wind to calculatedata is available the time seriessince ofthe potential year 2000 transport. at Ostend The Airport wind sensor (middle is located of the orientation. Hourly wind data is available since the year 2000 at Ostend Airport (middle of the 4coastline m above and the surface.approximately Figure 122 km shows inland) the measuredand is used wind to datacalculate at the the weather time series station of of potential Ostend coastline and approximately 1 km inland) and is used to calculate the time series of potential Airporttransport. for The the periodwind sensor 2000–2017 is located in the 4 form m above of a windthe surface. rose. The Figure red line22 shows represents the measured the mean wind direction data transport. The wind sensor is located 4 m above the surface. Figure 22 shows the measured wind data ofat thethe Belgianweather coastline.station of Ostend Measured Airport over for that the period, period a2000–2017 large west in tothe southwest form of a componentwind rose. The of thered at the weather station of Ostend Airport for the period 2000–2017 in the form of a wind rose. The red windline represents is found. the West mean to southwestern direction of the winds Belgian are obliquecoastline. onshore Measured to longshore over that period, with respect a large to west the line represents the mean direction of the Belgian coastline. Measured over that period, a large west Belgian coastline.

J. Mar. Sci. Eng. 2019, 7, 357 17 of 25

to southwest component of the wind is found. West to southwestern winds are oblique onshore to J. Mar. Sci. Eng. 2019, 7, 357 17 of 24 longshore with respect to the Belgian coastline.

Figure 22. Measured wind speed (Ws) and wind direction at Ostend Airport weather station for the periodFigure 2000–2017.22. Measured The wind red line speed represents (Ws) and the wind Belgian direction coastline at Ostend direction. Airport The potential weather transportstation for drift the isperiod also given. 2000–2017. The red line represents the Belgian coastline direction. The potential transport drift is also given. Figure 23 shows the annual potential transport and transport direction for the period 2000–2017 derivedFigure from 23 measurements shows the annual at Ostend potential Airport transport weather and stationtransport based direction on a grain for the size period of 310 2000–2017µm and coastalderived orientation from measurements of 57◦ (section at Ostend 103). Figure Airport 23 indicatesweather station considerable based temporalon a grain variability, size of 310 caused µm and by annualcoastal variationsorientation in of wind 57° (section climate. 103). Annual Figure variations 23 indicates are approximatelyconsiderable temporal between variability, 0.5 times and caused 1.6 timesby annual the mean variations transport in wind rate, climate. meaning Annual that potential variations transport, are approximately occasionally, between is three 0.5 times times larger and for1.6 sometimes years the mean than transport other years. rate, The meaning average that direction potential of potential transport, transport occasionally, over the is three period times of 17 larger years for is 260some◦ with years a standardthan other deviation years. The of 14average◦, implying direction that of the potential direction transport is constant. over Potential the period transport of 17 years has anis 260° oblique with onshore a standard character deviation with respectof 14°, implying to the coastline that the (Figure direction 22). is The constant. larger parallel Potential component transport (longshore)has an oblique of the onshore potential character transport with drift isrespect directed to towardsthe coastline the northeast (Figure (the22). Netherlands),The larger parallel while thecomponent normal component (longshore) (onshore) of the ispote directedntial transport towards thedrift southeast. is directed towards the northeast (the Netherlands),Interpreting while the the results normal on component a decadal timescale, (onshore) allis directed potential towards transport the ratessoutheast. are cumulatively summed. Potential dune volume changes because of aeolian sediment transport appears to vary linearly in time with a constant rate, which in return explains the decadal linear dune growth (Figure 24). Considering all coastal dune sections within the period of 2000–2017, Figure 25 indicates that onshore potential transport maximum ranges to 9 m3/m/year (average = 5.2 m3/m/year), while longshore potential transport could reach up to a maximum of 20 m3/m/year (average = 18.7 m3/m/year), assuming dry beach sand. In return, this gives a mean total potential transport of approximately 20 m3/m/year. Longshore transport is relatively constant along the coastline (4.9% deviation of the mean). Onshore transport has some longshore variations due to a varying coastline orientation, especially behind the protruding point of Wenduine (43 km), where transport is lower due to a more north–northeast coastal orientation. It also implies that longshore transport is almost three to four times higher than onshore transport. This would explain why brushwood fences are so efficient for enhancing dune growth locally.

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FigureFigure 23. (A 23.) Annual A) Annual potential potential longshore longshore transport transport for fo ther the period period 2000-2017. 2000-2017. B) (B )Annual Annual potential potential cross-shorecross-shore (or normal) (or normal) transport transport for for the the period period 2000-2017. 2000-2017. Positive Positive is offshore offshore transport, transport, negative negative is is onshoreonshore transport. transport. (C) Annual C) Annual potential potential transport transport (PT) (PT) for for the the period period 2000–2017. 2000–2017. All All transport transport rates rates are J. Mar.given Sci. Eng. inare m given 20193/m,/ year.in7, 357m³/m/year. (D) The D) angle The angle of potential of potential transport transport is is fairly fairly constant constant withwith an an average average value value of of19 of 25 260 degrees260 degrees to the to north. the north.

Interpreting the results on a decadal timescale, all potential transport rates are cumulatively summed. Potential dune volume changes because of aeolian sediment transport appears to vary linearly in time with a constant rate, which in return explains the decadal linear dune growth (Figure 24). Considering all coastal dune sections within the period of 2000–2017, Figure 25 indicates that onshore potential transport maximum ranges to 9 m³/m/year (average = 5.2 m³/m/year), while longshore potential transport could reach up to a maximum of 20 m³/m/year (average = 18.7 m³/m/year), assuming dry beach sand. In return, this gives a mean total potential transport of approximately 20 m³/m/year. Longshore transport is relatively constant along the coastline (4.9% deviation of the mean). Onshore transport has some longshore variations due to a varying coastline orientation, especially behind the protruding point of Wenduine (43 km), where transport is lower due to a more north–northeast coastal orientation. It also implies that longshore transport is almost three to four times higher than onshore transport. This would explain why brushwood fences are so efficient for enhancing dune growth locally.

FigureFigure 24. 24.Decadal Decadal evolution evolution of of potential potential dune dune growth growth at at section section 50 50 of of the the Belgian Belgian coast. coast.

Figure 25. Annual potential longshore and onshore aeolian sediment transport, based on the period between 2000–2017.

4.4. Correlation Between Potential Transport and Dune Volume Changes

4.4.1. Correlation on Annual Timescales More wind induces more aeolian sediment transport and intuitively, therefore, more dune growth is expected [4]. To find a correlation between variation in potential transport and dune volume change on an annual timescale, the Pearson correlation coefficient is calculated. In this procedure, three methods are applied to calculate the correlation coefficient: • Method 1: The first method compares dune erosion and dune growth with corresponding potential transport. Net potential transport is calculated based on the full wind rose (onshore and offshore winds).

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Figure 24. Decadal evolution of potential dune growth at section 50 of the Belgian coast. J. Mar. Sci. Eng. 2019, 7, 357 19 of 24

FigureFigure 25. 25.Annual Annual potential potential longshore longshore and and onshore onshore aeolian aeolian sediment sediment transport, transport, based based on on the the period period betweenbetween 2000–2017. 2000–2017. 4.4. Correlation between Potential Transport and Dune Volume Changes 4.4. Correlation Between Potential Transport and Dune Volume Changes 4.4.1. Correlation on Annual Timescales 4.4.1. Correlation on Annual Timescales More wind induces more aeolian sediment transport and intuitively, therefore, more dune growth is expectedMore [4wind]. To findinduces a correlation more aeolian between sediment variation tran insport potential and transport intuitively, and therefore, dune volume more change dune ongrowth an annual is expected timescale, [4]. theTo Pearsonfind a correlation correlation between coefficient variation is calculated. in potential In this transport procedure, and threedune methodsvolume arechange applied on toan calculate annual timescale, the correlation the Pearso coefficient:n correlation coefficient is calculated. In this procedure, three methods are applied to calculate the correlation coefficient: Method 1: The first method compares dune erosion and dune growth with corresponding potential •• Method 1: The first method compares dune erosion and dune growth with corresponding transport.potential Nettransport. potential Net transport potential is transport calculated is based calculated on the based full wind on the rose full (onshore wind rose and o(onshoreffshore winds).and offshore winds). Method 2: The second method compares dune erosion and dune growth with corresponding • potential transport based solely on onshore winds. It is assumed here that offshore winds do not extract sediment from the dune seawards. As mentioned before, dunes are, besides aeolian processes, also influenced by marine erosive processes. Method 3: The third and last method compares only the periods with dune growth with the • corresponding potential transport based solely on onshore winds. Offshore winds are not used in the calculation procedure.

A positive correlation implies that an increase in potential transport causes an increase in dune volume. A negative correlation implies the opposite. However, the three methods gave similar results. No significant correlation (positive or negative) is found between potential transport and dune volume change on an annual timescale, considering all sections with coastal dunes. The poor correlation could be attributed to:

According to Equation (1), potential aeolian sediment transport is proportional to the cube of shear • velocity, meaning that high winds above the threshold contribute exponentially to the annual sum of potential transport (Equation (4)). Although these winds are able to transport large amounts of sediment, they tend to create high water levels and long wave run-ups as well. J. Mar. Sci. Eng. 2019, 7, 357 20 of 24

Between consecutive LiDAR flights, dune erosion and dune growth can occur. Even during strong • aeolian transport events, a single dune-erosion event can undo any dune growth that happened before that storm. The period between two consecutive LiDAR flights is also too long, resulting in a coarse • spatial–temporal data set. Dune volume changes can be influenced by the sediment supply and sediment availability from • the marine zone [4,16]. Transport limiting factors, such as sediment size and distribution, beach J. Mar.geometry Sci. Eng. 2019 and, 7 moisture, 357 can be of more importance than the driving wind speed. 21 of 25

4.4.2. Correlation on Decadal Timescales valuesDecadal are higher timescales than the are observed of interest values. because When it is the the managed most appropriate zones are timescaleexcluded from for engineering the data set, purposesthe best andfit line coastline slope development.is approximately On decadalone, and timescales, the RMSE potential (Root Mean dune Square volume Error) appears value to grow is 2.9 linearlym³/m/year. with Most time, (90%) with aof similar the predicted magnitude data to are the with observedin a factor dune of volumetwo of the (see measured Section 4.3 values.). It is This of particularis a high interestscore, given if both all are variabilities correlated and at this non-uniformities spatial–temporal involved scale. For in aeolian all coastal processes dune sections, (wind field, the decadalsand composition, trend of potential bed relief, dune volumesurface changesroughness). is based The onfinding linear analysis.of potentially Linearity stronger is calculated correlations for thecompared entire period to the between literature 2000–2017. (e.g., [4,18]) It is at found the Belgian that in 93%coast of are all coastalmost likely dune caused sections, by thethe potential generally dunewider volume beaches trend (between has a correlation 150 m and higher 400 thanm). Keijsers 0.90. et al. [18] also found stronger correlations betweenFigure the 26 timeshows series the comparisonof potential betweensediment observed transport and and predicted dune volume dune on development wider beaches on decadal(>200 m) timescales.at the Dutch The coast. yellow This dots indicates show the placesthat natural were regular dune behavior management can activitiesbe predicted are carried with a out. reasonable About 75%accuracy of all predictedon decadal data time arescales, within and a factor it suggests of two that of the annual measured differences values. in Interpreting forcing and the transport results showlimiting that conditions the modified (wind Bagnold speed model and basedsurface on conditio Equationns) (1) only [38 ]have yields a aslight good effect performance on the whenoverall observedvariability values of dune are comparedvolume trends. to predicted dune development rates.

Figure 26. Comparison between observed and predicted linear dune development. Diagonal lines Figure 26. Comparison between observed and predicted linear dune development. Diagonal lines represent the one-to-one correspondence. Red dashed lines show the factor of two variance. Yellow represent the one-to-one correspondence. Red dashed lines show the factor of two variance. Yellow dots represent the locations where managing activities are carried out. dots represent the locations where managing activities are carried out. Figure 27 shows the alongshore extent of the observed and predicted dune development rates along the Belgian coastline. The variability in potential transport is well correlated to the variability

J. Mar. Sci. Eng. 2019, 7, 357 21 of 24 in dune volume changes at the considered spatial–temporal scale. The general variability, on a decadal timescale, between observed and predicted rates could be partly attributed to the management activities at certain sections along the coast. It is uncertain how many of the dune regions are managed. An attempt has been made to know which dune regions are managed. Based on historical images from Google Earth, approximately 50% of the coastal dunes are managed. Most of these activities include regular plantation of brushwood fences, especially where observed linear dune behavior is higher than the predicted values (under the line of perfect agreement). Other discrepancies include the ones discussed in Section 4.4.1. The locations including dune foot protection measures, dune foot reinforcements, and dune blowouts, are mostly the locations where predicted values are higher than the observed values. When the managed zones are excluded from the data set, the best fit line slope is approximately one, and the RMSE (Root Mean Square Error) value is 2.9 m3/m/year. Most (90%) of the predicted data are within a factor of two of the measured values. This is a high score, given all variabilities and non-uniformities involved in aeolian processes (wind field, sand composition, bed relief, surface roughness). The finding of potentially stronger correlations compared to the literature (e.g., [4,18]) at the Belgian coast are most likely caused by the generally wider beaches (between 150 m and 400 m). Keijsers et al. [18] also found stronger correlations between the time series of potential sediment transport and dune volume on wider beaches (>200 m) at the Dutch coast. This indicates that natural dune behavior can be predicted with a reasonable accuracy on decadal timescales, and it J. Mar. Sci. Eng. 2019, 7, 357 22 of 25 suggests that annual differences in forcing and transport limiting conditions (wind speed and surface conditions) only have a slight effect on the overall variability of dune volume trends.

Het Zwin Zwin Het Zeebrugge Zeebrugge

Nieuwpoort De Haan Wenduine Blankenberge Middelklerke Middelklerke Oostende De Panne

Figure 27. Annual predicted and observed dune volume change along the Belgian coast on aFigure decadal 27. timescale. Annual predicted Red and and black observed bars dune indicate volume the observedchange along and the predicted Belgian coast values on ofa decadal dune development,timescale. Red respectively. and black bars indicate the observed and predicted values of dune development, respectively. 5. Conclusions 5. ConclusionsLong-term temporal and spatial dune trends along the Belgian coast are analyzed using a data set of annualLong-term and bi-annual temporal beach–dune and spatial LiDAR dune trends surveys, alon conductedg the Belgian between coast 1979–2018.are analyzed Furthermore, using a data potentialset of aeolianannual sedimentand bi-annual transport beach–dune along the Belgian LiDAR coast surveys, is calculated conducted on the basisbetween of a wind1979–2018. data setFurthermore, from 2000–2017. potential On the aeolian basis sedi of thisment analysis transport we concludealong the that: Belgian coast is calculated on the basis of a wind data set from 2000–2017. On the basis of this analysis we conclude that: 1. Along the Belgian coast, concerning the coastal sections with vegetated dunes (approximately half of the coast), it is found that the dunes grow at a constant rate. Linear regression analysis shows that 80% of the dune sections have linear correlations higher than 0.9. There is alongshore variability in linear dune growth rates at the Belgian coast and they are found to be in the order of 0–12.3 m³/m/year. An average dune growth of 6.2 m³/m/year has been found. 2. Considering all coastal dune sections within the period of 2000–2017, onshore potential aeolian sediment transport ranges up to 9 m³/m/year (average = 5.2 m³/m/year), while longshore potential aeolian sediment transport could reach up to 20 m³/m/year (average = 18.7 m³/m/year). This means that total potential transport along the Belgian coastline is, on average, 20 m³/m/year. The main direction of aeolian sediment transport on the Belgian coast is from west to southwest. West to southwestern winds are oblique onshore to longshore with respect to the Belgian coastline. The larger parallel component (longshore) of the potential transport drift is directed towards the northeast (the Netherlands), while the normal component (onshore) is directed towards the southeast (hinterland). 3. There was no significant relationship between annual wind and dune volume change in the alongshore direction. However, a significant correlation is found between potential and observed dune volume development on a decadal timescale, indicating that dune growth is primarily caused by aeolian sediment transport from the beach. Most of the predicted data are within a factor of two of the measured values. The finding of potentially stronger correlations at the Belgian coast are most likely caused by the wider beaches (between 150 m and 400 m) due

J. Mar. Sci. Eng. 2019, 7, 357 22 of 24

1. Along the Belgian coast, concerning the coastal sections with vegetated dunes (approximately half of the coast), it is found that the dunes grow at a constant rate. Linear regression analysis shows that 80% of the dune sections have linear correlations higher than 0.9. There is alongshore variability in linear dune growth rates at the Belgian coast and they are found to be in the order of 0–12.3 m3/m/year. An average dune growth of 6.2 m3/m/year has been found. 2. Considering all coastal dune sections within the period of 2000–2017, onshore potential aeolian sediment transport ranges up to 9 m3/m/year (average = 5.2 m3/m/year), while longshore potential aeolian sediment transport could reach up to 20 m3/m/year (average = 18.7 m3/m/year). This means that total potential transport along the Belgian coastline is, on average, 20 m3/m/year. The main direction of aeolian sediment transport on the Belgian coast is from west to southwest. West to southwestern winds are oblique onshore to longshore with respect to the Belgian coastline. The larger parallel component (longshore) of the potential transport drift is directed towards the northeast (the Netherlands), while the normal component (onshore) is directed towards the southeast (hinterland). 3. There was no significant relationship between annual wind and dune volume change in the alongshore direction. However, a significant correlation is found between potential and observed dune volume development on a decadal timescale, indicating that dune growth is primarily caused by aeolian sediment transport from the beach. Most of the predicted data are within a factor of two of the measured values. The finding of potentially stronger correlations at the Belgian coast are most likely caused by the wider beaches (between 150 m and 400 m) due to the massive sand nourishments to keep the sediment budgets along the Belgian coast positive. It also suggests that annual differences in forcing and transport limiting conditions (wind speed and surface conditions) only have a slight effect on the overall variability of dune volume trends.

Author Contributions: G.S. conceived the overall concept and approach of this study. G.S. lead the design and development of the methodology and calculated decadal dune trends and potential aeolian transport. R.H. calculated dune volume from the airborne surveys. G.S. prepared the original draft of the manuscript with input from all co-authors. R.H. and P.R. also contributed to the manuscript revisions and editing until the final version was created. P.R. supervised the entire study. Funding: This publication was made possible through funding support of the KU Leuven Fund for Fair Open Access. Acknowledgments: This publication is the result of research part of the project CREST (Climate REsilient coaST), funded by the Strategic Basic Research (SBO) program of the Flanders Innovation and Entrepreneurship. We thank the support of Coastal Division of the Flemish Government, Department of Mobility and Public Works, to make it possible to use the data of dune volume and wind. Conflicts of Interest: The authors declare no conflict of interest. The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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