Recent Coastal Dune Development: Effects of Sand ... - BioOne

Journal of Coastal Research

28

3

587–601

West Palm Beach, Florida

May 2012

Recent Coastal Dune Development: Effects of Sand Nourishments Marcel A.J. Bakker{, Sytze van Heteren{, Laura M. Vonho¨gen{, Ad J.F. van der Spek{, and Bert van der Valk1 {

TNO Geological Survey of the Netherlands P.O. Box 80015, NL-3508 TA Utrecht, The Netherlands [email protected]

{

Deltares Unit Subsurface and Groundwater Systems P.O. Box 85467, NL-3508 AL Utrecht, The Netherlands

www.cerf-jcr.org

1

Deltares Unit Marine and Coastal Systems P.O. Box 177, NL-2600 MH, Delft, The Netherlands

ABSTRACT ¨ GEN, L.M.; VAN DER SPEK, A.J.F., and VAN DER VALK, L., 2012. Recent BAKKER, M.A.J.; VAN HETEREN, S.; VONHO coastal dune development: effects of sand nourishments. Journal of Coastal Research, 28(3), 587–601. West Palm Beach (Florida), ISSN 0749-0208. Much of the Dutch coast has been subject to structural erosion. From 1990 onward, sand nourishments have been used under a government policy of dynamic preservation. Annual monitoring and field inspections show that the structural erosion has decreased or even turned into coastal progradation after 1990. The monitoring data concern only morphodynamics and thus supply limited information on system-related geological processes driving the observed changes. Recently acquired ground-penetrating radar (GPR) data help establish the origin of sedimentary elements within the beach-foredune area, determine their decadal-scale preservation potential under the present nourishment policy, and demonstrate temporal and spatial accretion/erosion variability along nourished coasts. GPR images from a nonnourished retrograding barrier section show historical storm surge deposits within the eroding foredune and accumulations of natural eolian sediment farther landward. GPR images from a heavily nourished, prograding site show that the accreted foredune and beach consist of nourishment embankments (20%), wind-blown units derived from nourished sand (70%), and progradational beach deposits (10%). The net volume of accretion at this site is approximately 200 m3/m. Remarkably, almost all sand nourished before 2000 has been washed away, except for embankments constructed in 1990. Analysis of meteorological data suggests that 1999 storm surges are responsible for this erosion. The relative longevity of post-2000 nourishments can be attributed to a combination of shoreface nourishment and favorable meteorological conditions. During a storm surge in 2007, water-lain embankments proved to be more resistant against wave erosion than nourished sand redistributed by wind, indicating the importance of grain size, roundness and packing in the durability of nourishments.

ADDITIONAL INDEX WORDS:

Foredune, beach, coastal zone management, storm surge, meteorology, sediment

dynamics.

INTRODUCTION The Dutch coast has been subject to persistent erosion and landward retrogradation throughout at least the last centuries, presumably for up to 1500 years. The long-term erosional trend and the damage caused by a series of storm surges in 1990 prompted the definition of the ‘‘Reference Coast Line,’’ which corresponds roughly with the position of the coastline at that moment. The Reference Coast Line concept defines a minimum sediment volume in the beach-foredune area that is to be maintained. Maintenance takes place in the form of sand nourishments. Locations and volumes of these nourishments are determined on the basis of year-to-year trends as derived from coastal monitoring. A precondition is that, in controlling structural erosion, the natural behavior of the sandy coastline is maintained as much as possible. The so-called ‘‘dynamic preservation’’ of the coastline is achieved by the application of shoreface and beach nourishments (Rijkswaterstaat, 1990). At DOI: 10.2112/JCOASTRES-D-11-00097.1 received 20 May 2011; accepted in revision 24 October 2011. Published Pre-print online 19 March 2012. ’ Coastal Education & Research Foundation 2012.

places where safety is not at risk, nourishments are rarely or not applied, and limited breaching, scarp formation, and blowout of foredunes is allowed or even stimulated. On an annual basis, about 12 Mm3 of sand has been nourished at selected places along the Dutch coastline since the year 2000. Before 2000, this volume was about 6 Mm3. This sand was used for raising and widening the beach itself and for constructing embankments (erosion buffers or ‘‘shoulders’’) up against the foredunes. After later insights (evaluated by Nourtec, 1997), sand has since also been placed on the shoreface around the 5–6-m isobath (adjoining the outer beaker bar), allowing currents, waves and wind to redistribute the sand to adjacent areas. Since 1965, the development of the coastal zone has been monitored by means of coastal profiling (called Jarkus). Decadal shoreface, beach, and foredune development can be analyzed using this morphometric database. The Jarkus database contains elevations for cross sections spaced 250 m apart (corresponding with the beach-pole grid). These elevations are measured annually along the entire Dutch coast (cf. Arens and Wiersma, 1984). The profiles stretch from about 1000 m seaward of the low-tide line up to and including the

588

Figure 1.

Bakker et al.

The study sites of Heemskerk and Bergen along the southern North Sea coast.

frontal dune. The supratidal part of the profiles has been measured by airborne laser altimetry (LIDAR) since 1996. Earlier profiling of the land part was achieved by topographic levelers and aerial photography. Annual measurements extend landward to at least the top of the (fore)dune. Year-to-year variability in measuring and processing techniques occasionally complicate straightforward analyses of the annual net changes in profiles (Arens, 2009). Additionally, the Jarkus database solely represents annual registrations of the topography of the beach–dune profile. Hence, little can be said on the actual processes resulting in the observed year-to-year variability. This paper addresses these processes by linking Jarkus with ground-penetrating radar (GPR) and meteorological observations. This approach was applied to data from a nonnourished beach that has been subject to long-term net erosion and from an actively and frequently nourished beach that has experienced recent coastline progradation. The primary aims of this study are to determine (1) the origin of sedimentary elements within the upper beach and foredune area, (2) the fate of the sand volumes nourished in the past, and (3) the decadal-scale preservation potential of sand in the dune foot and on the adjacent backshore in light of the established nourishment policy. A secondary aim is to demonstrate temporal and spatial accretion and erosion variability along nourished coasts.

STUDY SITES The two study sites discussed in this paper are located along the west coast of the Netherlands, in a region where the coast is orientated roughly north–south (Figure 1). The tidal range is ,2 m, with spring tide around 1.2 m above mean sea level (+MSL). Prevailing winds, mostly moderate in strength, are from the southwest. Strong winds, occasionally gale force or stronger, reach the coastline from southwesterly to northwesterly directions. Storm surges, with significant water level setup, accompany westerly to northwesterly storm wind regimes. Water level monitoring shows extremes of 3.50 to 3.85 m +MSL, with associated offshore significant wave heights of approximately 7 m.

Site 1. Nonnourished Beach (Heemskerk aan Zee) Longshore redistribution of past nourishments has likely influenced any stretch of beach in one way or another, even when beaches and the adjacent shoreface have not been nourished themselves. This is the case for the Heemskerk site (Figure 1). Nourishments were carried out just south of the study site in 1996–97 and just north of the site in 2005. However, the first nourishment at the study site itself was not put into place until after completion of our study. In a situation of limited sediment availability, coastline retrogradation has

Journal of Coastal Research, Vol. 28, No. 3, 2012

Recent Coastal Dune Development

589

Figure 2. Foredune near Heemskerk aan Zee (view to the north on November 22, 2007). In the absence of nourishments, erosion has caused a steep, slumping foredune front that is subject to wind erosion and wave attack during water levels above 2 m +MSL. The back-dune drop has become oversteepened because of sand entrapment in the marram grass tussocks.

taken place, resulting in narrowing and steepening of the foredune (Figure 2) and, thus, in landward exceedance of the local Reference Coast Line. Because no economically valuable built structures or infrastructure are present near the coastline at this site and because the total dune area is wide and high enough to ensure safety, this coastline recession has not been considered a problem to date. During and immediately after each storm surge event, erosion is taking place in the form of bluff formation and slumping, exposing older eolian and storm surge deposits that date from the 17th and 18th century (Cunningham et al., 2011; Van Heteren et al., 2008). The coastline recession near Heemskerk aan Zee and the formation of large blowouts in the foredune are a consequence of the nonnourishment policy at this beach section and of the practice of dynamic preservation. It opens up the dune area to sand being blown inland from the present-day beach and from the exposed foredune (Figure 2).

pronounced after the impact of storm surges (Figure 3). The historical development of the coastline near Bergen is best demonstrated by the reconstructions published by Schoorl (1990) showing the retrogradation of the coastline at Egmond aan Zee, a village about 5 km south of Bergen aan Zee. The storm surges of early 1990 destroyed part of the Bergen aan Zee boulevard (Figure 3). Sand nourishments were applied shortly after these erosional events. They include a large embankment (dune face nourishment). Additional nourishments were put into place in 1992, 1995, 1997, 1999, 2000, and 2005 (see Table 1 for details). The nourishments before 2000 were carried out solely on the beach, whereas those of 2000 and 2005 were placed in part on the shoreface. Although the main goal of the nourishments is maintenance of the position of the coastline, they also result in a wider beach for the benefit of recreational activities.

METHODOLOGY Site 2. Heavily Nourished Beach (Bergen aan Zee) The coastal resort of Bergen aan Zee (see Figure 1 for location) experienced strong and prolonged coastal erosion in the past. Twentieth century aerial photographs of Bergen typically show a distinct escarpment along the foredune, most

This paper focuses on data from two main methodologies. GPR data from transects perpendicular to the coast, starting at the upper beach and extending into the area behind the frontal dune, are compared with Jarkus profiles across the same environments. The resulting dataset is analyzed to understand


590

Bakker et al.

Figure 3. Oblique aerial photo of Bergen aan Zee looking north, taken shortly after the storm surges of 1990 (source: photo archive Rijkswaterstaat). Part of the boulevard was destroyed. At the beach, in front of the boulevard, Roman-age peat beds were exposed (dark patches). The positions of images and crosssections shown in Figures 6–8 are indicated; Jarkus transect 32.750 is represented by the line labeled Figure 8.

the role of actual processes in nourished beach-foredune environments. Ground-penetrating radar has proven to be a significant technique in coastal studies, as demonstrated in many papers (e.g., Jol, Smith, and Meyers, 1996; Van Heteren et al., 1998). In this study, GPR data were collected using PulseEkko PRO equipment (Sensors & Software Inc.), using unshielded 100and 200-MHz antennae and 250-MHz shielded antennae. The acquisition settings are listed in Table 2. The velocity of the radar signal in the sediment, needed to convert the measurements (conducted in two-way travel time) to depth, was established in various ways. Velocity information was obtained directly by analyzing the shape of diffraction hyperbolae. This information was supplemented by data from indirect comparisons of reflection travel time and reflector (reflecting feature) depth. The depth to prominent reflecting

features (lithological units and the groundwater table) was determined by hand augering. Common-midpoint surveys were not conducted. The unsaturated zone was marked by signal velocities of 0.11–0.12 m/ns. Below the groundwater table, the velocity typically dropped to 0.07–0.08 m/ns. Penetration depth was generally on the order of 12–15 m for the unshielded 100-MHz profiles, 7–8 m for the unshielded 200MHz profiles, and 6–7 m for the shielded 250-MHz profiles. In very steep terrain, the penetration depth was reduced. In the processing of radar data, topographic correction is needed to assess the geometry and depth of subsurface units. Position and elevation data were gathered by a real-time kinematic global positioning system (RTK-GPS) with centimeter accuracy. Additional processing steps included dewowing, horizontal and vertical filtering, and the application of an automatic gain control to better view deeper reflections.



591

Table 1. Overview of sand nourishments at Bergen aan Zee (source: Rijkswaterstaat, Waterdienst). Volumes are given both per kilometer (as an indicator of nourished sand in the kilometer bordering the study site) and per nourishment, as a typical nourishment extends along the coast for several kilometers. The maximum elevations (top surface) of embankments are given; the elevations of the beach nourishments are unknown. No nourishments were carried out in Bergen before 1990. Beach nourishment 3 (June 1994) was placed only along the southern part of Bergen aan Zee. Dates of Jarkus profiling refer to the onshore part of the monitoring.

Number

Type

Volume (m3)

Volume (m3/km)

1a 1b 2 3 4 5 6 7a 7b 8a 8b

Beach Embankment Beach + embankment Beach Beach Beach Beach Beach Shoreface Beach + embankment Shoreface

386,000 60,000 1,473,000 101,000 306,000 352,000 206,000 225,000 994,000 300,000 1,500,000

257,000 40,000 115,000 168,000 306,000 144,000 165,000 450,000 497,000 200,000 320,000

After processing, the GPR images were interpreted on the basis of radar facies identified on the profiles, following the concept of Van Heteren et al. (1998). In this concept, radar facies are defined as spatial units within GPR profiles in which the reflection configuration is uniform. The lithological characteristics of the radar facies were verified by hand augering and are listed in Table 3, which includes a short description of their appearance, indications of their position and an environmental interpretation. The second research methodology involves analysis of Jarkus coastal profiles through time and comparison of the observed changes with the sedimentary architecture visible on GPR images. After this comparison, the observations are linked to processes—driven largely by meteorological conditions—that are known to have an effect on sand-nourished beach-foredune environments.

RESULTS Site 1. Nonnourished Beach (Heemskerk aan Zee) GPR Observations A steep dune scarp, the result of a significant storm surge in November 2007 that created a local water level of 3.13 m +MSL, provided a (laterally discontinuous) exposure of older beach and eolian sediments (Cunningham et al., 2011; Van Heteren et al., 2008). Over a length of about 1 km, relatively coarsegrained, shell-bearing paleostorm surge deposits were exposed at various elevations, intercalated with wind-blown sand. At beach pole 49.500 km (52u309570 N, 4u359400 E) near Heemskerk aan Zee, the foredune scarp was partly cut into a large blowout in the frontal dune, facilitating access for GPR surveying on top of the exposed sequence. This enabled direct comparison between the GPR images and exposed strata. The exposures were used to ground-truth the corresponding GPR Table 2.

Elevation (m +MSL)

Date of Placement

6.00 3.50

— 3.75 —

May–Jun 1990 May–Jun 1990 May–Nov 1992 Jun 1994 May 1995 Jun 1997 Apr–May 1999 Jun 2000 Apr–Aug 2000 Apr 2005 Aug–Sep 2005

Date of Jarkus Profiling in Year of Placement

Apr 14 Apr 14 Apr 25 Apr 25 Mar 30 May 22 Apr 17 Nov 4 Nov 4 Apr 24 Apr 24

images, linking subsurface reflections and reflection configurations (facies) with exposed boundaries and strata. This information supported the interpretation of GPR transects perpendicular to the shoreline. GPR profiles measured perpendicular to the escarpment start at the scarp and extend across the steep foredune, to a maximum elevation of 18.5 m +MSL, and landward down into the adjacent blowouts with lowest points around 3 m +MSL. The seaward side of the foredune is dominated by bare sand and was temporarily oversteepened as a result of the scarp formation. The vegetated lee side of the foredune is particularly steep, with angles up to 37u (Figure 2). The steep slope reflects strong fixation by European marram grass (Ammophila arenaria). Inclined and listric (spoon-shaped) reflections occur in the higher parts of the foredune (Figure 4, right-hand part). Radar facies at the lee sides are dominated by subparallel, inclined reflection sets (facies FDL, Table 3, Figure 4 in yellow) dipping landward. Locally, diffractions are visible. High-amplitude, continuous and subparallel reflection sets (facies SB, Table 3, Figure 4) are indicated in red. Below this unit a continuous subhorizontal reflection at ,1.5 m +MSL (facies WT, Table 3, Figure 4 in blue) is present.

GPR Interpretation The inclined and listric reflections in the higher parts of the profile are attributed to eolian bounding surfaces within the foredune (facies FDC). Unit FDL is interpreted as an eolian deposit that formed in the wind shadow of the migrating foredune, deposited at the angle of repose. Local diffractions can be caused by debris that washed up on the shore and was transported farther inland by storm winds. The scarp exposure revealed a link between unit SB and convoluted shell-bearing beds deposited in storm surge conditions (cf. Cunningham

Overview of GPR settings applied.

Frequency

Antennae Spacing

Pulser Voltage

Step Size

Stacks

Time Window

100 MHz 200 MHz 250 MHz

1.0 m 0.5 m 0.4 m

400 V 400 V 165 V

20 cm 10 cm 5 cm

16 16 16

300 480 350


592

Table 3. Unit

Bakker et al.

Overview of radar facies visible on GPR profiles of beach and foredune deposits at Heemskerk and Bergen aan Zee. Facies Description

Position

Interpretation

WBN

Long, subparallel reflections, dipping seaward; erosive lower boundaries; diffractions

On beach against foredune and embankment

WBS

Long, subparallel reflections, dipping seaward; erosive lower boundaries High-amplitude, subparallel reflections Subparallel, landward-dipping reflections; bounding surfaces; diffractions Steeply dipping, subparallel reflections; local diffractions Mostly transparent facies; blanket

On beach against foredune and embankments Against former scarp, below unit WBN

High-amplitude, continuous and subparallel reflection Subhorizontal, single reflection Tabular top-and-bottom reflection set; horizontal; transparent Westward-dipping low-angle reflections, locally hummocky

About 3–6 m +MSL

Embankment Foredune core, diffractions from root concentrations (marram grass) and debris (in top part only) ‘‘Avalanche’’ deposits of drifting wind-blown sand, diffractions from modern debris ‘‘Fall-out’’ of wind-blown sand, derived from beach (nourishments) and from exposed foredune scarps Shell-bearing storm surge beds

From 1–2 m +MSL Near MSL/groundwater table

Groundwater table Peat beds

Between 0.5 and 2 m –MSL.

Prograditional upper (storm) beach deposits, local eolian deflation and infilling

EM FDC FDL TR

SB WT P PF

On lee side of foredune At the surface behind foredune

et al., 2011). Hand augering at the landward dune foot confirmed the lateral, landward continuation of this unit. Additional GPR surveying showed that this unit extends more than 750 m inland. Reflection WT is interpreted as the groundwater table.

Jarkus Analysis and Comparison with GPR Imaging Jarkus profiles from selected years demonstrate a distinct landward shift of both the seaward foredune foot and foredune crest and a general oversteepening and narrowing of the foredune (dashed lines in Figure 4). Over the period 1965– 2008, the dune crest migrated ,0.5 m/y and the dune foot (beach side) ,1.0 m/y. It is interesting to note that the dune crest remained more or less on the same topographical level. Figure 4 also shows a profile recorded with RTK-GPS shortly after the storm surge of November 2007 (marked A). Comparison with the Jarkus profiles of spring 2007 and spring 2008 (marked S) shows that the severity of scarping is not captured in the annual monitoring series.

Wind-blown sand (nourishment-derived), brown, diffractions from modern debris incorporated in the sand Wind-blown, white, fine-grained sand

Site 2. Nourished Beach (Bergen aan Zee) GPR Observations GPR transects collected on the heavily nourished beachforedune system of Bergen aan Zee reflect a complex nourishment history. The positions of these transects are indicated on Figure 3, which shows the presence of a prominent scarp after a 1990 storm surge. The present-day situation is visible in Figure 5: a generally gentle seaward slope covered with marram grass. The remains of the 1990 scarp can still be recognized locally as a subtle morphological break in the slope. Near the base of the current slope, sparsely vegetated, ephemeral, and hummocky dunes are present. The GPR data show shore-perpendicular transects from the dune foot over the gentle slopes toward the hinterland in the immediate vicinity of the Jarkus profile at beach pole 32.750 km (52u399 580 N 4u379460 E, situated in the northern part of the town). Figure 6 presents a 100-MHz transect from the dune foot over the foredune crest (see Figure 3 for location). At

Figure 4. Combined image of GPR profile (200 MHz, unshielded) across the foredune at beach pole 49.500 near Heemskerk aan Zee and corresponding Jarkus profiles from selected years. Profiles marked S are standard Jarkus profiles recorded in spring; the profile marked A (solid line) was recorded with RTK-GPS shortly after the storm surge of November 2007. The dune crest has migrated ,0.5 m/y and the dune foot (beach side) ,1.0 m/yr, resulting in narrowing and oversteepening of the foredune. The groundwater table (in blue) and historical storm surge beds (in red) are clearly visible on the GPR profile. A set of strong and inclined subparallel reflections marks eolian deposits on the landward side of the profile.



593

Figure 5. Situation at Bergen aan Zee in October 2007. The position of the 1990 scarp is indicated in black, along with the position of the GPR profile (in light gray) shown in Figure 7. Most striking is the gentle slope from the former scarp toward the present-day foredune foot.

,1.0–2.0 m +MSL, a continuous subhorizontal reflection is visible (reflection WT, Table 3). On the seaward side, it is situated near the top of westward-dipping, low-angle oblique reflections (unit PF). Farther landward, the subhorizontal reflection is masked by a set of high-amplitude reflections (unit P). Farthest inland, subparallel, mostly landward-dipping reflection sets and bounding surfaces (unit FDC) occur. This unit can also be recognized in Figure 7. Unit FDC is generally bounded on its seaward side by a sharp, seaward-dipping discontinuity at an angle of about 32u. This erosional boundary is commonly characterized by a continuous reflection that is caused by a marked change in lithological and sedimentological properties. The discontinuity is covered with several sedimentary units. These units are marked by a dominance of long, subparallel reflection sets, dipping seaward and exhibiting erosive lower internal boundaries. Situated against the base of the discontinuity, unit EM (Table 3) shows high-amplitude, subparallel reflections across a limited lateral distance. The overlying unit WBS is dominated by long, subparallel reflections, generally dipping seaward.

The facies succession discussed above is also visible on additional GPR profiles, such as in Figure 7. Hand-augered cores were used to assign lithological characteristics to GPR facies EM, WBS, WBN, and FDC. The sequence is similar to that presented in Figure 6: unit EM resting against a dipping discontinuity, topped by unit WBS.

GPR Interpretation An extensive hand-augering campaign was conducted to establish the sedimentary character of the radar facies units. Reflection WT is interpreted as the groundwater table. Unit PF was correlated to shell-bearing sand and is interpreted as progradational beach deposits. Unit P was positioned too deep to be sampled by hand augering but is believed to consist of remnants of a peat bed that was partly exposed at the beach after the storm surges of 1990 (Figure 3). Hand augering unit EM was difficult because of the presence of very dense compacted (sub)angular brown sands with


594

Figure 6. Table 1).

Bakker et al.

Section at beach pole 33.160 (100-MHz GPR) fronting Bergen aan Zee (see Figure 3). Seaward is to the left. Several GPR facies are visible (see

abundant shells (mostly Donax sp.) and shell fragments. Unit EM is interpreted as the remains of the embankment, or dune face nourishment, that was put up as a quick fix to repair the storm surge damage of 1990. Hand augering has proven two overlying units to be eolian in origin (judging from the wellsorted character of the little-compacted sand and the presence of fine wind-blown debris). Patchy unit WBS (Table 3) was correlated to fine subrounded white-yellow sands, and unit WBN to subangular brown sand. The white-yellow and brown wind-blown sands produce identical radar facies that could only be distinguished by augering. Unit FDC was linked to white-yellow, fine-grained, and subrounded sands of Baltic (northern) origin poor in, or devoid of, CaCO3. Units FDC and WBS are bounded by a sharply inclined discontinuity, dipping seaward. Comparison with aerial imagery and Jarkus data shows that it represents the 1990 scarp (comparable to the recent Heemskerk aan Zee situation) that is buried in the present-day foredune. Locally, the reflection terminates at the position of a morphological break in the seaward slope of the foredune. The indistinct nature of the

reflection marking the scarp could be due to slumping of the steep slope during and immediately after the 1990 storm surges. Additionally, the scarp could be obscured by secondary slide planes within the foredune sand body. These slide planes are created under storm wave attack, when entire blocks of dune sand, held together by marram grass, slide into the surf zone as the steep-sided frontal dune is undermined. Bristow, Chroston, and Bailey (2000) encountered similar subsurface scarps in a GPR study of the coastal foredunes at north Norfolk (U.K.), attributing them to the 1953 and (likely) older storm surges. Analysis of the GPR profiles at Bergen aan Zee shows that the net accumulation of post-1990 sand adds up to 180–210 m3/m. The toe on the North Sea side of the foredune has migrated seaward over some 18 m during the period 2000–2008 and about 35 m in the period 1990–2008.

Jarkus Analysis and Comparison with GPR Imaging The Bergen aan Zee GPR profiles are situated close to Jarkus transect 32.750. Annual data for this transect are available



Figure 7. Detail of a 250-MHz GPR section at beach pole 32.630 (see Figure 3), with borehole data (A, B, and C) ground-truthing radar facies SH, WBS, WBN, and FDC.

over the period 1965–present. Most profiling of the dry part, relevant in the present study, has been done in spring. Beach nourishment will usually take place during the summer months, shortly after the annual Jarkus profiling. In 2000, however, measurements were taken on November 4 after substantial shoreface and beach nourishment earlier in the year (Table 2). For 2002, no Jarkus data are available; the profile was established by interpolation between 2001 and 2003. When all Jarkus profiles of a selected location are analyzed, it is possible to visualize stacked sediment volumes (deposited in different years) making up the sand wedge that has accumulated in front of the 1990 scarp. Figure 8 shows that this sand wedge is composed of sand volumes from only a limited number of years. The 1990 profile represents the most inland position of the dune front in the recorded period. All subsequent profiles are positioned farther westward (i.e., at a higher position), except the western part of the 1993 profile. Even though the profiles in the period 1990–99 were positioned seaward of the 1990 line, only sediment deposited in front of the scarp in 1990–91 has survived one or more erosive events that took place before the Jarkus profiling of 2000. The GPR surveying has shown that it concerns unit EM, the 1990–91 embankments. In the period 2000–08, gradual accumulation pushed the profile seaward. Sand nourishments were conducted in 2000 and 2005 only, but all other post-2000 years have contributed to the sand wedge that fronts the 1990 scarp (mostly GPR facies WBN). Figure 9 shows the data of Figure 8 in numerical form (between positions 290 and 2145 m). Sediments from 1992 and the period 1994–99 have not been preserved at Jarkus transect 32.750. The largest volumes that make up the sand wedge fronting the scarp originate from 1990–91 (the 1990 poststorm

595

embankment), 1992–93, and 2000 (the large nourishment). The gradual accumulation after 2000 is a result of redistribution by wind of nourishment sand located on the dry beach, as evidenced by field observations, GPR reflection configurations, and sedimentology of hand-augered material. Analysis of year-to-year gross volumetric changes at Jarkus transect 32.750 provides important additional information on system morphodynamics (Figure 10). Indicated are net changes for each year in the period 1990–2008, compared with the previous year, of the higher part of the beach plus the foredune front (between positions 290 and 2145 m in Figure 8). Erosion occurred primarily in 1989–90, 1991–92, and 1998–99. Major accumulation occurred not only in 1990 (the 1990 poststorm fix) and 2000 (the largest nourishment to date), but also in 1996 (second-largest beach nourishment to date). According to the Jarkus data at transect 32.750, there is an overall net growth of 233 m3/m during the period 1990–2008. The acquired GPR surveys complement this information, showing accreted volumes varying from 180 to 210 m3/m over a lateral distance of 530 m. From the GPR data, it can be concluded that 21% of this volume originates from the 1990–91 embankments, 11% from 1993, 36% from the 2000 nourishments, and the remaining 32% from the period 2001–08. The addition of sand has resulted in an average seaward shift of the foredune toe of about 35 m.

Understanding Processes Reworking Beach Nourishments It is relatively easy to determine the geometry, composition (origin), and age of nourishment-related sand deposits at the upper parts of the beach and the adjacent, windward part of the foredune at Bergen aan Zee. Linking these observations to actual processes in the coastal environment is more difficult. Along coasts marked by frontal dunes, wind is the most important parameter affecting erosion, transport, and deposition. It has an indirect effect on subtidal and intertidal areas by driving waves and currents, and a direct effect on intertidal and supratidal areas through various eolian processes. In the study area, shore-parallel sediment transport prevails during periods of strong southerly to southwesterly winds. Vast amounts of sand are set in motion under these conditions. Inland sand transport occurs mostly when strong westerly winds blow perpendicular to the coast. Numerous studies have shown how these coastal processes act on beach sand (e.g., Van der Wal [2004] for eolian sand transport and Pool [2009] and Roelvink et al. [2009] for wave-induced erosion and associated processes). Stuyfzand, Arens, and Oost (2010) additionally demonstrated that the nourishment-derived sand has been transported up to 300 m inland, affecting ecology. A complete understanding of the effect of these processes on the coastal profile requires analysis of meteorological and marine observations from the time period under consideration. Water level data and marine observations are available for two nearby sites (see Figure 1). The duration of water levels above 2 m +MSL for each year in the period 1989–2008 at Petten Zuid and IJmuiden Buitenhaven (Figure 11) is a relevant parameter in understanding the direct impact of waves on the coastal profiles of Heemskerk aan Zee and Bergen aan Zee.


596

Bakker et al.

Figure 8. Analysis of Jarkus profiles for transect beach pole 32.750. Preserved sand volumes originating from different years are shown in different gray tones (situation 2008, derived from the Jarkus data). The largest volumes originate from 1990 and 2000.

Although the exact water level at the dune foot is determined in part by beach profile and breaker bar geometry, which affect wave run-up, the 2 m +MSL water level, as measured at the two monitoring stations, is a practical indicator for the threshold that needs to be reached at both study sites before waves can saturate and erode the dry beach and foredune. Water levels above 2.0 m +MSL are reached only during significant storm surges; the normal spring high-tide level is around 1.05 +MSL (Petten) and 1.25 +MSL (IJmuiden). The maximum levels reached during 1990–2008 at IJmuiden Buitenhaven were 2.67 m +MSL in 1990 and 3.13 m +MSL in 2007. The highest level recorded within the instrumental monitoring time series is 3.85 m +MSL in 1953. Aside from the surge-related water levels, wave run-up must be taken into account. Under specific conditions, wave run-up can reach elevations of more than 4–5 m +MSL (Pool, 2009). Storm surge period 1 (Figure 11) occurred in 1990 and caused the damage shown in Figure 3, creating the most landward coastal profile to date. Storm surge 2, in 1993, eroded

Figure 9.

the beach to even lower levels than in 1990, but the embankments constructed in 1990 protected the dune foot and stayed mostly intact. The 1999 storm surges eroded all sediment accumulated between 1993 and 1999, including the beach nourishments of 1994, 1995, and 1997. Storm surge 4, in November 2007, resulted in the highest water levels since 1976 but had little effect on the profile at Bergen aan Zee. The erosive effects of the storm surges, as indicated in the volumetric balances of Figure 10, are more than offset by the sand accumulation effects resulting from the nourishments. In 2000, the largest beach nourishment to date was carried out. An even larger shoreface nourishment was put in place around the same time. A 7-year period of marked meteorological calmness followed; water levels above 2.0 m +MSL were very rare and short-lived. Despite limited coastal erosion during this time, additional beach (including embankment) and foreshore nourishments were carried out in 2005. As a result of the period devoid of significant storm surges, and reinforced by the 2005 nourishment that provided a new source of sediment, eolian sand accumulation against the foredune has occurred ever since the year 2000, without any significant intermittent erosion. Figure 12 shows wind frequencies for the period 1990–2008 at IJmuiden. Indicated are hours with wind speeds above 10 m/s, which are taken here as a measure of potential sand transport by wind acting on a typical western Netherlands beach. For natural and nourished sands with nonuniform grain sizes, critical wind velocity is not a single value but a threshold range (cf. Nickling, 1988). The threshold value of 10 m/s for Bergen aan Zee gives an upper-end value of the critical range. It is a function of grain size distribution of the local (nourished) beach sediment (cf. Van der Wal, 1998). The frequencies in Figure 12 are provided for winds from the 180u–270u quadrant (shore-parallel sediment transport dominant), the 270u–360u quadrant (inland sediment transport dominant) and their sum (180u–360u). Southeasterly to northeasterly winds are not considered because these are rare and do not usually reach 10 m/s. Even when strong easterly winds occur, (seaward) sediment transport is limited by vegetation on the landward side of the foredune.

Post-1990 sand volumes present in 2008 and their year of origin at Jarkus transect 32.750. About 233 m3/m sand has accumulated since 1990.



597

Figure 10. Sediment dynamics at Jarkus transect 32.750. Indicated are interannual volume changes. Profiles from successive years are compared with calculated volumes of erosion or sedimentation.

Figure 11. Duration (in hours) of water levels above 2.00 m +MSL for each year in the period 1990–2008 in Petten Zuid (north of the study areas) and IJmuiden Buitenhaven (south of the study areas). Average high-water level at these locations is 0.80 (1.05) and 0.95 (1.25) m +MSL, respectively (maximum spring tide levels in brackets). Highest levels are usually reached at IJmuiden. Significant storm surges occurred in 1990, 1993, 1999, and 2007. The maximum water levels observed during these extreme events are indicated above the bars in the graph (m +MSL).


598

Bakker et al.

Figure 12. Duration (in hours) of wind speeds .10 m/s for southwest and northwest circulations (and their sum) in the period 1990–2008 (wind directions indicated). Data from IJmuiden. Most hours with potential sand transport occurred in 1990, 1998–2000, and 2008. Calm years were 1996 and 2003. Note that windy years do not necessarily coincide with storm surge years (cf. Figure 11).

Potential sand transport occurs during 2000 hours (i.e., more than 80 full days) in an average year. Two exceptionally calm years were 1996 and 2003. Years with high potential wind transport do not necessarily coincide with storm surge years, which might explain why no direct links with the observed interannual profile changes could be established. It is clear, however, that given sediment availability on the dry beach and in nonvegetated foredune parts, redistribution of sand by wind will be significant almost every year.

Storm Surge Field Observations During the storm surge of November 2007, field observations were conducted in both Heemskerk and Bergen aan Zee. Along the Dutch coast, the northwesterly storm winds were not extreme along the coast (8–9 Beaufort, average wind speed 18–24 m/s). Nevertheless, the highest water levels since 1976 were recorded. The height of the storm surge was determined in the northern part of the North Sea, far offshore, where hurricane-force winds associated with a slow-moving depression were active across an extremely long fetch. These conditions generated large and high swells that moved into the southern part of the North Sea before reaching the coastline (maximum wave heights of 7–8 m near IJmuiden). During high tide, the water easily reached the dune foot, saturating the sand. Waves attacking the dune front resulted in large-scale scarping. In most places, the dune foot was

eroded back between 2 and 3 m (typical value for Bergen aan Zee) and 10 m (maximum value for Heemskerk). The amount of erosion depended on the presence or absence of nourishment sand, nearshore bathymetry, height and width of the beach, and other local conditions (e.g., Brodie and McNinch, 2009; Houser, Hapke, and Hamilton, 2008; Pool, 2009). In Heemskerk, the scarped dune face formed fresh exposures of foredune sand with intercalated shell-bearing, paleostorm surge deposits. In Bergen aan Zee, water-lain nourishment sands (including embankments) could be recognized in fresh exposures as generally subhorizontally stratified, densely packed sediments. The water-lain nourishment sands are subangular and poorly sorted. They are marked by silty admixtures, shell concentrations, and local clay balls and also contain plastic, rope, and other types of anthropogenic debris. The subangular nature and dense packing of these sands make them more resistant against erosion than the loosely packed, well-rounded eolian sands. This increased resistance could explain part of the observed difference in dune foot erosion between Heemskerk and Bergen aan Zee. It reduces slumping, which takes place at relatively steep angles. Eolian strata are more easily washed away and slump at considerably gentler angles. Although an additional explanation for the difference in dune foot erosion is the high elevation of the prestorm beach at Bergen aan Zee (Figure 8), it is likely that type and composition of nourished sediment and the way it is introduced into the beach system are important elements in the durability of beach nourishments.



DISCUSSION Observations and analyses linking coastal profile changes to natural processes and human interference on an annual to decadal timescale are rare. Our study corroborates patterns reported in the literature and supplements this knowledge by providing quantified process-response relationships explaining such patterns. A regional study of foredune behavior in The Netherlands was recently completed by Arens, Van Puijvelde, and Brie`re (2010) using laser altimetric data and Jarkus profiling. To quantify the effects of the nourishment policy on the dunes, they analyzed total volumetric changes of the frontal dune along the entire Dutch coast. They conclude that sand accretion to the frontal dunes has shown an overall increase since the establishment of the nourishment policy. Analysis of data from the area just south of Bergen aan Zee (between beach poles 34.000 and 37.000, 1–4 km south of the GPR transects) shows a trend break around 1997, from net erosion to net volumetric growth. For beach pole 49.500 in the Heemskerk area, Arens, Van Puijvelde, and Brie`re (2010) also observed a change around 1997, from a net strong erosional state to a more or less stable foredune volume. At this location, a landward shift of the dune foot coincides with volumetric growth at the landward side of the foredune, as confirmed and imaged by our data (Figure 4). Arens, Van Puijvelde, and Brie`re (2010) see the observed volumetric trend breaks as indirect proof of the success of the nourishment policy. The overall pattern is overprinted by local effects, such as sandbar migration, blowout formation and—in general terms—a decrease of storminess after 1990. Our study shows that the latter is true for storm surges, but not for stronger winds in general (Figure 12). As a result, sand loss through dune scarping has been rare, whereas sand accumulation on the landward side of the frontal dune has been significant. The effects of dune scarps or bluffs, like the one at the Heemskerk aan Zee, can be twofold. In a study on the influence of coastal morphology on the transport of shoreface sediment in the Canadian Beaufort Sea, He´quette et al. (2001) noted the importance of downwelling, near-bottom currents. The numerical model used in their study indicated that offshore sediment transport is strongest where beaches are backed by bluffs. Under these conditions, strong seaward-directed horizontal pressure gradients drive powerful offshore bottom currents. Low barriers that are easily breached experience lower set-up and run-up during storm surges, and eroded sediment is mainly transported obliquely offshore. Importantly, He´quette et al. (2001) suggest that sediment eroded from bluff-backed beaches might be transported offshore to depths that are beyond the influence of fair-weather waves needed to return the eroded material to the beach. Consequently, the presence of coastal features that act as barriers to onshore-driven surface water could result in a greater loss of sediment from the coastal system than the absence of such features. In a study on the Gulf coast of northwestern Florida, Houser, Hapke, and Hamilton (2008) found the reverse to be true. They noted that in areas with a relatively wide dune belt behind a foredune, most of the sediment eroded from beach and dunes during storm surges is deposited on the upper shoreface. This sediment is returned to

599

the beach during fair-weather conditions, when nearshore bars weld to the coast. Areas with large foredunes and a wide dune belt erode less rapidly than areas with smaller foredunes and greater sediment fluxes through overwash. Houser, Hapke, and Hamilton (2008) additionally suggested that the presence or absence of transverse ridges in the nearshore zone affect the storm surge level at the beach. In determining the response of a stretch of coastline to the next extreme storm for coastal management purposes, the differential influence of nearshore bathymetry should be taken into account. In addition to beach profiling and GPR imaging, field observations on sediment transport on shoreface, nearshore zone, beach, and foredune are needed, particularly during and after extreme events. The overall trends at Bergen aan Zee (steady seaward foredune migration) and Heemskerk (slow dune foot erosion), as observed by Jarkus monitoring, are obscured by the effects of major storm surges and by subsequent periods of relatively rapid beach and foredune recovery. A limitation in separating the erosive effects of storm surges from intervening recovery periods and from the long-term trend is the low (annual) temporal resolution of the Jarkus measurements. Pre- and postsurge measurements will include not only storm surge effects, but also the effects of all other coastal processes that took place during the year between subsequent measurements, including entrapment of sand by dune grass planted at the dune foot. Earlier, Zhang, Douglas, and Leatherman (2002) found a similar pattern as the ‘‘Heemskerk’’ scenario of slow, long-term erosion in their analysis of shoreline data from the U.S. East Coast. They noted that even after the most damaging storms, such as the Ash Wednesday Storm of 1962, beaches recover to positions matching their overall, century-scale trend. Zhang, Douglas, and Leatherman (2002) concluded that barrier beaches would not experience long-term erosion, even when experiencing frequent major storms, if sediment supply were sufficient to keep up with the effects of relative sea-level rise. This conclusion is confirmed by our observations from Bergen aan Zee, which show prenourishment erosion up to 1990 but slow nourishment-related foredune progradation since then, interrupted, but not reversed, by brief erosive events. Although the two sites analyzed as part of this study represent both a nourished and a nonnourished stretch of coast, our results must be validated by observations from other locations. Validation is possible where monitoring data of nourished and natural foredunes within single coastal cells are available. Where profile data are absent, GPR can be used by itself to contrast the sedimentary record and behavior of nourished and adjacent nonnourished beach-dune systems. The resulting data need to be supplemented by meteorological observations and by sediment transport measurements in the entire coastal tract, particularly during extreme events. Following this approach, morphological and ecological effects of nourishment programs will become clearer, allowing finetuning of nourishment strategy.

CONCLUSIONS In revealing the internal architecture of foredunes, GPR helps to establish the origin and preservation of stacked


600

Bakker et al.

sedimentary units that make up the foredunes. Thus, GPR profiles are instrumental in explaining Jarkus profiling data in terms of responsible coastal processes. The combination of GPR and Jarkus can be used to optimize coastal management policies. At the Heemskerk aan Zee site, sand nourishment has never been implemented. Jarkus profiling shows that during the period 1965–2008, the foredune foot migrated landward at an average rate of 1.0 m/y. Scarp development has revealed bounding surfaces in eolian strata resting on shell-bearing storm surge beds. Both elements can be traced in a landward direction using GPR surveying. Landward accretion of the foredune can also be imaged. It is seen that the morphodynamics of the upper beach and foredune are not captured by the lower temporal (i.e., yearly) resolution of the standard profiling measurements. At Bergen aan Zee, Jarkus and GPR data show a distinct, nourishment-related net growth of the foredune volume over the years, with high year-to-year variability. The dune foot has migrated 35 m in a seaward direction over the period 1990–2008. Integration of Jarkus and GPR data makes it possible to establish the year of origin of sand volumes and to attribute textural and compositional properties to these volumes. The data show that 21% of the accreted volume originates from water-lain embankments constructed in 1990, 11% from 1992–93 beach sands, 36% from year 2000 nourishments (partly water-lain and therefore in situ, mostly redistributed by wind), and the remaining 32% from the period 2001–08 (entirely wind-redistributed nourishment sand). The net volume of accumulation ranges between 180 and 233 m3/m over a shore-parallel distance of 530 m. Almost all sand of the nourishments applied before 2000 has been washed away. Analysis of meteorological and marine data suggests that the 1999 storm surges are most likely responsible for erosion of all post-1990 nourished material still present on beach and foredune at that time. After 2000, structural accumulation has taken place in the form of wind-blown nourishment sand, not only in the nourishment years 2000 and 2005. This accumulation can be attributed to consistent, longterm sediment supply from shoreface nourishments and to favorable meteorological conditions. Field observations during a significant storm surge in 2007 indicate that wind-blown nourishment sands are more prone to wave erosion than water-lain nourishment sands. Type and composition of the sand are very important and can be used not only to predict the durability of nourished sediment volumes, but also to assess ecological effects in the frontal dunes. The analysis of process-response relationships governing the behavior of the Dutch foredune system profits from the combined power of Jarkus profiling and GPR surveying. Whereas Jarkus profiling supplies volumetric data over long time spans, GPR profiles visualize the net effects of profile changes and reveal the contribution of natural processes and specific nourishments to sand volume gained by nourished coastal sections. Whereas the Jarkus profiling is restricted to 250-m intervals and limited to measurements made each spring, the GPR profiling can be carried out at any place and time. Assessment of net variability along the coast at the highest spatial and temporal resolution is essential when understanding the behavior and preservation of nourished sand volumes in light of meteorological conditions. Therefore,

combining a diversity of techniques and approaches, including GIS based studies, Jarkus measurements, GPR, and meteorological reconstructions, in beach-nourishment evaluations more often is recommended.

ACKNOWLEDGMENTS Hoogheemraadschap Hollands Noorderkwartier and PWN Water Supply Company North Holland gave us permission to access the sites and provided logistical support; we especially thank Menno van den Bos, Paul van der Linden, and Luc Knijnsberg. Alastair Cunningham and Piet Kok provided field assistance. Stijn van Puijvelde contributed to the analysis of meteorological and marine data. Xavier Comas and two anonymous reviewers are thanked for their critical comments. This work was funded by Deltares projects Coastal Maintenance (‘‘Kustlijnzorg’’) and Coastal Systems and is a contribution to IGCP project 588 ‘‘Preparing for coastal change: A detailed process-response framework for coastal change at different timescales.’’

LITERATURE CITED Arens, S.M., 2009. Effecten van suppleties op duinontwikkeling; geomorfologie. Rapportage fase 1 (Effects of Nourishments on Dune Development, Geomorphology). Report Arens BSDO, RAP2009.2, 68p. [in Dutch]. Arens, S.M.; Van Puijvelde, S.P., and Brie`re, C., 2010. Effecten van suppleties op duinontwikkeling, geomorfologie. Rapportage fase 2 (Effects of Nourishments on Dune Development, Geomorphology). Report Arens BSDO, RAP2010.3, 126p. [in Dutch]. Arens, S.M. and Wiersma, J., 1984. The Dutch foredunes: inventory and classification. Journal of Coastal Research, 10(1), 189–202. Bristow, C.S.; Chroston, P.N., and Bailey, S.D., 2000. The structure and development of foredunes on a locally prograding coast: insights from ground-penetrating radar surveys, Norfolk, UK. Sedimentology, 47, 923–944. Brodie, K.L. and McNinch, J.E., 2009. Measuring surf-zone morphodynamics, beach volume change and runup during storms: new methodology identifies beach cross-shore sediment flux metric. In: AGU Fall Meeting (San Francisco, California, American Geophysical Union), NH11A-1111. Cunningham, A.C.; Bakker, M.A.J.; Wallinga, J.; Van Heteren, S.; Van der Valk, L.; Schaart, D.R., and Van der Spek, A.J.F., 2011. Extracting storm-surge data from coastal dunes for improved assessment of flood risk. Geology, G32244, 1063–1066. He´quette, A.; Desrosiers, M.; Hill, P.R., and Forbes, D.L., 2001. The influence of coastal morphology on shoreface sediment transport under storm-combined flows, Canadian Beaufort Sea. Journal of Coastal Research, 17, 507–516. Houser, C.; Hapke, C., and Hamilton, S., 2008. Controls on coastal dune morphology, shoreline erosion and barrier island response to extreme storms. Geomorphology, 100(3–4), 223–240. Jol, H.M.; Smith, D.G., and Meyers, R.A., 1996. Digital ground penetrating radar (GPR): a new geophysical tool for coastal barrier research (Examples from the Atlantic, Gulf and Pacific Coasts, U.S.A.). Journal of Coastal Research, 12, 960–968. Nickling, W.G., 1988. The initiation of particle movement by wind. Sedimentology, 35, 499–511. Nourtec, 1997. Innovative Nourishment Techniques Evaluation, Final Report. The Hague, The Netherlands,: Coord. Rijkswaterstaat, National Institute for Coastal and Marine Management/ RIKZ Report MAS2-CT93-0049. 105p. Pool, A.D., 2009. Modelling the 1775 Storm Surge Deposits at the Heemskerk Dunes. Delft, The Netherlands: Technical University Delft/Deltares, Master’s thesis, 92p. Rijkswaterstaat, 1990. A new coastal defence policy for the Netherlands. The Hague: Ministry of Transport en Public Works, 103p.



Roelvink, D.; Reniers, A.; van Dongeren, A.; Van Thiel de Vries, J.; McCall, R., and Lescinski, J., 2009. Modelling storm impacts on beaches, dunes and barrier islands. Coastal Engineering, 56, 1133–1152. Schoorl, H., 1990. Kust en Kaart (Coast and Map; in Dutch). Schoorl, the Netherlands: Pirola, 160p. Stuyfzand, P.J.; Arens, S.M., and Oost, A.P., 2010. Geochemische effecten van zandsuppleties langs Hollands kust (Geochemical Effects of Sand Nourishments Along the Coast of Holland). Water Research Institute, Report KWR 2010.048, 71p. [in Dutch]. Van der Wal, D., 1998. The impact of the grain-size distribution of nourishment sand on eolian sand transport. Journal of Coastal Research, 14(2), 620–631.

601

Van der Wal, D., 2004. Beach-dune interactions in nourishment areas along the Dutch coast. Journal of Coastal Research, 20, 317– 325. Van Heteren, S.; Bakker, M.A.J.; Oost, A.P.; Van der Spek, A.J.F., and Van der Valk, L., 2008. Sedimentary signature of a storm surge unit in the western Netherlands coastal dunes. Geophysical Research Abstracts, 10, EGU2008-A-10264. Van Heteren, S.; FitzGerald, D.M.; McKinlay, P.A., and Buynevich, I.V., 1998. Radar facies of paraglacial barrier systems: coastal New England, USA. Sedimentology, 45, 181–200. Zhang, K.; Douglas, B., and Leatherman, S., 2002. Do storms cause long-term beach erosion along the U.S. East Barrier Coast? Journal of Geology, 110(4), 493–502.
