Intertidal Bar Dynamics and Aeolian Sediment Transport in
the Intertidal Zone on a Micro Tidal Beach; Vejers, Denmark.
Arjen van den Ouden
Department of Physical
Geography
University of Utrecht
2007
Contents
Figures 4 Tables 5 Preface 6 1 Introduction 7 2 Theoretical Background 9 3 Study Site 17 4 Methods 21 4.1 Field Methods 21 4.2 Data Analysis 24 5 Results 275.1 Intertidal Bar Dynamics 29
5.1.1 Morphologic Evolution of the Intertidal Beach During Low Energy Wave
Conditions 37 5.1.2 Morphologic Evolution of the Intertidal Beach During a Storm Event 46
5.2 Intertidal Bar Cycle 57
5.3 Aeolian Transport in the Intertidal Area 62
6 Discussion 67
7 Conclusion 69
8 Further Remarks 71
References 73
Figures
Figure 2-1: Variations in the cross-shore sediment transport rate and direction for bars at
different depths. (Masselink et al. 2006)... 12
Figure 2-2: Photo of an intertidal bar at low tide. ... 13
Figure 2-3: Sediment transport and wave height during two storms in 1996 at Skalinngen ... 14
Figure 2-4:Mean cross shore current, sediment transport and water level during a storm at Skallingen... 14
Figure 3-1: Two maps of the west coast of Denmark. ... 17
Figure 3-2: Satellite image of the fieldwork site... 18
Figure 3-3: cross shore profile of line 6200, showing the three coastal zones based on the Mean High Water Level and the Mean Low Water Level... 19
Figure 3-4: Cross shore profiles of DCA line 6170 on 15 September 2005 and 12 October 2006... 20
Figure 4-1: Photo of a fully equipped data frame (station 4). ... 22
Figure 5-1: Wind speed, significant wave height and water level during the field campaign, from 26 September 2006 to 18 October 2006. ... 27
Figure 5-2: A wind rose showing direction of the wind in percentage of occurrence during the field campaign.. ... 28
Figure 5-3: Cross shore profiles (6200) and photo of the intertidal area. The photo in the left panel depicts the lower intertidal bar. ... 29
Figure 5-4: Cross shore profiles of the intertidal zone from 26 September 2006 until 5 October 2006... 30
Figure 5-5: Four plots showing profiles before and after the storm of 7 October 2006. ... 31
Figure 5-6: Photo showing the rip channel and the bars north and south of Profile 6200... 33
Figure 5-7: Cross shore profiles of the central line (6200) on 10, 11, 12 and 13 October 2006. ... 34
Figure 5-8: Cross shore profiles from the central line (6200) on 15, 16, 17 and 18 October. .35 Figure 5-9: Three panels showing the southern, central and northern cross shore profiles for 16, 17, 18 and 19 October. ... 36
Figure 5-10: Sediment concentration, water depth and wave height at S2, S3 and S4 on 5 October 2006. ... 38
Figure 5-11: Cross-shore and longshore flow velocity at S2, S3 and S4 on 5 October 2006.. 39
Figure 5-12: Wave types at the intertidal beach at 5 October 2006... 40
Figure 5-13: Bed morphology at the intertidal beach at 5 October 2006... 41
Figure 5-14: Detailed cross shore profiles on 5 October 2006 at 5 different times. ... 42
Figure 5-15: Bed level changes in m at the intertidal beach at 5 October 2006. ... 44
Figure 5-16: Cross shore profiles of the intertidal area at 08:20 h and 18:45 h on 5 October 2006... 44
Figure 5-17: Photo of high water event. Surge water nearly reaching the dune foot during a storm on 7 October 2006... 46
Figure 5-18: Cross shore Profiles of DCA line 6200 showing the erosion of the berm on 7 October 2006. ... 47
Figure 5-19: Sediment concentration, water depth and wave height at S2 and S4 on 7 October 2006... 48
Figure 5-20: Wave types at the intertidal beach at 7 October 2006... 49
Figure 5-21: Cross- and long-shore flow velocity at station S2 and S4 on 7 October 2006.... 50
Figure 5-24: Wave type and high frequency cross shore profiles of the berm on 7 October
2006... 55
Figure 5-25: Day-to-day profile development (Line 6200).. ... 58
Figure 5-26: Day-to-day delta z development (Line 6200)... 58
Figure 5-27: Bar crest locations through out the field campaign... 59
Figure 5-28: Wave height and water level in the period before the field campaign (1 to 25 September 2006) ... 60
Figure 5-29: Photo of aeolian sediment transport to and from the berm on 2 October 2006. . 63
Figure 5-30: Photo of long shore aeolian sediment transport on the intertidal area on 5 October 2006. ... 64
Figure 5-31: Photo of offshore directed aeolian sediment transport on 9 October 2006. ... 65
Figure 7-1: Profiles with water levels over the 3 measurement stations at two different times on 5 October 2006. ... 71
Tables
Table 2-1: Overview of the characteristics of the subtidal, the intertidal and the supratidal zones... 10Table: 5-1: An overview of the mean values for wind speeds, wave height and wave period for the three time periods specified. ... 28
Preface
This MSc thesis is the final product of the Masters in Physical Geography, more specific in coastal morphodynamics. The fieldwork formed an important part of this Master study and was executed on the beach of Vejers on the west coast of Denmark. The study has been done in collaboration with the University of Copenhagen. I am thankful for the opportunity that was given to join in this field campaign. I would especially like to thank Dr. Aart Kroon who has been supportive from the first time we spoke about this particular field work. Throughout the research Aart Kroon has been present and working together has proven to be very
interesting as well as constructive.
I would also like to thank Dr Troels Aagaard and the University of Copenhagen, Geografisk Institut, for accommodating us and sharing the data that were collected by the measurement stations during the field campaign.
Furthermore, I would like to thank my fieldwork buddy Erwan Verkaart for being supportive before and during the field period. Besides, I’d like to thank Laust Nielsen, Kristine Boas, Michael Hughes, Brian Greenwood, David Mitchell, for their support, help and presence during the campaign.
I would like to thank the department of Physical Geography at the University of Utrecht, especially Chris Roosendaal who has been very helpful in supplying us with the right tools and instruments. I would also like to thank Prof. Piet Hoekstra for supervising the thesis, being responsible for the final product of the Master research.
1 Introduction
Intertidal bar dynamics at a sandy beach is the central topic of this study. This thesis focuses on morphological changes of the intertidal bar due to changing weather conditions and changing hydrodynamic processes. Besides, it tries to determine long term transport cycles in the intertidal zone. The fieldwork for this study was conducted in September and October 2006 on Vejers beach, Denmark. This fieldwork was part of a larger scale research conducted for the Geographic Institute of the University of Copenhagen by Dr. Troels Aagaard and Dr. Aart Kroon and co-workers. The field campaign is a continuation and extension of the field research that has been done at Skallingen since 1994 on sediment transport and
beach/shoreface morphodynamics (Aagaard et al., 2004 a/b).
The longshore, cross-shore and gross sediment transport in the sub-tidal near shore area of sandy beaches has received a lot of the attention in the past (e.g. Bailard, 1981; Wright and Short, 1984; Thornton and Kim, 1993; Aagaard and Greenwood, 1994; Beach and Sternberg 1996; Aagaard et al., 1998a; Grasmeijer and van Rijn, 1999; etc). Sediment transport rates and morphological changes in the intertidal zone are less studied, despite the relatively easy accessibility of the intertidal zone and its prominent role in beach protection. Normally, waves break and dissipate their energy in this area during storms (Masselink et al., 2006), and the physics behind the intertidal bar dynamics will highlight the erosion processes and might help to prevent beach erosion. Quite often under low-energy wave conditions, the intertidal bars migrate in landward direction and finally weld to the supratidal beach, apparently transporting sand up the beach profile (e.g. Wijnberg and Kroon, 2002; Aagaard et al., 2004; Masselink et al., 2006). This sand can so be available for aeolian transport. Sediment in the intertidal zone is thus both landward and seaward transported. The daily monitoring of the intertidal bar and berm behaviour will highlight sequences in the intertidal zone, with its specific relaxation times.
What causes the intertidal bars to weld to the beach? iv) When and how is sand mobilised for aeolian transport in the intertidal area?
2 Theoretical
Background
The coastal area can be divided into three zones: 1) the subtidal zone, 2) the intertidal zone and 3) the supratidal zone. This study is focused on the intertidal zone, but it is useful to revise all zones because they are all interlinked. Table 1 gives a summary of the forcings and process characteristics, the sediment budgets and morphological changes in all zones. Intertidal bar dynamics is influenced by wave conditions and the tidal amplitude. The wave energy largely determines the amount of sediment transport within the intertidal zone. The tide determines the water depth along the profile and therefore the type of wave action present at different location on the profile, i.e. the tide moves different wave induced processes along the profile in time. The rate at which this happens is called the tidal translation rate
(Masselink and Turner, 1999).
The sediment transport processes that occur in general in the area are undisputable. A lot of research has been done for quite some time on this matter. The problem is that no convincing evidence has been found telling where what mechanisms are dominant, with the small
Table 2-1: Overview of the characteristics of the subtidal, the intertidal and the supratidal zones.
Subtidal zone Intertidal zone supratidal zone
offshore onshore External forcing aves ind rocess characteristics elative small torm conditions alm conditions ediment budget alm conditions orphological change igration Tide W W P Relative large water depth (h) R water depth (h) S C S Storm conditions C M Formation m important; shifting idal
ined with tide
hoaling and Moderate
d
pending
sediment
ar: Convergence of
igration of subtidal
Very important; shifting
e .
uring low
hoaling, breaking
wash, bores, currents
e by undertow e bar in ra h/ ing -
nly way of transport after
maller fetch length, larger
ment r
e aeolian
d
ransport limited
rom exposed intertidal bar hydrodynamic
processes over subt bars
omb
hydrodyn. Process over intertidal bars exposing them at low tide.
ombined with tid C
driving the hydrodyn processes.
exposed d If
tide aeolian transport may initiate - O threshold velocity is reached S
moisture content: reduced aeolian transport
upply of dry sedi S
from intertidal bar, large fetch length: increase in aeolian transport
trong winds, larg S
transport or surge and therefore wave processes Depending on threshold
e shear stress and wind spe Max. transport. Supply limited
T
conditions
-
F
berm and beach, depending on wind direction c driving the hydrodynamic processes - S
wave breaking, weak undertow depending on morphology Intensive breaking an little shoaling waves, strong undertow, depending on morphology Like large h de on location em Id nshore O transport by wave skewness/offshore by undertow Onshore sediment transport by wave skewness/offshore by undertow b sediment transport M bars: offshore/onshore depending on location S waves, undertow S
induced by filling and draining of trough features, exposed bars
ike large h depending L on location em Id ffshor O (possible. flattening bars!)/ onshore 3D bathymetry Onshore sediment transport by surfzon processes/ offshore sediment trans. dominant undertow
ebuilding of R
There is a difference between subtidal bars and intertidal bars on the basis of the processes
ed
ars and the intertidal bars may vary in morphology from place to place
ces
tertidal bars may experience most wave processes consecutively, going from swash and
ch profile it is
to affect a
ay be different for every type of hydrodynamic process.
reement with most hydraulic models- that under calm wave
a they are subjected to. Subtidal bars are affected by the flow field induced by breaking and shoaling waves, whereas intertidal bars are affected by swash processes and currents induc by the filling and draining of trough features dictated by the rise and fall of the tide (Wijnberg and Kroon, 2002).
Both the nearshore b
and depend on specific hydrodynamic conditions. They may also depend on pre-existing morphology of the bed (Wijnberg and Kroon, 2002). The beach slope for example influen the hydrodynamic behaviour which in turn has its influence on the behaviour of the bars. A gentle slope with relatively large water depths promotes landward sediment transport due to weak to moderate wave breaking.
In
bores to breaking and shoaling waves during a full neap to spring tidal cycle. However, the dominance of a process is determined by the location on the bea
acting on. The deepest part of the intertidal profile will be dominated by breaking and shoaling waves while the highest part will be dominated by bores and swash.
The tidal range determines the residence time, the time available for a process
specific location. The smaller the tidal range (e.g. neap tide) the longer processes can be active and change the morphology.
The direction of sediment transport m
The net sediment transport direction during rising tide may shift from onshore by swash and bores, to offshore due to a return current generated by breaking waves in the surf zone (Figure 2.1). Then the net sediment transport may shift back to onshore again by shoaling waves. The sequence is reversed during falling tide and this process will continue with the rise and fall of the tide (Masselink et al., 2006).
Masselink et al., (2006) say -in ag
Figure 2-1: Variations in the cross-shore sediment transport rate and direction for bars at different depths. (Masselink et al. 2006).
The formation of intertidal bars is not completely understood and is subject to much
speculation. The formation of intertidal bars is associated with high energetic circumstances during storms and generally there are two hypotheses concerning their generation (Masselink et al. 2006). Kroon (1994) pursues the hypothesis that the beach is eroded during a storm and this sediment will be transported offshore and stored in the low tide area or in the inner nearshore trough. Swash processes form a small ridge around low tide level that will migrate gently upslope as an intertidal bar in the subsequent days after the storm. The other hypothesis is supported by Aagaard et al. (2004) amongst others. They claim that intertidal bars are part of a bigger cyclic pattern, where bars generated through wave breaking in the subtidal
domain, subsequently migrate onshore into the intertidal zone changing from nearshore bar to intertidal bar.
The intertidal bar is generated due to the convergence of sediment transport in both situations, despite the difference between the two mechanisms. Convergence of sediment transport takes place because the undertow and the infragravity oscillatory sediment transport decrease in larger water depth. The two transport mechanisms decrease in intensity and are directed oppositely (current divergence) so that almost no net sediment transport takes place. Intertidal bar aggradation takes place as a result, slightly landward of the trough (Aagaard et al., 2004; Masselink et al., 2006).
The intertidal bar will migrate on-or offshore after its formation determined by the sediment transport mechanism it is subjected to. In literature swash is mostly described as the
waves, bores and swash action. As soon as the sediment laden up rush flows over the bar crest, the flow magnitude is reduced and sedimentation will take place landward of the crest, thereby shifting the whole bar in the process towards the beach (Figure 2-2). The water is drained through the channel behind the bar, lacking the energy to mobilize the sediment.
Figure 2-2: Photo of an intertidal bar at low tide.
The effectiveness of the swash is largest for onshore bar migration when the crest of the bar is elevated just above high tide level. This will ensure that most swash events will overtop the bar crest during high tide (Masselink et al., 2006).
During storm conditions the water level over the intertidal bars rises, changing the dominant sediment transport mechanisms. According to Kroon (1994) morphological change occurs when the surf zone processes are shifted towards the intertidal domain, with net offshore transport when a significant breaker height of 0.4 m occurs over the bar crest and its seaward slope. The reason for this offshore directed sediment transport is the increase in magnitude of the bed return flow induced by the waves, and is strongest at the bar crest region.
However, onshore migration may persist even during storms, when the bathymetry of the intertidal bars is not two dimensional but three dimensional instead.
At the study site of Skallingen the incident waves had dissipated enormously in shallow waters due to wave breaking, so that the only sediment transport occurred through the mean flows that were generated by the bathymetry of the rip cell (Aagaard et al., 2004). Weak undertows were only measured during periods of high tide when the water depth over the crest was relatively large. During the research campaigns in 1995 and 1996 the welding of the inner bar to the beach was observed as an effect of several moderate storm events. The
primary driving force behind the landward bar migration was the current pattern induced by the alongshore- varying bathymetry creating a positive morphodynamic feedback.
During high tide the largest fluxes occur and are driven mainly by the mean current. These currents only occur when the water level exceeds the level of the bar crest (Figure 2-4). Again the three dimensional bathymetry is responsible for the cell driven mean currents.
Figure 2-3: Sediment transport and wave height during two storms in 1996 at Skallingen. In the lower panel the significant offshore wave height is depicted (hs solid lines) and the water level relative to DNN (z, dashed lines) The upper panel shows the cross-shore suspended sediment flux recorded at the crest of the inner near shore bar (Aagaard et al. 2004).
Figure 2-4: Mean cross shore current, sediment transport and water level during a storm at
Skallingen. Lower panel: mean cross-shore current velocity (Solid lines) and suspended sediment transport rate (dashed lines) at the seaward slope of the intertidal swash bar through. The upper panel shows the mean water level and the crest elevation (Aagaard et al. 2004).
Aagaard et al. (2004) conclude that the behaviour of the intertidal beach is also strongly dependent on morphology.
In several studies it appears that there is a cyclic development in the intertidal domain with phases of accretion and erosion. The process starts with a nearshore bar migrating in landward direction and eventually evolving into an intertidal bar. The intertidal bar migrates further landward and successively the runnel behind the crest will be filled in. Then erosion takes place followed by new intertidal bar formation around MWL, restarting the cyclic event. Increased hydrodynamic energy during storms seems to stimulate greater morphodynamic change whereas calm conditions take longer to modify the bathymetry. This however is not necessarily the case. The relaxation time of an intertidal bar, the time needed for
morphological change after a change in hydrodynamic conditions, is more than just a matter of energy (Masselink et al. 2006). Especially for the intertidal zone the magnitude of sediment transport driven by the dominant process for that specific location at that time, is crucial in determining whether there is on- or offshore directed sediment transport.
Once an intertidal bar has reached a point when it will not be inundated during high tide the bar has welded to the beach and may form a beach berm when it is not eroded (e.g. Aagaard et al. 2004). When the former intertidal bar is no longer overtopped swash/backwash action steepen the seaward facing slope thereby forming a berm. Another mechanism for berm formation is described by Weir et al. (2006). Here the process of berm formation is often linked to the spring–neap tidal cycle. A small berm formed around the maximum uprush of the swash during high tide at lower tidal ranges, is pushed landward and upwards with the rise to spring tides (Hughes and Turner, 1999; Weir et al., 2006). The upward growth of the berm is largely dependent on the largest incoming waves because their swash goes completely over the berm and deposits the sediment on top. The sedimentation can be quite large resulting in a down sloping landward berm face. However, when waves get too large for too long (during a storm event) in combination with increased water levels, they may cut back the berm which eventually leads to complete erosion of the berm (Bascom, 1953 cit. in Komar 1998).
3 Study
Site
Vejers beach is located on the west coast of Jutland in Denmark, just north of Blavands Huk (Figure 3-1). The latter is a distinct bend in the coast line and continues seaward as a sub-aqueous reef partly made of glacial deposits. This reef, Horns Rev, has a crest at about 6 m below Mean Sea Level and protrudes seaward in a WSW direction over 30 km. Horns Rev dissects two different coastal systems: south the peninsula of Skallingen and the Wadden Sea, north the sandy beaches of Vejers.
Figure 3-1: Two maps of the west coast of Denmark. The left panel is a large scale map adapted from Kystdirectoratet (2001). The right panel gives an overview of the study area (red square) and is adapted from Clemensen et al. (2001).
The study site of Vejers Strand is situated just a few kilometres to the north of Blavands Huk. The littoral drift gradient is strongly positive at this location and as an effect the shoreline has been pro-grading in the past. The sand is coming from the north and the net alongshore transport rate is about 2 million m3 per year (Kystderectoratet, 2001).
stronger than winds from eastern directions. The wind direction is ranging from
south-westerly in autumn and winter, and south-westerly in spring and summer. However, storm winds of force 9 and higher (Beaufort scale), are most common from western and north western directions throughout the year (DMI, 1999, Appendix I).
The direction of the wind is of great importance to the processes that play a role on different zones of the beach profile.
The beach of Vejers is wave dominated with an offshore mean annual wave height (Hs) of 1.2
m with wave periods between 4 and 6 seconds. The maximum annual offshore Hs can be
about 6.5 m during intense storms (Aagaard et al., 2007). An attendant effect to strong winds coming from sea, are storm surges. Due to these surges water levels may reach up to 2 to 2.5 m above mean sea level on this beach (Clemensen et al., 1996).
The tidal regime in this area is semidiurnal and can be called micro-tidal, with a mean tidal range of 0.7 m increasing to 1 m at spring tides (www.frv.dk).
The study site is about 200 meters long and about 200 meters wide. The beach is orientated approximately north–west (Figure 3-1). The central line through the study area is a cross-shore profile at line 6200 established by the DCA (Figure 3-2).
There are 3 subtidal bars at respectively ca. 400 m, 700 m and 1000 m from the reference point (Figure 3-3). One or two intertidal bars were present during the field campaign. The inner intertidal bar, closest to shore at ca. 220 m, was at the upper limit of the intertidal zone and welded to the berm. The outer intertidal bar was present at the lower limit of the intertidal zone at around 280 m from the reference point. A berm was situated around 210 m from the reference point at the start of the field campaign and shifted to around 220 m. The supra tidal beach generally slopes up from 1.5 m above DNN to 2.0 m at the dune foot. The dunes are up to 10 m above DNN. 0 200 400 600 800 1000 1200 1400 -6 -4 -2 0 2 4 6 8 10 cross-shore distance (m) el e v a ti o n w ith r e s p ec t to DNN (m ) Profile 6200 MHWL MLWL supratidal zone Intertidal zone Subtidal zone
Figure 3-3: cross shore profile of line 6200, showing the three coastal zones based on the Mean High Water Level and the Mean Low Water Level.
The shore face, as defined by Niedoroda and Swift (1991), has a very gentle slope with β = 0.002 (from ca. 1200 m to ca. 2000 m). The intertidal zone has a slope of β = 0.02 (from ca. 175 m to ca. 300 m). Accordingly the beach is modally dissipative (Komar, 1998).
0 200 400 600 800 1000 1200 1400 1600 1800 -8 -7 -6 -5 -4 -3 -2 -1 0 cross-shore distance (m) e lev ati o n w ith r esp e c t to DN N (m ) Profile 6170 15 September 2005 12 October 2006
Figure 3-4: Cross shore profiles of DCA line 6170 on 15 September 2005 and 12 October 2006. Cross-shore distance is set to 0 m at the end of the measured lines roughly corresponding to the intertidal area.
4 Methods
4.1 Field Methods
The data that have been used for this thesis have been collected from several sources. Morphological measurements have been done nearly every day during the field work and hydrodynamic data such as wave heights, currents and water depth were recorded on specific days. Furthermore external data sources such as the Danish Coastal Authority (DCA) have been used to complement the field measurements.
Data of offshore wave heights and water levels that were continuously recorded by the DCA are used for this thesis. The wave height was recorded by an offshore wave rider buoy at a depth of 15.4 m off the coast at Nymindegab, situated about 25 km from the study site (Figure 3-1, left panel). The measurement interval was 30 minutes.
The water level has been recorded at Hvide Sande, 50 km north of the field site (Figure 3-1, left panel). The water level is measured in the harbour of Hvide Sande with an interval of 10 minutes. Both the data series are downloadable from the website of the DCA.
The wind speed is measured at a meteorological station on Skallingen about 12 km away from the study site (Figure 3-1, left panel). The wind speed and direction were automatically
recorded for every 10 minutes. The wind speed, wave height and water level data were obtained for the entire field campaign, starting on 26 September 2006 at 16:00 h and ending on 18 October 2006 at 23:30 h. Wave and water level data were also obtained for the period before the field campaign, from 1 to 25 September 2006.
m above the bed. The instruments were measured every low tide and adjusted if necessary. The output from the pressure sensor was used to determine local water depth and wave height.
Figure 4-1: Photo of a fully equipped data frame (station 4).
At specific intervals data were recorded from all three stations directly onto a computer. The recording sessions were monitored continuously to prevent errors. The data were recorded with a frequency of 8 Hertz.
Morphological surveys were conducted nearly every day to acquire an accurate record of daily changes to the beach profile. The profile was measured daily with an electronic theodolite along the DCA line 6200. A benchmark on this line provided the z location according to the Danish ordnance datum (DNN). The x and y data were also measured relatively to this reference point (benchmark). Possible measurement errors were caused by either not levelling/steadying the reflector (due to wind for example), and/ or especially the sinking into unconsolidated sediment of the reflector pall. These errors were in the range of a few centimetres. However, this was not problematic when we considered the natural change in morphology during one tidal cycle.
Two additional profiles were measured 50 meters north and 50 meters south of the main profile on most days. This gave a good impression of the spatial variability on the beach. Even two extra profiles in between the other three were measured on days with calm
m. The error on this alongshore y-axis was up to 5 m, but in general it did not exceed the insignificant 1 m.
A detailed transect with bed elevation rods was surveyed about every 30 minutes on two occasions to estimate the instantaneous rate of bed level change. The cross-shore spacing of the rods was about 6 m and the reference height of the rods was measured with the theodolite. The distance from the top of the rods to the sea bed was easily and accurately monitored. October the 5th was the first day to do detailed measurements of the rods. The first
measurements were done at 07:30 h and the last measurements were done at 18:45 h. The second day of rod measurements was on 7 October. The rod surveys on this day were done between 10:55 h and 17:55 h.
Hydrodynamic data were recorded as well at all three stations during the high frequency morphological measurements. Water depth and significant wave height were recorded from 07:30 h until 18:00 h at S2 and S3 on 5 October. The hydrodynamic measurements around 7 October already started at 20:00 h on the 6th and finished on the 8th at 20:00 h. The data recorded at data frame 3 on 7 October were unreliable due to the occurrence of very high spikes. Hence, they were not used in the data analysis.
4.2 Data Analysis
The background data on waves, water levels and winds were calibrated, but had to be stored in the right format in a database in order to use within Matlab. The programme Grapher (GoldenSoftware) was used to construct a wind rose from the wind data obtained from the meteo-station on Skallingen.
Other data that were collected were sometimes calibrated prior to further analysis. The hydrodynamic data measured with the instruments on the data frames were all calibrated in the field. The pressure exerted by the mass of water over the bottom was measured at each data frame. An ‘on the spot’ calibration of the pressure sensors was done by putting the sensors in a tube filled with water. The data were cross checked by physically measuring the height of the water column above the sensor. A simple transformation of the data to height (m) was done to extract the local water depth at each station. These data were further calibrated to correct for e.g. sinking of the sensors into the bottom. The wave heights were extracted from these data by applying four times the standard deviation. This approach is based on a Rayleigh distribution and results in the average wave height of the highest third part of the waves (Aagaard and Masselink, 1999).
The water level, sediment concentration and current velocities measured at the data frames were averaged over a 10 minute interval using the original 8 Hertz sampled series. This was done in Matlab.
Wave type and morphology of the bed were qualitatively determined at the cross-shore rod locations during the detailed measurements of the bed elevation. Examples have been plotted in Figures for 5 and 7 October.
successive profiles were subtracted. The total erosion or accretion was computed by
integrating the differences along the overlapping profile part. All volume calculations for the intertidal area on 5 October are made from 255 m to 302 m. the volume changes calculated for 7 October were done over the longest cross shore distance possible for each survey, ranging between 18 m and 26 m.
An echosounder was used to measure cross-shore profiles of the nearshore.The raw data of the echosounder were transformed into a file with four columns: latitude, longitude, depth (m) and time. The noise in the echosounding profiles caused by waves, was removed by a
5 Results
The field campaign had roughly three periods with different energy conditions (Figure 5-1). The upper panel in Figure 5-1 shows the wind speed (m s-1) in time. The central panel shows the deep water (offshore) significant wave height and the bottom panel shows the water level in time. The fluctuations in water level due to the tide and the large scale fluctuation due to wind effects are visible. The large peak in the middle of the graph is the water level during the storm on 7 October. The increased water level was a result of spring tide and a storm surge.
25-090 28-09 01-10 04-10 07-10 10-10 13-10 16-10 19-10 5
10 15
time (day, month)
w inds pe e d ( m /s ) 25-090 28-09 01-10 04-10 07-10 10-10 13-10 16-10 19-10 1 2 3 4
time (day, month)
s igni fi c a nt w a v e he ig ht ( m ) 25-09-1 28-09 01-10 04-10 07-10 10-10 13-10 16-10 19-10 0 1 2 -1
time (day, month)
w a te r l e ve l to DN N (m )
Figure 5-1: Wind speed, significant wave height and water level during the field campaign, from 26 September 2006 to 18 October 2006.
Average values of wind direction, wind speed, significant wave height and wave period are shown in Table 5-1 for the three different periods.
heights up to 7 m. The last period was one of calm conditions with hardly any wind and small waves (Table 5-1).
Table 5-1: An overview of the mean values for wind speeds, wave height and wave period for the three time periods specified.
Time period Av. wind direction Av. Wind speed (m s-1) Av. Significant wave height (m)
Av. Wave period (sec.)
26-9 until 6-10 south west 5.7 1.2 4.1
6-10 until 8-10 west-north-west 11.0 2.9 5.8
8-10 until 19-10 south east 3.9 0.7 3.7
The wind climate during the field campaign is summarized in Figure 5-2. This wind rose shows the dominant wind direction in the length of the bars. The colours indicate the wind speed in meters according to the classes specified in the legend. Clearly, the strongest winds came from the west and for a short while from the south. Calm conditions were prevailing when winds were blowing from the east and south east. Northerly winds have been absent during the field campaign.
Figure 5-2: A wind rose showing direction of the wind in percentage of occurrence during the field campaign. The classes indicate wind velocities in m s-1.
The mean daily offshore wave heights are depicted in Table 5-2. The highest daily values were reached on 6 and 7 October. The lowest average wave heights occurred on 14 and 15 October.
Table 5-2: An overview of the mean daily values of offshore wave heights during the field campaign.
Date 26-09 27-09 28-09 29-09 30-09 01-10 02-10 03-10 04-10 05-10 06-10
Hs 0.79 1.04 0.93 0.89 0.99 1.44 1.51 1.10 1.43 1.75 2.18
Date 07-10 08-10 09-10 10-10 11-10 12-10 13-10 14-10 15-10 16-10 17-10
5.1 Intertidal Bar Dynamics
The wave energy, together with the tide, influences the amount and direction of the sediment transport and thus the intertidal bar dynamics. This paragraph will discuss the impact of the three distinctive periods as determined by weather and wave energy conditions, on the intertidal bar dynamics.
Water levels were only three times low enough to measure the nearshore bar during our field period. Three cross-shore profiles with two intertidal bars and a nearshore bar are shown in Figure 5-3. The difference in size between this relatively small nearshore bar and the two intertidal bars was obvious. The lower and larger intertidal bar ranged in height between about 0.2 m and 0.45 m throughout the field campaign. The height was measured from the deepest part of the onshore located trough towards the crest. This bar had a convex seaward slope and developed a slip face under calm conditions.
The inner intertidal bar had a height of maximum 0.12 m and therefore was much smaller. This intertidal bar was characterized by a rather long, concave seaward slope in contrast to its height. The lower intertidal bar was exposed during most low tides. The upper intertidal bar was located (and migrating up) around the maximum reach of high water level and was therefore not inundated during neap high tide.
100 150 200 250 300 350 400 450 -3 -2 -1 0 1 2 3 Cross-shore distance (m) el e vati o n w it h r e s p ec t to D N N (m ) profile 6200 11-Oct-06 12-Oct-06 15-Oct-06
The profiles of the DCA transect 6200, measured from 26 September until 5 October 2006 are depicted in Figure 5-4. The cross-shore distance from the reference point is plotted on the x-axis and the height according to DNN on the y-x-axis. The left side of the Figure is landward; the right side of the Figure is seaward. The hydrodynamic measurement stations S2, S3 and S4 are indicated in the Figure as black circles.
The beach had a typical summer profile (Komar, 1998) at the start of the campaign (26 September). The berm was well developed with a steep seaward sloping face and an intertidal bar was situated just beneath the berm. A flat plain was present around DNN which did not match the properties of an intertidal bar, i.e. no clear ridge nor trough was present.
On 27 September the berm had grown and the upper intertidal bar had migrated onshore up the slope. This intertidal bar had merged with the berm and was no longer overtopped by swash action during high tide.
A lot of sedimentation had taken place on the plain where a trough was formed on the
landward edge (approx. 25 m2; Figure 5-4). This formation coincided with a small peak in the significant offshore wave height (Table 5-2, Figure 5-1) and a decrease in the tidal amplitude. A relatively smaller water depth and a relatively larger wave height may have caused this deposition. 150 200 250 300 -1.5 -1 -0.5 0 0.5 1 1.5 2 cross-shore distance (m) el ev at io n w it h r es p ect t o D N N ( m ) S4 S3 S2 Profile 6200 26-Sept-06 27-Sept-06 30-Sept-06 150 200 250 300 -1.5 -1 -0.5 0 0.5 1 1.5 2 cross-shore distance (m) el ev at io n w it h r es p e ct t o D N N ( m ) S4 S3 S2 Profile 6200 01-Oct-06 02-Oct-06 04-Oct-06 05-Oct-06
Figure 5-4: Cross shore profiles of the intertidal zone from 26 September 2006 until 5 October 2006.
However, three days later the profile was modified and was flattened. A second intertidal bar had been formed with only a small runnel between both intertidal bars on 1 October. These intertidal bars were very small compared to the intertidal bar system that emerged on 2
energy conditions caused by a small storm event. Significant wave heights were at least 0.5 m larger and around 2 m. Significant wave heights before this period were around 1 m (Table 5-2, Figure 5-1).
The morphology in the cross-shore profile 50 m north of the 6200 line developed in the same way. However, 50 m to the south the large runnel started to form already on 30 September. The new intertidal bar system increased in size at least until 5 October in all three profiles along the beach. Specific morphologic changes over a tidal cycle on this day are discussed in detail in Paragraph 5.1.1. We could not measure the extended cross-shore profiles due to a storm on 6 and 7 October. This event was however recorded by the 3 data frames, together with high frequency morphological observations on the berm on 7 October (Paragraph 5.1.2). The first measurements of the central line after the storm were made on 8 October (Figure 5-5). The impact of the storm is clearly visible: the berm was completely eroded and the intertidal bar had migrated seaward and was somewhat flattened.
150 200 250 300 350 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 cross-shore distance (m) e le v a ti o n w it h r e s p e c t to DNN (m ) profile 6200S 04-Oct 09-Oct 10-Oct 150 200 250 300 350 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 cross-shore distance (m) e levati o n w ith r esp e c t to D N N (m ) profile 6200 S4 S3 S2 05-Oct 08-Oct 09-Oct 150 200 250 300 350 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 cross-shore distance (m) e le v a ti o n w ith r e s p e c t to DNN (m ) profile 6200N 04-Oct 09-Oct 10-Oct -60 -40 -20 0 20 40 60 250 260 270 280 290 300 310 320 long-shore distance (m) cr o ss -sh o re d ist a n c e ( m ) 04-Oct 09-Oct 10-Oct
The profile measurements of 9 October differed from those on the previous day. The irregular bed at the location of the intertidal bar was probably due to mega ripples that had formed here during the storm. However, the intertidal bar did not migrate offshore at the profiles north and south of the central line. Figure 5-5 (lower and upper left panel) shows that the bar even continued to migrate onshore, even though there were no profile measurements done between 4 and 9 October. Furthermore, the profiles measured on 10 October (north and south) showed that the old berm did not vanish completely, as in the central profile, but retreated some 30 meters inshore. Figure 5-5 (lower right panel) shows the location of the bar crest in the three different profiles just before and after the storm. The southern profile is plotted on the left (+50 m) and the northern profile on the right (-50 m). The crest of the bar was orientated parallel to the beach before the storm, on 4 October, i.e. the black dots were all in line. A lot had changed on 9 October, just five days later. The bar crest positions on the southern and northern profile shifted shoreward. No bar crest position was determined from the central profile due to the irregular bed morphology. However, the bar crest-positions on 10 October showed that the intertidal bar was no longer parallel to the shore. The intertidal bar was now positioned oblique to the shore and the bar crest in the southern profile became part of another intertidal bar. Where the bar crest on the central profile had moved backward (green dot), both the bar crests on the other profiles had moved forward. A rip channel was detected between the two oblique intertidal bars. This is the reason for the discrepancies between the profiles on such a short distance (lower right panel Figure 5-5).
The difference between the erosion on different locations along the berm can now be
explained by the change in morphology of the intertidal bar. The intertidal bar near the central line was partly eroded and changed into a rip channel. Therefore the wave dissipation at this location was smaller than near the neighbouring profile localities. More wave energy was thus dissipated on the berm in the central profile compared to the other profiles.
Figure 5-6: Photo showing the rip channel and the bars north and south of Profile 6200.
Profile 6200 Bar south
Bar north Rip channel
The change in morphology of the intertidal bar from shore parallel to shore oblique was the interlude for the last period in the sequence. Calm conditions prevailed with hardly any wind and small waves in the order of 0.7 m (offshore) during the last 11 days of the field campaign (Figure 5-1).
Figure 5-7: Cross shore profiles of the central line (6200) on 10, 11, 12 and 13 October 2006. The right panel is
a magnification of the area between 220 m and 260 m from the reference point. This panel shows the formation of an intertidal bar.
The lower intertidal bar was not completely eroded during the storm (Figure 5-5). The bar front started to migrate shoreward when the wave energy levels dropped after the storm. During the onshore migration of the bar, it only grew forward but no erosion took place on the seaward slope. The front of the bar became steeper and was migrating shoreward with more than 5 m per day from 10 till 12 October. Both intertidal bars continued to migrate shoreward in the next five days (Figure 5-8). However, the bar migration was not constant in time. Nearly nothing happened on the upper intertidal bar from 15 until 16 October, while the lower intertidal bar migrated almost 6 m in one day. Both bars migrated shoreward about 3 m from 16 till 17 October, and the upper intertidal bar continued to migrate onshore on 18 October. However the lower bar came to a stand still and the trough in front of the bar was quickly filled in.
200 220 240 260 280 300 320 -1.5 -1 -0.5 0 0.5 1 1.5 cross-shore distance (m) e lev ati o n w ith r e s p ect to DN N (m ) profile 6200 15-10-06 16-10-06 17-10-06 18-10-06
Figure 5-8: Cross shore profiles from the central line (6200) on 15, 16, 17 and 18 October. The profiles show the shoreward intertidal bar migration.
Figure 5-9 shows the cross-shore profile development between 16 and 19 October for the central line (top right panel) and the northern and southern lines (top left and bottom left). The lower intertidal bar was migrating onshore on 16and 17 October, and the trough was getting shallower. From 18 till 19 October the conditions slightly changed (Figure 5-1). The trough was filled in and the crest was eroded as a result. The intertidal profile was eroded -5.1 m2 over 80 m and the same process occurred north and south of the main profile. The process first started in the southern profile and was followed by the central and northern profile about three days later. The lower intertidal bar crest in the central profile was located 0.5 m lower and 20 m further seaward than in the other profiles.
220 240 260 280 300 320 -1 -0.5 0 0.5 1 cross-shore distance (m) el evat io n w it h r esp e ct t o DNN ( m ) profile 6200S 16-Oct 17-Oct 18-Oct 19-Oct 220 240 260 280 300 320 -1 -0.5 0 0.5 1 cross-shore distance (m) el ev at io n w it h r e sp e ct t o D N N ( m ) profile 6200 16-Oct 17-Oct 18-Oct 19-Oct 220 240 260 280 300 320 -1 -0.5 0 0.5 1 cross-shore distance (m) el evat io n w it h r esp ect t o D N N ( m ) profile 6200N 16-Oct 17-Oct 18-Oct 19-Oct
5.1.1 Morphologic Evolution of the Intertidal Beach During Low Energy Wave Conditions
The first period of the field campaign was characterized by moderate wave energy conditions. The average wave height was 1.2 m and the average wave period was 4.1 s. The average wind speed was 5.7 m s-1 while the wind was blowing from south western direction (Table 5-1). Waves varied only a little between 1.3 m and 1.4 m during 5 October at the end of the first period. Maximum wave heights that were recorded only barely reached 2.0 m. The waves were stably coming from the north-west throughout the day. The wind was blowing from the south west and steadily decreased from 9 m s-1 in the morning to 7 m s-1 in the evening. Low
tide was around 06:00 h and in the morning and 18:45 h in the evening. High tide occurred around 12:50 h. The tidal amplitude was about 1.0 m because spring tide was due in two days.
The sediment concentrations that were recorded on 5 October are plotted in Figure 5-10. The sediment concentrations at station 2 (most seaward) at three depths followed overall the same trend (top left panel). The sediment concentration increased with the falling tide and
decreased with the rising tide. The second low tide had overall higher sediment concentrations than the first low tide. The concentration at the top (Ct) showed some relatively high peaks during the rising tide from around 14:00 h to about 18:00 h.
The sediment concentration at S3 was low (Figure 5-10, top right). There was no particular trend and the tidal influence was minimal. However the concentrations shot up around high tide at the end of the day. Especially the sediment concentration at the middle (Cm) more than tripled. The concentration at the top was hardly affected.
The sediment concentration at S4 could only be measured between 10:30 h and 17:30 h because the water level was too low before and afterwards (Figure 5-10, bottom left). The sediment concentration overall at S4 is as high as at S2. The concentration is affected by the rise and fall of the tide in the same way as S2. However, the big difference lies within the magnitude of the rise in sediment concentration at the end of the day. The sediment concentration, especially at the bottom, shot up to more than 40 mg l-1.
depth completely, indicating a rise in the wave height towards the second low tide of the day. The offshore wave heights confirm a temporary rise of about 0.1 m between 15:30 h and 16:30 h. 800 1000 1200 1400 1600 1800 0 1 2 3 4 5 6 7 Sediment concentration S2 time (hours) c onc e n tr a ti on ( m g/ l) Ct Cm Cb 800 1000 1200 1400 1600 1800 0 0.5 1 1.5 2 2.5 Sediment concentration S3 time (hours) co n c en tr at io n ( m g /l ) Ct Cm Cb 800 1000 1200 1400 1600 1800 0 5 10 15 20 25 30 35 40 Sediment concentration S4 time (hours) c onc e nt ra ti on ( m g/ l) Ct Cm Cb 800 1000 1200 1400 1600 1800 0 0.5 1 1.5 h and H time (hours) he ight ( m ) S2h S3h S4h S2H S3H S4H
Figure 5-10: Sediment concentration, water depth and wave height at S2, S3 and S4 on 5 October 2006.
The cross-shore flow velocity U is plotted in the left panel of Figure 5-11 for the three data frames. S2 also has a current meter on the bottom. S4 and S2 all correlated in the same way with the tide determined water height h (Figure 5-10), despite relatively large fluctuations. The trend in U was negative during falling tide and positive during rising tide.
S3 however was opposite to the trend in h. S3 was located in the trough and therefore reacted differently. There was little water in the trough during ebb tide and only coast directed bores plunged over S3. It was not deep enough for an undertow to develop and the excess water was being drained through the trough. The cross-shore current was therefore influenced by
smaller influence of the bores (deeper water) and a larger drainage through the channel. The cross shore current at the bottom of S2 was negligible and hardly varied through the course of the tide.
The undertow (return flow) was largest at S4. This area was most strongly influenced by breaking waves during high tide and with a large enough water depth it had the largest undertow. The cross-shore current was directed onshore at all three stations around 16:30 h when the sea was retreating to the second low tide. This coincided with the increased sediment concentrations in the upper panels in Figure 5-10.
The longshore flow velocity V is plotted in Figure 5-11 (right panel) for all three frames. A general rising trend was present throughout the day at all three frames despite large
fluctuations. The shore parallel current was largest in southern direction at S3, where it decreased until an hour before high tide. This was a result of drainage of water through the trough in southern direction. The current almost became zero shortly before high tide at S2. U stayed steady with minor fluctuations after high tide. U was falling during rising tide at S4. Only around high tide the falling trend reversed but then started to fall again just after high tide. The long shore current went up fast after 14:30 h to become a north directed velocity just after 16:00 h. 800 1000 1200 1400 1600 1800 -0.25 -0.2 -0.15 -0.1 -0.05 0 0.05 0.1 0.15
Cross shore flow velocity
time (hours) U (m /s ) S2 S3 S4 S2 bottom 800 1000 1200 1400 1600 1800 -0.5 -0.4 -0.3 -0.2 -0.1 0 0.1
Long shore flow velocity
time (hours) V ( m /s ) S2 S3 S4
Figure 5-11: Cross-shore and longshore flow velocity at S2, S3 and S4 on 5 October 2006.
Detailed rod measurements were done simultaneously with the hydrodynamic recordings, between 07:30 h and 18:45 h.
The cross-shore distance is plotted on the y-axis; time is plotted on the x-axis. The colours indicate the type of wave that was present on a specific location each time. The development of the waves throughout a tidal cycle is visible from this figure. Bores propagated over the seaward slope of the bar during low tide in the morning. The bores reformed into shoaling waves in the trough behind the bar indicated by the red colours. The shoaling waves collapsed into swash closely behind the trough because of the low water level. The sequence of wave types as described above shifted landward during rising tide. The bore zone shoreward of the trough (nearly non-existent during ebb) thereby widened significantly. This bore zone was reduced and vanished again during falling tide.
time (hours) cr o ss-s h o re d is tan ce ( m ) 800 1000 1200 1400 1600 1800 200 220 240 260 280 300 dry swash bores br waves sh waves
Figure 5-12: Wave types at the intertidal beach at 5 October 2006. The colours illustrate the different wave types. From top to bottom: Shoaling waves, breaking waves, bores, swash and dry beach. The colours in between indicate transition zones. Cross-shore distance in meters seaward of the reference point.
time (hours) cr o ss -sh o re d is tan ce ( m ) 800 1000 1200 1400 1600 1800 200 220 240 260 280 300 dry flat bed ripples m ripples
Figure 5-13: Bed morphology at the intertidal beach at 5 October 2006. The colours illustrate the different bed morphologies. From top to bottom: Mega ripples, ripples, flat bed and dry beach. Cross-shore distance in meters seaward of the reference point.
Overall, 5 October was a day with relatively low wave energy. Sediment concentrations at all three stations were low and therefore not much sediment transport occurred. At the end of the day during the falling tide however, sediment concentrations went up. This coincided with increased wave heights, and onshore directed currents at all three stations. The result could be seen in the morphology which will be discussed next.
230 240 250 260 270 280 290 300 -0.5 0 0.5 S4 S3 S2 08:20 10:15 11:45 230 240 250 260 270 280 290 300 -0.5 0 0.5 S4 S3 S2 cross-shore distance (m) e leva ti o n w ith r e sp ec t to D N N (m ) 11:45 17:45 18:45
Figure 5-14: Detailed cross shore profiles on 5 October 2006 at 5 different times.
There was only slight erosion on the intertidal bar and little sedimentation in the trough and on the berm face between 08:20 h and 10:15 h. The sediment transport during this period resulted in net sedimentation of 0.5 m2. The tide was rising and the mean water level was around 0.0 m DNN during this period. Although the sediment concentrations at all levels were pretty low, the water level above the intertidal bar was high enough for bores or small
breaking waves to erode the top of the bar a little. The accretion just in front of station 4 coincided with the location where bores collapsed into swash due to a sudden decrease in water level behind the trough (Figure 5-12).
The next profile was measured 1.5 hours later. A total of -1.1 m2 erosion occurred over the intertidal area from 10:15 h to 11:45 h. The tide was still rising and reached its maximum between 12:00 h and 14:00 h. The sediment deposited in front of station 4 was eroded again at this time. Instead, a small ridge of new deposited sediment is located higher up the profile (ca. 240 m from the reference point) showing the new swash-back wash zone.
Figure 5.12 (right panel) shows that the intertidal bar had increased in size and had migrated towards the beach when the tide had reduced again at 17:45 h. The landward side of the trough had been eroded while at the front of the intertidal bar accretion had taken place. Although profile measurements were lacking during high tide, the measurement stations continued to record the hydrodynamic data. From these data, it can be estimated that not much had changed in the first two hours after the water level dropped in the afternoon. However, from around 16:00 h wave heights increased a little while the water level was still dropping. This could immediately be seen in the sediment concentrations. All sediment concentrations went up quite a lot and especially at the bottom sensor of S4. Even in the trough at station S3 sediment concentrations went up by a lot. At first the undertow at stations S2 and S4
increased a little due to the falling water level but then decreased rapidly when the shore directed movement of the waves and bores became dominant. The increased sediment concentrations and shore directed mean currents reduced just before 18:00 h. The net flux of sediment towards the beach was 0.2 m2 in six hours.
Only deposition took place (0.9 m2) mostly on the intertidal bar but also just landward of the
trough from 17:45 h to 18:45 h.
time (hours) cr o s s-sh o re d is tan ce (m ) 800 1000 1200 1400 1600 1800 200 220 240 260 280 300 -0.05 0 0.05 0.1 0.15
Figure 5-15: Bed level changes in m at the intertidal beach at 5 October 2006. The first observation of the day is the reference profile to all changes. Cross-shore distance in meters seaward of the reference point.
The net result after one sequence of low to low tide was a slight sediment gain in the intertidal area of 0.5 m2. The beach profile had changed even though not much sediment was brought into the system. The intertidal bar system had migrated 1-2 m shoreward (Figure 5-16).
260 270 280 290 300 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 cross-shore distance (m) e le v a ti o n w ith r e s p e c t to DN N (m ) S4 S3 S2 08:20 18:45
5.1.2 Morphologic Evolution of the Intertidal Beach During a Storm Event One period with high energy conditions occurred during the field campaign, starting on 6 October and ending on 8 October. This period started with wind speeds of 8 -10 m s-1. Wind speeds were around 10 m s-1 until noon in the morning of 7 October. The wind speed steadily increased in the afternoon and had its peak from 14:00 h until 18:00 h with average wind speeds between 13 m s-1 and 15 m s-1. The wind was predominantly north-westerly and the incoming waves were coming from south-westerly direction.
The incoming significant wave heights were around 2 m until 13:00 h. A steady rise in significant wave height resulted in waves of 4 m high on average and with a maximum wave height of almost 7 m after 13:00 h. The wave period increased to 5 s during the morning. The maximum wave periods of 7 s were reached during the peak of the storm after 13:00 h. This storm event coincided with springtide on 7 October. A storm surge occurred during this storm, pushing the water about 1 m higher up the beach. The storm surge combined with springtide and wave set up made the water almost reach the dune foot (Figure 5-17).
Figure 5-17: Photo of high water event. The surge water nearly reached the dune foot during a storm on 7 October 2006.
100 150 200 250 300 -1 0 1 2 3 4 5 cross-shore distance (m) e lev ati o n w ith r e s p ec t to D NN (m ) S4 S3 S2 07-Oct 08-Oct 150 200 250 -1 -0.5 0 0.5 1 1.5 2 2.5 3 cross-shore distance (m) e le v a ti o n w it h r e s p e c t t o DNN ( m ) C07-Oct C10-Oct N07-Oct N10-Oct S07-Oct S10-Oct
Figure 5-18: Cross shore Profiles of DCA line 6200 showing the erosion of the berm on 7 October 2006. The left panel shows the entire central profile. The right panel shows the berm for the profiles north, central and south on 7 and 10 October 2006
Most of the erosion had taken place on the berm. The total erosion that took place on the central line was -19 m2 from 150 m to 242 m over one day. The profiles 50 m to the north and south were less eroded, -14 m2 and -12 m2 respectively. Although the active berm was
completely eroded in all profiles, the old berm was conserved in the profiles north and south of line 6200 (Figure 5-18 right panel). The dashed lines in the right panel of Figure 5-18 illustrate the berm morphology as it was before the storm. The solid lines illustrate the situation after the storm. The active berm is located to the right (seaward) of the old berm. Erosion of the berm has been recorded in detail during the bed elevation rod measurements. Hydrodynamic data were recorded at S2 and S4 throughout the entire event representing 2 tidal cycles (Figure 5-19).
To be able to link the sediment concentrations to the wave height (H1/3) and water depth (h)
these are both plotted in Figure 5-19 (bottom right panel). Water depth and wave height both increased during the day. The difference between the first and second high tide of the day was for both stations about 1 m. The water depth over both stations at the second low tide was almost as large as during the first high tide.
different trends. The sediment concentration at the bottom (Cb) is nearly continuously high throughout the storm event except for some downward peaks during the highest high tide. Heavy wave breaking on the lower intertidal bar most likely caused the high sediment concentration at the bottom of S2. The biggest downward peak corresponds to the highest water level reached during the measurements (2.4 m over S2). The high water level shifted the intense wave breaking zone shoreward thereby temporarily reducing the sediment
concentration at S2. The sediment concentration at the top is relatively low at all times except for one peak around 18:30 h where values more than double.
0 500 1000 1500 2000 0 20 40 60 80 100 120 Sediment concentration S2 time (hours) co n cen tr at io n ( m g /l ) Ct Cm Cb 0 500 1000 1500 2000 0 2 4 6 8 10 12 Sediment concentration S4 time (hours) c onc e nt ra ti on ( m g/ l) Ct Cm Cb 0.350 0.4 0.45 0.5 0.55 0.6 2 4 6 8 10 12 H/h c onc e nt ra ti on m g/ l
Relation H/h, sed. concentr S4
Ct Cm Cb 0 500 1000 1500 2000 0 0.5 1 1.5 2 2.5 h and H time (hours) h e ight ( m ) S2h S4h S2H S4H
Figure 5-19: Sediment concentration, water depth and wave height at S2 and S4 on 7 October 2006.
Figure 5-19 (lower left panel) depicts the relation between the relative wave height (H/h) and the sediment concentration. At all three levels at S4 the relation was more or less exponential meaning that as waves became higher relatively to the water depth, the sediment
water where concentrations vary greatly over the vertical profile (Figure 5-19, upper left panel).
The wave types present at the bed elevation rods were observed during each survey on 7 October. Figure 5-20 shows the wave type, illustrated by the colours, at each rod location trough out the day. Shoaling waves occurred in the trough behind the berm until around 14:30 h when the only rods that could still be measured were situated in the bore and swash zone. The seaward rod locations could not be reached because large breaking waves made it impossible to measure. time (hours) cr o ss -sh o re d is tan ce ( m ) 800 1000 1200 1400 1600 1800 200 220 240 260 280 300 dry swash bores br waves sh waves
Figure 5-20: Wave types at the intertidal beach at 7 October 2006. The colours illustrate the different wave types. From top to bottom: Shoaling waves, breaking waves, bores, swash and dry beach. The colours in between indicate transition zones. Cross-shore distance in meters seaward of the reference point.
Then the undertow sharply increased in magnitude to -0.25 m s-1 just after 13:30 h. This was caused by the erosion of the berm explained in detail below. The undertow fluctuated around -0.25 m s-1 with peaks up to -0.33 m s-1 throughout the rest of the day. The undertow stayed strong during the day because no channel was present anymore (formerly behind the berm) to drain the excess water. In general a decreasing trend was present.
At S2 a different trend occurred during the storm event. At the start the undertow at S2 was quite strong (ca. -0.3 m s-1). The return current was decreasing in magnitude until high tide and then increased again towards low tide. During the next rising tide the water level got so high that the mean current became almost zero at 17:20 h. Shortly after the maximum high tide the undertow increased rapidly reaching velocities of at least -0.4 m s-1 at 18:10 h.
0 400 800 1200 1600 2000 2400 -0.5 -0.4 -0.3 -0.2 -0.1 0 0.1
Cross shore flow velocity
time (hours) U (m /s ) S2 S4 S2 bottom 0 400 800 1200 1600 2000 2400 -0.5 -0.2 0.1 0.4 0.7 1
Long shore flow velocity
time (hours) V ( m /s ) S2 S4
Figure 5-21: Cross- and long-shore flow velocity at station S2 and S4 on 7 October 2006. U is positive in seaward direction, V is positive in northward direction.
Although the longshore current (V m s-1) developed a bit different at S4 and S2, the trend at both stations was decreasing, going from north- to south flowing (Figure 5-21). At S4 the long shore current correlated well with the tidal motion. During falling tide the trend was negative and during rising tide the trend was positive. Although the maximum extent of the tide was not reached at 14:00 h near S4, the longshore current started to decrease rapidly to almost zero with 0.5 m s-1 in one hour. The flow direction turned the other way shortly afterwards. After reaching -0.2 m s-1 at 16:30 h the trend became positive again even though the tide was still falling. At S2 the development of the longshore flow velocity was clear. At 01:00 h the current was directed north with a magnitude of 0.8 m s-1. This current kept
The bed level changes that were measured on 7 October are depicted in Figure 5-22. Red indicates accretion whereas all the other colours show gradations in the amount of erosion. Clearly the largest measured changes occur just after 14:00 h.
time (hours) cr o s s-sh o re d is tan ce ( m ) 800 1000 1200 1400 1600 1800 200 220 240 260 280 300 -0.35 -0.3 -0.25 -0.2 -0.15 -0.1 -0.05 0 0.05
Figure 5-22: Bed level changes in m at the intertidal beach at 7 October 2006. The first observation of the day is the reference profile to all changes. Cross-shore distance in meters seaward of the reference point.
time (hours) c ro s s-sh o re d ist a n ce ( m ) 800 1000 1200 1400 1600 1800 200 220 240 260 280 300 dry flat bed ripples m ripples
Figure 5-23: Bed morphology at the intertidal beach at 7 October 2006. The colours illustrate the different bed morphologies. From top to bottom: Mega ripples, ripples, flat bed and dry beach. Cross-shore distance in meters seaward of the reference point.
The rod measurements started at 10:55 h, just after ebb. Nevertheless, the minimal water level in the morning was still about 0.6 m. Nearly the entire intertidal zone was therefore flooded from the start, except for the berm.
Figure 5-24 shows the profile of the berm at different times during the storm on 7 October. The y axis depicts the height above DNN and the x-axis the cross-shore distance from the reference point. The profiles are plotted for 7 different times.
