Analyses of Amelander Nourishments Sander van Rooij Deltares, 2008
Prepared for: Kustlijnzorg Analyses of Amelander Nourishments Sander van Rooij Report December 2008 Z4582.12
Client Kustlijnzorg Title Analyses of Amelander Nourishments Abstract The morphological impact of nourishments at the island of Ameland during the period 1998-2007 has been analyzed. The development of the sand spit de Haak has a very large influence on the coastline of Ameland. Its recent development involves a straightening of the coastline and erosion of the western and northern part of the spit. Sedimentation is taking place at the eastern side of the spit. With the straightening of the coastline sand waves have developed and migrated from the ebb delta alongshore in front of the Ameland coast. The transport rates are high at the Ameland coast. Sediment losses of over 2 million m 3 per year were observed in an area of 10 km alongshore and 1600 m cross-shore. Sediment losses in the order of a million m 3 per year are occurring over several years. Sand waves are migrating along the coastline from the ebb delta with alongshore velocities in the order of almost 300 meter per year. Sedimentation rates are also high on the eastern side of the sand spit de Haak. It is not clear if this long term erosion of the Ameland coast with roughly 1 Mm3/year is continuing in the future or that it part of a bigger cycle. It is recommended to investigate a longer time span, because the 60 year cycle of the outer deltas may have an influence on the behaviour of the morphology of the central part of the island and its sand balance. A quick scan over the period 1927-2005 does not show an erosion trend of more than 0,2 Mm3/year References Ver Author Date Remarks Review Approved by Sander van Rooij December 2008 J.G.de Ronde Project number Z4582.12 Keywords Shoreface nourishment, Ameland, effectiveness Number of pages 60 Classification None Status Final
Contents List of Figures 1 Introduction...1 2 Area description...2 2.1 The island coast...2 2.2 Overview of nourishments...3 2.3 Wave climate...5 3 Available data...8 3.1 Approach...8 4 2D Data analysis...9 4.1 Cross-shore bar system dynamics...9 4.2 Beach width...20 5 3D Data analysis...25 5.1 Pre-nourishment morphodynamics...25 5.2 Morphodynamic response after the nourishment...29 6 Volume changes...48 7 Discussion...53 7.1 2D morphological changes...53 7.2 3D morphological response of the bar system...53 7.2.1 Pre-nourishment...53 7.2.2 Post-nourishment...53 7.2.3 The decay phase...54 7.2.4 Bar switching...54 7.2.5 Interaction with other nourishments...54 7.3 Sand spit and sand wave development...56 7.4 Beach evolution...56 7.5 Volume development...56 8 Conclusions and recommendations...58 9 Literature...60 Deltares i
List of Figures Figure 2.1 location of Ameland in The Netherlands...2 Figure 2.2 bathymetry of Ameland and surrounding area...3 Figure 2.3 Overview of nourishments at the Ameland coast for the period 1991-1995...4 Figure 2.4 Overview of nourishments at the Ameland coast for the period 1996-2000...4 Figure 2.5 Overview of nourishments at the Ameland coast for the period 2001-2006...5 Figure 2.6 wave height...6 Figure 2.7 occurrence of wave heights above 4 meter...6 Figure 2.8 wave height and direction above 3 meter in the period 2001-2006...7 Figure 4.1: Overview of transects analyzed....9 Figure 4.2: Time series transect 1000...10 Figure 4.3. plot of profiles at transect 1000 for the period 1997 to 2003...11 Figure 4.4 plot of profiles at transect 1000 for the period 2003 to 2007...12 Figure 4.5 Time series transect 1200...12 Figure 4.6 Time series transect 1400...13 Figure 4.7 Time series transect 1600...14 Figure 4.8 profile plot of transect 1600. The nourishment bar height increases after completion of the nourishment (1999)...15 Figure 4.9 Time series transect 1800...16 Figure 4.10 Time series transect 2000...17 Figure 4.11 Time series transect 2200...18 Figure 4.12 profile plot of transect 2200...19 Figure 4.13 Beach width evolution in the nourishment area. The beach width is determined as the distance between the +3 m (dune foot) and -1 m NAP (low water)....20 Figure 4.14 Beach width evolution in the nourishment area. The beach width is determined as the distance between the +3 m (dune foot) and 0 m NAP....21 Figure 4.15 Beach width evolution of the dry beach in the nourishment area. The beach width is determined as the distance between the +3 m (dune foot) and +1 m NAP (high water)....22 Figure 4.16: Assembly of close-ups on the profiles of transects 1000, 1200, 1400 and 1600. In two transects; 1200 and 1400, the onshore movement of the bar near the low waterline can be related to bar behavior resulting from the nourishment....23 Figure 4.17: Assembly of close-ups of profiles of transects 1800, 2000 and 2200....24 Figure 5.1 Jarkus bathymetry of 1995...25 Deltares List of Figures
Figure 5.2 erosion and sedimentation between 1995 and 1996. Sedimentation takes place east of the sand spit (circle). while the outer rim of the spit erodes. Bars migrate offshore....26 Figure 5.3 Jarkus bathymetry of 1996...27 Figure 5.4 sedimentation and erosion map of the period 1996 to 1997....27 Figure 5.5 Jarkus bathymetry of 1997...27 Figure 5.6 sedimentation and erosion map of the period 1997 to 1998...28 Figure 5.7 Jarkus bathymetry of 1998...29 Figure 5.8 Erosion and sedimentation between 1995 and 1998....29 Figure 5.9 Bathymetry of November 1998 showing the nourishment placed between transects 1300 and 2100....30 Figure 5.10 Erosion and sedimentation between spring 1998 and autumn 1998 showing placement of the nourishment....31 Figure 5.11 Jarkus bathymetry of 1999. The circle shows an initiation of a bar switch of the outer bar and nourishments bar. The dashed lines show a bifurcation....31 Figure 5.12 Erosion and sedimentation between 1998 and 1999 (both Jarkus measurements)...32 Figure 5.13 Bathymetry of August 1999...33 Figure 5.14 Jarkus bathymetry of 2000...34 Figure 5.15 erosion and sedimentation between 1999 and 2000...34 Figure 5.16 Bathymetry of July 2000...35 Figure 5.17 Jarkus bathymetry of 2001. Migrating sand waves are indicated with the arrow....36 Figure 5.18 Erosion and sedimentation between 2000 and 2001....37 Figure 5.19 Jarkus bathymetry of 2002...38 Figure 5.20 Erosion and sedimentation between 2001 and 2002...38 Figure 5.21 Jarkus bathymetry of 2003...39 Figure 5.22 Sedimentation and erosion between 2002 and 2003...39 Figure 5.23 Jarkus bathymetry of 2004...40 Figure 5.24 Sedimentation and erosion between 2003 and 2004...41 Figure 5.25 Jarkus bathymetry of 2005...42 Figure 5.26 Sedimentation and erosion between 2004 and 2005...42 Figure 5.27 Jarkus bathymetry of 2006...43 Figure 5.28 Erosion and sedimentation between 2005 and 2006....44 Figure 5.29 Jarkus bathymetry of 2007...45 Figure 5.30 Sedimentation and erosion between 2006 and 2007...46 Figure 5.31 Difference in bathymetry between 1998 (pre-nourishments) and 2007 (postnourishments)....47 Deltares List of Figures
Figure 6.1 Segments for volume calculations...48 Figure 6.2 Location of areas for volume calculations...49 Figure 6.3 Volume changes 6 different sections and total volume change. The three shoreface nourishments are indicated with the red dashed lines. The 2003 shoreface nourishment is not located in the center sections, but only in section 1....50 Figure 6.4: Volume development within the nourishment area (within fixed boundaries)....51 Figure 6.5 MKL position in several transects (1000 to 2200) relative to the BKL...52 Deltares List of Figures
1 Introduction After the analysis of nourishments along the Holland coast, it has become possible to make guidelines for designing nourishments. The Holland coast has however a very different behaviour than the coast at the wadden islands or at the south-western delta. In this analysis a shoreface nourishment is examined at the coasts of Ameland. The morphological behaviour is analyzed based on the Jarkus data and data from morphological surveys, which are specially made to monitor the nourishment behaviour. Deltares 1
2 Area description 2.1 The island coast The Island of Ameland is in the eastern part of the Wadden Sea. The location is shown in Figure 2.1. Ameland has a round head in the west with a long tale. The shape of the island is shaped by the tidal delta in the west, visible in Figure 2.2 and the alongshore currents and oblique wave attack creating the elongated shape. The ebb tidal delta in the west had an important influence on the island because of the ebb delta shoals that migrate onto the island coast. Israël (1998) found a repetitive cycle of shoal formation, migration, attachment to the island and decay. The cycle takes about 50 to 60 years to complete. Ameland Figure 2.1 location of Ameland in The Netherlands Deltares 2
Figure 2.2 bathymetry of Ameland and surrounding area 2.2 Overview of nourishments Several nourishments have been placed at the Ameland coastline. Figure 2.3, Figure 2.4 and Figure 2.5 give an overview of the nourished transects per year, and the type of nourishment. The nourishment of 1998 is the first shoreface nourishment on the island, 2.5 Mm 3 of sediment was placed in the area between transects 1300 and 2100. A second shoreface nourishment was carried out during 2003 between transects 940 and 1370 of approximately 1.3 Mm 3. The nourishment was placed partially seawards of the outer breaker bar (about 200000m 3 ) and mostly in the landward trough. For this purpose a trench was dug through the breaker bar, in order to reach the trough. A third shoreface nourishment was placed in 2006 with a volume of 1.5 Mm 3 between transects 1300 and 1700. Deltares 3
Analyses of Amelander Nourishments Z---- December 2008 Figure 2.3 Overview of nourishments at the Ameland coast for the period 1991-1995 Figure 2.4 Overview of nourishments at the Ameland coast for the period 1996-2000 Deltares 4
Analyses of Amelander Nourishments Z---- December 2008 Figure 2.5 Overview of nourishments at the Ameland coast for the period 2001-2006 2.3 Wave climate As indication the wave characteristics over the period 2001-2006 are given below. In the winter of 2002-2003 the wave climate was relatively calm. In the winter 2003 2004 there were more high waves, as shown in Figure 2.6 and Figure 2.7. Wave direction during storm condition varies in this area between southwest to northwest as shown in Figure 2.8. This figure also shows that the highest waves come from west to northwest, while waves coming in from the southwest are usually lower than 5 meter. Deltares 5
golfhoogte 800 700 600 500 cm 400 300 200 100 0 01-01-01 01-04-01 Figure 2.6 wave height 01-07-01 01-10-01 01-01-02 01-04-02 01-07-02 01-10-02 01-01-03 01-04-03 01-07-03 01-10-03 01-01-04 datum 01-04-04 01-07-04 01-10-04 01-01-05 01-04-05 01-07-05 01-10-05 01-01-06 01-04-06 01-07-06 01-10-06 optreden van golfhoogten boven 4 meter 900 800 700 golfhoogte (cm) 600 500 400 300 3-6-2001 3-12-2001 3-6-2002 3-12-2002 3-6-2003 3-12-2003 3-6-2004 3-12-2004 3-6-2005 3-12-2005 3-6-2006 3-12-2006 tijd Figure 2.7 occurrence of wave heights above 4 meter Deltares 6
hoogte en richting van golven 900 800 700 600 500 400 300 0 45 90 135 180 225 270 315 360 r i c ht i ng v a n gol v e n i n gr a de n Figure 2.8 wave height and direction above 3 meter in the period 2001-2006. Deltares 7
3 Available data The measured bathymetries used for the morphological analysis are based on Jarkus measurements and additional morphological surveys. The used bathymetries are shown in Table 1. Some of the Jarkus measurements showed to be the same as the morphological survey data. The Jarkus data are available both as (transect) profiles and gridded coverage with a 20x20 meter resolution. The morphological survey data is only available as gridded coverages with 20x20 meter resolution. number measurement date Data type 1 1995 spring Jarkus 2 1996 spring Jarkus 3 1997 spring Jarkus 4 1998 spring Jarkus 5 1998 November Extra survey 6 1999 spring Jarkus 7 1999 April Extra survey 8 1999 August Extra survey 9 2000 spring Jarkus 10 2000 July Extra survey 11 2001 spring Jarkus 12 2002 spring Jarkus 13 2003 spring Jarkus 14 2004 spring Jarkus 15 2005 spring Jarkus 16 2006 spring Jarkus 17 2007 spring Jarkus Table 1: Measured bathymetries used for the Ameland nourishment analysis. 3.1 Approach In this study we will focus on the 1998, 2003 and 2006 shoreface nourishments carried out along the Ameland coast. The data are presented and analyzed in different ways, similar to the approach in the VOP 2007 (research program RWS and WL Delft Hydraulics). First, profiles are used to examine cross-shore changes in morphology. Second, the 2D bathymetry maps are used to show the two-dimensional behaviour of the nourishment and the bar/trough system. The morphological analysis is followed by volume calculations to get insight in the sediment transport directions and rates. The analysis aims at answering the following questions: How does the nourishment change in time? Where does the sand go to? What influence does the nourishment have on local morphology? What influence do the morphodynamics have on the development of the nourishment? Deltares 8
4 2D Data analysis 4.1 Cross-shore bar system dynamics The profiles that are analyzed are located at the standard Jarkus transect locations. The transects used are shown in Figure 4.1. The nourishment is visible in the centre of the image within the white line. Of each of the transects the morphological development from 1997 until 2007 will be briefly described. Figure 4.1: Overview of transects analyzed. Deltares 9
Figure 4.2 shows the development of the cross shore profile at the location of transect 1000 in the period 1985 to 2007. The transect is outside the area of the 1998 nourishment. Bar migration is indicated with black lines. Until 1995 there is a steady offshore migration of bars. In 6 years time 3 bars emerge at the shoreline and migrate offshore. The life span of each bar is about 6 years, before it flattens out. After 1997 the profile becomes more irregular and sequences are hard to follow. In the period 2000 to 2007 a two bar system keeps a steady position. After 2004 a new bar appears far offshore, migrating shoreward over 400 meters in 3 years. This is a sand wave related to the ebb tidal delta, as will be shown in chapter 5. Figure 4.3 shows the emergence of a new bar in 1998, at the same time of the placing of the nourishment. In 2003 a shoreface nourishment is placed in the transect area. The bar gets a double top after this, but it is more likely that this is caused by the ebb tidal delta movement, than by the nourishment. Figure 4.4 shows the changing cross shore profile after the 2003 nourishment. Volumes keep increasing. In chapter 5 will be shown that these changes cannot be contributed to the 2003 nourishment alone, but that they are mostly caused by alongshore import of sediment in the area. Figure 4.2: Time series transect 1000 Deltares 10
Figure 4.3. plot of profiles at transect 1000 for the period 1997 to 2003 Moving sand waves Deltares 11
Figure 4.4 plot of profiles at transect 1000 for the period 2003 to 2007 Figure 4.5 with transect 1200 shows a similar pattern as transect 1000. The first period has offshore moving bars between 1985 and 1997. In 1998 there is a rearrangement of the bars. A stable 2-barred system is formed, which moves shoreward until 2004. Note that this transect is outside the nourished area of 1998, but that we see some possible effects of the nourishment in this area. From 2004 onwards the outer bar is bigger, as a result of a second shore face nourishment in 2003. In 2005 bar position changes 100 meter offshore and starts migrating onshore again after that. In 2006 a third shore face nourishment was placed, further away from the transect, so this is not visible in the transect data. nourishment Figure 4.5 Time series transect 1200 Deltares 12
Figure 4.6 shows the development of the cross shore profile in transect 1400. Between 1985 and 1998 there is a steady offshore migration of bars and emergence of bars near the shore. Between 1998 and 2000 the bar positions change to a more pronounced bartrough morphology as a response to the nourishment in 1998. The nourishment reforms into a bar. The former outer bar is stable until 2005, then migrates shoreward. The profile keeps a stable shape. The nourishments of 2003 and 2006 can not be distinguished from the cross-shore morphology. Figure 4.6 Time series transect 1400 Deltares 13
Figure 4.7 shows the profile development in transect 1600. The offshore bar migration is visible until 1996. The 1998 nourishment creates a new bar. Between 2000 and 2005 bar migration is offshore, opposite of the bar behavior in transect 1400. In 2005 the (1998) nourishment bar has mostly disappeared. Figure 4.8 also shows a plot of the profile changes. Remarkable in this case is that the nourishment height increases significantly after 1999. In 2003 a nourishment is placed, mostly in the trough landward of the outer bar. The nourishment is not clearly visible. A third nourishment is placed in 2006, the 2007 profile shows increased bar/trough morphology compared to 2006. Figure 4.7 Time series transect 1600 Deltares 14
Figure 4.8 profile plot of transect 1600. The nourishment bar height increases after completion of the nourishment (1999). Deltares 15
Figure 4.9 shows the cross shore profile development of transect 1800. Bar migration is offshore between 1985 and 1997. A new bar is formed from the 1998 nourishment, but the landward trough does not become very pronounced. Around 2005 the 1998 nourishment bar is almost gone. The bar systems responses to the nourishment with a landward shift, which turns to offshore migration again in 2004. The 2003 and 2006 nourishments are not in the transect area. Figure 4.9 Time series transect 1800 Deltares 16
Figure 4.10 shows the cross shore profile development of transect 2000. Bar migrations is offshore between 1985 and 1995. The profile is relatively stable between 1995 and 2000, possibly due to the 1998 nourishment. After 2000 offshore migrations continues. Figure 4.10 Time series transect 2000 Deltares 17
Figure 4.11 shows the cross-shore profile development of transect 2200, this transect lies outside the nourishment areas. Between 1985 and 1989 bar migration is offshore. In the period 1990 to 1992 the outer bar stabilizes and the inner bar moves shoreward. Between 1994 and 1998 the profile is very stable without any bar migration in cross shore direction. After the nourishment in 1998 a bar is formed that migrates offshore, but note that transect 2200 is outside the nourishment area (which extends to transect 2100). The effect may be a downstream effect of the nourishment. The bar around 600 meter offshore (from RSP) keeps a stable position. The distance between bars consequently increases, and a plateau is formed in between. Figure 4.12 shows the profile plot between 1997 and 2003. Although no nourishment was placed at this location, a bar is formed in 1999. The outer bar then moves offshore, the landward bar remains in place. Figure 4.11 Time series transect 2200 Deltares 18
Figure 4.12 profile plot of transect 2200. Deltares 19
4.2 Beach width The morphological response of the beach to shoreface nourishments is usually not very clear due to the influence of alongshore currents and changing wind and wave conditions. To find effects of bar migration on the beach morphology, the transect locations that were used before in the 2D cross-shore analysis are used here to find trends in beach width evolution as a response to the nourishment. Figure 4.13 shows the beach width evolution of 4 transects within the nourishment area, and 3 transects (without connecting lines) outside the nourishment area. The beach width is determined as the cross-shore distance between the dunefoot (at +3 meter NAP) and the low water line (at -1 meter NAP). Figure 4.14 and Figure 4.15 show the beach width evolution with a seaward boundary at 0 meter NAP and +1 meter NAP. The vertical blue lines in Figure 4.13 to Figure 4.15 show the starting of the shoreface nourishments in 1998 and 2006. There is no uniform response of the beach width. In transect 1000 the beach width increases a lot in the last years, this is caused by migration of the sand spit into the transect area. The spreading of beach width in Figure 4.13 reduces in 1999, but that only occurs in that year. Figure 4.15 shows a drop in the width of dry beach (above +1m NAP) in 1999, followed by recovery the next year. beach width Ameland between +3 and -1 m NAP 500 450 400 beachwidth(m) 350 300 250 200 150 1000 1200 1400 1600 1800 2000 2200 nourishment 100 50 0 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 time(y) Figure 4.13 Beach width evolution in the nourishment area. The beach width is determined as the distance between the +3 m (dune foot) and -1 m NAP (low water). Deltares 20
beach width Ameland +3 to 0 m NAP 250 200 beach width (m) 150 100 1000 1200 1400 1600 1800 2000 2200 nourishment 50 0 1990 1992 1994 1996 1998 2000 2002 2004 2006 time(y) Figure 4.14 Beach width evolution in the nourishment area. The beach width is determined as the distance between the +3 m (dune foot) and 0 m NAP. Deltares 21
Beach width Ameland measured from +3 to +1 m NAP 120 100 beach width (m) 80 60 40 1000 1200 1400 1600 1800 2000 2200 nourishment 20 0 1990 1992 1994 1996 1998 2000 2002 2004 2006 time(y) Figure 4.15 Beach width evolution of the dry beach in the nourishment area. The beach width is determined as the distance between the +3 m (dune foot) and +1 m NAP (high water). Deltares 22
The nearshore bar close to the waterline is difficult to distinguish in the cross-shore profiles, because of its smaller appearance compared to the bars offshore. Figure 4.16 and Figure 4.17 show close-ups of the profile evolution near the beach of all the previously used transects. It was observed before in transect 1200 (Figure 4.5) and transect 1400 (Figure 4.6) that the bars moved shoreward (transect 1200) or maintained position (transect 1400) after the nourishment. In Figure 4.16 is visible that the most landward bar follows this behavior. Figure 4.16: Assembly of close-ups on the profiles of transects 1000, 1200, 1400 and 1600. In two transects; 1200 and 1400, the onshore movement of the bar near the low waterline can be related to bar behavior resulting from the nourishment. Deltares 23
Figure 4.17: Assembly of close-ups of profiles of transects 1800, 2000 and 2200. Deltares 24
5 3D Data analysis 5.1 Pre-nourishment morphodynamics For an overview of the morphodynamics before the nourishment, Jarkus data from 1995 to 1998 is used. Figure 5.1 shows the natural bar system of Ameland with a 2 bar system under an angle to the shoreline. There is also an intertidal bar at some locations. In the west, a sand spit the Haak is visible. Israël (1998) earlier showed that the development of this sand spit shows a cyclic behavior of growth and erosion of about 50 to 60 years. The area of influence of this morphological phenomenon is several kilometers and is likely to influence the nourishment area. At the east of the island the Pinkegat tidal inlet can be found. Figure 5.1 Jarkus bathymetry of 1995 Figure 5.2 shows the erosion and sedimentation taking place between 1995 and 1996, resulting in the 1996 bathymetry as shown in Figure 5.3. The bar system shows a seaward migration and the eastern most tip of the sandspit de Haak has sedimentation. The outer rim of de Haak is eroding. The bathymetry of 1997 in Figure 5.5 shows an increase in height of the outer bar at some points and a further sedimentation east of the sand spit, accompanied by erosion of the outer rim of the spit. Seaward of the spit sand dunes are becoming visible. The centre of the spit remains a low area with little change. The increasing curvature towards the coast locally gives a lowering of the beach and erosion of the dune food (see arrow). The inner bar along the coastline is fragmented. The changes are also visible in Figure 5.4, where the increase in height of the bar and deepening of the trough is visible as Deltares 25
sedimentation (red) and erosion (blue). The sedimentation is taking place at the location of the bar, or alongshore of it, showing alongshore sediment transport rather than cross-shore movement of bars. Figure 5.2 erosion and sedimentation between 1995 and 1996. Sedimentation takes place east of the sand spit (circle). While the outer rim of the spit erodes. Bars migrate offshore. Deltares 26
Figure 5.3 Jarkus bathymetry of 1996 Figure 5.4 sedimentation and erosion map of the period 1996 to 1997. Erosion of the beach and dune foot. Figure 5.5 Jarkus bathymetry of 1997 Deltares 27
The sedimentation and erosion between 1997 and 1998 is shown in Figure 5.6. The sand spit migrates further east. The centre of the spit is filling up with sediment and erosion on the Westside of the spit continues. North of the spit migration of sand waves to the east is visible. Figure 5.6 sedimentation and erosion map of the period 1997 to 1998 Deltares 28
Figure 5.7 Jarkus bathymetry of 1998 Figure 5.8 shows the sedimentation and erosion over the period 1995 to 1998. This figure illustrates the morphodynamics over a period of years. The sand spit de Haak shows eastward migration with erosion at the channel side and sedimentation eastward on the spit. Sand waves related to the ebb delta migrate alongshore eastward. The bar system shows offshore migration under an angle to the coast and there are many changes in bar height, without migration. This is probably the result of alongshore sand transport. Figure 5.8 Erosion and sedimentation between 1995 and 1998. 5.2 Morphodynamic response after the nourishment The first shoreface nourishment in Ameland is placed during 1998. Figure 5.9 shows the bathymetry after completion of the nourishment. Between transect 1400 and 2200 an extra bar is created. Around transect 1400 the two bar system in the western part, connects to the three bar system at the nourishment location. There is no clear alongshore connection between the nourishment bars itself, and the natural bar system in the west. The nourishment is also clearly visible in the erosion and sedimentation map in Figure 5.10. Although the present bar-trough morphology shows steeper slopes, there is little landward bar migration. In 1999 a bar switch is initiated and the outer bar around transect 1400 is more aligned with the nourishment bar, as shown in Figure 5.11(in circle). A full switch is not present yet. A bifurcation is present at the inner bar (dashed lines). The overall effect of the nourishment is best seen in the sedimentation and erosion map in Figure 5.12, which compares the pre-nourishment Jarkus bathymetry of 1998 with Jarkus 1999. The bar system shows major shifts in bar position and alongshore alignment with the nourishment bar. Bars located northeast of the sand spit also Deltares 29
connect alongshore to the bar system. The erosion previously seen at the western side of the spit is also increasing at the northern edge of the spit. Figure 5.9 Bathymetry of November 1998 showing the nourishment placed between transects 1300 and 2100. Deltares 30
Figure 5.10 Erosion and sedimentation between spring 1998 and autumn 1998 showing placement of the nourishment. Figure 5.11 Jarkus bathymetry of 1999. The circle shows an initiation of a bar switch of the outer bar and nourishments bar. The dashed lines show a bifurcation. Deltares 31
Figure 5.12 Erosion and sedimentation between 1998 and 1999 (both Jarkus measurements) The additional survey made in August 1999, shown in Figure 5.13, shows shoreward bar migration. This is typical for summer conditions, when wave conditions favor onshore migration. Deltares 32
Figure 5.13 Bathymetry of August 1999 Figure 5.14 shows the bathymetry of 2000. The spit continues to migrate to the east along with sand waves just north of it. Erosion of the spit is focused at the northwest, as also shown in the sedimentation and erosion map in Figure 5.15. Bar-trough morphology is more pronounced in 2000, with a higher nourishment bar, and a clear trough being formed landward of the nourishment bar. The bar switch around transect 1400 is completed. The outer bar west of the nourishment now fully connects to the nourishment bar in the east and the sand spit in the west. The bifurcation of the inner bar shown in 1999 (Figure 5.11), resulted in a bar switch, but there is no clear trough formed on the landward side. Figure 5.15 also shows sedimentation offshore of the nourishment bar, and landward erosion, showing some offshore movement of the bar. Deltares 33
Figure 5.14 Jarkus bathymetry of 2000 Figure 5.15 erosion and sedimentation between 1999 and 2000. Deltares 34
In the end of July 2000 a second survey is made. The bathymetry is shown in Figure 5.16. Compared to the earlier Jarkus measurement the bars lie closer to the shore. The shoreward bar migration is common during summer periods. Figure 5.16 Bathymetry of July 2000 Deltares 35
Figure 5.17 shows the bathymetry of 2001. Around transect 2200 bars are migrating landward, which is also visible in the sedimentation and erosion map in Figure 5.18. The sedimentation pattern around transect 1400 lies under an angle with the coast, indicating bar switching. The sand spit de Haak has erosion in the northwest and sedimentation in the east. The cross shore diameter of the spit is decreasing, but the alongshore length is increasing. Migration of sand waves to the east along the coastline is visible seaward of the spit (see arrow). In the east of the island, around transect 2200, landward bar migration occurs. Sand waves 2 Figure 5.17 Jarkus bathymetry of 2001. Migrating sand waves are indicated with the arrow. Deltares 36
Figure 5.18 Erosion and sedimentation between 2000 and 2001. Figure 5.19 shows the bathymetry of 2002. Sand continues to accumulate at the eastern side of the sand spit de Haak. Around transect 1000 and 1200 the bars have moved landward, possibly related to the changes at the sand spit. The landward movement is also visible in the erosion and sedimentation map in Figure 5.20. Sand wave migration along the coastline at the location of the spit is reaching transect 1000. The effects are also visible in the profiles at transect 1000 in Figure 4.2. The trough landward of the nourishment has deepened. The nourishment bar itself is not connected anymore on the eastern side with the ebb delta. The bar ends around transect 1800. Around transect 1800 the inner breaker bar is moving landward, but further to the east bar migration is offshore. The inner breaker bar extended eastward and from the intertidal area a third bank has detached. Deltares 37
Figure 5.19 Jarkus bathymetry of 2002 Figure 5.20 Erosion and sedimentation between 2001 and 2002 Figure 5.21 shows the bathymetry of 2003. The nourishment is lowered between transects 1200 and 1600 while sediment is still moved in by spit migration east of transect 1200. There is some erosion and sedimentation in the area, ranging from Deltares 38
troughs being filled to some patches with landward bar migration. This is also shown in the erosion and sedimentation map in Figure 5.22. Figure 5.21 Jarkus bathymetry of 2003 Figure 5.22 Sedimentation and erosion between 2002 and 2003 Deltares 39
Figure 5.23 shows the bathymetry of 2004. In the west the sand spit is growing east around transect 1000, along with the sand waves. At the same time the alongshore curvature of the spit further decreases. The nourishment bar is decaying, which is also visible by the erosion in the sedimentation and erosion map in Figure 5.24. Between transect 940 and 1370 nourishment was carried out during 2003 of approximately 1.3 Mm 3. The nourishment was placed partially seawards of the outer breaker bar (about 200000m 3 ) and mostly in the landward trough. For this purpose a trench was dug through the breaker bar, in order to reach the trough. The extra volume is visible around transect 1000 as sedimentation in Figure 5.24. There is also local landward migration of the inner bar. Around transect 1200 a bifurcation is present (circle in Figure 5.24), probably the result of nourishing in the trough. The breaker bar landward of the 1998 nourishment is connecting to the shoreline at this location. The 2003 nourishment is difficult to distinguish. Between transect 1600 and 2000 the inner bar is moving offshore. Between transect 2000 and 2200 migration is landward. Figure 5.23 Jarkus bathymetry of 2004 Deltares 40
Figure 5.24 Sedimentation and erosion between 2003 and 2004 Figure 5.25 shows the bathymetry of 2005. The 1998 nourishment has disappeared. The sand spit has a straighter coastline with sedimentation eastward. The bar system around transect 1000 is not well connected to the bars around the sand spit or the bars along the eastern part of the coast. The nourishment of 2003 between transect 940 and 1370 is also influencing morphology, but it is difficult to distinguish it. The sedimentation and erosion map in Figure 5.26 shows erosion where there was sedimentation the year before. There is a bifurcation between transects 1400 and 1600, with a scour hole separating the western outer bar from the outer bar in the east. The outer bar, east of the bifurcation connects to the shoreline, as it did the previous year, but shifted eastward. The yearly amount of erosion and sedimentation in 2004-2005 (visible in Figure 5.26) is larger then in the previous seven years. The adaptation of the bar system to the disappearance of the nourishment is very clearly visible. Deltares 41
Figure 5.25 Jarkus bathymetry of 2005 Erosion of nourishment-bar Offshore bar Figure 5.26 Sedimentation and erosion between 2004 and 2005 Figure 5.27 shows the bathymetry of 2006. The sand spit continues to straighten alongshore and sand transport is directed eastward as visible by migration of sand Deltares 42
waves and sedimentation east of the sand spit. This is visible in the erosion and sedimentation map in Figure 5.28. The bar position is mostly the same as in 2005. Around transect 1400 there is a scour hole. Figure 5.27 Jarkus bathymetry of 2006 Deltares 43
Figure 5.28 Erosion and sedimentation between 2005 and 2006. Deltares 44
Figure 5.29 shows the bathymetry of 2007. A straightening of the coastline of the sand spit continues and sand wave migration is to the east. The outer bar has made a bar switch around transect 1600. The connection to the shoreline is gone and the bar is now connected to the outer bar formed near the spit; around transect 1200. At this location a new bifurcation is formed (in circle). During 2006 (after Jarkus 2006 was made) a nourishment was placed between transect 1200 and 1700 of 1.5 Mm 3. In the sedimentation and erosion map in Figure 5.30 the nourishment is visible as sedimentation between transect 1400 and 1800. In the 2007 bathymetry the nourished area has a filled trough and a slightly more seaward outer bar. Sand wave Figure 5.29 Jarkus bathymetry of 2007 Deltares 45
Figure 5.30 Sedimentation and erosion between 2006 and 2007 A different way to show the morphological changes is by showing the difference between the 1998 bathymetry and the bathymetry of 2007. This difference is shown in Figure 5.31. This figure shows the large scale changes of the sand spit with erosion in the west and northwest and sedimentation on the eastern side of the spit. Sand wave migration has led to sedimentation over an area of about 3 km. The sand wave indicated by the arrow in Figure 5.17, and in Figure 5.29 is the same one. Alongshore sand wave migration from 2001 to 2007 is some 1700 meter, giving a migration rate of over 280 meter per year. The yearly fluctuating bar positions and heights give large differences as well. Deltares 46
Figure 5.31 Difference in bathymetry between 1998 (pre-nourishments) and 2007 (postnourishments). Deltares 47
6 Volume changes To be able to estimate the life span and effectiveness of the nourishment and to determine where the sediment goes, the area is divided in different sections, for which separate volume calculations are made. Figure 6.1 shows the sections for volume calculations, and Figure 6.2 the location of the sections on the island. The sections follow the nourishment, which is under an angle to the coastline. As Figure 6.2 shows, the most landward section is only partially over the grid. The outer breaker bar region fits well into section 3. 1 2 3 4 5 6 Figure 6.1 Segments for volume calculations Deltares 48
Figure 6.2 Location of areas for volume calculations. The volume changes are shown in Figure 6.3. Section one lies west and Section 6 east of the nourishment. They are made to estimate the influence of the alongshore transport gradient. Section 2, 3, 4 and 5 are sections at the nourishment location, where section 2 is most landward, 3 is the wave breaker zone, 4 the nourishment and 5 lies seaward of the nourishment. The volume in the nourished section had a large increase in 1998 during the nourishing. After this period the volume increases slightly. This is also visible in the total volume, which shows a peak in 1999, rather than in late 1998. The maximum gain of volume in the total area is 2.5 Mm 3 compared to early 1998. The nourished volume is also 2.5 Mm 3 according to Rijkswaterstaat, but the volume increase within section 4 (the nourishment) is only about 1.4 Mm 3. This is probably because the nourishment is partially in section 3 as well. Between august 1999 and the Jarkus measurement of 2000 the volume in section 4 decreases approximately 1.6 Mm 3 and levels off. After 2001 the volume decreases further until in 2004 it reaches the lowest point at over -2 Mm 3, compared to 1995. Then the volume increases again and stabilizes around the same volume as in 1995. Section 5, seaward of the 1998 and 2006 nourishments has increased volumes of sediment after the 1998 nourishment, of about 0.5 Mm 3 until 2004. After this, the volume decreases to about the pre-nourishment volume. Section 3, landward of the 1998 and 2006 nourishments, has a rapid response to the 1998 nourishment with an increase in volume in the same period and a slow continuous decrease until 2004. After 2004, the volumes decrease rapidly to over -2 Mm 3. After 2006 the volume increases again, due to the 2006 nourishment. During the placement of the 1998 nourishment and the year after, the volume of sand in section 2 decreases. This is the most landward section at the nourishment location. This followed by a short period of increase, which in 2000 results in volumes that are normal again. This is followed by a steady decrease until the last measurement in 2007. The volumes of section 1 and 6 show an overall decrease after the 1998 nourishment. Just after the nourishment, the volume in section 6 increases slightly, but this is very temporary. The 2003 nourishment is located mainly in section 1, which can be seen from the slight volume increase in that section after 2003. Deltares 49
The total volume in the area shows an increase due to the 1998 nourishment and a peak volume in the year after the nourishment. After this the volume decreases rapidly until 2006, when the second large shoreface nourishment is placed in the area. The 2003 nourishment is hardly visible in the overall volume figure. The loss off sediment in the period 1999-2006 is 6.5 Mm 3, giving an average yearly decrease of 0.9 Mm 3 and a peak of 2.7 Mm 3 between 2001 and 2002. Figure 6.3 Volume changes 6 different sections and total volume change. The three shoreface nourishments are indicated with the red dashed lines. The 2003 shoreface nourishment is not located in the center sections, but only in section 1. The volume development of the nourishment has also been calculated using a more exact polygon to give a better estimation of the volumes of the nourishment. The graph is given in Figure 6.4. The volume development is similar to the one in Figure 6.3. The primary objective of nourishments is to maintain the coastline position. The effect of the nourishment on the present coastline position (MKL) determines its success. Therefore the MKL is compared to the standard coastline (BKL) which serves as a reference. The MKL position relative to the BKL position is given for each transect in Figure 6.5. Most transects show a positive trend in the first years after the nourishment; except for transect 1400 and 2000. Deltares 50
Figure 6.4: Volume development within the nourishment area (within fixed boundaries). Deltares 51
Raai 1000 1200 20 25 10 0-10 -20-30 1998 1999 2000 2001 2002 2003 2004 Raai 1000 20 15 10 5 0-5 -10 1998 1999 2000 2001 2002 2003 2004 1200 1400 1600 20 60 15 50 40 10 5 1400 30 20 10 1600 0 1998 1999 2000 2001 2002 2003 2004 0 1998 1999 2000 2001 2002 2003 2004 1800 2000 35 30 25 20 15 10 5 0 1998 1999 2000 2001 2002 2003 2004 1800 40 30 20 10 0-10 -20 1998 1999 2000 2001 2002 2003 2004 2000 2200 20 15 10 5 0-5 -10-15 -20 1998 2000 2001 2002 2003 2004 1999 2200 Figure 6.5 MKL position in several transects (1000 to 2200) relative to the BKL Deltares 52
7 Discussion 7.1 2D morphological changes The cross-shore profiles shown in chapter 4 show a bar system with offshore migration of bars in the period 1985 to around 1993. Between 1993 and the moment of nourishment there is a decrease in bar movement, and in some transects the cross shore movement is completely halted ( Figure 4.11). The effect of the 1998 nourishment is the formation of a new bar. A bar is also formed at transect 1000 and 1200, where no nourishment was made. In transects 1000 to 1400 the bar movement is shoreward or stable. In profiles 1600 to 2200 there is an initial response with landward migration in the first year after the nourishment, but after that bar movement is offshore again. The inner bar around -3 m NAP is hard to distinguish in the profiles because of its smaller scale and irregular pattern. Sometimes the trough is gone and it is not possible to distinguish the bar from the normal slope. It usually follows bar migrations of the offshore bars. The nourishments of 2003 and 2006 are hardly recognizable in the profiles. In transect 1000 there are some major changes in the profile after 2003, but these changes are also caused by sedimentation of the sand spit. Morphological features on the beach like ridge and runnel systems do not seem to respond to a landward migration of bars. With a 5 meter spacing in measurement points within the transects, the beach morphology is hard to capture. A clear relation is hard to find, if present at all. 7.2 3D morphological response of the bar system 7.2.1 Pre-nourishment The pre-nourishment morphodynamics are characterized by small seaward bar migration. The sand spit de Haak shows eastward migration with erosion at the channel side and sedimentation eastward on the spit and in front of it. Sand waves related to the ebb delta migrate eastward alongshore. The bar system shows offshore migration under an angle to the coast. Both in the transect analysis and in the bathymetries a large variation in bar height and steepness is visible, even when cross shore movement of bars is small. Outside the nourished area, transects 1000, 1200 and 2200 respond to the nourishment with formation of a bar connecting to the nourishment. Along with the changes of the sand spit De Haak, this illustrates the large alongshore influences of sediment transport in this area. 7.2.2 Post-nourishment After placement of the nourishment the adjacent bar system connects the outer bar to the nourishment bar by bar switching. The initial reaction is that the outer bar west of the nourishment aligns with the nourishment bar. This is also visible in the transects outside the nourished area as bar formation. The bar system at the nourishment location responds with a landward bar migration of the former outer bar in the first year. The nourishment is reworked into a bar with a pronounced trough at the landward side around the year 2000. After this the nourishment slowly erodes until it completely disappears in 2005. The eastern and most offshore part of the bar erodes the fastest. After 2000 the bar migration in the nourishment area is offshore again. The effect of the nourishment on bar migration thus lasts about a year. Deltares 53
7.2.3 The decay phase The bar system responds to the eroding of the nourishment bar with large scale outward migration of the outer bar in 2004. The nourishment has disappeared after approximately four years, but the landward push of the bar system lasts only about a year. This illustrates the large dynamics in the area. The nourishment only has a small influence within the total dynamics of the system. The system also responds to the disappearance of the nourishment with bar switching; restoring the old alignment of bars along the coastline. Complicating factors are the nourishments of 2003 and 2006 in the same area. 7.2.4 Bar switching When bar switches occur, the local bar-trough morphology is smoothed and a Y shaped bifurcation is formed, where (for example) the outer bar is connected to both the nourishment bar and the former outer bar landward of the nourishment. The inner bar often shows an arcshape with the bar curving seaward at the location of the outer bar switch. At some locations a scour hole, or erosion pit, forms on the landward side at the tip of the outermost bar. A bar switch does not occur in this situation. The development of bar switches as observed at Ameland due to the nourishment is schematized in Figure 7.1. At the end phase of the nourishment the system switches back to the old setting. Bar switches occur between transect 1000 and 1600. East of that, the bars fan out into the ebb tidal delta east of Ameland. 7.2.5 Interaction with other nourishments The many influences of sand spit, the coastline curvature, sand waves, bars and bar switches make the area between transect 1000 and 1200 more difficult to interpret in terms of morphodynamics. There have been two nourishments in 2003 and in 2006 in this area. The 2003 nourishment was placed largely in the trough. The exact shape of the nourishment is not visible in the bathymetry. The bifurcation at the location (visible in Figure 5.23) is most likely made with the nourishment. One year after the nourishment the trough is present again, and the bifurcation is gone, although a new one is formed 2 km east. It seems that the effect of the 2003 nourishment is less visible due to the large natural morphodynamics. A similar nourishment in a trough at the island of Terschelling also showed that a trough is restored again within one year (Grunnet, Walstra en Ruessink 2004). This was accompanied by onshore migration of the landward bar. At Ameland this effect is not visible, because of the large sedimentation caused by the sand spit (see sub chapter 7.3 below). For the 2006 nourishment the effects might be similarly small, but the measuring period is too short to be conclusive. Deltares 54
a) Nourishment Bar migration is offshore. Placement of the nourishment. b) Nourishment (bar) Nourishment is reworked into a bar. A trough is formed landward. Initiation of first bar switch by a bifurcation. Erosion pits can be formed blocking bar switch. Landward bar curves seaward. c) Nourishment bar Bar switch of outer bar completed. A trough is formed landward. Inner bar makes bifurcation d) Bar switch of inner bar completed. The effect on the beach is unclear. Nourishment bar Deltares Figure 7.1 Schematized development of bar switching due to nourishment at Ameland. 55
7.3 Sand spit and sand wave development The morphological development of the sand spit called de Haak is part of a cyclic process on a much larger timescale then the nourishment. Israël (1998) describes a period of 50 to 60 years for the development and erosion of a spit at Ameland. The sand spit influences the morphological development of the island coast as a whole. Between 1997 and 2000 parts of the beach and dunes were eroded as a result of a meandering (ebb tidal) channel on the spit, that was pushed towards the beach in the western part of the island. The spit also provides sediment to the near shore zone along the island coast. Sedimentation occurs at the east and on top of the spit in the eastern side, while it erodes along the tidal channel and on the north side. In the observed period the erosion shifted from mainly on the western side (tidal channel) to mainly on the northern side. The sedimentation of the spit is difficult to use as an indication for alongshore transport, but the sand waves that keep pace with the eastward expansion give an estimate. These sand waves have become better developed and moved into the area since 1995. The decreasing curvature and erosion of the northern part of the sand spit should have made it easier for alongshore currents to pass the spit and form these sand waves. This is probably why sand waves are moving into the area only now. Between 2001 and 2007 sand waves migrate alongshore in eastward direction about 1.7 km, resulting in 280 meter per year of alongshore movement. If the migration rate is considered more or less constant, the most easterly sand wave of 2007 (see Figure 5.29) will reach transect 1200 in 2 years. In Figure 4.2 and Figure 4.3 it is visible that in 2007 a sand wave has moved into the area of transect 1000 and reached a distance from the outer bar of some 200 meter, which might be close enough to start influencing the bar system in the coming years. 7.4 Beach evolution Figure 4.13 shows that in 1999 the spreading in beach width along the nourished transects reduces. This is the result of a bar being pushed landward to just below the intertidal area. This is only for a short time, as in 2000 spreading is larger again. In the evaluation of Jarkus data at Texel (Deltares memo Kustlijnzorg 2008) a similar response has been observed, but for a longer period. In the situation of Ameland, this short lived morphological change can not be attributed to the nourishment in a conclusive way, because of the short life span and large natural dynamics. Together with the Texel results it does however suggest that there might be a tendency of the bar system to form stable bars just below or at the low water level, as part of the landward migration of the bar system and that these bars have a stabilizing effect on the beach width. 7.5 Volume development There is an increase in sediment volume during, and directly after the nourishment in the section of the nourishment and the section landward. The landward section probably shows this increase because part of the nourishment is placed in this section. In the nourished area (section 4) the sediment volume decreases rapidly in 1999-2000 and 2001-2004. The decrease in volume in 1999-2000 is also visible in the sedimentation and erosion map in Figure 5.15. Most erosion is occurring on the landward side of the nourishment, where a trough is formed. There is no equivalent volume of sediment being deposited in the area. In the period 2000 to 2001 there is only little sediment transport, this is visible in the sedimentation and erosion map in Figure 5.18. The volumes remain constant during this period Deltares 56