Sand nourishment - Hondsbossche Dunes, NL
Location: Coastline between Camperduin and Petten (NL)
Date: 2014-2015 (construction), 2015-2018 (monitoring and innovation research project)
Involved parties: Waterboard Hollands Noorderkwartier, Rijkswaterstaat, Ecoshape (HKV, Witteveen en Bos, WUR, Arcadis and Deltares)
Technology Readiness Level: 9 – actual system proven in operational environment
Environment: Sandy shores
Keywords: Constructed dune area, dune lake, sand nourishment, innovation research project, habitat development, sand transport.
The Hondsbossche and Pettemer sea dike (see Traditional Design) did no longer meet current safety standards. Therefore the dike was reinforced in 2015 with a soft, natural barrier of 30 million cubic metres of sand on the seaside of the dike. The resulting area was renamed 'Hondsbossche Dunes'. The construction presented the unique opportunity to study the development of dunes and nature following a large sand nourishment.
The design consists of a soft shallow foreshore (the beach) and a varied artificial dune landscape that has the potential to develop in valuable Nature 2000 habitats. Together, these connected systems make up the primary flood defence and provide the desired spatial quality. The Ecoshape innovation project, is a three-year interdisciplinary study of the area focussing on the development and perception of nature and the morphological development of the area. The aim of the innovation project is to learn more about efficient sand-nourishment with added value for nature and leisure. The research programme includes three themes:
A. Predictability in the development of engineered habitats
B. Optimizing design and morphological evolution
C. Perception of local community and visitors
An improved understanding of these issues is crucial to the elaboration of an evaluation framework for decision-making about sandy solutions to coastal fortifications. The project also offers a clearer picture of the possibilities for the design, management and maintenance of these sandy structures.
Both the Hondsbossche and Pettemer sea dike and parts of the sandy dunes failed the test. The wave overtopping criteria were not met and also the stability of the rear slope grass layer was questionable.
There were several alternatives (see sketches below) to improve the dyke safety in this area: crest raising, foreshore nourishment, making the dyke overtopping-resistant and constructing dunes with a foreshore. The decision to construct dunes with a foreshore prevailed (option 4 in below image). Choosing a sandy solution is in line with the national coastal policy to apply a soft sea defence made of sand where possible. Furthermore, fighting structural erosion by replenishment of sand is also a proven method.
Alternative designs: (from top to bottom) crest raising, foreshore nourishment, making the dyke overtopping-resistant, and creation of dunes in combination with foreshore nourishment.
The other three design alternatives have some negative points which include the following:
- Crest raising would cause too much impact on inside of dike (option 1)
- Only foreshore nourishment would not provide adequate safety (option 2)
- Upgrading of hard protection on seaside and land side of dike would not be sustainable and intensify erosion of foreshore (option 3)
A sandy defense structure such as the Hondsbossche dunes (option 4) has multiple benefits (random order):
- Creates opportunities for nature, recreation and tourism
- Has minor impact on the environment
- Small risk of project delay
- Broad support from stakeholders
- Ready before the end of 2015
- Easy to adapt (future climate changes)
- Verification: straight forward
- In line with our governmental policy
- Proven method
Planning and design
The project had a dual objective: improve the flood safety and spatial quality. The budget that was made available for construction plus 20 years of maintenance was 250 million euros. Several alternatives were considered, but a sandy solution was chosen because of multiple benefits.
On the right you see an overview of the projectplan of the new sea defence. In the plan a spatial zonation is applied. In the middle section nature development prevails with a wet dune valley and a large habitat for birds and plants. At both sides there is space for recreation and tourism. These areas are situated close to the existing villages of Petten and Camperduin.
Natural quality and recreation
The design aims for four habitat types of Natura2000 namely: foreshore habitat (permanently submerged sandbanks, H1110), embryonic dunes (H2110), dunes with sea buckthorn (Hippophaë rhamnoides, H2160) and a dune slack (lowering between the dunes where the water table lies close to the surface, H2190). Dune slacks were included in the design to improve natural quality of the dune area. The design also provided recreational opportunities for citizens and visitors, like bicycle- and footpaths and a horse track.
Nature development at the Hondsbossche Dunes
In English: Grey seal (grijze zeehond), black seaduck (zwarte zee-eend), sword sheath (zwaardschede), big tern (grote stern), dwarf tern (dwergstern), ringed plover (bontbekplevier), natterjack toad (rugstreeppad), beach plover (strandplevier), dune pearl butterfly (duinparelmoervlinder), sand lizard (zandhagedis), nightingale (nachtegaal)
white dunes (witte duinen): sand oats (zandhaver), marram grass (helm), sea milk thistle (zeemelkdistel), blue sea thistle (blauwe zeedistel), sea wind (zeewinde), sea wolf's milk (zeewolfsmelk),
moist dune valley (vochtige duinvallei): green tuber orchid (groen knolorchis), button band (knopbies), kneeling pangassia (knielparnassia), yellow heart (geelhartje), duinrus (duinrus)
dune strudel (duinstruweel): sea buckthorn (duindoorn), wild privet (wilde liguster), single hawthorn (eenstijlige meidoorn), barberry (zuurbes)
The construction contains measures to capture the sand and reduce sand transport. Apart from planting marram grass, these measures included willow screens. Artificial Relief Features (ARF) were introduced to increase the natural dynamics in the area.
Local measures: Willow screens (left) and Artificial Relief Feature (right) (image December 2016)
In the initiation phase, the concerns and preferences of the local citizens were collected via official letters asking for their reaction to the plan. It turned out that for the local citizens, a main concern was that the new dune area would lead to sand transport into the hinterland of the dike (into ‘their backyard’). In a response to the local people’s concerns, the design was adapted to minimise sand transport behind the dike. This was done by creating a ‘buffer area’ between the dunes and the dike and by putting straw bales into this buffer area to capture the sand. Furthermore, the marram grass and dune lake would trap most of the sand within the dune area.
The construction works started with foreshore nourishment using medium size Trailing Suction Hopper Dredgers (TSHD), followed by beach and dune nourishment with jumbo size hoppers. Sand was taken from several offshore borrow areas with varying water depth between 20-28 m and sailing distance between 6 and 11 nautical miles to the project site. By April 2015 all the sand was in place, and the coastal section was officially declared ‘Up to safety standard’ by the Dutch minister of Public Works.
Foreshore nourishment (left) and beach nourishment (right) (image start of 2015)
The Weak Links project consists of the design, construction and 20 years maintenance of the new coastal protection between Camperduin and Petten. The area has a total coastal length of ~11km. The main objective of the construction was the installation of ~35 million m3 of sand. Most of this volume was needed to create a new dune and beach system that fulfils modern safety standards. Approximately 30% was needed to compensate for settlements,hydraulic and aeolian sand losses and a smooth connection to the adjacent shorelines. Additionally, the dune area was partially planted with marram grass, and amongst others a bicycle path, walking path and other recreational objects were constructed. The nourishment works started on March 3, 2014.
Foreshore nourishment was placed by means of ‘rainbowing’ by medium TSHD (left figure). The sand for beaches and dunes were discharged to site with a sinker-line (right figure).
The Hondsbossche Dunes were constructed from Camperduin (south) to Petten (north), following the dominant northern sediment transport direction. The picture below shows the location of the bulldozers and shovels, the hydrodynamic excavator and the dredging ships (hoppers) during the construction works.
Top view and cross section during construction of the Hondsbossche Dunes
In English: foreshore (vooroever), pour front (stortfront), beach and dune (strand en duin), existing HPZ (bestaande HPZ)
bestaande situatie (existing situation), pour set (stortset), hydraulic excavator (hydr. graafmachine), trailing suction hopper dredger (sleephopperzuiger)
Operation and maintenance
The maintenance works for the coastal section is part of the project. The main objective of the maintenance is keeping the coastal protection system up to the initial (safety) requirements of the project.
The maintenance works will continue until 2036. The maintenance tasks within a yearly cycle consist amongst others of:
- Annual bathymetry and topography monitoring
- Quarterly visual inspections
- Sand safety volume and beach width assessment
- (if required) Sand (re-) nourishments
- Pro-active measures against undesirable aeolian deposits
- Monitoring of (quality of) vegetation
- Monitoring of water quality of the recreational lagoon in Camperduin
- Inspections of recreational and other objects
Every year, the monitoring and maintenance program is evaluated and adjusted if required.
Monitoring equipment used for maintenance of the Hondsbossche Dunes
The objective of the Ecoshape innovation project is to learn more about efficient sand-nourishment, with added value for nature and leisure. The research themes are:
A. Predictability in the development of engineered habitats
B. Optimising design and morphological evolution
C. Perception of local community and visitors
In the read more you will find the lessons learned from the Hondsbossche Dunes within the three themes.
A. Predictability in the development of engineered habitats
In the Netherlands, since the late 90s, several sandy reinforcements have been realised. Examples are the ‘Kennemerstrand/duinen’, ‘Spanjaardsduin’, ‘Zandmotor’ (the so-called Sand Engine) and the ‘Hondsbossche Duinen’ which is addressed in this study. A number of other projects such as ‘Waterdunen’ and the ‘Prins Hendrik Zanddijk’ are still under construction. It is envisaged that in the near future more reinforcement projects will follow to compensate for the effects of sea-level rise in order to preserve a safe water defence. All these projects have been initiated for a different reason. Each individual design provides a combination of the three functions, namely coastal safety, nature, and recreation.
While the predictability of the morphological developments with respect to coastal safety is generally good where it concerns the direction and type of developments, it is rather moderate in terms of actual development speed. The (theoretical) controllability of morphological processes seems to be favourably although this assessment could not be made for all areas.
Regarding ecology however, e.g. habitat and nature development, the processes are generally well understood, but can be chaotic. The bandwidth in which (cascade) effects can occur, can be large. To counter undesirable developments, of course measures can be defined, but such an intervention would go against the overall nature concept and the original idea behind the projects, i.e. Building with Nature.
Since the relevant processes have long timescales (up to decades) it is not yet possible to draw conclusions, i.e. judge after only a few years (being the duration of this project). Therefore, whether interventions will be needed on a larger scale to maintain sand dynamics is not clear yet.
B. Optimising design and morphological evolution
The lessons-learned on the morphological evolution and design optimisation of the newly built dune area ‘Hondsbossche Duinen’ are mainly based on data of the dry beach and dune area, therefore determined by aeolian transport of sediment. The development of the underwater area was analysed briefly based on yearly transect data (Bodde et al., 2018b).
The most relevant findings of this study are addressed for a number of aspects, namely (i) physical processes, (ii) the impact of design measures and (iii) the impact of flood safety and subsequent design optimisation.
With respect to the dominant physical processes four main conclusions have been defined.
The total volume of sand blowing into the dunes is determined by the supply of dry sand from the beach rather than by the local geometry of the dune body itself. In other words, the local geometry has only impact on the distribution of the supplied volume.
Alongshore variation in net accumulation of sediment in the study area was observed. The orientation of the dune foot showed a correlation with the total accumulation of sand in the dunes where more accumulation was observed on locations where the shore orientation is closer to perpendicular to the dominant wind direction. The beach width also plays a role in sediment supply to the dunes. But because there is such an abundance of sand on the beach the beach width has not become a limiting factor in sand transport towards the dunes yet. However, locally a reduced beach width was found to reduce the dune growth rates. In other words, the large-scale configuration of the beach/dune area determines the supply of sand.
Morphological development in two years after construction of beach/dune area Hondsbossche Duinen.
Measurement and image taken on May 24, 2015 (left) and August 11, 2017 (right).
An average net dune growth rate (measured above the NAP+3 m level) of 33 m3/m1/year was observed in the first three years after realisation of the nourishment. Later observations correspond with the estimates that were defined during the project design phase. Based on the observed dune growth rates, estimates for future nourishments can now be made with a higher reliability since this additional dataset supports the findings of De Vries et al. (2012) and Van der Wal (2004). It is expected however that sediment accumulation rate will decrease in the coming years due to a reduction in the supply of especially fine sediments from the beach due to natural processes.
The underwater area was not part of the scope of this research project and was analysed only briefly based on transect data, see the figure below. It was found that at least 50% of the total volume increase of the dune area originates from the underwater area below mean high water (MHW). Furthermore, the total dune growth volume is equal to approximately 1/3 of the total observed volume loss from the beach and underwater area.
Volume change of the shallow foreshore, intertidal area, beach and dune on the basis of Jarkustransects over the period 2015-2017 (red – erosion; green – accretion)
Impact of design measures
With respect to the impact of a number of design measures the following conclusions have been derived.
Alongshore variability in the design results in alongshore variable evolution after construction, and therefore in an alongshore variable dune landscape. This is expected to have a positive impact on ecological values (theme A) and public perception and appreciation of the project (Theme C) as well.
On an average cross-shore profile, most of the sediment (70 %) was deposited on the seaward side of the foredune complex, mainly below the NAP+8 m level. The rest (25-30%) of the volume is deposited on the top of the dune, mainly on the seaward part. Only small amounts are deposited on the landward side of this first dune. This typical cross-shore deposition pattern is present for different cross-shore geometries. Only the exact deposition location (height in the profile) of the ‘70 % deposit’ on the seaward side varied.
Typically observed cross-shore pattern of sand deposition
The distribution of sediment deposits in the cross-shore can be used in the design phase to optimise the spatial design of the dune. Habitats or functions which require a dynamic system should be located near the seaward front, whereas functions which require limited or no sand supply should be placed at the landward side. In case dynamics are desired further landward, design measures can be optimised to stimulate transport of sand further into the dune area (f.e. dune slacks).
The overall dune geometry can be optimised as well. Sand is mostly deposited in the lower parts of the profile, so a dune including a low dune on the seaward side creates a wider zone with sand deposits, creating a wider zone with dynamic development. On the other hand, from a safety perspective, a narrower, but high dune is optimal, because the dunes increase very little in height after construction, so the anticipated dune level should be present from the start. In, time, the aeolian deposits on the seaward side of the dune will lead to increased safety level.
Variations in large scale geometry and local measures like brushwood screens, artificial relief features and vegetation can be applied to:
- Influence the locations of sediment deposition;
- Retain sand for coastal safety;
- Prevent nuisance to users and local communities;
- Stimulate differentiation in morphological development.
Depending on whether less or more sediment is required in more seaward or landward dune locations, one can choose between different measures and cross-shore profiles, which will trigger less or more changes in certain areas.
Artificial Relief Features (ARF) are local, unvegetated depressions in the dune with an area in the order of 20-400 m2 and a depth up to 2 m. They trap sand and facilitate an increase in local sand dynamics. By stimulating local sand dynamics, they create height differentiation and varying boundary conditions for habitat development (theme A). An ARF is most effective when situated on the lower dune or on the seaward part of the top of the dune, where the supply of sand is higher. The initial shape/pattern of the ARF remains visible, at least in the first years after construction. To further stimulate a more natural look, the pattern and shape of constructed ARF should be as varied as possible.
Vegetation or another measure like brushwood screens are very effective to locally trap large volumes of sand and are essential to retain the nourished sand in the dune and to capture sand supplied by aeolian transport. In the absence of vegetation less sand is captured, and a larger volume is transported further in cross-shore or alongshore direction and deposited elsewhere. The ARF's are mostly unvegetated and create variations in sand transport further into the dune, showing that a variable vegetation pattern stimulates differentiation in the morphological development in the dune.
Flood safety and design optimisation
With respect to the impact of flood safety and design optimisation, the next conclusions can be drawn (Leenders et al., 2018):
The Hondsbossche Dunes were designed for the current, legally required safety level, including a compensation volume for subsidence and local sea level rise of 0.3 m in the next 50 years. As a consequence, the initial design has a positive safety surplus.
Based on the observed dune growth rates in the first three years after construction and the projected sediment deposition the coming years, it is expected that the natural dune growth rate keeps pace with the rising sea level and subsidence up to 2050, maintaining the initial high safety level.
As such, the compensation volume for 0.3 m sea level rise that was applied in the design, might have been omitted. An important assumption is that the foreshore, intertidal area and beach are maintained at their current level by nourishments, which is standard practice in the Netherlands.
Therefore, in future, similar projects, it may be possible to achieve a volume reduction in the design by anticipating on the natural dune growth rate. Since there is a large uncertainty in sea level rise scenarios, the need for additional nourishments in case of increased sea level rise, can still be assessed as part of the yearly monitoring programme and adaptive management, allowing for a flexible and adaptable coastal defence.
Since windblown sand is mostly deposited in the lower parts of the profile the initial construction of narrow, high dunes at the seaward boundary is favourable. Over time, the aeolian deposits on the seaward side of this dune will lead to a gradual increase of the safety level.
C Perception of local community and visitors
Within this third theme two questions have been addressed, namely (i) how do stakeholders experience sand transport and sand deposition behind the dike, and how do their experiences relate to the monitored sand transport and sand deposition and (ii) what is the effectiveness of measures to reduce sand transport and sand deposition behind the dike (Lagendijk, 2016)?
Monitored and experienced sand transport and deposition
Before the Hondsbossche Dunes were constructed, 18 inhabitants and other stakeholders have expressed their concerns about sand deposition behind the dike.
Both interviews and monitoring results show that the amount of deposited sand behind the dike reduced in the years after construction. The amount of sand blowing towards the dike and over the dike was highest during the construction phase (2014 and 2015). Interesting fact is that the amount of blown sand near Camperduin was already decreasing from the year 2014. This is because substantial sand transport was present before the actual construction of the Hondsbossche Dunes. Creating these dunes and (especially) planting marram grass helped to trap the sand and thus reduce the sand transport from an early stage onwards.
With respect to the cross-shore distribution, sand transport rates are highest at the seaside of the dune area and decrease in the direction towards the dike (see also theme B). Over time, the accumulation of sand at crests of the most inland dunes (the high dunes) decreases, indicating that sand is transported less far inland as excepted. Measurements show that almost no sand is blown over the original dike, and that the amount of deposited sand behind the dike decreases exponentially with inland distance. Also, the interviewed stakeholders confirmed that the highest sand transport occurs near the beach and the first dune row. Here, blown sand can give nuisance for the beach restaurants. Within the dune area, sand has sometimes been found to accumulate on the cycle paths. In the nature area behind the dike (the Harger- and Pettemerpolder) almost no blown sand has been found.
With respect to the alongshore distribution, the amount of sand transport has been highest in the southern part of the Hondsbossche Dunes. This is also confirmed in the LIDAR measurements, which show that sand accumulation in the dunes is higher in the south than in the north. Near the dune valley, the sand gets trapped into the water and does not reach the inner dunes. The coarser sand in the northern dunes causes the amount of sand transport to stay limited, however. The municipalities both north and south of the Hondsbossche Dunes have received no or almost no complaints about blown sand. So, in the interviews there was no difference between the north and south part of the area.
With respect to the local effects it was found that on several places sand accumulation has occurred, particularly on the cycle path in the Hondsbossche Dunes. Especially in the first year after construction, the amount of sand on the cycle path was substantial. Often this occurred near a non-vegetated spot in the dunes that was located seaward from the cycle path (so called Artificial Relief Features). Several stakeholders called the sand accumulation on the cycle paths a nuisance, sometimes causing the cycle path to be closed off. Not only the cycle path, but also the foot paths that are used to access the beach were sometimes covered in sand. This caused high costs for the municipalities, who are responsible for the maintenance of these paths. The amount of sand on the cycle path has decreased since the first year after construction and the expectation is that this amount will decrease further over time.
Effectiveness of measures to reduce sand transport and sand deposition
Because of the concerns of the stakeholders that the construction of the Hondsbossche Dunes could cause sand to be blown behind the dike, the design included measures to trap the sand and reduce sand transport rates. Below, the effectiveness of these measures is described based on measurements and interview responses.
The valley between the dike and the high dune forms a buffer zone to trap blown sand. Although some sand has accumulated at the landward side of the high dune, only a small amount of sand has been deposited on the dike. And incidentally some sand has been recorded on top of the dike or on the landward side of the dike. Sand accumulation on the dike has been higher in the south than in the north. In conclusion, the buffer zone seems to be effective in trapping the sand.
Straw bales were placed in the buffer zone between the high dune and the dike, to trap sand. Because the amount of deposited sand in this valley was so low, however, we cannot conclude if these bales are effective in trapping sand. They did work as wind break and created a sheltered area that stimulated vegetation growth.
Paper pulp has been used to reduce sand transport directly after construction and before the marram grass had been planted. The paper pulp was deposited on the sand to reduce sand transport. One year later, some remainders of this paper pulp were still found between the shrubs and marram grass. The measure was considered partly effective; the effectiveness had been reduced significantly by breaking the pulp cover due to driving over it with heavy machinery.
Marram grass has been planted in the dunes to trap the sand. The marram grass looks more vital in the south part of the Hondsbossche Dunes than in the northern part. At the sea side, the marram grass can hardly keep up with the high amounts of accumulating sand, allowing for the sand to be blown further land inward. All stakeholders have confirmed that the marram grass has been effective in trapping the sand, already from the moment of planting. Some stakeholders note that the less vital marram grass in the north traps less sand compared to the vital marram grass in the south. Therefore, it is recommended to consider what the best conditions are to plant marram grass, to make sure the plants will remain vital.
Several species of shrubs have been planted at the landward side of the high dunes. Most of these shrubs have died, however. Only sea buckthorn has survived. These shrubs are more vital in the south than in the north. Stakeholders mention that the shrubs indeed help to trap the sand.
The sheltered depressions trapped more sand after construction than the vegetated parts of the dunes. They cause local wind dynamics and influence the sand transport locally. Often these sheltered depressions show both accumulation and erosion and cause sand deposition further landwards (downwind). In cases where the depression is located close to a cycle path, it can cause sand deposition on the cycle path. This effect can be reduced by planting part of the sheltered depression, but it would be better not to construct any of these depressions close to cycle paths (or other parts where sand deposition is not appreciated).
Screens made from willow branches have been used to stimulate sand deposition. The screens have been very effective and were often fully covered in sand (after which they do no longer trap sand). They could have been used in a more effective way by placing new screens after the first ones were covered, and by placing screens on strategic locations to reduce nuisance (e.g. near beach restaurants or paths).
Overarching lesson learned
When a project combines different functions trade-offs have to be made. The Hondsbossche Dunes combine water safety, recreational value and nature value. In several occasions you will have to choose which function prevails. Therefore, even though the different functions might be valued equally, on some occasions you will have to choose between the functions.
Bodde, W., Jansen, M., Smit, M., Scholl, M., Lagendijk, G., Kuiters, L., Vries, de D., Kramer, H., Smits, N. and Leenders, J. (2018a) Monitoringsrapportage 2017 (in Dutch)
Vries, de S., Southgate, H.N., Kanning, W. and Ranasinghe, R. (2012) Dune behavior and aeolian transport on decadal time scales, Journal of Coastal Engineering
Wal, van der D. (2004) Beach-Dune interactions in Nourishment areas along the Dutch coast, Journal of Coastal Research, vol 20, pp 317-325
Arens, B., Smit, M. and Valk, van der B. (2015) expertsessie verslag 01-07-2015
Arens, B., Groot, de A., Smit, M. and Valk, van der B. (2015) expertsessie verslag 07-12-2015
Bodde, W. (2017) expertsessie verslag 12-07-2017
Groot, de A. (2015) Veldobservaties Hondsbossche Duinen 11-08-2015
Groot, de A. (2015) Veldobservaties HPZ 02-10-2015
Valk, van der B., Arens, B. and Groot, de A. (2016) expertsessie verslag 08-08-2016
Valk, van der B. (2016) expertsessie verslag 14-12-2016
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