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Sand nourishment - Hondsbossche Dunes, NL
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.
Toggle Cloak exclusive true id Abstract
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.
In 2004, the coast was designated by Rijkswaterstaat (Directorate-General of Public Works and Water Management in the Netherlands) as one of the Weak Links along the coastline. Which is a qualification to tackle the problem with priority. However, it still took until 2011 before the upgrading of the dike was addressed as an integral project within the framework of the HWBP (the High Water Protection Program). In the meantime, a number of temporary measures were taken because the coastal defense of the area was even weaker than expected in 2003. These temporary measures included a sheet wall and increased slope roughness.
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:
A sandy defense structure such as the Hondsbossche dunes (option 4) has multiple benefits (random order):
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.
The design of the Hondsbossche dunes had several project requirements. First of all, the safety requirements had to be met within the project boundaries. The coastal defense had to be able to withstand 1/10.000 year stormconditions. Secondly, the dunes had to be designed for 50 years taking into account sea level rise. Furthermore, the dune top should not exceed 12 m + NAP and the coastline must be smooth. An important part of the design was the incorporation of environmental quality (recreation, infrastructure and nature) and taking into account uncertainties (see the tool:
In the end, a design was made based on the minimum profile needed to meet safety requirements, and a buffer to compensate for losses during the maintenance period. It makes optimal use of available sand using a feeder beach, providing maximal technical and spatial quality and minimal hindrance.
Cross-section of a part of the Hondsbossche Dunes
Photo of the Hondsbossche Dunes just after creation with some of the main features defined
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.
Morphological development in two years after construction of beach/dune area Hondsbossche Duinen.
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.
Typically observed cross-shore pattern of sand deposition
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):
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.
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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