Sand nourishment - Sand Engine Delfland, North Sea, NL
Location: Coast of Delfland (Netherlands)
Date: March 2011 – November 2011, monitoring from 2011-2016 and from 2016-2021
Involved parties: Rijkswaterstaat, Province of South Holland, Ecoshape, DHV, Deltares, Van Oord, Boskalis
Technology Readiness Level: 9 (Technology available)
Environment: Sandy shores
Keywords: mega-nourishment, innovative, safety, space for nature
|Building with Nature design||Traditional design|
To assess the feasibility of mega-nourishment as an innovative measure to create long term safety conditions in combination with extra space for nature and recreation, a pilot project "Sand Motor Delfland" has been initiated.
Location of Sand Motor
(source Project bureau Pilot Sand Motor)
The coast of Delfland, a coastal stretch of about 14 km between Hook of Holland and The Hague (Netherlands) is characterized by dunes and a net northward transport of sand, driven by predominantly south-westerly winds. The coast is maintained by regular supplies of sand, formerly mostly in the form of beach nourishment, more recently in the form of foreshore nourishment, typically once every 4 or 5 years. The nourishment needed for the Delfland coast is in the order of 300.000 to 500.000 m3 annually.
Sea level rise will lead to a substantial increase in nourishment need for two reasons. One is that erosive processes will intensify, whence keeping the coastline in its present position will require more nourishment. The other reason is the Dutch policy to maintain the entire coastal profile, down to the 20-meter depth contour, because also the deeper part of the coastal profile (the coastal foundation) is considered to be vital to keeping the coast in place. Maintaining the coastline and the coastal foundation of the Delfland stretch while sea level rises, is expected to lead to a nourishment need in the order of 20 million m3 for the coming 20 years.
Considering present nourishment practice, the sea level rise will lead to more frequent nourishment. Beach and foreshore nourishment disturb the (underwater) ecosystem to a large extent and, given the system’s recovery time, a higher nourishment frequency will bring the system into a more or less permanent state of disturbance. This raises the question whether the practice of periodic small-scale nourishment is the most environmentally friendly way of coastal maintenance. The objective of the project was to investigate the feasibility of a practice based on mega-nourishment (‘sand motors’), each enough for a few decades.
An alternative to these periodic nourishments is a mega-nourishment applied every 15 to 30 years. The main advantage of a mega-nourishment over periodic smaller-scale nourishment is less ecosystem disturbance. Moreover, the unit price of the large amount of sand is likely to be less than that of smaller amounts at a time, nature does most of the distribution work and there are additional benefits (recreation, increased nature value, extension of the dune area). Whether this outweighs the costs of the earlier capital investment, however, remains to be seen.
To investigate the effectiveness of a mega-nourishment, a pilot and demonstration project “Sand Motor” was proposed for the Delfland Coast. It involved depositing a large amount of sand (21.5 million m3) on the foreshore and let the forces of nature (waves, tide, wind) distribute it over the coastal profile and along the shore. In this way mega-nourishment gradually feed the dune ridge over a long stretch of coast and over a time span of a few decades, thus contributing to safety against flooding. Large nourishment also create opportunities for nature development and recreation, important additional goals of a mega-nourishment.
The Province of South-Holland initiated the Delfland Sand Motor, driven by the wish to give nature and recreation in the area a boost and to have an icon of innovation. The Province prepared the pilot in close collaboration with various parties, such as the Ministry of Infrastructure and Environment - Rijkswaterstaat (responsible for long-term coastal safety by maintaining the coast line and the sediment volume of the coastal foundation), the Water Board of Delfland (responsible for flood defense system maintenance), the Westland municipality, the municipalities of the Hague and of Rotterdam, Milieufederatie Zuid Holland, the World Wildlife Fund and Ecoshape.
Costs and benefits
Considering only the design and construction costs in the light of the primary function (maintaining the coastal flood defense system), the traditional periodic nourishment practice might be more cost-efficient than a Sand Motor. Yet, there was a strong preference for a mega-nourishment, as additionally, this would create an island or peninsula that would create new possibilities for recreation and nature development. These possibilities, the showcase, the learning experience and the fact that the area might be clear of maintenance for the next 20 years (less frequent disturbance of environment) weighed more than cost-effectiveness in the light of the primary function. Whether eventually the Sand Motor will turn out to be a better deal, economically and ecologically, is a point of investigation.
The first ideas for the Sand Motor date back to the beginning of this century. Initiators were the Province of South Holland and the Ministry of Infrastructure and Environment (Rijkswaterstaat).
A number of developments were brought together in the initiation phase which started in 2007:
- the 'Geluk' parliamentary resolution of 2003, requiring the exploration of ‘an integral, multi functional and sustainable, phased expansion of the coast between Hoek van Holland and Scheveningen’,
- the advice of the Tielrooij Committee of the Province South Holland, in their ‘coast booklet’ (In Dutch: ‘kustboekje’) of 2006, and
- the development of the idea of mega-nourishment in the Rijkswaterstaat innovation program (WINN), reflecting the ambition of Rijkswaterstaat to explore methods to upscale coastal nourishment.
Planning and design phase
The planning and design phase consisted of a pre-feasibility study after which the planing phase stated, taking into account the process of Environmental Impact Assessment (EIA), which emphasized on safety, recreation and knowledge development for the different scenarios.
The Province of South Holland led the exploration phase of the Sand Motor. In a pre-feasibility study (Bruens et al, 2007) different shapes and locations of a Sand Motor were proposed and investigated. Consultant Royal Haskoning was hired to guide the process towards an ambition agreement between the main actors and to design a project development process. In April 2008 the ambition agreement was signed and the planning phase started.
Alternatieve nourishment schemes
The planning phase included the Environmental Impact Assessment (EIA) process. A strategic impact assessment (Grontmij, 2008) and an Environmental Impact Assessment (DHV, 2009) were carried out. In preparation, several alternative designs of a Sand Motor were studied and several scenarios for sustainable long-term nourishment strategies evaluated (Mulder et al., 2010). The EIA procedure was meant to identify the most feasible and environmentally friendly alternatives of nourishment-based coastal management.
Four alternatives were considered, each with a construction volume of 20 million m3 of sand:
- the original nourishment regime (4 to 5 year frequency) with larger amounts of sand,
- a large foreshore nourishment,
- a detached island 1 km off the coast, and
- a peninsula (different locations and shape) attached to the coast.
The preferred alternative: a Peninsula
The design challenge was to locate and design a cost-effective mega nourishment that would serve coastal management and long-term coastal protection and offer opportunities to nature and recreation without having negative impacts on existing nature areas and recreation. Morphological and ecological processes were studied in order to assess the costs and the impacts of these alternatives.
The peninsula had the best scores in the EIA as far as aspects such as safety, recreation and knowledge development were concerned. The shape of a shore-attached hook gives more variation than an island or a foreshore nourishment, as a sheltered zone is created between the hook and the beach which is likely to develop into a temporary lagoon.
Several locations were considered for the peninsula. Given the nature of the pilot, the dynamic character of the Sand Motor and the existing functions and infrastructures, the Solleveld reach (between Kijkduin and Ter Heijde) was selected.
At this location the Sand Motor would have a limited (direct) effect on areas with extensive recreational functions, whereas the area is still accessible for recreation and is “central enough” to supply sediment to most of the Delfland Coast.
The Sand Motor is a pilot project. It is the first mega-nourishment of this size and form, with an anticipated functional lifespan of more than 20 years. So far the coast was maintained with periodic nourishment parallel to the coast or on the beach, the peninsula-shape is a novelty. The largest periodic nourishments are in the order of 5 to 7 million m3, with a functional lifespan of 5 to at the most 10 years.
Since form, location and volume of the Sand Motor are determined by its functioning in the first 20 years after construction, extensive morphological studies were conducted in order to underpin the initial design, realizing that long-term predictions include a large uncertainty bandwidth. Several aspects critical to beach safety, such as beach development and rip current formation, were handled with smaller-scale morphological models. Issues that could not be predicted or dealt with in the design were included in management and monitoring programmes.
After the location and volume were determined in the EIA process, the Sand Motor was contracted with a set maximum budget. The Design and Construct contract was awarded to the contractor that could deliver the largest volume of sand for the set price.
Knowledge development in the design phase of the Sand Motor experiment focuses on three different activities: design development, design assessment to select the preferred alternative and the optimization of the preferred design. Integrating design and assessment activities led to a multifunctional design contributing to coastal safety, nature, recreation and knowledge development. Further information on the knowledge developed in this phase of the Sand Motor process can be found under here.
Important enabling factors for realization of the Sand Motor in the planning phase turned out to be:
- Involvement of a broad representation of actors and stakeholders in the project organization. The ambition agreement was signed by the Province, national ministries, municipalities, a environmental NGO and the Delfland Waterboard. In the project team these parties are represented and complemented with consultants and knowledge institutes (Deltares), NGOs and other parties relevant to the process. This broad and intensive involvement of actors and stakeholders turned out to positively influence the general support for the project.
- The design alternatives for the Sand Motor resulted from the pre-feasibility study (Bruens et al, 2007) and three design workshops held in the summer of 2008. In the study as well as the workshops experts from several disciplines were involved: morphologists, ecologists and dredging operation specialist. During the workshops all stakeholders were represented. This integral approach was instrumental to achieving the necessary multi-functional design.
- The location of the Sand Motor was subject to few legal restrictions, the only one being that the environment should not be negatively affected. Moreover, there were no immediate coastal safety issues and no specific targets concerning nature or recreation. The open formulation of the goals and the absence of legal restrictions contributed to feasibility of the experiment.
Next video, Dutch spoken, illustrates the predicted coastal- and potential ecological development of the Sand Motor as presented at the end of the design stage.
In 2010 an international tender took place, which was won by the consortium of Boskalis and Van Oord. Construction started in March 2011.
After selecting the location (the Solleveld reach, a natural dune area in between the recreational beach areas of Kijkduin in the North and of Ter Heijde in the South) and the shape (hook-shaped peninsula), the dimensions of the Sand Motor were determined.
The peninsula extends 1 km into the sea, and has a longshore dimension of 2 km. Its maximum level is 5 m above chart datum (NAP), which means that part of the surface remains above sea level even under storm conditions. A total amount of 21,5 million m3 of sand was nourished, of which 19 million in the Sand Motor proper. The other 2,5 million m3 were placed in two foreshore nourishments, one at each side. These two foreshore nourishments will serve the parts of the coast to which redistribution of sands from the Sand Motor will not contribute in the initial few years.
The 19 million m3 of sand created an emerged area of initially 100 ha. The main part of the nourishment is within the active zone (which on this coast, is down to the 10 to 12 m depth contour), i.e. that part of the foreshore where wave-induced sand movement is most active. After redistribution of the sand, eventually the Sand Motor is expected to generate a total of 35 ha of new dunes.
In the base of the peninsula a 8 ha lake was created. Apart from creating extra morphological and ecological variation, this lake contributes to maintaining the original groundwater level in the existing dune area, thus safeguarding conditions for the commercially exploited groundwater reservoir in the Solleveld reserve behind the dunes.
In a late stage, some (initially underestimated) potential problems appeared related to the commercial drinking water exploitation in the Solleveld dune area. Generally speaking, the gradual extension of the dune area induced by the Sand Motor will increase the fresh water reservoir of the dunes, which at first sight is positive for drinking water exploitation in the longer run. Yet, the very specific conditions at Solleveld made this to be problematic, as in the existing dunes north of the Sand Motor an amount of polluted rubble was dumped and buried after the war. Widening of the dunes will lead to a seaward expansion of the exploited watershed and may bring it into contact with the polluted rubble. In order to prevent this, a system of groundwater pumps was installed around the polluted area, in order to artificially lower the groundwater table and to prevent groundwater flow from the polluted area into the exploited watershed (for more information see this video - Dutch spoken).
In November 2011 the total of 21.5 million cubic meters of sand had been dredged and put in place. The Sand Motor was officially opened on November 24th, 2011.
Before construction started slight adjustments were made to the position of the Sand Motor, based on discussions with the municipalities of Westland and The Hague. This resulted in a slight northward shift of the location, such that the Sand Motor is located in both municipalities. They agreed, however, that the management of the entire Sand Motor will be in the hands of the municipality of Westland.
During construction regular management and user meeting were organized to inform the stakeholders. To ensure swimmer and beach safety the lifeguard was closely involved and organized information meetings.
Operation and Maintenance phase
In November 2011 the construction of the Sand Motor was completed. A total amount of 21,5 million m3 of sand had been placed in front of the Delfland coast, with the objectives to provide long-term safety, to create extra space for recreation and natural development and to learn as much as possible from it. Are mega-nourishments a good alternative for smaller-scale periodic nourishments? The first results of the experiment will be assessed 5 years after construction. To this end an extensive monitoring program is in place.
During the first year after construction the Dutch coast was exposed to a number of heavy south-westerly storms. As a consequence, the morphological evolution of the Sand Motor proceeded faster than expected, but the shape developed according to the expectation, with the tip of the initial hook extending northward and bending over to the shore, thus creating a tidal lagoon (see this video).
In spring 2012, the extension of the Sand Motor’s tip enclosed a lagoon and an alongshore channel through which the lagoon filled and emptied every tide. Worried about the further development of this channel and its effects on beach slopes and swimmer safety, the Province of South Holland, the authority responsible, decided to close it off with rock and create by another feeder channel, more remote from the beach. During the summer, the dynamic coastal processes drastically changed the lay-out of the channels. As swimmer safety was no longer endangered, the majority of the rock was removed again in September 2012.
In view of the pilot character of the project an extensive monitoring program was set up concerning the morphological and ecological development of the Sand Motor (RWS, Province Zuid Holland et al. (2013).
Considering that the Sand Motor is meant to redistribute alongshore over the stretch of coast between Hoek van Holland and Scheveningen, additional maintenance on this stretch for the coming 20 years, is expected to be limited (Mulder and Tonnon, 2010).
The time-evolution of the Sand Motor is carefully monitored year-round. The first step was to map the shape of the seabed and the ecosystem before construction. The Sand Motor was monitored during construction and is being monitored after construction, for management and operational purposes and to evaluate whether this innovative method of coastal protection actually works.
The monitoring program focuses on six areas: 1) Weather, waves and currents, 2) Sand distribution, 3) Groundwater table and quality, 4) Flora and fauna, 5) Recreation, and 6) Management (see Infographic Sand Motor Monitoring).
Early October 2012 a 40 m high Argus-mast has been placed on the Sand Motor. It is equipped with video cameras which will register the changes of the Sand Motor continuously; directly by visual monitoring of the emerged parts, and indirectly by monitoring wave behaviour (propagation, breaking).
The research program is divided into two phases: 2011-2016 and 2016-2021. This program is coordinated jointly by the Ministry of Public Works and EcoShape and is funded partly by the European Union under the European Program for Regional Development (EFRO).
End of life
Eventually the Sand Motor will be redistributed along the coast by wind, waves and currents. It will induce gradual dune formation along a larger stretch of coastline over a period of a few decades, thus contributing to flood safety and dune nature. Since the Sand Motor is a sacrificial coastal maintenance measure, no significant maintenance is foreseen.
The main lesson learned of the monitoring period 2011-2016 can be found through this link, other lessons learned can be read below.
- Mega-nourishments are tailor-made; to be successful the relevant site-specific coastal processes, such as hydrodynamics, morphodynamics and ecosystem dynamics, need to be understood.
- The specific properties of a particular location will also determine the design of a mega-nourishment.
- Define all (ecosystem) functions and related stakeholders in the area considered. These are crucial to the various project phases, as the selection of alternatives is seldom based on cost-effectiveness alone: socio-economic and political considerations always play a role.
- When considering a mega-nourishment it is important to discuss the merits, costs and effects of various alternative strategies. Focus on comparing the option of a mega nourishment with more traditional alternatives (periodic small-scale nourishment) and identify plusses and minuses of each alternative.
- When considering a mega-nourishment it is best to discuss design (location, position and volume), implementation and management as one comprehensive set, including the way in which monitoring and risk management can take place.
- Mega-nourishments are generally not the most cost-effective in terms of getting sand on the beach, and will only become a preferred strategy if other benefits are taken into account, such as recreational potential and avoiding frequent ecosystem disturbances.
- The form of a mega-nourishment is a matter of choice, governed by a combination of preferences. A temporary island is less suitable for recreation, since it is difficult to reach, very dynamic and may create conditions that are unsafe for swimmers; on the other hand an island is ideal for birds and sea mammals, as it cannot be reached by terrestric predators like foxes. A peninsula helps forming a lagoon, which is an asset for recreation as well as nature development. Foreshore nourishment is ideal if the existing beach conditions are already optimal. Beach nourishment may be the preferred option if beach sports are to be facilitated that require wide open beaches, but also in situations where tidal currents close to shore make offshore alternatives less cost-effective
- A logical choice is to nourish as much as possible within the active zone, as sand within this zone is most susceptible to onshore transport. If the costs of foreshore and beach nourishment do not differ much, there is a strong preference for beach nourishment, of which the losses to deeper water are less.
- Sand properties: using readily available sand is financially often the most attractive. If one may choose between different grain sizes, there is a preference to use the coarser sand below the low water mark, where it will create a steeper stable slope and therefore requires a smaller total volume. The finer fractions then can be kept for dune formation. If wind-blown sand constitutes problems, e.g. because of nearby roads and houses, coarser fractions may be preferable.
- Most mega-nourishments will be quite dynamic, whence their longer-term development is difficult to predict. Close monitoring, adaptive management, scientific supervision and a well defined communication strategy to the stakeholder community therefore are necessary.
- In order to take maximum profit from a large-scale experiment like this, monitoring, data management and generic research need to be specified, arranged and funded before construction.
- Read the results of the first monitoring period 2011-2016 here.
- NatureCoast (website offline), a scientific research program with 12 PhD students and 3 postdocs will deliver its results in 2017.
- Aarninkhof, S.G.J., J.A. van Dalfsen, J.P.M. Mulder and D. Rijks, (2010), Sustainable development of nourished shorelines; Innovations in project design and realisation, Proc. PIANC MMX Congress Liverpool UK 2010
- Bruens, A. et al., (2007), Globaal voorontwerp Zandmotor., WL| Delft Hydraulics report. Project nr Z4459 (in dutch)
- Fiselier, J. (2010), Projectnota MER Zandmotor Delflandse Kust, rapport DHV en H+N+S Amersfoort; dossier C 6158-01.001; 303 pp. (EIA-report; in dutch)
- Grontmij (2008), Pilot Zandmotor Delflandse Kust, Startnotitie Milieu effect rapportage, Grontmij Nederland, Houten februari 2008; 13/99088668/RJJ (in dutch)
- Mulder, J.P.M. , and J. van Dalfsen, (2011), Current Challenges in Coastal Management in the Netherlands: Examples of Pilot Projects, DOI: 10.1061/41190(422)88, ASCE Conference Proceedings , 1088 – 1101
- Mulder, Jan P.M. and P.K. Tonnon, (2010), “Sand Engine”: Background and design of a mega-nourishment pilot in the Netherlands, Proceedings Of The International Conference On Coastal Engineering, No. 32(2010), Shanghai,China. Paper #: management.35.
- Mulder J.P.M. en M.J.F. Stive, 2011, Zandmotor maakt bouwen met de natuur mogelijk, Land + Water nr. 5 ( mei 2011); 20-22 (in dutch)
- RWS , Province Zuid Holland et al. (2013), The Sand Motor; pilot project for coastal protection, Rijkswaterstaat, Provincie Zuid Holland, Ecoshape, Kansen voor West, EFRO, STW Monitoring and Research Sand Motor Brochure 15 pp.
- RWS, Province Zuid Holland et al. (2013), Infographic Sand Motor Monitoring, 1p
- RWS, Province Zuid Holland et al. (2013), Monitoring Fact Sheet Zand Motor. April 2013, 2 pp.
- RWS and Province Zuid Holland. (2012), Fact Sheet Sand Motor, June 2012, 2 pp.
- Stive, Marcel, Matthieu de Schipper, Arjen Luijendijk, Roshanka Ranasinghe, Jaap van Thiel de Vries, Stefan Aarninkhof, Carola van Gelder-Maas, Sierd de Vries, Martijn Henriquez, Sarah Marx, (2013),The Sand Engine: a solution for vulnerable Deltas in the 21st century ? , Proceedings Coastal Dynamics 2013, 1537 – 1545
Tonnon, P.K. (ed.), L. van der Valk, H. Holzhauer, M.J. Baptist, J.W.M. Wijsman, C.T.M. Vertegaal, S.M. Arens, (2011), Uitvoeringsprogramma Monitoring en Evaluatie Zand Motor., Rapport 1203519-000-ZKS-0034 | C172/10, Deltares / Imares, 150 pp. (Monitoring and Evaluation Program – in dutch)
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