1 Paper III-21
ZANDMOTOR (SAND MOTOR): BUILDING WITH NATURE
JAN P.M. MULDER1 AND MARCEL J.F.STIVE21Deltares, Delft, the Netherlands and Universiteit Twente, Enschede, the Netherlands 2Delft University of Technology, Delft, the Netherlands
Our coast essentially consists of sand from the bottom of the North Sea that was laid there by rivers in previous interglacial periods. Increasing sea levels have meant that this source has gradually almost ceased to exist with build-up reversing to break down. The balance between the supply and demand for sediment is currently negative but sand supplementation can restore this balance. By scaling up supplementation we can ‘Build with Nature’ and so grow in a natural way as the sea level rises and even if it rises more quickly. Along the Delft coastline there is currently an ongoing experiment with a large scale ‘Zandmotor’ which will ensure longer term safety in an innovative way and also provide space for nature and recreation.
DEMAND AND SUPPLY
Our dune coastline is always different. In calm weather waves and currents bring sand from the surf zone to the beach and the wind blows beach sand against the dunes which cause the beach and dunes to grow. When there is a storm, sand is removed from the coastal strip and this mainly disappears under water while some is also blown onto the inner dunes. The dune front is being broken down: the sea gives and the sea takes away. The balance in this game of supply and demand determines the coastal dynamic that we observe through the seasons.
If we leave seasonal variations for what they are and consider the longer term, covering years, then trends of both sand deposition and erosion become apparent. As regards this, it was agreed in 1990 that erosion would be compensated for with sand supplementation, and this was done successfully.
As far as the longer term and larger scale are concerned, the development of our coast will mainly be determined by the relative increase in sea level – which is the combined effect of the increase in the water level (the absolute increase in sea level) and subsidence - and the availability or supply of sediment.
When the water level rises the coast has the tendency to encroach on the land. How strong this effect is depends firstly on the speed of the rise and the type of coast. If the coast slopes gently or has tidal basins, coastal loss will be greater than if the coast is steep and has no tidal inlets. But that isn’t everything. The final effect also depends on the extent to which coastal ‘drowning’ can be compensated for by the supply of sediment from elsewhere. This makes it complicated because an increase in the water level doesn’t just cause the coast to recede because it also changes the possibilities for the supply of sediment. After all, an increase in the water level changes the availability of sources of sediment and the capacity of water and wind to move sediment.
This complicated, large-scale process can also be interpreted as a game of supply and demand. A (relative) increase in the water level causes the depth to increase and in doing so creates extra potential areas for sediment deposition: there is demand for sediment. The supply is determined by the availability of sediment from rivers and by its interchange between the seabed and other parts of the coast and with the Waddenzee and estuaries.
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It is the supply of sand from the North Sea seabed in particular that has been a controlling factor concerning the availability of sediment for the geological make up of our coast and the Low Netherlands. The gradual reduction of this supply of sand – the sea level has slowly increased so much that a large part of the North Sea seabed is now out of the reach of the waves – also explains the erosion of our coast over the last centuries to a large extent. The balance between demand and supply is negative. Earliest estimates indicate an annual shortfall of 20 million cubic metres with the current sea level increase of 2 mm per year. That is about the same as filling the Feijenoord stadium completely full of sand about 25 times per year.
COASTAL MANAGEMENT IS SAND MANAGEMENT
The understanding that the structural recession of the coast is due to a shortfall in the sediment balance led to the introduction of what is known as the Dynamisch Handhaven Beleid (Dynamic Maintenance Policy) in the nineties. Since then the coast line has been maintained by supplementing the shortfall of sand with sand that has been dredged from the North Sea seabed: the geological process of sand deposition has been artificially imitated.
Coastal management is sand management and it would appear that this management is successful: the coast is growing as the sea level rises, the number of locations where the coastline is overrun each year has fallen from 32 % in 1990 to less than 10% after 2001 and the volume of the dunes has grown in most places. Over the entire Dutch coastline the dune foot has shifted between 15 and 20 metres on average toward the sea since 1990 which has increased the land area by around 200 hectares.
Having sand-based coastal management makes it possible to respond flexibly to changing circumstances and understanding which is an important plus point, the value of which can already be seen in practice. The annual sand supplementation volume can be adjusted in parallel with the increasing understanding of the scope of the sand shortage. The initial 6 million cubic metres had already been increased to 12 in 2001, while in 2009 the National Water Plan (Nationaal Waterplan) announced it was investigating a further increase to 20 million cubic metres.
ZANDMOTOR
This announcement appears to be justified. Not only do we currently have a better understanding of the sand shortage given the current speed of the rise in sea level, but there also doesn’t seem to be any question that the sea level is only going to rise faster in the future. Growing at the same rate is only possible then if the volume of supplementation also increases. But do we actually know the optimum way to turn the ‘sand motor’ up a gear?
Our experience is limited to 12 million cubic metres per year. A policy choice in the National Water Plan to follow the rise in sea level – which entails a possible increase to 20 million cubic metres per year – means that it almost doubles. A policy choice for a proactive approach as proposed in recommendation 4 by the Delta Commission 2008, would lead to an annual volume of 85 million cubic metres which is seven times greater than our experience horizon. We will have to gain experience in scaling up supplementation.
Remaining questions:
• if increased sand supplementation is used, what is the most natural way of dynamically maintaining or developing our coast?
• do we consider very large supplementation to be a sand motor – an excess of sand that can freely distribute itself along the coast so that the coast grows naturally? What should this motor look like, where should it be and when, how often and with what intensity should it operate?
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And the questions aren’t limited solely to the technical aspects because the role of social acceptance is at least as important.
PILOT ZANDMOTOR DELFT COAST
The Provinc of Zuid-Holland, in cooperation with the Ministry of Infrastructure and the Environment, has taken the initiative of implementing a pilot sand motor off the Delft coast in order to gain experience in coastal development on an unprecedented scale. The main objective of the pilot sand motor is to combine longer term safety inland from the Delft coast with more room for nature and recreation. Research has shown that the residents of the south wing of the Randstad experience a considerable shortage of these amenities. An important secondary aim is innovation and the development of knowledge. The development phase has two main points of attention in addition to the questions mentioned above. Particular attention has been given to new ways of dealing with coastal maintenance and safety and to the process of cooperation between parties in developing the coastal zone as regards functional invitations to tender and financing forms among other things.
In developing the project the initiators collaborated with the Delfland District Water Control Board (Hoogheemraadschap van Delfland) and the municipalities of The Hague, Westland and Rotterdam. In addition to this, the Stichting Ecoshape was a member of the project group and consultant agencies Grontmij and DHV, research institute Deltares and the World Nature Funds were involved as advisors concerning content.
The design of the sand motor has been gradually refined. In the pre-phase 13 sub variations were considered and these were reduced to 4 in the exploratory phase, each having a construction volume of 20 million cubic metres: (1) an elongated underwater supplementation along the Delft coast with three variations off Ter Heijde, each having a height of 3 metres above Amsterdam Ordnance Datum, namely (2) a delta-shaped peninsula, (3) a streamlined spit and (4) a small island. The main themes for the assessment were coastal safety, nature, recreation, other functions and innovations and feasibility. The dynamic character of the sand motor – the continuously changing form over the longer period – was one of the bigger challenges of the assessment.
A spit to the north of Ter Heijde off the Solleveld area of dunes was put forward in the Environmental Effect Report (Milieu Effect Rapport: MER) as an alternative that achieved the best scores over the entire line – this is known as the Preferred Alternative (VoorkeursAlternatief: VKA). This spit, which would extend into the sea for about 1.5 kilometres and be about 2 kilometres wide, had the best scores as far as effects such as recreation, the development of knowledge and safety were concerned. The spit formation offers the most variation. A sheltered zone is created between the spit and the beach where the island meets the coast, which could gradually become a temporary lagoon. In contrast with this the dynamic in the sea is very strong at that point.
In the definitive design a new lake in the dunes will be created at the base of the spit between the existing dunes and the actual spit. As well as the extra morphological and ecological variation that this creates, it also moderates the negative effects on the ground water flow in the Solleveld drinking water area which is behind the dunes.
A comprehensive monitoring plan has been set up in order to be able to follow the development of the sand motor effectively. The management plan pays particular attention to guaranteeing the safety of swimmers and other holiday-makers.
The official start of the sand motor - the ‘first splash’ – was on 17 January last in Ter Heijde and construction began in March. The sand motor will be constructed as quickly as possible so that storms and currents won’t cause dispersion during dredging and pumping. The bulk of the work should be completed by the end of 2011.
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LEARNING BY TRYING
We have twenty year’s experience in the implementation, monitoring and research of the effects to thank for our existing knowledge of supplementation. The pilot Zandmotor Delfland offers the chance to use this approach in practice with the necessary increase in the scale of supplementation. The pilot can teach us to take care of safety over the longer period and to provide space for nature and recreation in an innovative way.
