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, RHDHV, Deltares, Van Oord, Boskalis
Technology Readiness Level: 9 – actual system proven in operational environment
Environment: Sandy coasts
Keywords: mega-nourishment, innovative, safety, space for nature
A surplus of sand (in case of the pilot project Sand Motor, order 20 million m3; 10,000 m3 per linear m of coastline) is put into the natural system and is expected to be re-distributed along shore and into the dunes through the continuous natural action of waves, tides and wind. In this way mega-nourishment gradually induce dune formation along a larger stretch of coastline over a period of one or more decades, thus contributing to coastal safety against flooding over a longer period of time and giving less disturbance to the coastal ecosystem while creating opportunities for nature and recreation.
A traditional design of a sand nourishment in the Netherlands, has the primary objective of shoreline maintenance using a medium volume of sand (2-5 million m3; typically 200- 400 m3/m). The lifespan of the nourishment is in the order of 5 years. This means that every 5 years the nourishment has to be repeated, resulting in a frequent disturbance of the ecosystem.
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.
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.