Haringvliet Sluices Ajar: mitigating ecological impacts of the Delta-works in the Netherlands
By Hugo Coops (RIZA-Institute for Inland Water Management and Waste Water Treatment), Stan Kerkhofs and Kees Storm (Rijkswaterstaat Directorate Zuid-Holland)
Why were dams built in the Delta?
Large part of the Netherlands is situated in the delta of the rivers Rhine and Meuse. The low-lying position next to the sea has been advantageous for the society, resulting in a high population density and a thriving economy. On the other hand, flooding risks from both the rivers and the sea have always been high. This became obvious when on 1 February 1953 a disastrous flood occurred in the southwestern part of the country, killing nearly 1800 people.
In the wake of the disaster the Delta-project was started, intended to forever safeguard the land from such disaster. After the design phase, the plan was adopted in 1958.
The Delta project consisted of damming off all estuarine branches except those forming the entrances to the harbours of Rotterdam and Antwerpen. Several straight dams were completed in the period 1960-1969, followed by the Haringvliet dam in 1970. The latter included a complex of 17 sluice-gates to regulate discharge from the rivers to the sea.
The final dam to be built, in the Oosterschelde branch, roused large-scale protest in the 1970's because of increased environmental awareness: as in the other estuarine branches, the unique tidal ecosystem would be lost, and anoxic conditions, smelly algal blooms, and loss of the profitable musselbeds would occur. The debate ended with a high-tech solution of a huge storm-surge barrier, that was ultimately completed in 1986.
Impacts of the Haringvliet-dam
Back to the Haringvliet-dam: the main estuarine branch of the Rhine and Meuse rivers, with a large, unique area of brackish and freshwater tidal marshes, that was important as a habitat for many species of flora and fauna and a major migratory pathway for important fish species such as salmon, shad and sturgeon. These values had already been in decline due to river regulation, embankment and pollution, but the Haringvliet dam gave the final strike.
Among the negative impacts of the Haringvliet sluices were:
1) A reduction of the tidal amplitude from 2 m or more to a relic tide of 0.3 m, and reduction of the intertidal area from 6550 ha to a mere 300 ha. Parallel to this was the loss of shorebird feeding areas and transition of the intertidal vegetation into extensive ruderal vegetations.
2) Near disappearance of bulrush, reed and willow coppice cultures and estuarine fisheries.
3) The former brackish transition zone was completely lost, including habitat for estuarine fish and macro-invertebrates and characteristic brackish flora.
4) Extensive erosion of lower shoreline zones due to the stabilized water level.
5) Large-scale sedimentation filling up former estuarine channels with highly polluted silt (heavy metals and PCB's, to mention a few).
6) Transition of a foodweb dominated by pelagic production to benthic production (oligochaetes, zebra mussels), with associated risks of toxic compounds from the sediment entering the food chain.
7) Extended residence time of the water in the estuary resulting in algal blooms and oxygen deprivation in deeper water.
Although the impacts on the ecosystem were negative, the Haringvliet Sluices have had also some acclaimed positive effects:
1) reliable freshwater resource for drinking water and agriculture
2) tide-free navigation connection between the harbours of Rotterdam and Antwerpen
3) opportunities for recreation and road infrastructure
4) morphological development in front of the estuary
Reopening the Haringvliet Sluices?
Considering the negative impacts of the Haringvliet dam on the estuarine ecosystem, and the changed attitude towards damming off estuarine branches, as exemplified by the Oosterschelde storm-surge barrier, alternative management of the existing sluices was taken up in an Environmental Impact Study during the years 1994-1998. The aim of the study was to assess the restoration potential of the estuary without affecting the safety level for which the dam was built in the first place. The conclusions of the study were that the operation of the Haringvliet Sluices as a storm-surge barrier, could lead to the redevelopment of estuarine communities in the region. However, the negative effects on the various functions would need many costly compensatory measures. Therefore, intermediary alternatives called "Broken Tide" and "Controlled Tide", with progressive degree of sluice opening, were also studied.
In the 4th National Policy Document on Water Management (1998) it was decided to implement the "Controlled Tide" alternative with gradual steps. This option involves the sluices to be 1/3 open for 95% of the year. The first step called "Sluices Ajar" (10% open for 75% of the year) has the goal of partial rehabilitation of a brackish transition zone in the Haringvliet and a free migration pathway for fish. However, this first step will not have much effect on tidal movement or sediment transport in the larger area.
In 2001 the plan for implementation of "Sluices Ajar" was ready, including de definitions for a new operational sluice management scheme, safeguards for freshwater intake points, and the decision-making and communication processes involved.
In 2002-2003 the project was dramatically influenced by the changed government and a bad economic situation. A reconsideration took place because of the risk of exeeding bugets, and declined interest in environmental issues. Last november the government decided to implement the "Sluices Ajar" option in 2015, bound to a strict budget of 35 Mε and without binding to further steps.