Veilige Vecht exploratory study: water security through water systems? 

The dykes on the Vecht river between Dalfsen and Zwolle no longer meet legal flooding standards. Under assignment to the WDODelta water authority, and in collaboration with them, Royal HaskoningDHV, and Land-iD, Witteveen+Bos conducted a comprehensive variant analysis for the Veilige Vecht (‘Safe Vecht’) project. Its goal was to determine the best way to restore water security to this part of the Overijssel province.

The Veilige Vecht project is part of the national HWBP flood protection programme, which aims to ensure that all dykes in the Netherlands meet legal standards by no later than 2050. The exploratory phase of the project is intended to result in a preferred variant. Many of the dykes and floodplains involved lie in protected nature reserves: the Natural Ecological Network and the ‘Uiterwaarden Zwarte Water en Vecht’ Natura 2000 area. This protected status and the underlying natural and cultural values of the locations are constant factors in decisions made during the studies.

Catchment area

Central to the task at hand is the fact that approximately 60 % of the Vecht river’s catchment area is in the Netherlands – the river enters the Netherlands near Gramsbergen and empties into the Zwarte Water river near Zwolle. The Dinkel and Regge rivers drain their water into the Vecht. This creates the potential for water-system measures to be influential in mitigating flood surges.

In our exploratory study for Veilige Vecht, we investigated whether water-system measures throughout the Vecht catchment area could contribute to water safety between Dalfsen and Zwolle. Examples of such measures include retaining water in the catchment area, inhibiting and retaining water at ground level along the Vecht, and increasing discharge. These could reduce flood surges on the Vecht, in turn reducing the need to raise the dyke.

On the study, calculations were based on a flood surge that might occur once every thousand years. This is significantly more water than in the period of high water levels in December 2023 and also a much higher level than in 1998, when much of this part of Overijssel was flooded. A ‘thousand-year flood surge’ represents a movement of 500 m³ of water per second.

Whether water system measures could act as a (partial) alternative to raising existing dykes was thoroughly examined. The examination incorporated other functions in the area, such as agriculture, nature and recreation, and housing. The study also examined whether the measures could help mitigate water shortages during dry periods, defining areas within and outside the floodplain using parameters such as ‘feasibility without major interventions’ and ‘non-intensive use’. An additional 25 cm of storage was assumed on top of regular water levels during wet periods.

The study ultimately concluded that water-system measures alone do not provide the capacity required to substantially reduce flood surges. To achieve real impact, it would be necessary to dedicate approximately 10 % of the catchment area to water storage. In the short term, this is not possible. The height of the flood surge was partly accounted for in the models by a storm. Storms cause water to be pushed up the Vecht from the Ijsselmeer, which cannot be addressed by system measures. Applying such measures to future-proof the Vecht dykes – with the exception of the Vechterweerd side channel – has therefore been eliminated as an option. The study did, however, provide valuable insights for fulfilling long-term safety obligations further upstream in a more climate-adaptive manner.

Reinforcing sand dykes

A logical conclusion based on the exploratory study’s outcome is that water security must be achieved by modifying the dykes themselves. The Vecht dykes, however, are generally less accessible than those along the Rhine, Waal and IJssel rivers. Also, as they are sand rather than clay dykes, they require a specific approach to reinforcement.

When preparing the environmental impact assessment and integrated variant analysis, we worked towards a preferred alternative for dyke reinforcement that ensures water security while being feasible, affordable, and compliant with permit requirements. Assessment criteria included:

1. Soil reinforcement where possible, rather than through construction, as this is sustainable, circular, easier to expand (and thus more future-proof), and durable. Also, it is easier to inspect and manage and generally cheaper to realise.
2. Sufficient space to properly incorporate the solution. Dyke reinforcements must not have a disproportionate effect on existing local values and functions (e.g. housing, essential infrastructure, or natural values). Reinforcements must also not result in a fragmented appearance – a unified image over long stretches is the goal.
3. Avoid riverbed modifications as much as possible to keep the riverbed climate-robust and future-proof. It was first considered whether sufficient capacity for reinforcement was available on the land side of the dykes. In the event of obstructions (e.g. houses or major infrastructure) or sufficient capacity on the riverbed, reinforcement can be effected on the other side. When selecting a preferred alternative, the fact that solutions must do justice to the dykes’ unique locations, characteristics and histories was also taken into account.

Varying dyke design

The study resulted in two promising alternatives, the best elements of which were combined – following in-depth analysis – into a preferred alternative. As a result, there is not one uniform dyke design along the entire Dalfsen-Zwolle route; rather, the design varies to suit the local environment, doing justice to the Vecht dykes’  traditions.

When designing the reinforcements, we worked to maintain the appearance of the dyke from past and present – a sandy dyke with gentle slopes and the type of plants that feel at home there. Adding sand increases biodiversity and other natural values, with solutions that allow the appearance of the dyke to remain compact also being applied. A key step involves adding clay to the dyke covering, which allows the dykes to retain their compact shape featuring a narrow crest and steep slope.

Preventing piping

To ensure the reliability of the dykes, a new solution was devised to prevent piping: ‘stoorlaagvermenging’ (‘interference layer mixing’). This involves mixing the thin interference layer (up to approximately 30 cm thick) with about 1.5 m of sand located directly below ground level. This makes the interference layer sufficiently permeable to allow seepage water to exit without taking sand with it, preventing piping. An additional advantage is that interference layer mixing is significantly cheaper and more sustainable than traditional measures and has less impact on the local environment. The technique can be used for approximately fifteen kilometres within the project area.

Combination opportunities

On the Veilige Vecht project, we also looked at combination opportunities: initiatives with goals other than water security that offer benefits and synergy to the area and are financially feasible. This led to a number of options that will be incorporated into the plan development. One example is the combination opportunity on the Zuidelijke Vecht dyke. Here, improving the cycling link is being investigated in combination with the construction of the Vechterweerd side channel. This side channel is a water-system measure that serves to compensate for impacts on the river caused by the outward strengthening of some sections of dyke. The side channel will also be designed to accommodate fish migration. This is combination opportunity is in line the Water Framework Directive (WFD), affording opportunities to improve flora, fauna and water quality.