Development of a framework to evaluate locations for tidal creek restoration, based on the effects on surrounding land use

Abstract

Tauw bv proposes tidal creek restoration to bring back the tide into the upland area of Zeeland in a safe and controllable manner. This will positively contribute to the Water Framework Directive, ecological values and attractiveness of the region. However tidal creek restoration may also lead to negative effects on the surrounding land use, current knowledge is merely focused on the success rate of the ecological restoration. Within this research a framework has been developed to evaluate locations for tidal creek restoration, based on the effects on the surrounding land use. This framework will be used to evaluate three locations, Zierikzee, Colijnsplaat and Sint Maartensdijk, which are selected by Tauw bv.

The framework is mainly formed by a literature study and expert meetings, and handles the relevant factors concerning evaluating locations for tidal creek restoration: safety preconditions and wishes of stakeholders, show the possibilities and restrictions for tidal creek restoration. The lateral extent of groundwater fluctuations and salinity intrusion combined with their valued effects and the surface area of the affected types of land use are the criteria which show the impact of tidal creek restoration on the surrounding land use.

The preconditions in the Netherlands are set by “Wet op de Waterkering” of 1996 and “Waterwet” of 2009 (www.wetten.overheid.nl). In order to ensure these safety standards, the incoming tide via the tidal creek needs to be controlled and damped by water constructions. Tidal creek restoration with the help of a water construction diminishes the wave energy, so the tidal energy is the most important factor from the tidal regime. The wishes of the stakeholders are set by expert meetings

To predict the lateral extent of groundwater fluctuations and salinity intrusion, landscape analysis is needed to find the relevant parameters. The lateral extent of groundwater fluctuations depends on whether the adjacent aquifer is a confined or unconfined aquifer. Unconfined aquifers (Colijnsplaat; ± 200 m) showed a far less extent of groundwater fluctuations compared to the confined aquifers (Zierikzee, Sint Maartensdijk; ± 1,000 m). All the locations showed that the amplitude of the groundwater fluctuations decreases exponentially with the distance. The lateral extent of the intrusion of salinity is not depended on locations characteristic, only on the tidal period (which is the same for all the three locations), and has a fixed distance of 60 meters.

The value of the effect is depending on the land use type, which for Zeeland ca be divided in the following groups: urban areas, agriculture and nature areas. Groundwater fluctuations have especially a negative influence to wooden foundations of constructions in urban areas, due to bacterial decay. The most prominent negative effects of an increase in salinity are on agriculture and most terrestrial nature areas as forests. Finally a quantification of the surface area of a certain land use type with the given valued effect is needed as it makes it able to compare different locations to each other.

Applying the framework to the three locations shows that the framework makes it easily visible to see which, how and how much of a land use type is affected, which can help policymakers to make their decision on which location is the “best option” for tidal creek restoration. However this framework is not meant to point out the best location, it is a tool in the evaluating process for policymakers.