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The Interreg IVB North Sea Region Programme

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Lippenbroek: Flood Control Area with Controlled Reduced Tide (FCA-CRT)

Measure description

The worldwide extent and abundance of intertidal habitat has greatly decreased, primarily due to human alteration of estuarine habitats. Impacts include construction of embankments or weirs, harbour expansion, deviation of freshwater inflow and conversion of tidal marsh for agricultural uses and urban and industrial development. The remaining habitat is often degraded by strong anthropogenic pollution or changing hydrodynamics. Loss of valuable habitat results in loss of associated function, such as estuarine biogeochemistry (C, N and P transformations and buffering of estuarine silica concentrations), energy dissipation (control tidal currents and waves) and mitigation of floods (protecting landward sea defences from scour and erosion).
Throughout the world, tidal marshes are restored to obtain natural protection against recurring storm surges and to preserve the goods and services these habitats provide. Managed realignment is a technique that consists of the removal or breaching of dikes to restore tidal influence. Elevation is a key factor in planning of managed realignment and for suitable site selection as it relates directly to frequency, height and duration of tidal inundation, which are the main factors in sedimentation patterns and vegetation development. Agricultural sites adjacent to estuaries have often lowered in elevation as a result of compaction. This typically leaves them below the levels of contemporary marshes within the same system, which have often increased in height with sedimentation (Temmerman et al. 2003). The difference in elevation rules out many sites for potential realignment, since this would result in entirely flooding of the site every tide and would thus result in development of completely non-vegetated intertidal mudflats, of which it is not certain if and how fast they could evolve to a vegetated marsh system.
An alternative solution is the installation of a restricted tidal exchange. This has the advantage of lowering the tidal wave in the site to an acceptable level, but the technique cuts out spring-neap tide variation needed for optimal habitat diversification.
Another solution is the installation of sites under controlled reduced tide (CRT). This could offer opportunities for tidal marsh restoration in combination with safety measures.

To protect the Sea Scheldt (Zeeschelde) for storm surges and in general for sea level rise, the Sigmaplan was developed. Initially the focus was restricted to flood reduction for which Flood Control Areas (FCA) were developed. European, international and national legislations like the Water Frame Directive (WFD), Habitat and Bird Directive (HD/BD), that dictate a ‘no net loss’ policy and the development of a ‘sound ecosystem’, had led to the development of the actualised Sigmaplan. The actualised Sigmaplan is the Flemish management plan for flood prevention that also has to reach the nature objectives in the Belgian part of the Scheldt estuary. The expansion of intertidal wetlands is part of the strategy to control flood risk. The new combined focus on both security and nature had led to the new concept of Flood Control Area with Controlled Reduced Tide (FCA-CRT), areas with a safety function but, with a special sluice system, also attributable to the restoration of important estuarine functions. The long term objective for the Scheldt estuary is the development of several thousands of ha of tidal restoration projects, of which ca. 1000 ha with the CRT technique. Eventually (by 2030) new CRT habitat will comprise 36% of tidal freshwater marshes along the Scheldt.
However, before implementation on a large scale, as the basal hypotheses whether this technique could successfully restore freshwater tidal marsh habitat on a lowered rural site, had to be checked. A research strategy was developed in three stages with increasing complexity. First, two mesocosmos experiments were executed to study the growth of Reed and the behaviour of heavy metals in soils and flood water in relation to flood frequencies and different soil textures. Lastly, a pilot project for a Flood Control Area with Controlled Reduced Tide (FCA-CRT) was developed in Lippenbroek. The main objective was to study the ecosystem functioning of the FCA-CRT. This pilot project will indicate if sustainable ecological structures and functions could develop in a FCA-CRT which are qualitatively and quantitatively similar to outer-dike intertidal habitat.

Pilot project LIPPENBROEK
Lippenbroek is an area of 10ha (8ha without counting the dike surface) with the aim to combine water storage during extreme high tides and estuarine wetland restoration. It is the first FCA-CRT in the world that is functional and acts as a pilot project in terms of habitat development for other flood control areas to come (eg. Kruibeke-Bazel-Rupelmonde). By creating the correct conditions (primarily a correct tidal regime), the aim is to develop a sustainable freshwater tidal marsh habitat on a lowered rural site with nature itself as the primary steering factor.
Lippenbroek concerns a low polder area, separated from the estuary by a lowered overflow dike with sluice system (high inlet culverts (Figure 3), low outlet valves) to introduce a wide range of inundation frequencies in this embanked site to restore estuarine functions and habitat (Figure 4 & Figure 5). Only with extremely high water, the water flows over the low dike to (partially) fill the FCA. The area is separated with a high dike (Sigma dike) from the hinterland for safety against flooding. This technique allows implementation of a restricted tidal regime with neap and spring tides.

Concept FCA-CRT
The development of intertidal wetlands (mudflats and marshes) in a FCA-polder, which normally only floods once or twice a year at storm tide, needs a sluice system that enables the exchange of Scheldt water at a daily basis. Mudflats will develop in parts of the polder that are flooded daily with Scheldt water, marshes on the other hand flooded only at neap tide. For the development of a diverse wetland ecosystem, differentiation in flooding frequency on the polder surface is essential and has to be similar to natural wetland areas.
A special sluice technique allows implementation of a restricted tidal regime with neap and spring tides, by the use of high inlet culverts and low outlet valves. Important, an inlet culvert which only lets in the top of the tidal wave is created. Consequently, water flows gravitationally out to a lower exit culvert when water levels in the river have again decreased (Figure 6). The site elevation and river tidal regime are thus detached. Fine-tuning of inlet culvert level permits installation of a tidal regime with a pronounced spring-neap variation. This aspect is of tremendous importance to restore the full range of intertidal habitats and has until now been lacking completely from other tidally restricted systems.
When a site has a pronounced topography, or an extended ditch system, the water volumes needed to create a tidal gradient are much bigger. This would theoretically lead to more culverts at mean high water neap level, but the problem can also be solved by adding some culverts with lower thresholds (Figure 7). Such a configuration permits bigger water volumes to enter every tide without increasing the number of culverts, which saves up on the building costs. Off course, in the latter case, precise fine tuning will be required according to the desired flooded surfaces at neap, average and spring high waters (Figure 7). Therefore, calculation of surface/volume relationships of the site and discharge estimates of the culverts is needed. After a few test runs and fine-tuning of the inlet sluice sill level, a reduced tidal regime was introduced from March 2006 on.

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