Project part-financed by the European Union (European Regional Development Fund)

The Interreg IVB North Sea Region Programme

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

Monitoring results

Spring-neap tide cycle

The spring-neap tide cycle is studied in detail because it determines, in combination with the elevation of the area, the inundation frequency and hence the marsh restoration potential. The Controlled Reduced Tide (CRT)-principle enables the creation of an intertidal habitat without a suitable elevation for normal dike removal. By using a system with high inlet and low outlet, a high variation of inundation frequencies is possible however the tidal curve will be with a stagnant phase instead of sinusoidal (Figure 9). The inlet construction has to be high enough (4.70 m at Lippenbroek) because only then enough variation in the water entering duration and –volume to enable the high variation in water levels in the polder.

Results on the spring-neap tide cycle at Lippenbroek
The tidal amplitude is reduced, on average, from 5.5 m in the Scheldt to 1.3 m in the pilot CRT and the high water (HW) level is about 3 m lower in the polder than in the Scheldt (Figure 10). The spring- neap tidal cycle is however maintained, the difference between spring and neap tide HW being approximately 1.0 m in the Scheldt and in the pilot CRT (Maris et al.). In contrast to estuarine marshes, in the CRT the low water levels are lowest during neap tides.
When analysing the characteristics of the inundations more in detail by calculating distribution of inundation frequencies (IF) for both the CRT and the adjacent estuarine marsh, very similar inundation frequencies (IF) are found (Maris et al.). For comparing both sites with different elevation, elevation is expressed relative to the level corresponding with an IF of 50% (Figure 11).

At the scale of one single tide, some important differences with the estuarine marshes occur: after the water has entered through high inlet culverts, a stagnant high water phase develops (1-2 hours), before it evacuates through the low outlet culverts (Maris et al.), as predicted by the theory (Cox et al. 2006). The length of this phase is dependent on the elevation difference of the site with the mean high water level. The seepage phase, when water gravitationally drains from the marsh during low tides, the end of outstream can be blocked by the next flood. Both are consequences of the gravitational outlet sluice, which only opens when the water level in the river is lower than the water level within the site. This stagnant phase, together with the extreme maximal dry droughts and flooding durations, could well have consequences on sediment biogeochemistry, plant and benthos colonization and nutrient exchanges, while hampered seepage could lower the exchange capacity with the estuary.
Also, on a larger temporal scale, some important deviations from the river tides appear (Maris et al.). During neap tides, the threshold of the inlet construction might not be reached, and no flooding at all occurs. This provokes drought events, which are longer (seasonal maxima 4 to 27 days) then in adjacent river marshes (seasonal maxima 12 to 14 days). Also, prolonged flooding events occur at storm tides, provoking extreme flooding durations much higher and more variable (seasonal maxima 12 to 35 hours) then in the river marshes (seasonal maxima 3 to 7 hours).

Lippenbroek is characterised by a wide range of inundation frequencies (IF): sites with high IF (4, 5, 6), intermediate IF (1, 2, 3), low IF (7, 10), constantly flooded (8, 9).

Topography: Sedimentation / erosion and creek formation

Sedimentation (import of fresh sediment) is important for the development of typical marsh morphology (eg. creeks) and the typical marsh soil, which is determining for the colonization of estuarine vegetation and macrobenthos. Inadequate sedimentation when removing/breaching a polder dike could lead to only bare mudflats without marsh development. This will for example occur when tidal dynamism is too high (Maris et al. 2008a). On the other hand can sedimentation also endanger the safety function of a Flood Control Area with Controlled Reduced Tide (FCA-CRT). Strong sedimentation can lead to a loss of water storage capacity and hence oppose flood protection.
A CRT risks more sedimentation because dynamism is low (completely embanked and with reduced tide). Also, during the stagnant phase the water is stable for some hours and suspended matter can deposit. Indeed, the results show net accretion at all sites (except for sites in a tidal pool: site 8 and 9), with a variation between 0.5 and 12.5 cm/year (Figure 12) (Maris et al. , Maris et al. 2008a). At Lippenbroek, a positive relation between average elevation and inundation frequency is observed; lower sites (eg. 4 and 5) are characterised by higher inundation frequency and hence higher sedimentation rates (Figure 13a) and elevation (Figure 13b) than higher sites (eg. 7 and 10). The highest accretion rates (at sites 4 and 5) are however decreasing over time while elevation increases and corresponding inundation frequency decreases. Overall, this had led to a fast flattening of Lippenbroek. The relationship between sedimentation rate and inundation frequency is similar on Lippenbroek and the adjacent river marsh (Figure 14). Hence, sites with comparable inundation frequencies show similar accretion rates in the CRT as on the river marsh. Sedimentation rates of 1.5 to 10 cm/year at marshes in the Sea Scheldt are observed and related to variations in elevation and inundation frequency. Hence, Lippenbroek acts similar as a natural marsh. The effect of sedimentation on the water storage capacity of Lippenbroek is however not clear yet.

Besides sedimentation, also erosion occurs at Lippenbroek which results in channel and creek formation. Already during the first months, small creeks developed more or less perpendicular to the main drainage ditches (Figure 15 & Figure 16). 1029 meters of new creeks have formed three years after installing the tidal regime (December 2008), mainly at the lower sites of Lippenbroek. Newly formed creeks are relatively width (circa 1m) and shallow (0.1 to 0.35 m deep). Creek density increased from 150 m/ha to 325 m/ha.

In the pre-existing central channel, erosion was observed near the sluices (-0.8m) and sedimentation near the head (+0.5m) (Figure 17). Most deepening and accretion occurred already in the first year after the introduction of the CRT. Also, meanders are developing, converting the former drainage ditch into a natural creek. New creeks incised the compacted agricultural soil, and many of them are still deepening (Figure 18). In the zones with high sedimentation rates, small creeks are more chaotic, sometimes disappearing and often changing shape. However, once a certain depth is established, they seem to consolidate and start eroding the accreted layers and underlying soils. Creek density and drainage capacity are not yet in equilibrium with the sites’ surface area and exchanged water volumes, as indicated by the on-going structural evolution. Further development will most likely result in a flat marsh platform (merlons), incised by creeks (krenels) (Figure 18).

After 2008, no significant changes in the thalweg elevation of the main creek were observed (survey 2010). This suggests that (1) after the implementation of Lippenbroek as a FCA with CRT in 2006, in a period of three-year time (2006-2008), some important morphologic changes occurred in the main creek near the sluice; (2) the main creek near the sluice has reached a kind of quasi-balanced state with respect to coupled sedimentation-erosion processes in the last two years (2008-2010). Lateral creeks however further developed: It can be clearly observed that erosion has been taking place close to the sluice in the lateral creek 1 (up to -0.15 m) and even a bit stronger in the lateral creek 2 (up to -0.25 m), while sedimentation has been going on in the lateral creek 3 for about 0.1-0.2 m.

Water and sediment quality

The influence of a CRT on water quality (oxygen and nutrients) is also monitored.

Oxygen: The pilot CRT never faced problems with anoxia because the inflow wave aerated the (often) oxygen poor Scheldt water (increase up to circa 60% oxygen saturation). This is followed by surface aeration: by spreading the water in a thin layer over the polder surface, a good contact with air results in a high oxygen transfer. In all seasons and weather condition, minimum oxygen saturation between 60% and 80% is reached. On the contrary, on sunny days, super saturation was recorded.

Nutrients: Lippenbroek plays, just like natural marshes, an important role as sink for nitrogen and source for dissolved silica. Retention of nitrogen and phosphorus is regarded as a potentially important process since it could partially compensate for excessive anthropogenic inputs of these nutrients. As detailed research on exchange mechanisms is ongoing, we can only conclude that monitoring results so far indicate no significant differences between the recorded CRT deliveries and earlier observations in river marshes (Van Damme et al. 2009) (Figure 19).

Heavy metals: Historical Scheldt sediment contaminated Lippenbroek with heavy metals. During the last decennia the sediment quality in the Scheldt had improved, the CRT functioning resulted in burying of contaminated sediment layers under a sediment layer with considerable lower heavy metal concentrations (Maris et al. 2008a). Furthermore, heavy metals that were present in the soil became less bioavailable because of changes induced by the floods.


After two years of controlled reduced tide, typical tidal freshwater vegetation has been restored in a former agricultural site. Establishment of an intertidal plant community in the CRT site was quick, with fast eradication of terrestrial species and colonisation by tidal freshwater marsh species already during the first few months. Moreover, communities which have mostly disappeared along the degraded Scheldt, but are described for several historic European references, are establishing in the pilot site. Presence of initial terrestrial vegetation slowed down establishment at higher as well as lower locations. The vegetation monitoring shows drastic movements in the favour of hydrophyte species. A dense vegetation cover with co-dominating ruderal species (Great hairy willow-herb (Epilobium hirsutum) and Stinging Nettle (Urtica dioica)) established on the abandoned and fertile agricultural land during the two year building period, and did this in a fairly homogenous way over the elevation gradient. After installation of the tidal regime, ruderal dominants were eliminated and replaced by a less dense hydrophyte and wetland pioneer vegetation. Colonisation by Purple loosestrife (Lythrum salicaria), Bulrush (Typha latifolia), Speedwell (Veronica), Common reed (Phragmites australis) and willow (Salix) has started (Figure 20), while at other sites the vegetation coverage moved for bare mudflat.

Vegetation communities seem to deviate from the nearby tidal freshwater marshes, but show similarities with tidal freshwater marshes described for the beginning of the 20th century. The CRT-technique provides strong potential for durable, adaptive restoration of tidal marshes on sites with low elevation. Wider implementation of the CRT technique could increase the total surface of tidal freshwater marshes with fully developed vegetation gradients.


The first estuarine macroinvertebrates were observed after three months of tidal inundation. After one year taxonomic richness on the frequently flooded sites exceeded the richness observed in the river sites, and alike the vegetation development, taxa which are absent in the river are observed to establish in the CRT. Oligochaete densities six fold exceed the river site densities. After three years, macroinvertebrate colonization continues, albeit with lower pace, on the less frequently flooded sites.


Flood control areas are important breeding, foraging and nursery areas for fish. A safe passage through the sluices of the CRT is therefore crucial. Apparently, fish mainly enters Lippenbroek via the outlet sluice (oxygen rich water stream). Because the outgoing discharge rapidly increases, fish passage is assumed to be possible only during a limited period of time after outflow starts. Adaptations of the tidal gate are investigated to broaden the time frame for fish passage.


Despite high disturbance (terrain visits, monitoring campaigns, cyclers and hikers on the dike), a clear positive evolution in the number and diversity of bird species was observed at Lippenbroek. In total, 98 species were inventoried, of which 30% aquatic bentivores (19 species), 54% wetland generalists (11 species), 8% terrestrial species (18 species) and 8% forest species (13 species).

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