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Tidal flat nourishment - Galgeplaat, NL
Tidal flats are valuable habitats and are important for coastal protection. The total area of tidal flats is decreasing worldwide due to various problems like sea level rise, coastal squeeze, subsidence by gas extraction and erosion initiated by man made constructions. The construction of a storm surge barrier and compartmentalization dams in the Eastern Scheldt in the 1980s is one example of a man made structure that resulted in a change in hydrodynamic conditions of the Eastern Scheldt estuary and hence the sediment equilibrium. As a result, channels are filling in and tidal flats inside the estuary are eroding. Nourishing tidal flats with sediment might be a promising solution to mitigate these effects.
To test this approach, a small area of the Galgeplaat, a tidal flat in the Eastern Scheldt, was nourished in 2008 with 130.000 m3 sand dredged from adjacent channels over a total area of 150.000 m2. The processes of sediment distribution on the flats and benthic recolonization are coupled and interact with each other. Therefore the design challenge is to find an optimum to reduce the initial impact of the nourishment on the benthic fauna, while optimizing the distribution of the sand over the tidal flat by wind and waves and the subsequent recovery of benthic life.
Planning and design
Primary purpose of the Galgeplaat pilot was to investigate whether nourishment can compensate the erosion of a tidal flat. Furthermore, the pilot offers the opportunity to gain a better insight into the biological and morphological development of an intertidal flat and the relationship between biotic and abiotic parameters. This will help improving the design of new measures with respect to the mitigation of the ongoing erosion of tidal flats in estuaries like the Eastern Scheldt in the future.
At the planning and design phase, the challenge was to meet a number of conditions:
One important argument in the design of the nourishment was the availability of sand from dredging activities in nearby navigation channels. Apart from this, the location for the nourishment on the Galgeplaat was chosen on the basis of the following criteria. It was:
From August to September 2008 130,000 m³ of sand was deposited on the Galgeplaat in a low circular mound, creating a kind of sand reservoir (figure 9).
First, a protective ring of sand was built, approximately 1 m high with a diameter of 450 m. The ring was made by bulldozers with sediment taken from the flat itself. At the south east side an opening was created for discharging excess water during the nourishment operation. At the inner side of the opening a settling-basin was excavated, in order to reduce the content of suspended matter in the effluent. The opening was oriented towards the nearby channel (Brabantsche Vaarwater).
This ring was filled with sand during the flood phase of the tidal cycle and spread by bulldozers during the ebb phase. The time phasing for the nourishment activities was based on the water level reaching the top of the ring wall. This allowed for a construction process with a controlled dredging-induced turbidity in the surroundings.
During and directly after the nourishment the concentration of suspended matter in the water around the tidal flat was monitored continuously at five locations, enabling an intervention if needed. The maximum sediment concentration in the water column near the commercial mussel beds was set at 50 NTU over and above the background sediment concentration in the water column. This maximum was exceeded twice during the nourishment activities. Both times the nourishment activities were allowed to proceed after an analysis of the situation by Rijkswaterstaat.
The sand for the nourishment, dredged from two adjacent channels, was slightly coarser than that on the tidal flat. This resulted in a small increase of the median grain size at the nourished location, but not in a change of the relative grain size distribution (Holzhauer and van der Werf et al., 2010).
The surface of the nourishment was not horizontal, but mildly sloping, with a maximum at NAP + 0.4 m and descending to NAP - 0.6 m in the south (figure 10). This was an unintended consequence of a lack of sediment at the end of the construction phase.
Operation and Maintenance
The development of the nourishment and its impact on the morphological and biological processes has been and will be further monitored for several years, primarily by Rijkswaterstaat. Additionally Building with Nature has taken extra samples of the benthic macrofauna and has installed an Remote monitoring of bio -and morphological developments (ArgusBio) for morphological imaging and close-up images of birds, and other ecological features like the presence of algal mats and microphytobenthos.
A detailed monitoring program was set up to follow the morphological and ecological development of the nourishment on the Galgeplaat in space and time by Ramaekers, et al., (2007).
Morphological developments were monitored every month in the first year and later every third month, through visual inspections at the edge of the nourishment, sedimentation-erosion plots at 14 locations along three transects and elevation measurements with RTK-DGPS with a spatial resolution of 25 m. Wave and current velocities were measured with an ADCP during a month just after placement of the nourishment. Additionally a Waverider was installed 200 m southwest of the Galgeplaat in order to measure the wave climate continuously. During construction suspended matter in the water column was measured in the channels around the tidal flat. Permanent hydrodynamic and meteorological measurement stations located in and around the Eastern Scheldt were used for data on water level, wind speed and wind direction respectively (Holzhauer and Van der Werf, 2009; Holzhauer et al., 2010, Van der Werf et al., 2011).
Ecological measurements included regular sampling of benthic macrofauna, sediment characteristics (grain size) and chlorophyll-a to track the benthic recolonisation in time. Sampling sites on the nourishment (n = 10) are compared with reference stations (n=6) in nearby undisturbed sediments. Macrofauna samples consisted of three cores (3 × 0.005 m2) pushed 30 cm into the sediment within a 1-m radius of the sample site, located with a GPS. The macrofauna samples were sieved over a 1-mm mesh, fixed with 4% buffered formalin and stained with Rose Bengal, after which specimens were determined to the species level. Sediment samples for grain size and chlorophyll-a concentrations were taken with a 1-cm diameter tube pushed 3 cm and 1 cm into the sediment respectively. Samples were taken in June 2008 (before the nourishment), and shortly after the completion of the nourishment in September and October 2008. In 2009 and 2010 sampling was done in April, July and October. In July and October 2009-2012 additional samples (n=25 in total) were taken on the nourishment to get a better picture of the spatial patterns of recolonisation on the nourishment (De Mesel and Ysebaert, 2011, De Mesel et al., in prep).
Morphological Modelling of the Galgeplaat
The processes occurring on the intertidal flats and their effects on accretion and erosion are still not well understood, due to their highly variable and dynamic nature. Therefore in order to gain more understanding on these processes and how they affect the morphological development of the estuary, a depth-averaged, two-dimensional horizontal (2DH) Delft3D-FLOW hydrodynamic model was created for the Galgeplaat within the context of the ANT project (figure 12). Delft3D-WAVE (SWAN) was used to simulate waves on the same computational grid which was nested in a larger wave domain. In their turn, these coupled hydrodynamic and wave models were nested in a larger domain, the KustZuid (South Coast) model (figure 13). This larger model simulates the hydrodynamics (including waves) of the southern part of the North Sea, Western Scheldt and Eastern Scheldt.
The sediment transport and morphology module can be run online with Delft3D-FLOW and supports both bed-load and suspended load transport of non-cohesive sediments and suspended load of cohesive sediments. Here, uniform sand fraction with a grainsize of 200µm was applied.
The nested models were set up for a spring and winter spring/neap cycle in April and November 2009, respectively, and for a longer period from September to December 2009 to investigate tje wind and wave forcing responsible for sedimentation and erosion. Once these forcing factors had been established, different simulations were set-up including the nourishment in the model, in order to examine the morphological effect at its current location and elsewhere on the Galgeplaat. Ecological effects also play a strong role on the morphological developments of intertidal flats. Oyster reefs and mussel beds modify current patterns and influence sediment movement. Algal and diatom mats play a role in stabilising the bed, especially during summer. Further model simulations were performed including these ecological effects, particularly investigating the roughness effects of biological features on current magnitudes. At locations where large patches of oysters exist, the simulation of current magnitudes in the model was improved with respect to measurements. This in turn reduced tje erosion on the intertidal flats. More details can be found in Cronin, 2012.
Several lessons-learned have been obtained on the field of morphology, ecology dynamics and (predictive) modelling
The nourishment has a relatively long-lasting effect on the height and emergence time at the nourished area itself. On the other hand, the nourishment only has a limited effect on the surrounding area on the flat. Several factors can explain this limited redistribution:
The nourishment buried and killed all benthic macrofauna. The recolonisation of the nourished area started directly after the nourishment was finished. After 2-3 years, the recolonisation appears to be most successful on the lower parts of the nourishment. The recolonisation seems to be driven by:
Although the emergence time was negatively correlated with benthic recolonisation on the Galgeplaat nourishment, this factor seems less relevant and correlated with the water content (dry vs wet areas) of the sediment. Shaping the nourishment with bulldozers is not recommendable, as this might additionally compact the sediment.
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