Effects of Possible Land Reclamation Projects on Siltation in the Rotterdam Harbour Area

Effects of possible land reclamation projects on siltation in the Rotterdam harbour area.

A model study.

J.M. de Kok <", A. van der Meulen <'>, Z. B. Wang^ and

M. Schroevers ^ ^ National Institute for Coastal and Marine Management/RIKZ, f.O. ^ox 20907, A%-2JOO&Y7%e J%zg^, 7%g WgrAer/aW^

Email: j. m. dkok@rikz. rws. minvenw. nl ^ Delft University of Technology and Delft Hydraulics, f.A #0% 777, A%-2600 M77Dg% 7%e ^rA^/aW^ Email: [email protected] ® Aqua Vision, Zaagmolenlaan 4,NL-3447 GS Woerden

Abstract

A three-dimensional numerical model system is used to study the effect of a possible large scale land reclamation on the motion of water and suspended sediment in the sea area around the Rhine outlet and in the inland channels and harbours of Rotterdam. The model is tide resolving and includes deposition and resuspension of cohesive sediment. The results of scenario model runs with different land reclamation designs sho- wed, that the shape of the coast line has a major influence on siltation rates in harbours and shipping channels in the Rhine estuary.

In one case siltation rates decreased dramatically as a result of a certain land reclamation. In another case, with an extension of the entrance channel to the West, siltation rates increased to more than the double. These differences are caused by chan- ging residual flow patterns. In the first case the natural alongshore flow of sediment was diverted by residual currents in an off shore direction, leading to a decrease of suspended sediment concentrations in the river mouth. In the second case the near bottom flood currents in the river mouth were much stronger and brought more sediment to the inland areas.

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Rhine

Figure 1 : Situation of the Rhine/Meuse estuary.

1.1 The Rhine outflow area

The harbours of Rotterdam are situated in the Rhine/Meuse estuary, where currents are dominated by the tide and the density gradients connected with the fresh water run off of the Rhine and Meuse(see fig.l). In the western part of the area the bottom depths of harbours and shipping channels are kept at a main- tenance depth of 25 m. The mouth of the estuary has been dredged out over a width of 1000 m resulting in a profile that is far beyond the morphodynamic equilibrium. As a consequence siltation rates are high and navigation depths have to be maintained by regular dredging. In the most western part of the estuary, called the "Maasmond" (see fig. 2), marine sands and fine cohesive sediments are deposited at a rate of about 6 million tons dry weight a year. Most of the deposited fine cohesive sediment (3 million tons/y) is of marine origin. The supply of marine cohesive sediments is maintained by tidal currents and strongly enhanced by the typical density structure of the sea water in the outflow plume of the Rhine. Within a radius of 20 km around the Rhine outlet a strong estuarine like residual current pattern exists with near bed currents directed towards the fresh water outflow point (fig. 2). In the Maasmond the strong density driven estuarine residual near bed current is transporting sea water and marine cohesive sediments further landward. Tidal pumping, exchange by density currents and horizontal eddies are responsible for high siltation rates in the adjacent harbours. The presence of fresh water from Rhine and Meuse is not only causing the strong landward near bed residual currents, but is also responsible for a strong salinity stratification in the whole estuary and in a large part of the outflow plume in the sea. This stratification causes a strong reduction of vertical turbulence eddy activity in the entire water column.

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Figure 2 : The most western part of the Rhine/Meuse estuary. Arrows indicate residual SPM flow.

This results in very slow vertical mixing of suspended sediment and as a consequence very high SPM (suspended matter) concentrations near the sea bed, re- gularly increasing to several grams per liter. At the same time the near surface concentrations are well below 50 mg/1. During the turn of the tide turbulent kinetic energy is almost absent and suspended particles are rapidly sinking to the bed, with average speeds between 0.2 and 1 mm/s. Near the bottom the high SPM concentrations cause vertical density gradients, resulting in a further reduction of turbulent kinetic energy, during a considerable part of the tidal period

(see Winterwerp et al., 1998). For these reasons most of the SPM transport is taking place near the sea bed, following the shoreward directed residual current pattern and leading to a high influx of marine cohesive sediments into the Maasmond.

1.2 Tidal motion and long term cohesive sediment transport

Tidal energies are high along the Dutch coast and are connected with tidal current amplitudes of 1.0 - 1.5 m/s near the surface and 0.2 - 0.5 m/s close to the bed. These currents cause rapid resuspension of fine sediments, deposited during the turn of the tide. Near bed measurements of SPM loads and current velocities indicate critical velocities for resuspension between 0.05 and 0.15 m/s, depending on wave activity. Only in areas sheltered from these tidal currents deposition of fine cohesive sediments for periods longer than the tidal period occurs.

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Figure 3 : Flow of suspended matter along the Dutch coast.

The maximum current velocities during flood time are almost everywhere north- eastward directed and are much higher than the maximum ebb velocities in most places, at least when winds are moderate. Only at the north-eastern side of pro- montories, such as the Maasvlakte (see fig. 2), ebb currents are on average stronger, resulting in anti-cyclonic residual eddies (see fig. 3). In the centre of a residual eddy suspended sediment can be caught, but it does not get the chance to settle for a longer time as a result of the high tidal current amplitudes that still exist over the entire area of the residual eddy. Because flood currents are dominant and also the residual current is directed towards the NE, the average flow of suspended sediment during quiet weather conditions is alongshore and NE directed. As a result of the near bed shoreward density current SPM concentrations increase towards the shore until values of 25 - 100mg/l.

1.3 Wind and waves

When significant wave heights 15 km offshore are higher than 3 m, almost everywhere critical bed stresses for erosion are exceeded, even after consolidation times of several weeks, with an exception for the Maasmond, where resuspension is very rare. Wave heights above 3 m occur on year average during 3% of the time (Roskam,1995). During the winter season roughly once in two weeks such an event occurs. They are correlated with strong SW, WSW and NW winds. As a result of erosion of the silt areas sheltered from tidal currents and after resuspension of small particles from sand/silt mixtures in the sea bed, patches of highly concentrated SPM are transported then to the NE by wind driven currents.

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These events are also correlated with an increase of rapid siltation in the Maas- mond. It is estimated, that roughly 30 % of the year total is connected with this rapid siltation.

Because the average strong wind is WSW and the average wind is SW, the year averaged alongshore SPM transport is NE directed. It amounts to 10 - 20 M ton- nes per year (van Alphen, 1990, Macmanus & Prandle, 1997).

2.1 Modeling of fine cohesive sediment

In the Rhine/Meuse estuary and in the adjacent sea the velocity, salinity and SPM distributions are highly three-dimensional and unpredictable, with haline stratification and strong density driven currents with opposite directions near the surface and near the sea bed. SPM profiles are governed by the periodicly chan- ging vertical diffusivities and show large gradients near the bed. Therefore the modeling of transport of suspended matter has to be performed with a 3D-computer code for water motion and SPM transport. In this study the effects of two possible land reclamation designs on SPM fluxes in the area are assessed using three-dimensional models for hydrodynamics and SPM transport. The model area covers the Rhine/Meuse estuary, the adjacent harbours and shipping channels and the adjacent part of the North Sea (see fig. 1).

For the hydrodynamic part the DELFT3D code of Delft Hydraulics was used. This code is based on the primitive equations for 3D incompressible free surface flow with Boussinesq approximation and hydrostatic assumption. The baroclinic pressure gradient terms in the momentum equations are coupled to the prognos- tic salinity field obtained by the solution of the advection-diffusion equation for salinity. Turbulence viscosity and diffusivity are computed with a k-c model (see Uittenbogaard et al.,1992). The computed space and time dependent vertical turbulence diffusivities are stored on the output file, together with the velocity and layer depth fields. A orthogonal curvilinear grid is used in the horizontal. Mesh widths range from 50 m in the harbour entrances to 1000 m at the sea boundaries. In the vertical a a-transformation is applied using 10 layers.

On the sea boundaries periodic water levels are imposed. These are based on the first 12 Fourier components of a realistic tidal signal. The salinity boundary conditions are based on the observed climatological mean. On the river boundaries constant (annual mean) fresh water discharges are imposed.

The computation of transport of suspended matter is done separately, using a periodic repetition of the hydrodynamic file covering one tidal period. In this way there is no back coupling of the SPM concentrations with the hydrodynamic model. In reality high SPM concentrations are affecting the hydrodynamics near the bottom in the Maasmond, where a fluid mud layer exists in winter and spring. The present model does not account for theflui dmu d motions. They have to be computed with a separate model, not discussed here. However, there is no resuspension from the fluid mud layer and therefore fluid mud motions do not influence the SPM concentration field.

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The SPM transport model is based on the 3 D-advection-diffusion equation with space and time varying diffusion coefficients, a constant (in time) fall velocity for SPM and in the lowest water layer a sink term for deposition and a source term for resuspension (see de Kok, 1992). A separate bottom layer is used for storage of deposited sediment. The numerical method used is based on the extended second moment method (see de Kok, 1992, de Kok & vd Meulen, 1997). This is a semi-Lagrangian method, especially suited for the reproduction of fronts, strong gradients, patches and elongated concentration patterns. This is necessary, as both in horizontal and vertical direction strong gradients and discontinuities exist in the SPM field.

2.2 Deposition and resuspension

Deposition of suspended sediment is modeled by a constant fall velocity, which is equal to the settling velocity when the computed current velocity for the lowest water layer is smaller in magnitude than the critical value for deposition. This critical value and also the fall velocity are space dependent. When current magnitudes exceed the critical value, no deposition is computed.

The space dependent fall velocities were chosen between 0.1 and 0.5 mm/s. Resuspension from the bottom layer is only possible if sediment has already been deposited there (Jago & Jones, 1998). It is transferred to the lowest water layer at a fixed rate of 0.5 g/nfs, when the magnitude of the current velocity in the lowest water layer exceeds the critical value for resuspension. This critical value was chosen between 0.0 m/s and 0.15 m/s depending on the area. The critical value for erosion was everywhere chosen equal to the critical value for resuspension.

In shallow areas (mean depth < 2.5 m) a constant wave activity is supposed to prevent all deposition and the critical current velocities were set to 0.0 m/s there. In areas with a supposed fluid mud layer all resuspension was suppressed. Because sedimentation and resuspension in the model area is always either in sand/silt mixtures or in wave affected areas or in fluid mud layers, the usual for- mulations by Krone and Partheniades were not used, but simplified to the simple on/off switches described above, using empirical coefficients.

3.1 Case studies : Present situation

With the model the effects of two land reclamation designs were assessed by comparison of the silt intrusion in the Maasmond with that in the present situation. In the undisturbed situation and with average weather conditions the model results show an alongshore flow of suspended sediment (fig. 3) with an anticy- clonic residual eddy north of the Maasmond. A part of the sediment flux is de- flected by density currents and enters the Maasmond, where deposition takes place amounting to 3 10* kg per tidal period. Most of this sedimentation occurs in the "fluid mud layer" in the most western part of the Maasmond (fig. 2). Figure 4 shows the depth mean SPM distribution during high tide.

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Figure 4 : Computed depth mean SPM concentrations in mg/l during high tide.

Weak SW wind. Present situation.

Figure 5 : Computed depth mean SPM concentrations in mg/l during high tide. Weak SW wind. Situation with south-westerly extension (case 1).

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3.2 Case study 1 : Effects of south-westerly land reclamation.

When the present "Maasvlakte" (see fig. 2) is extended towards the South-West

(see fig. 6), this will result in a new residual eddy west of Hoek van Holland and a deflection of the north-eastward residual current to the North (fig. 6). The model results indicate that the main alongshore flow of SPM will also be diverted from the Maasmond in offshore direction. This results in a much smaller silt intrusion in the Maasmond. The model predicts a decrease of 80 % under quiet weather conditions. Figure 5 shows the depth mean SPM distribution during high tide for this scenario.

Figure 6 : Residual flow of cohesive sediment in the case of a land reclamation extending the Maasvlakte in south-westerly direction (case I).

3.3 Case study 2 : Land reclamation in westerly direction

Another possible extension of the Maasvlakte is in westerly direction. This op- tion implicates also an extension and deepening of the entrance channel. Model results indicate that this results in a higher (+ 50 %) tidal exchange between the estuary and the sea and also in a higher (+100%) exchange by density currents.

Both phenomena are causing an increase (total + 100%) of the SPM intrusion and sedimentation in the Maasmond. As silt intrusion mainly takes place in the lower water layers, the increase of the density driven exchange has the most effect on the siltation rates, beacuse the density driven current is connected with a strong near bottom residual current. As can be seen in Figure 7, there is no major increase of depth mean SPM concentrations near the Maasmond. All effects seem to be caused by higher exchange of sea water with the estuary.

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Figure 7 : Computed depth mean SPM concentrations in mg/l during high tide. Weak SW wind. Situation with westerly extension (case 2).

4 Conclusions

Scenario studies with a three-dimensional hydrodynamic model and a coupled SPM transport model indicated major effects of changes of the coast line on siltation rates in the most western part of the Rhine/Meuse estuary and the adjacent harbour area.

In the case of a south-westerly land reclamation the computed sedimentation under quiet weather conditions is reduced to 20 % of the present one. In the case of a westerly land reclamation in combination with a deeper and longer entrance channel the computed quiet weather sedimentation is doubled.

Model results of both cases show that changes in the coast line south of Hoek van Holland can have a major impact on residual currents in the Rhine/Meuse estuary and the adjacent sea and on the supply of suspended sediment to the estuary.

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