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Abstract
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Introduction
Simulation Methodology and Calibration
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Earth Sciences
Oceanography

Numerical modelling of morphodynamics—Vilaine Estuary

Profile image of Bo Brahtz ChristensenBo Brahtz Christensen

2013, Ocean Dynamics

https://doi.org/10.1007/S10236-013-0603-7
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Abstract

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Figures (25)
Fig. 1 The Vilaine Estuary. The map shows key locations, bathymetry of 2007 and position of the measurement stations. Reference level is chart datum
Fig. 1 The Vilaine Estuary. The map shows key locations, bathymetry of 2007 and position of the measurement stations. Reference level is chart datum
model (Fig. 3), which is forced by the astronomical tide, along its open boundaries, defined by tidal constituents from the French Maritime Organisation (SHOM). In addition, the local model is forced by the wind measured at Arzal at the
model (Fig. 3), which is forced by the astronomical tide, along its open boundaries, defined by tidal constituents from the French Maritime Organisation (SHOM). In addition, the local model is forced by the wind measured at Arzal at the
dam. The upstream boundary condition of the local model is the release flow or run-off from the dam. Hourly discharge values from the actual recordings are applied. the sediment transport depend on both the currents and the wave conditions, which are determined from the wave sim- ulation with a fixed bed. The wave data are input each 900 s. The procedure is schematized in Fig. 11.
dam. The upstream boundary condition of the local model is the release flow or run-off from the dam. Hourly discharge values from the actual recordings are applied. the sediment transport depend on both the currents and the wave conditions, which are determined from the wave sim- ulation with a fixed bed. The wave data are input each 900 s. The procedure is schematized in Fig. 11.
measurements of 0.05 to 7 mm/s collected during March 2007. w is the settling velocity, c, the near bed sediment concen- tration calculated according to Teeter (1986), 7, the bed shear stress due to skin friction, T,g the critical shear stress for deposition, Ep the erosion rate and 7, the critical shear stress for erosion.
measurements of 0.05 to 7 mm/s collected during March 2007. w is the settling velocity, c, the near bed sediment concen- tration calculated according to Teeter (1986), 7, the bed shear stress due to skin friction, T,g the critical shear stress for deposition, Ep the erosion rate and 7, the critical shear stress for erosion.
Fig. 5 Overview of equations, modified from Lumborg and Pejrup (2005)
Fig. 5 Overview of equations, modified from Lumborg and Pejrup (2005)
maintained. To achieve this, it was realised that a spatial distri- bution of the critical order to describe the distinction between separated by slashes For layer 2 as examp 0.6 and 0.5 Pa. The shear stress for erosion was required in heterogeneous character of the sediment behaviour. The definition of the spatial distribution was guided by the information of sand and mud contents as well as a deeper water and the inter-tidal flats. Figure 7 shows the spatial distribution of the critical shear stress for layer 2. The distribution is defined by the four values as shown in Table 2 for layers 1 and 2. ©, Tee IS defined by the values 0.85, 0.7, higher values are applied in the external estuary and the deeper channel, while the lower values are applied in the interna no spatial distribution part and over the mudflats. For layer 3, was applied. It should be mentioned that with the applied settings, layer 1 is less significant for the results. It either rapid is transferred to layer y erodes or its material consolidates and 2. The necessity of applying relatively high values for 7,. in the mouth of the estuary and tidal channel may be explained by the so-called drag reduction. Suspended cohesive sedi- ments may cause damping of the turbulent fluctuations in flowing water and may alter the apparent bed roughness, thus leading to a drag reduction Toorman et al. (2002). In Winterwerp and Kesteren (2004), the impact of high- concentration mud suspensions (0.1 g/l to several grammes per litre) and their effect on the hydrodynamics are thor- oughly discussed. The sediment—fluid interactions can lead to drag reduction or saturation of the water column. Similar to what is reported in Winterwerp and Kesteren (2004), for estuaries with high concentration mud suspensions, it was found that a relatively low friction was required to calibrate the hydrodynamic model (Manning number n= 0.0143 s/m) for the Vilaine Estuary indicating that this phenomena may be present. The model does not include the effect of drag reduction. This would require a coupling of the hydrodynamic friction and the bed shear stress calcula- tion in the sediment transport model. The effect of drag reduction was therefore compensated for by a higher value Of Toe.
maintained. To achieve this, it was realised that a spatial distri- bution of the critical order to describe the distinction between separated by slashes For layer 2 as examp 0.6 and 0.5 Pa. The shear stress for erosion was required in heterogeneous character of the sediment behaviour. The definition of the spatial distribution was guided by the information of sand and mud contents as well as a deeper water and the inter-tidal flats. Figure 7 shows the spatial distribution of the critical shear stress for layer 2. The distribution is defined by the four values as shown in Table 2 for layers 1 and 2. ©, Tee IS defined by the values 0.85, 0.7, higher values are applied in the external estuary and the deeper channel, while the lower values are applied in the interna no spatial distribution part and over the mudflats. For layer 3, was applied. It should be mentioned that with the applied settings, layer 1 is less significant for the results. It either rapid is transferred to layer y erodes or its material consolidates and 2. The necessity of applying relatively high values for 7,. in the mouth of the estuary and tidal channel may be explained by the so-called drag reduction. Suspended cohesive sedi- ments may cause damping of the turbulent fluctuations in flowing water and may alter the apparent bed roughness, thus leading to a drag reduction Toorman et al. (2002). In Winterwerp and Kesteren (2004), the impact of high- concentration mud suspensions (0.1 g/l to several grammes per litre) and their effect on the hydrodynamics are thor- oughly discussed. The sediment—fluid interactions can lead to drag reduction or saturation of the water column. Similar to what is reported in Winterwerp and Kesteren (2004), for estuaries with high concentration mud suspensions, it was found that a relatively low friction was required to calibrate the hydrodynamic model (Manning number n= 0.0143 s/m) for the Vilaine Estuary indicating that this phenomena may be present. The model does not include the effect of drag reduction. This would require a coupling of the hydrodynamic friction and the bed shear stress calcula- tion in the sediment transport model. The effect of drag reduction was therefore compensated for by a higher value Of Toe.
Fig 7 Horizontal distribution of 7,. of layer 2. The spatial distribution is defined by the values 0.85/0.7/0.6/0.5 Pa Table 2, third column, gives the range of values of 7. one would expect according to Fig. 6. It is seen that these values are in accordance with the range of calibrated values given in the fourth column of Table 2. Table 2 also gives the erosion coefficient Ey, which is almost constant for the three layers.
Fig 7 Horizontal distribution of 7,. of layer 2. The spatial distribution is defined by the values 0.85/0.7/0.6/0.5 Pa Table 2, third column, gives the range of values of 7. one would expect according to Fig. 6. It is seen that these values are in accordance with the range of calibrated values given in the fourth column of Table 2. Table 2 also gives the erosion coefficient Ey, which is almost constant for the three layers.
Thus, as the period March 2007 to March 2008 contains a combination of all the typical morphodynamic events, it wil be applied as a reference year, which can be scaled tc represent other combinations of the strength of waves/winds and dam release flow.
Thus, as the period March 2007 to March 2008 contains a combination of all the typical morphodynamic events, it wil be applied as a reference year, which can be scaled tc represent other combinations of the strength of waves/winds and dam release flow.
Fig 10 The evolution of seabed level (metres) (observed and modelled) at three Altus stations (A black, B red and C green) on Strado mudflat
Fig 10 The evolution of seabed level (metres) (observed and modelled) at three Altus stations (A black, B red and C green) on Strado mudflat
Table 4 Relative occurrence of the nine combinations of classes of events over a period of 33 years The occurrence in the number of years for combinations of high, medium and low waves and run-off over 33 years is presented. The relative occurrence is given as well the expected number of occurrences for a period of 10 years
Table 4 Relative occurrence of the nine combinations of classes of events over a period of 33 years The occurrence in the number of years for combinations of high, medium and low waves and run-off over 33 years is presented. The relative occurrence is given as well the expected number of occurrences for a period of 10 years
Table 3 Classification of the years 1970-2009 based on run-off and wave conditions
Table 3 Classification of the years 1970-2009 based on run-off and wave conditions
Fig. 11 Morphological update procedure from year n to year n+1
Fig. 11 Morphological update procedure from year n to year n+1
surveyed and difficulties in interpolation make the differential maps uncertain. This is indicated by hatching Fig. 12 Comparison of morphodynamic simulation (Jower) and differentional maps from surveys (upper) for the period from 1998 to 2005. Areas along coast and towards Bay of Vilaine have not been
surveyed and difficulties in interpolation make the differential maps uncertain. This is indicated by hatching Fig. 12 Comparison of morphodynamic simulation (Jower) and differentional maps from surveys (upper) for the period from 1998 to 2005. Areas along coast and towards Bay of Vilaine have not been
Table 6 Sequence of forcing for the 10-year morphodynamic simula- tion with alternating high and low run-off The scaling factors are all relative to the reference year April 2007- April 2008
Table 6 Sequence of forcing for the 10-year morphodynamic simula- tion with alternating high and low run-off The scaling factors are all relative to the reference year April 2007- April 2008
A 10-year long morphodynamic simulation has been pre- pared. The occurrences of high, medium and low river run- off and waves listed in Table 4 are used. The scenario has been defined to alternate between dry and wet years and respecting occurrences of the different joint events. The scaling factors in Table 6 are all relative to the reference year April 2007—April 2008. The applied scaling factors for run-off have been set to 1.1, 0.7 and 0.25 for high, medium and low, respectively.
A 10-year long morphodynamic simulation has been pre- pared. The occurrences of high, medium and low river run- off and waves listed in Table 4 are used. The scenario has been defined to alternate between dry and wet years and respecting occurrences of the different joint events. The scaling factors in Table 6 are all relative to the reference year April 2007—April 2008. The applied scaling factors for run-off have been set to 1.1, 0.7 and 0.25 for high, medium and low, respectively.
Fig. 14 Yearly deposition—erosion in metres of sediment thickness by the end of each simulation year (30 March 2008). Initial conditions year 2007
Fig. 14 Yearly deposition—erosion in metres of sediment thickness by the end of each simulation year (30 March 2008). Initial conditions year 2007
Fig. 15 Yearly deposition—erosion in metres of sediment thickness by the end of each simulation year (30 March 2008). Initial conditions yea 2005 The yearly erosion—deposition patterns for the nine fol- lowing years and starting from t he bathymetry of year 2007 and 2005, respectively, are shown in Figs. 14 and 15. The years with low run-off (years 2, 4 and 6) show deposition in the internal estuary similar to the differential survey maps for 2001-2003 and 2003-2005. For the low run-off (year 2), the model predicts an import of about 120,000 tons to the internal part. This is compara ble to estimates based on The 10-year simulation starts with the reference year. Figure 13 shows the accumulated bed-thickness changes and sediment fluxes between the different compartments at the end of the first year. The high run-off during the refer- ence year gives an export in the order of 260,000 tons from the internal part and 420,000 tons from the intermediate
Fig. 15 Yearly deposition—erosion in metres of sediment thickness by the end of each simulation year (30 March 2008). Initial conditions yea 2005 The yearly erosion—deposition patterns for the nine fol- lowing years and starting from t he bathymetry of year 2007 and 2005, respectively, are shown in Figs. 14 and 15. The years with low run-off (years 2, 4 and 6) show deposition in the internal estuary similar to the differential survey maps for 2001-2003 and 2003-2005. For the low run-off (year 2), the model predicts an import of about 120,000 tons to the internal part. This is compara ble to estimates based on The 10-year simulation starts with the reference year. Figure 13 shows the accumulated bed-thickness changes and sediment fluxes between the different compartments at the end of the first year. The high run-off during the refer- ence year gives an export in the order of 260,000 tons from the internal part and 420,000 tons from the intermediate
Fig. 16 Accumulated deposition and erosion after 5 years of simulation with the initial bathymetry of 2007 and 2005
Fig. 16 Accumulated deposition and erosion after 5 years of simulation with the initial bathymetry of 2007 and 2005
that together result in strong bed shear stresses. Despite these shear stresses, the sediment is not resuspended in nature to the extent that one would expect for freshly deposited sediment. This could be due to drag reduction not accounted for in the sediment transport model. Due to the lack of site specific near sea bed observations in the centre of Bay of Vilaine, the exact cause cannot be identified. the morphological trends from 1998-2005, it was necessary to reduce the scaling factors, calculated from the measures of the annual energy, with respect to run-off.
that together result in strong bed shear stresses. Despite these shear stresses, the sediment is not resuspended in nature to the extent that one would expect for freshly deposited sediment. This could be due to drag reduction not accounted for in the sediment transport model. Due to the lack of site specific near sea bed observations in the centre of Bay of Vilaine, the exact cause cannot be identified. the morphological trends from 1998-2005, it was necessary to reduce the scaling factors, calculated from the measures of the annual energy, with respect to run-off.
Fig. 18 Scenario 2. Accumulated deposition and erosion by the end of five dry years of simulation with the initial bathymetry of 2007 and 2005 It is emphasised that an accurate description of the hy- drodynamics, although trivial, is crucial to be able to simu- late the sediment behaviour. Cohesive sediment transport modelling of natural systems is, despite scientific progress, As the objective was to estimate the long-term morphodynamics, a relatively simple model has been devel- oped. This is in accordance with the philosophy of Roelvink and Reniers (2012), who argue (albeit for non-cohesive coastal morphology) that adding more physics will improve
Fig. 18 Scenario 2. Accumulated deposition and erosion by the end of five dry years of simulation with the initial bathymetry of 2007 and 2005 It is emphasised that an accurate description of the hy- drodynamics, although trivial, is crucial to be able to simu- late the sediment behaviour. Cohesive sediment transport modelling of natural systems is, despite scientific progress, As the objective was to estimate the long-term morphodynamics, a relatively simple model has been devel- oped. This is in accordance with the philosophy of Roelvink and Reniers (2012), who argue (albeit for non-cohesive coastal morphology) that adding more physics will improve

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A large scale morphodynamic process-based model of the Gironde estuary
Catherine Villaret

We present here our effort to develop a morphodynamic model of the Gironde estuary, using a process-based approach. In this complex and strongly dynamical environment, internal coupling between the flow (Telemac-2d) and sediment transport models (Sisyphe) allows the representation of detailed sediment processes and interactions with the bed, using a bed roughness feedback method. In this complex and highly heterogeneous environment, the sediment bed composition had to be schematized assuming 3 classes of bed material. Model results could be further improved by accounting for the cohesive sediment (consolidation processes). Thanks to parallelization of the codes, the simulation takes only 18 hrs on 8 processors (linux station) to calculate the 5-year bed evolution, at the basin scale (150 km).

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Related topics

  • Geology
  • Oceanography
  • Ocean Dynamics
  • Fine Sediments

    • Cited by

    Man-induced regime shifts in small estuaries—II: a comparison of rivers
    Alexander Van Braeckel

    Ocean Dynamics, 2013

    This is Part II of two papers onman-induced regime shifts in small, narrow, and converging estuaries, with focus on the interaction between effective hydraulic drag, fine sediment import, and tidal amplification, induced by river engineering works, e.g., narrowing and deepening. Paper I describes a simple linear analytical model for the tidal movement in narrow, converging estuaries and a conceptual model on the response of tidal rivers to river engineering works. It is argued that such engineering works may set in motion a snowball effect bringing the river into an alternative steady state. Part II analyses the historic development in tidal range in four rivers, e.g., the Elbe, Ems, Loire, and Scheldt, all in northwest Europe; data are available for many decades, up to a century. We use the analytical model derived in Part I, showing that the effective hydraulic drag in the Ems and Loire has decreased considerably over time, as anticipated in Part I. We did not find evidence that the Upper Sea Scheldt is close to its tipping point towards hyperturbid conditions, but risks have been identified. In the Elbe, tidal reflections against the profound step in bed level around Hamburg seem to have affected the tidal evolution in the last decades. It is emphasized that the conceptual picture sketched in these papers is still hypothetical and needs to be validated, for instance through hind-cast modeling of the evolution of these rivers. This will not be an easy task, as historical data for a proper calibration of the models required are scarce.

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