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Modelling the morphodynamic impact of offshore sandpit geometries
Abstract
We investigate the hydrodynamic effects and morphodynamic impact of large-scale offshore sand extraction, for a variety of pit designs. We use a process-based idealized model for flow, sediment transport and bed evolution in a tide-dominated environment. Legislation and other practical considerations motivate our assumption that sandpits are both wide (horizontal dimensions of the order of kilometres) and shallow (the ratio of pit depth to water depth being small). This results in a semi-analytical tool that, unlike previous studies, enables a quick and extensive study into the effects of varying the physical characteristics as well as the pit design parameters. These parameters include pit length, width, and orientation with respect to the tide.The model results show that sandpits experience flow contraction, which is enhanced by friction–topography interactions and also affected by Coriolis effects. As a result, sandpits trigger the morphodynamic instability associated with the formation of large-scale bed features known as tidal sandbanks. It implies a gradual deepening and deformation of the pit itself, as well as the appearance of adjacent humps. The time scale of this behaviour is of the order of decades to centuries.A sensitivity analysis is then carried out to determine the effects of pit geometry on the model results. The morphodynamic response is found to be strongest for sandpits elongated in the preferred direction of sandbank formation (giving the largest area of morphodynamic influence), and weakest for pits perpendicular to this direction. The migration of the pit is shown not to depend on the pit geometry, but rather on the ambient flow conditions.
... Regarding the morphodynamic response of tidal sand ridges to sand extraction, found that a ridge recovers on decadal time scales after a local removal of sand. The study by Roos et al. (2008) revealed that the presence of sand pits on a flat shelf bottom triggers a morphodynamic instability that causes formation of new tidal sand ridges. Brière et al. (2010), who modelled a 30 year morphodynamic evolution of the depression area that formed on top of the Kwinte Bank, did not find any recovery of this area during the modelling time period. ...
... Following Roos et al. (2008), the area of morphodynamic influence is defined as the surface area where the absolute differences between bed levels of the cases with and without intervention (| |) exceed 1% of the mean (undisturbed) depth of the shelf ( ∼ 21 m). Mathematically, this is expressed as ...
... Thus, introducing a local disturbance has a global impact on the shelf morphodynamic evolution. Note that the results of the sand extraction experiments extend those of Roos et al. (2008) to offshore sand pits of finite extent and depth of a shelf that is already covered by tidal sand ridges. Nevertheless, inspection of the difference plot of Fig. 12a, with superimposed the induced residual flow field, shows that the bottom perturbations induced by the pit behave quite similar as those in Roos et al. (2008). ...
Tidal sand ridges are large-scale bottom features that occur on continental shelves with strong tidal currents and abundance of sand. These ridges have longshore spacings of several kilometres, heights of tens of meters, they evolve on centennial time scales and their crests are cyclonically rotated with respect to the direction of the dominant tidal currents. The coupled Delft3D-SWAN numerical model is used to study the natural morphodynamic evolution of tidal sand ridges and their response to interventions (sand extraction and construction of islands) in a setting that resembles the Belgian shelf. New aspects in this study are that a sophisticated wave model (SWAN), a shelf with a sloping bottom and state-of-the-art formulations for sediment transport (including suspended load) are considered.
Starting from an initially flat bed, model results show that sand ridges develop on the shelf that have similar characteristics as those of observed tidal sand ridges. Results further show that ridges recover after sand extraction, i.e., the original sand volume of the ridge crests is recovered on decadal time scales. Deepening of the troughs provides sand for this recovery. The recovery of the ridges is weaker when pits are located further offshore and/or are deeper. Ridges recover faster when waves are included. If a sand pit is too deep (>4 m), the ridge loses sand in the first tens of years prior to its recovery on longer time scales. This initial sand loss seems to agree with the observed loss of sand volume from a tidal sand ridge on the Belgium shelf (Kwinte Bank), which has been subject to intense extractions.
Model results further show that islands lose sand on decadal time scales until they eventually disappear. This sand loss is deposited on the crests and in the troughs. The presence of islands causes the surrounding ridges to break into smaller ridges that have smaller orientation angles than those of the natural case. Using different island configurations (location, geometry and number of islands, longshore distance between islands) does not lead to qualitative changes of the model results.
... Regions with erosion and sedimentation were observed resulting in a variety of ecological responses (Bonne, 2010;de Backer et al., 2014). Due to the orientation of the sandbanks and sand extraction on the crests which are parallel to the tidal current, channelling of the flow can occur resulting in higher flow velocities and bed shear stresses (Roos et al., 2008;Werf van der and Giardino, 2009). The Belgian sand extraction cases are therefore excluded from our present study because flow channelling cannot be predicted with the two-dimensional quadratic friction law. ...
... Water depths are smaller (15 m) and large tidal amplitudes are present which makes the area prone to the influence of waves resulting in higher bed shear stress values that cannot be predicted with the applied 2D approach. The orientation of the borrow pit and dredge furrows are parallel to the tidal current and instead of decreased current velocities due to the increased water depth, flow contraction may occur (Roos et al. 2008) which may lead to higher current velocities and bed shear stress values. Instead of sediment deposition as in the MV2 borrow pit, erosion may occur. ...
... Due to continuing sand extraction on the Dutch Continental Shelf (DCS), a mega-scale trench may emerge. With the orientation of the trench parallel to the tidal current, flow contraction may occur (Roos et al., 2008) resulting in increased flow velocities and bed shear stresses. Instead of sedimentation, erosion of the seabed may even occur. ...
On the Dutch continental shelf, approximately 26 million m3 of marine sand is extracted each year which may increase up to 40 to 85 million m3 to counteract sea-level rise. For Maasvlakte 2 (MV2), a seaward harbour extension of the Port of Rotterdam 220 million m3 of sand was used. The Dutch authorities permitted 20 m deep sand extraction depth instead of the common 2 m to decrease the surface area of direct impact.
We studied the effects of deep sand extraction (20 m) and compared these with Dutch sand extraction case studies with intermediate and shallow extraction depths. We observed significant changes in faunal species composition and sediment characteristics in the deep areas of the MV2 borrow pit. Biomass of macrozoobenthos increased 7-12 fold and demersal fish biomass increased 20-fold in the deep areas. Macrozoobenthos and demersal fish correlated with sediment and hydrographic characteristics and time after cessation of sand extraction.
Ecological and bed shear stress data were combined and transformed into Ecosystem-based design (EBD) rules which can be used in the design phases of future borrow pits in order to simultaneously maximise the sand yield and decrease the surface area of direct impact.
... Based on these 2 assumptions the safety area of 500m is evaluated with the Twente morphological sand extraction model developed by Roos and Hulscher (2004) and adjusted by van der Veen (2008); this model is described in appendix E. This is realised by using an extreme situation based on Roos et al. (2008) who concludes that the morphological influence area around the sandpit depends on pit size, shape, orientation (with current direction), current velocity and water depth. Larger pits or a high current velocity will increase the area of morphological influence. ...
... Based on this information the parameters in The M2 velocity and water depth are based on simulations with the WAQUA 'SIMONA-kuststrookfijn-1999-v4' model. The angle of the pit with respect to M2 current, length and width are based on Roos et al. (2008). ...
... Due to the fact that the model uses a symmetric tide, pit migration is not possible and there is an under estimation in the morphological influence of the pit. Roos et al. (2008) has already updated the Twente model, this update takes also pit migration into account. The Roos et al. (2008) model is developed in FORTRAN and it is expected that this model can be implemented in the same way in MapTable. ...
... Intense shipping increases the risk of cargo loss (like containers), which enlarges the probability of undetected objects on the sea floor. Human intervention in the natural processes of shallow sandy seas (factor 4) include dredging, sand mining and dumping, land reclamation, and the placement of objects on the sea floor (like wind farms) [Van der Veen, 2008;Roos et al., 2008]. Human intervention in natural processes also have an indirect effect that may cause long-term changes in depth until tens of kilometres away from the location of the intervention. ...
... Present morphodynamic models allow for the qualitative prediction of the effect of human intervention in the natural morphological processes, like areas where accretion or removal of sediment is expected [Van der Veen, 2008;Roos et al., 2008], or sand wave growth and migration [Besio et al., 2008;Németh et al., 2007]. This knowledge gives opportunities to adapt resurvey frequencies based on human interventions, past or planned, in combination with observed sea floor dynamics (factor 5). ...
... The resulting recommendations for changes in the survey policy are of higher detail than recommendations from recent exploratory surveys or remote sensing techniques, but they are based on linear extrapolations of trends. To get a better insight into the present and future morphodynamics than by a linear extrapolation, a coupling with process-based models should be made, especially for areas where the indirect effect of human interventions could pose a threat to navigation [Van der Veen, 2008;Roos et al., 2008]. Such a coupling has already been made between the trend analysis method of Wüst [2004] and the process-based Landau equation for sand wave growth, by Knaapen et al. [2005]. ...
Safe nautical charts require a carefully designed bathymetric survey policy,
especially in shallow sandy seas that potentially have dynamic sea floor patterns.
Bathymetric resurveying at sea is a costly process with limited resources,
though. A pattern on the sea floor known as tidal sand waves is clearly present
in bathymetric surveys, endangering navigation in the Southern North Sea because
of the potential dynamics of this pattern. An important factor in an
efficient resurvey policy is the type and size of sea floor dynamics. The uncertainties
of measurement and interpolation associated with the depth values
enable the statistical processing of a time series of surveys, using deformation
analysis. Currently, there is no procedure available that satisfies the Royal
Netherlands Navy requirements. Therefore, a deformation analysis procedure
is designed, implemented and tested in such a way that the procedure works on
bathymetric data and satisfies the Royal Netherlands Navy requirements. Also,
it is necessary to develop a procedure that translates the results into changes of
the resurvey policy, taking into account their confidence intervals.
To describe the sea floor statistically, we assume the sea floor to consist
of a spatial trend function (or characterization) and a residual function (or
dispersion). Such a description is called a representation. The covariances
between positions are expressed in a covariance function, based on the residual
function. The covariance function is used by Kriging, an interpolation procedure
that propagates the variances and covariances of the data points to variances of
the interpolated values. This approach is used widely for spatial analyses, like
the interpolation of a bathymetric data set.
The method that we propose uses Kriging to produce a time series of grids
of depth values and their variances. Subsequently, it uses deformation analysis,
a statistical procedure based on testing theory. Our application of deformation
analysis is particularly aimed at the detection of dynamics in areas with tidal
sand waves, resulting in parameter estimates for the sea floor dynamics, and
their uncertainty. We apply the method to sea floor representations both with
and without a sand wave pattern. A test scenario is set up, consisting of a
survey of an existing area in the Southern North Sea, for which dynamics are
simulated. The results show that the proposed method detects different types
of sea floor dynamics well, leading to satisfactory estimates of the corresponding
parameters.
We show results for the anchorage area Maas West near the Port of Rotterdam,
the Netherlands first. The area is divided into 18 subareas. The results
show that a sand wave pattern is detected for most of the subareas, and a shore ward migration is detected for a majority of them. The estimated migration
rates of the sand waves are up to 7.5 m/yr, with a 95% confidence interval that
depends on the regularity of the pattern. The results are in confirmation with
previously observed migration rates for the Southern North Sea, and with an
idealized process-based model.
Thereafter, we analyze several other areas for which a time series of surveys is
available in the bathymetric archives of the Netherlands Hydrographic Service,
to study the spatial variations in sea floor dynamics. We present results for
several sand wave areas and a single flat area. In some of those areas, dredging
takes place, to guarantee minimum depths. The results indicate sand wave
migration in areas close to the coast, and bed level changes of the order of
decimeters. The dominant wavelength of the sand waves varies. We compare
our results to literature of the same sand wave areas, in which we find similar
migration rates, and different wavelengths.
By formulating four indicators, recommendations are made for the resurvey
policy on the Belgian and Netherlands Continental Shelf. These indicators follow
from the estimates for sea floor dynamics. We present a concept for the
shallowest likely depth surface, on which we base two of the indicators. The
other two indicators act as a warning: they quantify the potentially missed
dynamics, which makes the procedure more robust in case of complicated morphology.
We show clear differences in recommended resurvey frequency between
the five analyzed regions.
We conclude that the designed method is able to use a time series of bathymetric
surveys for the estimation of sea floor dynamics in a satisfactory way.
Those dynamics may be present on the scale of the sea floor, it may be a local
effect, or it may be due to a tidal sand wave pattern. Also, the results are successfully
reduced to a set of four indicators, used to improve a resurvey policy.
Based on these conclusions, we formulate recommendations on the extrapolation
of the results in space and time, on potential adaptations to the designed
procedure, and on implementation of the procedure.
... Previous studies focussed on the hydrodynamics and morphodynamics of extraction areas with horizontal dimensions of the order of km and smaller. We distinguish trenches, characterized by a length that is much larger than the width (Jensen et al., 1999a(Jensen et al., , 1999bvan der Kreeke et al., 2002;Ribberink et al., 2005), and pits, for which these scales are of comparable magnitude Roos et al., 2005Roos et al., , 2008Walstra et al., 2002). Alternatively, investigating the morphodynamic impact of sand extraction from large-scale marine bed forms such as tidal sandbanks requires a model approach accounting for the dynamics of these features (Brière et al., 2010;Roos and Hulscher, 2007;van der Veen and Hulscher, 2009). ...
... The cross-basin tidal current amplitude at Q thus decreases, indicating that the cross-basin tidal current inside the trench is mainly governed by continuity (i.e. the increased water depth leads to decreased tidal currents). In contrast, the along-basin tidal current amplitude at Q increases, suggesting effects of flow contraction (Roos et al., 2008). The largest changes in the velocity amplitudes occur at (Table 3). the trench boundaries y = y 1 and y = y 2 , because of the abrupt change in water depth at these locations (see Figs. 6c and 7c). ...
We investigate the effects of a mega-scale sand extraction trench (length ~200 km, width ~10 km, depth ~10 m, located in front of the Dutch coast) on tidal dynamics in the Southern North Sea. To this end, an idealized modeling approach is adopted, extending the classical problem of Kelvin wave reflection in rectangular semi-enclosed basins to account for lateral depth variations and the presence of a trench. The model results indicate changes in tidal range (zones of decrease and increase of the order of several cm), phase and currents (order cm/s) throughout the whole basin, which emphasizes the importance of a model domain on the scale of the basin. The changes in currents may structurally change sediment transport rates by several percents, which is relevant to coastal morphology. It is argued that this idealized model can be used to support a companion study carried out with a more detailed numerical engineering model. This is because of its ability to (1) provide insight into the physical mechanisms, (2) reveal the order of magnitude and area of influence of the effects and (3) allow for a quick sensitivity analysis with respect to trench geometry.
... Regions with erosion and sedimentation were observed resulting in a variety of ecological responses (Bonne, 2010;De Backer et al., 2014). Due to the orientation of the sandbanks and sand extraction on the crests which are parallel to the tidal current, channelling of the flow can occur resulting in higher flow velocities and bed shear stresses (Roos et al., 2008;Werf van der and Giardino, 2009). The Belgian sand extraction cases are therefore excluded in our present study because flow channelling cannot be predicted with the two-dimensional quadratic friction law. ...
... Due to continuing sand extraction on the Dutch Continental Shelf (DCS), a mega-scale trench may emerge. With the orientation of the trench parallel to the tidal current, flow contraction may occur (Roos et al., 2008) resulting in increased flow velocities and bed shear stresses. Instead of sedimentation, erosion of the seabed may even occur. ...
The demand for marine sand in the Netherlands as well as globally is increasing. Over the last decades, only shallow sand extraction of 2 m below the seabed was allowed on the Dutch Continental Shelf (DCS). To guarantee sufficient supply and to decrease the surface area of direct impact, the Dutch authorities started to promote sand extraction depths over 2 m for sand volumes over 10 million m3. The ecological effects of deep sand extraction, however, are still largely unknown. Therefore, we investigated short-term effects (0–2.5 y) of deep sand extraction (20–24 m) and compared these with other case studies such as, regular shallow sand extraction on the DCS (2 m) and an 8 m deepened shipping lane. For intercomparison between case studies we used tide-averaged bed shear stress as a generic proxy for environmental and related ecological effects. Bed shear stress can be estimated with a two-dimensional quadratic friction law and showed a decrease from 0.50 to 0.04 N m−2 in a borrow pit in 20 m deep water and extraction depths up to 24 m. Macrozoobenthos in a borrow pit with a tide-averaged bed shear stress of around 0.41 N m–2 is expected to return back to pre-extraction conditions within 4–6 year. When tide-averaged bed shear stress decreases below 0.17 N m−2 enhanced macrozoobenthic species richness and biomass can occur. Below a tide-averaged bed shear stress of 0.08 N m−2, increasing abundance and biomass of brittle stars, white furrow shell (Abra alba) and plaice (platessa platessa) can be expected. Below 0.04 N m−2, an overdominance and high biomass of brittle stars can be expected whereas demersal fish biomass and species composition may return to reference conditions. Next to changes in faunal composition, a high sedimentation rate can be expected.
... Intense shipping increases the risk of cargo loss (like containers), which enlarges the probability of undetected objects on the sea floor. Human intervention in the natural processes of shallow sandy seas (factor 4) include dredging, sand mining and dumping, land reclamation, and the placement of objects on the sea floor (like wind farms) [Van der Veen, 2008; Roos et al., 2008]. Human intervention in natural processes also have an indirect effect that may cause long-term changes in depth until tens of kilometres away from the location of the intervention. ...
... Present morphodynamic models allow for the qualitative prediction of the effect of human intervention in the natural morphological processes, like areas where accretion or removal of sediment is expected [Van der Veen, 2008;Roos et al., 2008], or sand wave growth and migration [Besio et al., 2008;Németh et al., 2007]. This knowledge gives opportunities to adapt resurvey frequencies based on human interventions, past or planned, in combination with observed sea floor dynamics (factor 5). ...
Bathymetric resurveying at sea is a costly process with lim-ited resources, yet necessary for adequate nautical charts and therefore crucial for safe navigation. An important factor in an efficient resurvey policy is the type and size of sea floor dynamics. By formulating four indicators, we make recommendations for the resurvey policy of the Hy-drographic Service of the Royal Netherlands Navy on the Belgian and Netherlands Continental Shelf. The continen-tal shelf of these two countries is characterized by a sandy sea floor covered with rhythmic patterns and by limited depth. These indicators follow from the estimates for sea floor dynamics, as given by applying the statistical method of deformation analysis. We present a concept for the shal-lowest likely depth surface, on which we base two of the indicators. The other two indicators act as a warning: they quantify the potentially missed dynamics, which makes the procedure more robust in case of complicated morphology. We show clear differences in recommended resurvey prior-ity between the five analyzed regions, which currently have equal resurvey frequencies.
... The presence of a bathymetric anomaly (a bump or a trough) affects the both the non-linear evolution of the featuresand, surprisingly, numerical simulations show that a large bathymetric anomaly can change the wavelength of the pattern developing down-or up-current. Similarly, Roos and colleagues (Roos and Hulscher, 2004;Roos et al., 2008; but also see Blondeaux and Vittori, 2008) use linear stability analysis to explore the morphodynamic impact of wide and shallow perturbations (representative of large-scale offshore sand extraction). Results of the analysis shows that small and medium scale pits can trigger the formation of sand waves (Blondeaux and Vittori, 2005) while huge pits can trigger the formation of sand banks (Roos et al., 2008). ...
... Similarly, Roos and colleagues (Roos and Hulscher, 2004;Roos et al., 2008; but also see Blondeaux and Vittori, 2008) use linear stability analysis to explore the morphodynamic impact of wide and shallow perturbations (representative of large-scale offshore sand extraction). Results of the analysis shows that small and medium scale pits can trigger the formation of sand waves (Blondeaux and Vittori, 2005) while huge pits can trigger the formation of sand banks (Roos et al., 2008). These studies deal with infinitesimal perturbations and neglect non-linear effects so that no insight can be gained on the evolution of these perturbations and the mechanism leading to their steady state. ...
Many studies focus on the emergence and development of rhythmic landscape patterns. In this contribution we explore the different behaviors found as patterns evolve; the trajectories that patterns exhibit as they transit from infinitesimal-amplitude perturbation to a statistically steady state (or in some cases to continued statistical evolution). The variety of behaviors observed, either through field and laboratory experiments or numerical modeling, can be reduced to four classes: (a) simple stabilization where predictions based on the initial growth of small perturbations corresponds with the characteristics of patterns observed in nature; (b) significant pattern coarsening en route to saturated wavelength, where non-linear interactions between finite-amplitude pattern elements change the geometric properties of a pattern as it approaches steady-state; (c) perpetual coarsening where the wavelength associated with the emerging pattern continues to grow over time and is only limited by physical boundaries or external constrains; (d) slow evolution toward a different attractor, a novel behavior observed in numerical modeling that involves profound temporal changes in pattern characteristics. Within these classes we also observe generalizable non-linear behaviors: dependence on initial conditions, the emergence of pattern-scale variables such as pattern defects, and the presence of multiple stable states. Predicting the shape of patterns to come remains a challenge – one that we suggest requires a range of modeling approaches to address both initial instabilities and the emergent properties of evolving patterns, which involve disparate forms of non-linear interactions. Consideration of generic system behaviors at the pattern scale could enhance future pattern formation studies, facilitating appropriate pairings of analysis approaches and pattern-evolution modes. Copyright © 2013 John Wiley & Sons, Ltd.
... Human intervention in the natural processes of shallow sandy seas (factor 4) includes dredging, sand mining and dumping, land reclamation, and the placement of objects on the sea floor (such as wind farms) [24]. Human intervention in natural processes may also indirectly trigger longterm changes in depth until tens of kilometres away from the location of the intervention [18]. ...
... Present morphodynamic models allow for the qualitative prediction of the effect of human intervention in the natural morphological processes, such as areas where deposition or erosion of sediment is expected [18,24], or sand wave growth and migration [1,17,23]. This knowledge gives opportunities to adapt resurvey frequencies based on human interventions, past or planned, in combination with observed sea floor dynamics (factor 5). ...
Bathymetric resurveying in shallow seas is a costly process with limited
resources, yet necessary for adequate nautical charts and therefore
crucial for safe navigation. An important factor in an efficient
resurvey policy is the type and size of sea floor dynamics. We propose
four indicators, which we use in a procedure to recommend changes to the
resurvey policies of the Belgian and Netherlands Continental Shelf
(BNLCS). These shallow regions are characterised by a sandy sea floor
covered with rhythmic patterns. The proposed indicators follow from the
estimates for sea floor dynamics, as given by applying the statistical
method of deformation analysis. We present a concept for the shallowest
likely depth surface, on which we base two of the indicators. The other
two indicators may act as a warning: they quantify the potentially
missed dynamics, which makes the procedure more robust in case of
complicated morphology. Finally, applying our procedure to the BNLCS, we
show clear differences in recommended resurvey priority between the five
analysed regions, which currently have equal resurvey frequencies.
... Bank growth is driven by horizontal circulations that result from interactions between topography, bed friction, tidal motion, and Coriolis effects, as has been observed in the field (Caston & Stride, 1970), studied theoretically (Huthnance, 1973;Pattiaratchi & Collins, 1987;Robinson, 1981;Zimmerman, 1981), and reproduced in complex numerical models (e.g., Sanay et al., 2007). The initial instability may be triggered by natural topographic variation or excavation of the seabed (Nnafie et al., 2020;Roos et al., 2008). Importantly, linear stability analysis is only applicable when bank amplitudes are low with respect to the mean water depth. ...
Plain Language Summary
Sandbanks are large‐scale bodies of sand with rhythmic crests and troughs found in shallow seas. They grow in size due to the interaction of the tidal flow with the sand on the seabed until they eventually reach an equilibrium shape. They may also migrate when the tidal flow is asymmetric. Observations in the North Sea have revealed that sandbanks still exist when sand is scarce, but we do not yet understand how scarcity affects the equilibrium shape and the migration rate. Therefore, we have developed a mathematical model that simulates bank evolution of a small bank until equilibrium under increasingly scarce conditions. We find that sand scarcity leads to lower and slightly narrower sandbanks. The asymmetry of the shape also changes. The cross‐section is skewed towards the direction of a residual current (if present) when sand is readily available, but becomes increasingly symmetric when scarcity increases. Bank shapes may even be skewed in the opposite direction when sand is very scarce. Finally, the migration rate of sandbanks increases when sand becomes scarce, but decreases again when sand becomes very scarce. Our findings help to understand how sand scarcity (naturally or due to extraction) affects future marine sandbank landscapes.
... Dredging interventions can be seen as a local perturbation. In a flat bed this may trigger the instabilities associated with the formation of tidal sand waves (Roos et al., 2005) and tidal sand banks (Roos et al., 2008). In a finite amplitude setting, such a perturbation may trigger recovery mechanisms, as has been shown for shoreface-connected ridges (De Swart and Calvete, 2003) and tidal sand banks (Roos and Hulscher, 2006). ...
Maintenance of navigation channels in shelf seas with tidal sand waves usually requires repeated dredging operations. Optimizing these interventions is a difficult task, particularly complicated by the nonlinear morphodynamics of sand wave recovery after dredging. Here we present a process-based model study, incorporating different strategies of dredging in an existing nonlinear sand wave model. We consider ‘topping’ (removing sand from crest) and ‘swiping’ (same, but now placing it in the troughs), for a range of dredging depths. Starting point is a fully developed sand wave or sand wave field, as simulated in the model. Results indicate that sand wave recovery is slowest after swiping. Also, larger dredging volumes imply longer recovery times. Next, we study the maintenance intervals and (cumulative) dredging volumes resulting from adopting a typical temporal strategy: swipe to a prescribed dredging depth, as soon as the recovering sand wave crests have reached a critical depth. Maintenance intervals are found to depend on the dredging depth and, importantly, to vary over time, as well. This last result shows that sand wave recovery depends not only on height, but also on its shape, emphasizing the limitations of existing, empirical (Landau-type of) models based on amplitude (or height) only.
... The local and reach-scale impact of dredging has been widely discussed in the literature. Many researchers have sought to elucidate the impact of dredging by refining theoretical models of hydrodynamics and backfilling effects (Lagasse, 1986;Rijn, 1986;Fredsøe, 1979;Lee, Fu, & Song, 1993;Jensen, Madsen, & Fredsøe, 1999;Jensen & Fredsøe, 2001;Neyshabouri, Farhadzadeh, & Amini, 2002;Roos, Hulscher, & de Vriend, 2008). Other researchers have focused on the impact of dredging in terms of engineering practices within the context of river system development. ...
This paper presents a unique case study of Nanshi River in Taiwan which may be able to serve as the basis for decision making and river maintenance operations or as a reference for other cases. This study area is composed of regions from upstream alluvial channel to delta‐front and the Guishan Dam reservoir, with a surplus of material supply to transport capacity in the whole region. Dredging is a standard method by which to deal with the accumulation of sediment within a watercourse; however, our understanding of hydrodynamics and sedimentation is limited by the complexity of the sedimentation process. In addition, most of the similar geomorphological studies did not pay attention to the upstream effects of reservoirs on alluvial channels. Similar examples are more often observed in estuaries and coastal zones. Our objective was to evaluate the morphological impact of river dredging implemented in conjunction with downstream gate operations by employing a 2D hydraulic sediment model, SRH‐2D. With the assistance of the model simulations, we revealed that the deposition in the study area is under the interaction among three main mechanisms: headward deposition of the dam, channel width contraction in midstream, and progressive deposition of sediment from the confluence. Accordingly, this study suggested that dredging focuses on the creation of deep trenches in the midstream and upstream sections. It is also important to coordinate gate operations at Guishan Dam with peak flows in order to facilitate the transport of sediment to the reservoir for subsequent discharge via empty flushing. Regarding the effect of Guishan Dam removal, it would have a more pronounced effect on head cutting which leading to erosion in the reservoir and the downstream transport of substantial quantities of sediment, but its effect would be limited to a certain distance. However, dam removal is not considered a viable option considering the need for local water storage and the benefits of the dam in terms of tourism.
... Van Rijn [31] developed a mathematical model to predict the sedimentation of dredged channels by waves and currents. Roos et al. [32] developed an idealized process-based morphodynamic model for the evolution of large-scale sandpits in a tide-dominated offshore environment. Benedet and List [33] use a numerical model to evaluate the effects of bathymetric features, such as dredged borrow areas, on the formation of erosion hot spots. ...
This study is to investigate morphological changes in the Tamsui River Estuary in Taiwan driven by multiple physical processes, such as river flows, tides, waves, and storm surges, and then to study the impacts of sediment flushing operated at the Shihmen reservoir upstream on the river estuary. An integrated coastal and estuarine processes model (CCHE2D-Coast) (Center for Computational Hydroscience and Engineering Two-Dimensional-Coast) was validated by simulating these physical processes in the estuary driven by three historical typhoons in 2008. The site-specifically validated model was then applied to simulate morphological changes in the estuary in response to reservoir sediment flush scenarios from the upstream. For the impact assessment of sediment flushing, a synthetic hydrological event was designed by including a historical typhoon and a typical monsoon. It was found that during the typhoon, the sediments will be mostly deposited in the estuarine river reach of Tamsui and the Wazihwei sandy beach. During the monsoon period, most of the sediments tend to be deposited in the second fishing port of Tamsui, the northern breakwater, and the estuary, while the Wazihwei sandy beach in the river mouth would be scoured by backflow. Simulations of the complex flow fields and morphological changes will facilitate the best practice of sediment management in the coastal and estuarine regions.
... Several studies ( [38], and references therein) have been conducted to investigate the impact of offshore sand extraction on the local hydrodynamic and morphodynamic conditions. However, those studies either were limited in computation efficiency for simulations longer than a century or they employed simplified topography, i.e., a 1D configuration or a 2D configuration with perturbations of small amplitudes ( [59], and references therein). Sand extraction could be applied in the same way as that of Nnafie et al. [50], in which the effect of sand extraction on shoreface-connected sand ridges was investigated. ...
Tidal sand ridges are large-scale rhythmic bedforms that are observed on the offshore parts of shelf seas where sand is abundantly available. Spacings between successive ridges are several kilometres, they evolve on centennial time scales, and their crests are cyclonically rotated with respect to the direction of the principal tidal currents. Here, an overview will be presented of the current knowledge about these ridges with respect to their manifestation in different seas, their observed behaviour, the basic mechanisms that explain their initial formation and their evolution towards finite heights and the ability to model them. It will be shown that both tides, waves and changes in mean sea level have a profound impact on the evolution of the ridges.
... Sur des échelles de temps plus longues, la fosse résultant du dragage de sable peut migrer vers le rivage et agir comme un puits pour les sédiments du système littoral (Van Rijn & Walstra 2002). La gravité et la persistance de ces effets dépendent, entre autres facteurs, des conditions hydrodynamiques, de la nature du substrat, de la géométrie de la zone de dragage et de sa distance de la côte, et de l'échelle de temps de la perturbation des fonds marins (Cooper et al. 2007;Roos et al. 2008). Si l'extraction est opérée dans la zone de déferlement, l'impact est massif et immédiat. ...
In many countries, the increasing demand on sand for construction and development needs has driven users to exploit marine sand deposits. The opening of this market raises several questions, due to its impacts on environmental and socio-economic systems, in a context marked by coast erosion risks associated to the sea level rise, among other factors. In Morocco, as elsewhere, the challenge consists on the control of these impacts and their integrated management. In this paper, we give an overview on the governance of sand market in Morocco, in its relationship with the development of the marine sand dredging. We analyze the regulation framework governing the sector of marine sand extraction and we dissect the main elements of the environmental issues, neglected in the design of this framework. We also recommended some technical measures, which are likely to improve the governance of this activity on both technical and regulatory plans and on the environmental assessment of marine sand dredging projects.
... Sur des échelles de temps plus longues, la fosse résultant du dragage de sable peut migrer vers le rivage et agir comme un puits pour les sédiments du système littoral (Van Rijn & Walstra 2002). La gravité et la persistance de ces effets dépendent, entre autres facteurs, des conditions hydrodynamiques, de la nature du substrat, de la géométrie de la zone de dragage et de sa distance de la côte, et de l'échelle de temps de la perturbation des fonds marins (Cooper et al. 2007;Roos et al. 2008). Si l'extraction est opérée dans la zone de déferlement, l'impact est massif et immédiat. ...
... Lorsque l'extraction du sable est effectuée au large, la fosse résultant de l'extraction du sable marin peut migrer vers le rivage et agir comme un puits pour les sédiments du système littoral ( VAN RIJN & WALSTRA, 2002). La gravité et la persistance de ces effets dépendent, entre autres facteurs, des conditions hydrodynamiques, de la nature du substrat, de la géométrie de la zone de dragage et de sa distance à la côte, et de l'échelle de temps de la perturbation des fonds marins (COOPER et al., 2007 ;ROOS et al., 2008). Si l'extraction est opérée dans la zone de déferlement, l'impact est massif et immédiat. ...
Le dragage des matériaux marins au niveau des estuaires et en offshore au Maroc pour les besoins d'entretien et/ou de construction soulèvent des questions relatives à la gestion de leurs impacts sur l'environnement ainsi que la stabilité du littoral et des ouvrages qu'il héberge. Ce travail donne un aperçu sur les impacts physiques et écologiques des projets de dragage du sable marin et apporte quelques mesures techniques pouvant être prises en considération lors de l'évaluation de leurs études environnementales. Mots-clés : Dragage des matériaux marins, Impacts physique et écologique, Evaluation environnementale.
... Here U max is the maximum velocity of the basic flow → u 0 . Linear and nonlinear bed shear stress were also considered by Roos et al. (2008), but they did not systematically investigate the differences in the resulting growth characteristics of the bedforms. ...
Tidal sand ridges and long bed waves are large-scale bedforms that are observed on continental shelves. They differ in their wavelength and in their orientation with respect to the principal direction of tidal currents. Previous studies indicate that tidal sand ridges appear in areas where tidal currents are above 0.5 m s−1, while long bed waves occur in regions where the maximum tidal current velocity is slightly above the critical velocity for sand erosion and the current is elliptical. An idealized nonlinear numerical model was developed to improve the understanding of the initial formation of these bedforms. The model governs the feedbacks between tidally forced depth-averaged currents and the sandy bed on the outer shelf. The effects of different formulations of bed shear stress and sand transport, tidal ellipticity and different tidal constituents on the characteristics of these bedforms (growth rate, wavelength, orientation of the preferred bedforms) during their initial formation were examined systematically.
The results show that the formulations for bed shear stress and slope-induced sand transport are not critical for the initial formation of these bedforms. For tidal sand ridges, under rectilinear tidal currents, increasing the critical bed shear stress for sand erosion decreases the growth rate and the wavelength of the preferred bedforms significantly, while the orientation angle slightly decreases. The dependence of the growth rate, wavelength and the orientation of the preferred bedforms on the tidal ellipticity is non-monotonic. A decrease in tidal frequency results in preferred bedforms with larger wavelength and smaller orientation angle, while their growth rate hardly changes. In the case of joint diurnal and semidiurnal tides, or spring-neap tides, the characteristics of the bedforms are determined by the dominant tidal constituent. For long bed waves, the number of anticyclonically/cyclonically oriented bedforms with respect to the principal current direction increases as the ellipticity of the cyclonic/anticyclonic tidal currents increases. Besides, under anticyclonic tidal currents, the growth rate of cyclonically oriented long bed waves increases as the tidal ellipticity increases. The model was also used to provide a possible explanation for the fact that the Dutch Banks have a larger wavelength than that of the Flemish Banks in the North Sea.
Full text see the link (free download till 04-11-2016) http://authors.elsevier.com/a/1TjU5-JmAX3yM.
... Over longer time scales, the sandpit area can migrate towards the shore and may act as a sink for the sediments from the nearshore system (van Rijn and Walstra, 2002). The severity and persistence of such impacts depend, among others factors, on the hydrodynamic conditions, the nature of the substrate, the sandpit geometry and its distance to the shore, and the time-scale of the seabed perturbation (Cooper et al., 2007;Roos et al., 2008). In order to minimize these effects and nearshore coastal erosion, the mining areas need to be located offshore of the closure depth. ...
... Sur des échelles de temps plus longues, la fosse résultant du dragage de sable peut migrer vers le rivage et agir comme un puits pour les sédiments du système littoral (Van Rijn & Walstra 2002). La gravité et la persistance de ces effets dépendent, entre autres facteurs, des conditions hydrodynamiques, de la nature du substrat, de la géométrie de la zone de dragage et de sa distance de la côte, et de l'échelle de temps de la perturbation des fonds marins (Cooper et al. 2007;Roos et al. 2008). Si l'extraction est opérée dans la zone de déferlement, l'impact est massif et immédiat. ...
In many countries, the increasing demand on sand for construction and development needs has driven users to exploit marine sand deposits. The opening of this market raises several questions, due to its impacts on environmental and socio-economic systems, in a context marked by coast erosion risks associated to the sea level rise, among other factors. In Morocco, as elsewhere, the challenge consists on the control of these impacts and their integrated management. In this paper, we give an overview on the governance of sand market in Morocco, in its relationship with the development of the marine sand dredging. We analyze the regulation framework governing the sector of marine sand extraction and we dissect the main elements of the environmental issues, neglected in the design of this framework. We also recommended some technical measures, which are likely to improve the governance of this activity on both technical and regulatory plans and on the environmental assessment of marine sand dredging projects.
... This was because the length-scales of typical dredged pits are small relative to the length-scale required for a significant fraction of the suspended sediment to settle out of the flow due to the reduction in flow velocity. More recent studies have sought to refine these theoretical models by testing the average effects of parameters such as borrow-pit geometry (Lee et al., 1993;Jensen and Fredsøe, 2001;Roos et al., 2008), the pit location within a channel (Neyshabouri et al., 2002), and the pit orientation relative to dominant currents (van Rijn 1986;Jensen et al., 1999). ...
In-channel sand mining by dredge removes large quantities of bed sediment and alters channel morphodynamic processes. While the reach-scale impacts of dredging are well documented, the effects of the dredged borrow pit on the local flow and sediment transport are poorly understood. These local effects are important because they control the post-dredge evolution of the borrow pit, setting the pit lifespan and affecting reach-scale channel morphology. This study documents the observed morphological evolution of a large (1.46 million m3) borrow pit mined on a lateral sandbar in the lower Mississippi River using a time-series of multibeam bathymetric surveys. During the 2.5 year time-series, 53 % of the initial pit volume infilled with sediment, decreasing pit depth by an average of 0.88 m yr-1. To explore the controls of the observed infilling, a morphodynamic model (Delft3D) was used to simulate flow and sediment transport within the affected river reach. The model indicated that infilling rates were primarily related to the riverine sediment supply and pit geometry. The pit depth and length influenced the predicted magnitude of the pit bed shear stress relative to its pre-dredged value, i.e., the bed-stress reduction ratio (R*), a metric that was correlated with the magnitude and spatial distribution of infilling. A one-dimensional reduced-complexity model was derived using predicted sediment supply and R* to simulate patterns of pit infilling. This simplified model of borrow-pit evolution was able to closely approximate the amount and patterns of sediment deposition during the study period. Additional model experiments indicate that, for a borrow pit of a set volume, creating deep, longitudinally-shorter borrow pits significantly increased infilling rates relative to elongated pits. Study results provide insight into the resilience of alluvial river channels after a disturbance and the sustainability of sand mining as a sediment source for coastal restoration
... These modelling approaches had mainly been developed with the intention of serving as coastal management tools. Typical applications comprise scenario analyses such as the model-based prediction of the morphological impact of coastal engineering (Grasso et al., 2011;Grunnet et al., 2004;Lu et al., 2009;Roos et al., 2008). In the recent past, however, the geo-sciences have also recognised the advantage of numerical modelling: Amongst many local studies, which address site specific topics and characteristics (Dan et al., 2011;Deleu et al., 2004;Di Silvio et al., 2010;Ruggiero et al., 2009;Sutherland et al., 2004b;Winter, 2006a;Xie et al., 2009), others address more fundamental questions on the formation and dynamics of coastal environments. ...
The topic of sandy coast morphodynamics involves the mutual influences of coastal topography, local sedimentology, the driving meteorological and hydrodynamic boundary conditions, flora and fauna, and the activities of human beings: The latter as direct actors through coastal constructions and other interventions, as indirect actors through possible contributions to global change, but also as receiving agents - as living individuals confronted with the forces of the sea. The general aim of coastal research is to gain an as comprehensive as possible understanding of the different systems and their interaction in order to be able to evaluate their current state, assess their stability, explain past changes (in the geological record), and predict future developments under different conditions. Such systems dynamics involve a large bandwidth of spatial and temporal scales: from the microscopic interaction of turbulent fluid motions with single particles to meso-scale tidal dynamics of subaqueous bedforms to macro-scale seasonal adaptations of beach profiles or the meandering of tidal channels, to the mega-scale evolution of shorelines and shelf systems over decades to centuries. The process of understanding involves a continuous feedback of observations, abstractions, mathematical formulations, model development (ranging from conceptual models to mathematical formulations of processes, and to complex, process-based numerical modelling systems), and the testing of models on the basis of observations, new abstractions, and so forth. In the case of the morphodynamics of sandy coasts, the interaction of the physical processes involved in hydrodynamics, sediment dynamics, and their mutual adjustment to changing bed topographies seem most relevant, although biogeochemical processes play a (commonly underrated) additional role. This discourse presents an extended summary of the current state in the continuous process of gaining knowledge on coastal morphodynamics. It focuses on the dynamics of tidal channels and their main roughness elements: subaqueous compound bedforms. Methodological approaches involved are field measurements and numerical modelling, which are introduced and discussed.
... Os efeitos de persistência de uma escavação e a sua evolução, em particular a sua migração e taxa de enchimento, são ditados pelas características hidrodinâmicas e físicas do local, tal como a natureza do substrato, o método de dragagem usado, a geometria da escavação, a distância à costa e o tipo de comunidades bentónicas existentes no local (Hitchcock & Bell, 2004;Cooper et al., 2007;Roos et al., 2008). ...
RESUMO A extracção de sedimentos nas zonas costeiras é uma actividade de importância e prática crescentes há já vários anos e em diversos países. A evolução de uma zona de extracção depende essencialmente do tipo de sedimento e das características hidrodinâmicas do local. No presente trabalho pretendeu-se estudar a evolução morfológica e sedimentar de uma escavação ao largo de Vale do Lobo (Algarve, Portugal) com base em 3 campanhas batimétricas realizadas entre 2006 e 2008, antes e após a extracção, assim como numericamente, através da aplicação de um modelo morfodinâmico – MORSYS2D. Os dados batimétricos foram inseridos num sistema de informação geográfica (ArcGIS), para estimar os volumes de sedimento removidos e acumulados, analisar a variação morfo-sedimentar ocorrida durante o período de estudo e prever o tempo de persistência da escavação. Da análise dos dados batimétricos verificou-se uma suavização geral da área escavada com acreção no interior e erosão nos flancos e estimou-se um tempo de persistência da perturbação da batimetria, não inferior a 24 anos. Os resultados numéricos foram analisados a partir dos fluxos sedimentares e da evolução batimétrica induzida pelas correntes geradas pelo vento, maré e agitação marítima. Os resultados mostram um grande impacto dos eventos de tempestade com altura significativa de onda superior a 2,5m e ventos fortes na evolução morfológica da escavação e nas áreas em redor. A comparação dos resultados numéricos com a análise dos levantamentos batimétricos adquiridos permitiu avaliar a boa performance do modelo. Palavras-chave: Dragagem, Impacto Morfológico, Evolução de uma Escavação, Plataforma Continental, Modelo Morfodinâmico.
... The stability of bed forms with an orientation perpendicular to the tidal current was addressed with the help idealized stability models (linear and non-linear analysis after Sterlini, 2009), which obtained their tidal boundary conditions from a larger-scale model according to Roos (2008). The morphological calculations revealed that the preferred (stable) wave length of the bed forms is in the order 300-400 m. ...
Recent years have shown an ongoing need for marine infrastructure, however the development of large-scale infrastructure projects is associated with uncertainties and delays, increased awareness of the environmental effects and extensive demands on environmental management plans and monitoring programs. The combination of these tendencies drives innovations in project design and realisation, which are often characterized by integral, multidisciplinary approaches, increased contractor involvement and the application of Building with Nature type approaches. This paper presents two examples of such innovations in project design and realisation, both developed in the context of of sustainable development of nourished coastlines. The examples concern the application of mega-nourishments for coastal development and ecological landscaping in sand mining areas. The work is carried out as part of the Innovation Program Building with Nature, which aims, amongst others, to draft a manual for the ecodynamic design of marine infrastructure. We believe the use of ecodynamic design strategies for marine infrastructure will inspire ecologists and engineers to maximally explore environmental benefits of projects, thereby adding to the ecological and public appreciation of marine works.
... This was because the length-scales of typical dredged pits are small relative to the length-scale required for a significant fraction of the suspended sediment to settle out of the flow due to the reduction in flow velocity. More recent studies have sought to refine these theoretical models by testing the average effects of parameters such as borrow-pit geometry (Lee et al., 1993;Jensen and Fredsøe, 2001;Roos et al., 2008), the pit location within a channel (Neyshabouri et al., 2002), and the pit orientation relative to dominant currents (van Rijn 1986;Jensen et al., 1999). ...
Sand mining in alluvial rivers by hydraulic or bucket dredge causes a significant disturbance within the geomorphic processes controlling river form and function. While the reach-scale impacts of dredging associated with a general decline in sediment availability (e.g., channel incision) are well documented, the effects of the borrow pit on the local flow and sediment transport field are not well understood. These local effects are important because they control the post-dredge evolution of the borrow pit, setting the lifespan of the pit as well as affecting channel morphology on a reach-scale. This study documents the observed morphological evolution of a large (1.46 x 106 m3) borrow pit located on a lateral sandbar in the lower Mississippi River channel that was mined for coastal restoration purposes using a time series of multibeam bathymetric surveys. Observations show that within the 2.5 year study period, 53 % of the initial pit volume infilled with sediment, decreasing pit depth by 0.88 m/yr on average. To explore the possible controls of the observed pit infilling, a calibrated 3D hydraulic model (Delft3D) was used to simulate flow and sediment transport within the affected river reach. Results indicate that the observed infilling rates were closely related to predicted sediment supply rates and borrow pit geometry. The pit geometry (pit depth, length) influenced the predicted magnitude of the bed stress within the pit relative to its initial pre-dredged bed stress value, i.e., the bed stress reduction ratio (R*), a metric that was correlated with the magnitude and spatial distribution of infilling. A simple 1D model was derived using sediment supply and R* to simulate temporal and spatial patterns of pit infilling. This model was able to closely approximate the cumulative amount of observed infilling during the study period and reproduce realistic longitudinal infilling patterns. Additional infilling model experiments show that, for a borrow pit of a set volume located within a lower Mississippi river channel bar, creating deeper rather than longer borrow pits can reduce the time required to infill with sand by an order of magnitude. Study results provide insight into the resilience of alluvial river channels after a disturbance and the sustainability of sand mining as a sediment source for coastal restoration.
... Complex processbased models aim at resolving all processes (Jensen et al. 1999;Boers and Sea 2005;Hommes et al. 2007). Finally, idealized process-based models include only the essential physical mechanisms and focus on obtaining fundamental knowledge Roos et al. 2008). ...
Shoreface-connected sand ridges are rhythmic bedforms that occur on many storm-dominated inner shelves. The ridges span several kilometers, are a few meters high, and they evolve on a timescale of centuries. A process-based model is used to gain a fundamental insight into the response of these ridges to extraction of sand. Different scenarios of sand extraction (depth, location, and geometry of the extraction area; multiple sand extractions) are imposed. For each scenario, the response timescale as well as the characteristics of the new equilibrium state are determined. Results show that ridges partially restore after extraction, i.e., the disturbed bathymetry recovers on decadal timescales. However, in the end, the ridge original sand volume is not recovered. Initially, most sand that accomplishes the infill of the pit originates from the area upstream of the extraction, as well as from the areas surrounding the pit. The contribution of the latter strongly decreases in the subsequent time period. Depending on the location of the pit, additional sand sources contribute: First, if the pit is located close to the downstream trough, the pit gains sand by reduction of sand transport from the ridge to this trough. Second, if the pit is located close to the adjacent outer shelf, the ridge recovery is stronger due to an import of sand from that area. Furthermore, pits that are located close to the nearshore zone have a weak recovery, deeper pits have longer recovery timescales, wide and shallow pits recover most sand, while multiple sand pits slow down the recovery process.
... Over longer time scales, the sandpit area can migrate towards the shore and may act as a sink for the sediments from the nearshore system (van Rijn and Walstra, 2002). The severity and persistence of such impacts depend, among others factors, on the hydrodynamic conditions, the nature of the substrate, the sandpit geometry and its distance to the shore, and the time-scale of the seabed perturbation (Cooper et al., 2007;Roos et al., 2008). In order to minimize these effects and nearshore coastal erosion, the mining areas need to be located offshore of the closure depth. ...
... Kim et al. (2009) numerically investigated the wave reflection over a Gaussian-shaped pit, finding good agreement with the results of Bender and Dean (2003). Roos et al. (2008) developed a semianalytical model to investigate the hydromorphodynamic impacts of sand extraction in a tide-dominated environment for several pit designs. They showed that the sandpit causes a flow contraction, triggering the morphodynamic instabilities associated with the formation of tidal sandbanks. ...
The present paper explores the possibility of exploiting sandpits at intermediate and shallow water depths by analyzing both the hydrodynamic and the morphodynamic response in the proximity of a dredged area. More precisely, a laboratory investigation was carried out to analyze the effects of the wave-current interaction over a borrow area, particularly in terms of the morphodynamic evolution of the pit. The influence of both regular and irregular waves on the current was analyzed. It was found that the flow is significantly influenced by such interaction in the presence of the borrow area. The morphodynamic evolution of the sandpit is mainly affected by the waves, while the super- imposition of a current does not introduce qualitative modifications but increases the mobilization of sediments, with repercussions on the speed of the evolution process. The results of the experimental campaign suggested that the analysis of the morphodynamic evolution can be approached at a laboratory scale. Indeed, the volume infill rate resulted in agreement with the observations in real dredged sites (CNEXO pit); moreover, the same quantity can be interpreted in the light of literature relationships proposed to describe the time evolution of noncohesive sandy beds.
... The model uses the Reynolds depth-averaged, Navier-Stokes equations with hydrostatic assumption. Earlier morphodynamic studies (e.g., Roos, Hulscher, and De Vriend,, 2008;Sandpit, 2005) applies a depth-averaged flow module under similar marine morphodynamic conditions (offshore sandpits with h o . 20 m, b po /h o , 10, d po /h o , 0.2, tidal periods T 5 12.42 h). ...
Numerical modeling of dredged pits is conducted to investigate the hydrodynamic and morphodynamic interaction in offshore sand extractions. Based on an analytical formulation, a semianalytical numerical model (MEMPITS) has been developed to study the morphodynamic evolution of offshore (h0 > 20 m) sand borrow areas. The numerical model has been applied to study the morphodynamic evolution of two offshore sand borrow areas in the Balearic Islands (Spain).Field data allowed a detailed characterization of the evolution of the sandpits. Time series of local hydrodynamics have been obtained using generation models (hindcast) combined with local wave and flow models. A verification of the simple model has been carried out using relatively slight adjustments to the calibration factors. The simple model provides good estimates of the infill rate and migration velocities of the offshore pits on the scale of years. This semianalytical tool allows a quick systematic investigation of the physical mechanisms as well as a detailed sensibility analysis regarding the pit design parameters. These parameters include location (water depth), pit length, width, depth, and orientation with respect to the mean flow. A nondimensional analysis based on the model is also carried out to explore the role of the different variables involved in the evolution of offshore sandpits. Based on the field data and the nondimensional analysis, some basic design recommendations for offshore sandpits are proposed.
... In contrast with the tidal range pattern, the effects of the trench on tidal currents are more strongly confined to the region of the trench (figures 4 and 5). At the trench center Q, as introduced in figure 2d, the depth-averaged flow velocity amplitude in the along-basin direction increases by 2.1 cm/s, indicative of flow contraction (Roos et al. 2008). Flow contraction also explains the zone of increase immediately south and north of the trench. ...
We investigate the effects of a mega-scale sand extraction trench (length ~200 km, width ~10 km, depth ~10 m, located in front of the Dutch coast) on tidal dynamics in the Southern North Sea. To this end, an idealized modeling approach is adopted, extending the classical problem of Kelvin wave reflection in rectangular semi-enclosed basins to account for lateral depth variations and the presence of a trench. The model results indicate changes in tidal range (zones of decrease and increase of the order of several cm), phase and currents (order cm/s) throughout the whole basin, which emphasizes the importance of a model domain on the scale of the basin. The changes in currents may structurally change sediment transport rates by several percents, which is relevant to coastal morphology. It is argued that this idealized model can be used to support a companion study carried out with a more detailed numerical engineering model. This is because of its ability to (1) provide insight into the physical mechanisms, (2) reveal the order of magnitude and area of influence of the effects and (3) allow for a quick sensitivity analysis with respect to trench geometry.
The removal of reservoir silt and the restoration of existing reservoir capacities through land excavation and hydraulic sediment flushing have become necessary. Hydraulic sediment flushing discharge changes flow and sediment conditions of the downstream river channel. In the Tamsui River estuary in Taiwan, sediment flushing from the Shihmen reservoir upstream has potential impacts on the morphology of the navigation channels and the adjacent coasts. This study employed a validated coastal and estuarine processes model to investigate: (1) the influence of sediment flushing and tidal levels on morphological changes during flood and flushing-discharge operations of the reservoir, and (2) the differences in morphological changes on the estuary between monsoon and typhoon seasons. The prediction of the morphological changes was carried out by simulating hydrodynamic and morphodynamic processes under multi-year synthetic conditions combined by northeast monsoon and three historical typhoon events. The simulation results reveal that during the operation of sediment flushing when the peak discharge of river flood flow reaches the estuary section at ebb tides, more sediment can be transported to the open sea than that at flood tides. Additionally, the nature reserve area on the left bank of the estuary is eroded during monsoon and silted in typhoon seasons.
The 1D (cross-shore) nonlinear nourishment-as-a-perturbation model of Chen and Dodd (2019) is extended to include wave streaming and depth-varying mean return flow. This is shown to lead to physically consistent values for the migration rate of small amplitude bed perturbations in the whole nearshore region. The model is also extended to arbitrary wave incidence angle, and an alternative energy dissipation description is included. The model is applied to a background profile that includes multiple nearshore bars, and which also allows comparison with observations of nourishment development and migration. The chosen profile is that of the Terschelling shoreface nourishment. Over the initial nourishment region the linearised bed evolution equation predicts onshore small-amplitude bed perturbation migration rates in the troughs (where waves shoal) and offshore rates on the bars (where waves break). Sensitivity studies using the fully nonlinear model reveal wave height to be the primary controlling parameter for the resultant perturbed sediment flux. Perturbed sediment fluxes are onshore up to a threshold wave height; beyond this offshore fluxes predominate. Wave period and angle of incidence, and tide level are all secondary controlling factors, variation in which yields no change in perturbed sediment flux sign (for fixed wave height). Recorded wave and tide signals are used to drive the model to simulate six months of the initial nourishment evolution. It is shown that in calmer conditions the small amplitude migration rate of bed disturbances is a highly accurate predictor of the fully nonlinear migration rate (as described by the centre of mass of the nourishment); in storm periods the two predictors diverge. The observed overall migration rate is well described by the model. The details of the profile evolution show a moderately good reproduction of those observed (Brier Skill Score=0.52 ).
Extracting an annual amount of about 25 million m3 the Netherlands is leading in sand mining from offshore parts of the North Sea. The sand is needed for large-scale infrastructural projects, land reclamation and shore nourishments. The term 'mega-scale' emphasises the unprecedented spatial extent of the extraction trench. Indeed, the much smaller pits currently being created for the extension to Mainport Rotterdam, with horizontal dimensions of several kilometers, are already referred to as large-scale. Emphasis is laid on analytical and semi-analytical solution techniques, which generally lead to a quick tool. In turn, this allows for an extensive sensitivity analysis by systematically varying the model parameters. A sensitivity analysis with respect to trench geometry shows that the response is more sensitive to trench width and depth than to length. Apart from the obtained orders of magnitude, the main conclusion is that the mega-scale trench indeed feeds back onto the tidal system as a whole.
In recent years, promoted by tremendous demands from national economic construction, great progress has been made in geomorphology and Quaternary geology in China. This article generalizes the recent progresses, mainly those in the period 2006-2008, including the progress in the fields of tectonic landforms, fluvial landforms, eolian landforms, glacial landforms, estuary and coastal landforms and Quaternary environmental evolution. Furthermore some suggestions are proposed to develop the discipline in the following aspects: (1) Studies of basic theories; (2) Research into applied geomorphology and Quaternary geology concerning the key construction projects; (3) Experimental studies, both in laboratories and in field surveys; (4) Training of young geomorphologists and Quaternary geologists.
Sediment diversions on the Mississippi River are considered as potential solution for land building. This study investigates the impact of pulsed sediment diversions on the morphodynamics of the river channel. The intent is to operate the diversion during the flood season where sand is entrained to the upper layers of the water column. Recent field observations and numerical modeling experiments suggest that locating the diversion on top of lateral sand bar is favorable. However, little is known about the optimum sand/water ratio to minimize the impact on the sediment regime of the Lower Mississippi River. Further, the impacts on the sand bar should be investigated.
The Delft3D model is being used herein to conduct such investigation. The model is supported by a detailed field data collection program. The model reproduced the flow patterns and agreed well with the suspended and bed load sediment measurements.
We discuss two cases of international cooperation between hydrographic offices. The first case is the unified approach to populate the CATZOC indicator for bathymetric quality in nautical charts, in the presence of sand wave fields. The second case is the North Sea wide formulation of resurvey policies. The two cases can only be successful if it is understood and agreed how the sea floor develops. For both of the cases, it is very relevant to distinguish between the growth of a pattern, and its migration. Therefore, hydrographic offices need accurate morphodynamic models, an appropriate set of methods for the analysis of time series of bathymetric data, and versatile remote sensing techniques. The two cases are discussed using the example of a sand wave field near the Port of Rotterdam, in the Southern North Sea. The development of the field is potentially influenced by the maintenance of dredged channels and an emergency turning zone; extensive sand pits; and two infrastructural projects in the coastal vicinity: Maasvlakte 2 and the Zandmotor.
Long-term considerations of repeated and increasing sand extraction on the Netherlands Continental Shelf (North Sea) may lead to the creation of a mega-scale extraction trench in front of the Dutch coast (length hundreds of km, width over 10 km, depth several m). We investigate the impact of such a huge topographic intervention on tidal dynamics, which is a key aspect in hydrodynamics, and indirectly also affecting morphodynamics and ecology.
Because of the unprecedented extent of the extraction scenario, we follow a generic approach aimed at understanding the physical mechanisms behind the changes, the orders of magnitude and area of influence. Hence, rather than applying an existing numerical model, we develop an idealized model for tide propagation in semi-enclosed rectangular basins. The model geometry consists of three adjacent compartments with a realistic cross-basin depth profile, the trench being located in the second compartment, while assuming along-basin uniformity of depth within each compartment. The problem is forced by an incoming Kelvin wave. The solution in each compartment, satisfying the linear depth-averaged shallow water equations on the f plane including bottom friction, is written as the superposition of semi-numerically obtained wave solutions. A collocation technique is employed to satisfy no-normal flow across the basin's closed end as well as continuity of elevation and flux across the interfaces between the compartments.
The results indicate changes in tidal range, phase and currents throughout the whole basin, which shows the importance of a model domain on the scale of the basin. Changes in coastal tidal range show zones of decrease and increase (order cm). Changes in coastal shore-parallel tidal current amplitudes are of the order of cm s− 1. A sensitivity analysis for various trench geometries shows that the hydrodynamic impact generally increases with increasing extraction volume, being more sensitive to trench depth and width than trench length. The changes in tidal currents may structurally alter sediment transport rates with several percents. As the bathymetry and coastal morphology result from subtle balances in long-term sediment transport, the trench may indirectly affect various user functions and values (coastal safety, navigation, marine ecology, cables and pipelines) and, hence, be of concern to coastal management.
This brief comment of Roos et al.'s [Roos, P.C.. Hulscher, S.J.M.H. & de Vriend. H.J., 2008. Modelling the morphodynamic impact of offshore sandpit geometries. Coast. Eng. doi:10.1016/j.coastaleng.2008.02.019] paper is aimed at supporting the validity of idealized process-based models as a valuable tool for the design of sand pits. The discussion moves from a brief description of the capabilities of an already existing idealized process-based model [Blondeaux, P. and Vittori, G. 2005a Morphological development of shallow sand pits. Proceedings 29th International Conference Coastal Engineering 2004, World Scientific (Jane McKee Smith ed) ISBN 981-256-995-2 pp. 2581-2593; Blondeaux, P. and Vittori, G., 2005b. Flow and sediment transport induced by tide propagation: part 2: the wavy bottom case. J. Geophys. Res., Vol. 110, No. C8. C08003] which can predict the morphological time development of sand pits. Then, the differences between Blondeaux & Vittori's model and Roos et al.'s model are analysed to give to the potential user an idea of the range of applicability of the different models, also in relation to practical applications. (C) 2008 Elsevier B.V. All rights reserved.
Observations of the flow field over an elongated hollow (bathymetric depression) in the lower Chesapeake Bay showed tidally asymmetric distributions. Current speed increased over the landward side of the hole during flood tides and decreased in the deepest part of the hollow during ebb tides. A simple conceptual analysis indicated that the presence of a horizontal density gradient can generate the asymmetric spatial variations of flow structure depending on the sign of the horizontal density gradient. When water density decreases downstream, the velocity increases over the downstream edge of the hollow. Conversely when water density increases downstream, the flow decreases over the hollow more than a case without a horizontal density gradient. The conceptual analysis is confirmed by numerical experiments of simplified hollows in steady open channel flows and of an idealized tidal estuary. These hollows also alter the local current field of tidally averaged estuarine exchange flows. The residual depth-averaged currents over a hollow show a two-cell circulation when Coriolis forcing is neglected and an asymmetric two-cell circulation, with a stronger cyclonic eddy, when Coriolis forcing is included.
A process-based morphodynamic model is developed to investigate the nonlinear dynamics of tidal sandbanks. We focus on two types of dynamics: autonomous behaviour of a patch of sandbanks and the response of such a system to a local removal of sand. Simulations for asymmetric tidal conditions indicate that sandbanks gradually attain a larger wavelength than suggested by existing linear stability models. Sandbanks are found to recover from sand extraction. We discuss the time scales associated with these processes as well as the implications for our understanding of sandbank dynamics.
We extend the class of simple offshore models that describe large-scale
bed evolution in shallow shelf seas. In such seas, shallow water flow
interacts with the seabed through bed load and suspended load transport.
For arbitrary topographies of small amplitude we derive general bed
evolution equations. The initial topographic impulse response (initial
sedimentation and erosion patterns around an isolated feature on a flat
seabed) provides analytical expressions that provide insight into the
inherent instability of the flat seabed, the Coriolis-induced preference
for cyclonically oriented features, and bed load transport being a
limiting case of suspended load transport. The general evolution
equation can be used to describe sandbank formation, known as the result
of self-organization. Examples of human intervention at the seabed
include applications to a dredged channel and an offshore sandpit. An
outlook toward future research is also presented.
We investigate how a local topographic disturbance of a flat seabed may become morphodynamically active, according to the linear instability mechanism which gives rise to sandwave formation. The seabed evolution follows from a Fourier integral, which can generally not be evaluated in closed form. As numerical integration is rather cumbersome and not transparent, we propose an analytical way to approximate the solution. This method, using properties of the fastest growing mode only, turns out to be quick, insightful, and to perform well. It shows how a local disturbance develops gradually into a sandwave packet, the area of which increases roughly linearly with time. The elevation at the packet's center ultimately tends to increase, but this may be preceded by an initial stage of decrease, depending on the spatial extent of the initial disturbance. In the case of tidal asymmetry, the individual sandwaves in the packet migrate at the migration speed of the fastest growing mode, whereas the envelope moves at the group speed. Finally, we apply the theory to trenches and pits and show where results differ from an earlier study in which sandwave dynamics have been ignored.
HOMMES, S.; HULSCHER, S. In this paper, we focus on a complex management issue, namely the physical effects of a large-scale offshore sand extraction. For these kinds of issues there is no obvious morphological model available to answer all management questions. Therefore, we aim to answer as many management questions as possible, using a set of existing morpho-logical models parallel to each other. In this way, we can support governments to assess applications for licenses for large-scale sand extraction. We investigate whether this parallel modeling approach is significantly more helpful in addressing the management questions than a single modeling approach. The management questions are translated into quantifiable variables, known as Coastal State Indicators (CSIs). We focus on three coastal user functions: coastal safety and maintenance, offshore infrastructure, and navigation. The selected morphological models are assessed on (1) their applicability to the CSIs and (2) the reliability of their predictions. We quantify the predictive power of the models based on these two parameters. We conclude that by using a parallel modeling approach it is possible to address more management questions effectively in comparison with using just the best single model. The use of this parallel modeling approach increases the predictive power significantly, here 35%. ADDITIONAL INDEX WORDS: Management questions, coastal user functions, coastal state indicator (CSI), sand ridges, predictive power.
A non-linear morphodynamic model of a microtidal coastal shelf is used to study the response of shoreface-connected sand ridges and the net sand balance of the shelf to large-scale interventions. The model describes the interaction between storm-driven currents and the erodible bottom. The transport of sediment comprises both bedload and suspended load contributions and is due to the joint action of waves (stirring of sediment from the bed) and net currents (causing transport). Three basic types of interventions are studied: extracting sand from ridges, nourishing sand at the shelf and constructing navigation channels. The model results indicate that for all interventions studied a relatively fast local recovery (time scale of decades to centuries) of the disturbed bathymetry to its original pattern takes place. Readjustment of the global system to its original equilibrium state (the saturation process) occurs on a longer time scale (several centuries). During the adjustment stage, significant net sand exchanges between inner shelf and adjacent outer shelf and near-shore zone occur. The results further suggest that extraction of sand from the shelf and dredging of navigation channels have negative implications for the stability of the beach (its sand volume decreases).
Sandy shallow seas, like the North Sea, are very dynamic. Several morphological features are present on the bed, from small ripples to sand waves and large tidal sandbanks. The larger patterns induce significant depth variations that have an impact on human activities taking place in this area. Therefore, it is important to know where these large-scale features occur, what their natural behaviour is and how they interact with human activities. Here, we extend earlier research that compares the results of an idealized model of large-scale seabed patterns with data of seabed patterns in the North Sea. The idealized model is extended with a grain size dependency. The adaptations lead to more accurate predictions of the occurrence of large-scale bed forms in the North Sea. Therefore, grain size dependency and, in particular, critical shear stress are important to explain the occurrence of sand waves and sandbanks in the North Sea.
We extend the class of simple offshore models that describe large-scale bed evolution in shallow shelf seas. In such seas, shallow water flow interacts with the seabed through bed load and suspended load transport. For arbitrary topographies of small amplitude we derive general bed evolution equations. The initial topographic impulse response (initial sedimentation and erosion patterns around an isolated feature on a flat seabed) provides analytical expressions that provide insight into the inherent instability of the flat seabed, the Coriolis-induced preference for cyclonically oriented features, and bed load transport being a limiting case of suspended load transport. The general evolution equation can be used to describe sandbank formation, known as the result of self-organization. Examples of human intervention at the seabed include applications to a dredged channel and an offshore sandpit. An outlook toward future research is also presented.
The formation and the elementary behaviour (e.g. propagation, growth and decay, dispersion) of linear sand banks in shallow tidal seas are analyzed. The principal tool is a linearized mathematical model of three-dimensional topographical changes of a sandy bottom in a tidal environment.
The resulting insight is used to explain this class of sand banks, in general, and the sand bank system off the coast of Holland, The Netherlands, in particular.
Equations are derived for the displacement and deformation of the cross-section of a trench in a sandy bottom. The trench is subject to tidal currents and waves. Tidal currents are rectilinear and perpendicular to the longitudinal axis of the trench. The tidal velocity comprises the residual velocity M0 and the harmonic constituents M2 and M4. The effect of waves on the sediment transport is incorporated as stirring. Separate equations are derived for bed-load and suspended-load transport. The equations are averaged over the tidal time scale assuming that the morphological time scale is large compared to the tidal period. To determine the leading order terms, variables are scaled and the equations are written in dimensionless form. The relative magnitude of the terms is determined by a small parameter ε. ε is the ratio of trench depth and undisturbed water depth. Retaining only leading order terms, the equations reduce to advection–diffusion equations with constant coefficients. The cross-section migrates, widens and shallows. The velocity of migration is a function of the residual velocity and the amplitudes and phases of the M2 and M4 constituents. Widening and shallowing is a result of diffusion. For bed-load transport, diffusion derives from the effect of the bed slope on the sediment transport. For suspended-load transport, diffusion is assumed to be the result of velocity shear and vertical turbulent mass exchange (shear dispersion). When accounting for higher order nonlinear terms an initially symmetric cross-section becomes asymmetric. Application to the access channel to the Port of Amsterdam yields displacement velocities of 1.3 m yr−1 for bed-load transport and 1.8 m yr−1 for suspended-load transport. The initial rates of increase of the half-width of the channel are, respectively, 0.33 and 1.02 m yr−1.
The energy criterium used in the Lorentz linearization of a quadratically damped oscillator can be derived from the simplest nontrivial truncation of a renormalized perturbation expansion which gives the linearized friction coefficient as a function of forcing amplitude and frequency, together with the frequency-response functions of the fundamental harmonic and its odd multiples.
The three-dimensional model presented in this paper is used to study how tidal currents form wave-like bottom patterns. Inclusion of vertical flow structure turns out to be necessary to describe the formation, or absence, of all known large-scale regular bottom features. The tide and topography are treated as a coupled system, and the potential free instabilities in this system are investigated. A linear stability analysis is used to study the dynamics of bed forms. This yields information on the role of physical mechanisms; such information cannot be obtained by direct numerical nonlinear simulation. The basic state represents a horizontal elliptical flow, veering in vertical direction, over a flat bottom. It is shown that in such a system, modes can grow which have characteristics of both tidal sand banks and sand waves. The model predictions are compared with observations and show good agreement.
We consider a model where the fluid depth depends on both horizontal coordinates, quasi-steady depth-uniform non-divergent fluid flow is governed by inertial, pressure and bottom-frictional forces, sand transport is proportional to the cube of the instantaneous current but augmented by a down-slope component and by wind-wave action, and sand is conserved.It is found that low parallel banks grow fastest, so that in an extensive spatially uniform sea previous calculations for linear banks are appropriate. The inclination of banks to the tidal currents can be interpreted in terms of similarly inclined deposition bands resulting from vorticity generation and advection in flow over a small isolated hump. A small bump can evolve to an equilibrium bank (typically after an initial rapid extension across the tidal currents) provided that sand is sufficiently restricted and particularly if some wind-wave action prevents growth up to the sea surface. Sand banks are likely to be in a late stage of evolution, when the main change is a slow lengthening as the net current and transport along the bank side slows and turns around the bank end with net deposition. The equilibrium is apparently stable except when there is an overall bed slope in the direction of the tidal currents, or when sand is abundant.
A 3D investigation of sediment transport processes across long, straight channels exposed to an oblique incoming current is presented. For this purpose a detailed mathematical flow and sediment transport model is applied. The governing flow and sediment transport equations are solved in a curvilinear coordinate system. The sediment transport model is composed of bed and suspended load, applying a convection-diffusion equation for the latter. As the flow crosses a channel obliquely, it is refracted toward the channel alignment. The effect of the current refraction on the sediment transport is studied using both 2D (depth-averaged) and 3D models. Shortcomings of models based on depth-averaged descriptions are underlined through comparison with the 3D model. A reliable description of the backfilling process requires a how and sediment transport model capable of capturing the following aspects: (1) The direction and magnitude of the bed-shear stress; and (2) the adaptation of suspended sediment to local sediment transport capacities. Calculations of the morphological development of small-scale excavations (e.g., pipeline trenches) exposed to an oblique incoming current are presented.
A 3D investigation of how across long, straight channels aligned obliquely to the flow direction has been conducted. The applied mathematical model solves the Reynolds-averaged Navier-Stokes equations using a k-epsilon model for turbulence closure in a curvilinear coordinate system. The uniformity along the channel alignment allows the three momentum equations to be solved in a 2D computational domain. With respect to a steady current entering a channel obliquely, two important how features arise: (1) The flow will be refracted in the direction of the channel alignment, which may be described by depth-averaged models; and (2) a secondary flow will be introduced due to shear in the velocity profile. This can only be described using a 3D approach. The secondary flow will cause a horizontal deflection of streamlines over the vertical. Only by capturing the 3D flow behavior can the direction and magnitude of the bed shear stress be well modeled. When crossing a channel obliquely, the flow is gradually accelerated in the direction of the channel alignment. Results of the numerical flow model are compared with existing experimental data and good agreement is found.
Through an analysis of the interaction between end users and researchers participating in the Eurooean Union-funded CoastView project (EVK3-CT-2001-0054), this article illustrates some of the difficulties associated with end user-oriented research. A way of structuring and focusing discussion between end users and researchers, which was applied and further developed during the course of the project, is suggested as a method to deal with these difficulties. The analysis in this article indicates that successful specialist support of decision making is related to the use of a systematic frame of reference. This involves explicit definitions of both strategic and operational objectives applied in a four-step decision recipe consisting of (1) a quantitative state concept, (2) a benchmarking procedure, (3) a design procedure for measures or interventions, and (4) an evaluation procedure.
Many shallow tidal areas show the occurrence of ebb and flood surplusses, organized in cell-like structures or ‘tidal residual eddies’. They seem to arise from a nonlinear transfer of vorticity from the oscillating tidal field to the mean field, the irregular structure of the bottom topography or the coastline acting as a catalyst to produce the necessary vorticity gradients. The dynamical characteristics of the eddies assist our understanding of the geomorphology of the sea bed.
A class of compound mathematical models of transient morphological evolutions in shallow water is discussed, based on commonly applied horizontally two-dimensional formulations of the water and sediment motion in the coastal zone. Three major aspects of the composition of such models are considered, viz. the specific adequacy of the constituent models, the balance of the total model and the possibility of spurious interactions. The lacunae in the knowledge and the tools needed to judge a proposed model at these points are indicated.Examples of spurious interactions are described in further detail and techniques to trace and avoid them are given. These examples, concerning morphological evolutions without the influence of waves, show that the advective acceleration terms in the flow model play an essential part in the bottom evolution and its interaction with the current and that incorporation of the bottom slope effects on the sediment transport is indispensable in models with a local transport formulation.
A series of model intercomparisons, and model comparisons with field data, was carried out as part of the EU MASTIII SEDMOC Project (1998–2001). Initially, seven ‘research’ models were intercompared over a wide range of wave and current conditions, corresponding to both plane and rippled sand beds. These models included both one-dimensional vertical (1DV) formulations, varying in complexity from eddy viscosity and mixing length models to a full two-phase flow formulation, and also 2DV formulations capable of representing vortex shedding above sand ripples. The model results showed greatest convergence for cases involving plane beds, with predicted sand transport rates agreeing to well within an order of magnitude, and greatest divergence for cases involving rippled beds. A similar intercomparison involving (mainly) practical sand transport models, carried out over wide wave and current parameter ranges, also showed greatest variability in cases involving rippled beds. Finally, (mainly) practical models were compared with field data obtained at five contrasting field sites. The results showed that suspended sand concentrations in the bottom metre of the flow were predicted within a factor of 2 of the measured values in 13% to 48% of the cases considered, and within a factor of 10 in 70% to 83% of the cases, depending upon the model used. Estimates of the measured longshore component of suspended sand transport yielded agreement to within a factor of 2 in 22% to 66% of cases, and within a factor of 10 in 77% to 100% of cases. The results suggest that, at the present stage of research, considerable uncertainty should be expected if untuned models are used to make absolute predictions for field conditions. The availability of some measurements on site still appears to be a necessary requirement for high-accuracy sand transport predictions. However, for morphological modellers, the results may be viewed as more encouraging, since many of the present models exhibit agreement in their relative behaviour over wide ranges of wave and current conditions, which is a prerequisite to obtaining correct morphodynamic predictions.
A simple morphological model is considered which describes the interaction between a tidal flow and an erodible bed in a shallow sea. The basic state of this model describes a spatially uniform tide over a flat bottom where the flow vector is represented as a tidal ellipse. The linear stability of this solution is analysed with respect to bed form perturbations. Results are presented for both a uni-directional and circular tide. In the former case the wave-length and the orientation of the fastest growing bed mode agree well with those of tidal sand banks. However, this model only predicts the growth of large-scale sand ridges. With a simplified numerical model we tentatively show that the effects of secondary currents on the sediment transport trigger the formation of instabilities at an essentially smaller scale, viz, sand waves. Another limitation of a model with uni-directional tides is that no selective modes found are the first to become unstable if the model parameters are varied. In the case of a circular tide, critical model parameters are found below which the basic state is stable. We conclude that this provides a starting point for the development of a weakly non-linear analysis, which will yield information on the amplitude behaviour of marginally growing bed forms.
Asymmetric tidal currents (Huthnance, 1973) provide a fluid-dynamical basis for Caston's (1972) description of linear sand-bank maintenance by converging sand transport. We suppose (i) depth-uniform tidal currents, slightly inclined to the bank crest, (ii) bottom-drag, which retards the current more over the bank and (iii) a faster-than-linear increase of sand transport with current. Then over a sloping bank side the total tidal current having an upslope component and the associated onto-bank sand transport are stronger than the retarded reverse tidal current and transport coming off the bank.Supposing that (iv) sand is more easily transported ‘downhill’ shortwavelength perturbations on a level sea floor are suppressed. There is a maximum bed-form growth rate at a particular wavelength (typically 250 times the water depth) and orientation (relative to the tidal currents) which probably evolve and persist during subsequent sand-bank growth. The orientation is sensitive to the (uncertain) formulation of supposition (iv), and is probably also susceptible to (for example) the trend of an adjacent coastline.In the representative context of friction-dominated tidal currents, the banks evolve to an equilibrium profile which is flatter on top than a sinusoid owing to wind-wave erosion and the inclination to the tidal current. For a limited sand supply the banks narrow to about one-fifth of their separation; further restriction mainly reduces their height. A net sand-transport overall due to a stronger ebb tide (say) than flood, as occurs over the Norfolk Sandbanks, yields the observed steeper slope on the obliquely downstream side of the bank (as viewed by the stronger ebb current).
Large-scale, marine sand mining in front ofthe Dutch coast might be necessary in the near future in order to meet the needs of sand for regular engineering works and/or for a possible second Maasvlakte or a national airport on an artificial island. In order to make a socio-economically and environmentally sound decision with respect to location and magnitude of sandpits, one has to know the hydrodynamic and morphological consequences of such large-scale mining areas in advance. This study takes place in the research-program Coast*2000, coordinated by the National Institute for Coastal and Marine ManagementIRIKZ. This study is performed with the Delft3D modelling software. Numerical models of sandpits in all sorts of variants have been used to study the hydrodynamic and morphological behaviour of sandpits. The emphasis in the hydrodynamic part of this study is on the influence of Corio lis and on the orientation of the sandpit with respect to the main current direction. The morphological computations is performed over a period of 1000 years, which is necessary because of the time scale of such bottom disturbances. Aspects ofthe morphological simulations are the propagation of the sandpits, the developments of the slopes and the resemblance in behaviour between sandpits and tidal sand banks.
Effects of a deep sand extraction pit, final report of the PUTMOR measurements at the lowered dump site
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various pit designs. Report Z3223.30, WL|Delft Hydraulics. Delft, The Netherlands.
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Large-scale sandpits, Master's Thesis
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Linear evolution of sandwave packets An analytical model for the morphodynamics of a trench in the presence of tidal currents
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European overview of marine sand and gravel
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Fysische effecten van zeezandwinning
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