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R.J. Labeur
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1
A general theory for the exchange timescale of lock-exchange flows where both the lock and the ambient are stratified has not been established yet. Here, two-dimensional large-eddy simulations of the lock exchange are performed, to quantify the dependence of the adjustment time on linear stratification in the lock and ambient. After validation of the model, simulations show that stratification in the ambient decreases the adjustment time, which is related to an increase of the hydrostatic pressure difference driving the front traveling into the lock. Stratification in the lock also decreases the adjustment time, because a larger stratification is associated with faster internal waves, thereby highlighting the role of internal wave speed in the stratified lock exchange. A reduced model for adjustment time is developed, that extends known relations for front speeds in unstratified conditions to situations with a vertically stratified lock and ambient. Stratification is incorporated by combining its effects on hydrostatic pressure and internal wave speed. This reduced model reproduces the dependence of the adjustment time on the initial density field with reasonable accuracy.
...
A general theory for the exchange timescale of lock-exchange flows where both the lock and the ambient are stratified has not been established yet. Here, two-dimensional large-eddy simulations of the lock exchange are performed, to quantify the dependence of the adjustment time on linear stratification in the lock and ambient. After validation of the model, simulations show that stratification in the ambient decreases the adjustment time, which is related to an increase of the hydrostatic pressure difference driving the front traveling into the lock. Stratification in the lock also decreases the adjustment time, because a larger stratification is associated with faster internal waves, thereby highlighting the role of internal wave speed in the stratified lock exchange. A reduced model for adjustment time is developed, that extends known relations for front speeds in unstratified conditions to situations with a vertically stratified lock and ambient. Stratification is incorporated by combining its effects on hydrostatic pressure and internal wave speed. This reduced model reproduces the dependence of the adjustment time on the initial density field with reasonable accuracy.
Journal article
(2025)
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Mario van den Berg, Robert J. Labeur, Stefan G.J. Aarninkhof, Paul J. Visser
Coastal and fluvial flood defences currently rely primarily on existing (grey) infrastructure such as dikes. However, coastal flood risk is expected to increase substantially in the near future. This requires ever increasing efforts to strengthen dikes. To aid these conventional methods, Nature-based Solutions (NbS) are increasingly proposed, such as coastal wetlands. Coastal wetlands have many ecological benefits, but also aid flood protection, especially tidal marshes. Tidal marshes protect the dikes behind them through wave attenuation and reduce flood damage if the dike is breached. Meanwhile, flood risk assessment relies on dike breach modelling to estimate the breach discharges for inundation simulations. Yet, how these foreshores (e.g., tidal marsh) affect dike breach development is largely unknown. For this reason, we experimentally explore how different foreshores affect the dike breaching process. In this study we performed a series of breach tests with a 1.5-1.9 m high model sand-dike with and without a foreshore. We tested two types of foreshores, an erodible sand and low-erodible clay layer, acting as proxies for a sandy beach and unvegetated tidal marsh. Because dike breach flow closely resembles weir flow, the standard weir equation applies, which is also frequently used in breach discharge models. The observed foreshore effects are qualitatively evaluated using this weir equation. Depending on foreshore stability, we find that foreshores affect breach hydrodynamics which alters the weir shape, leading to reduced breach width growth and ultimately limits the specific discharge.
...
Coastal and fluvial flood defences currently rely primarily on existing (grey) infrastructure such as dikes. However, coastal flood risk is expected to increase substantially in the near future. This requires ever increasing efforts to strengthen dikes. To aid these conventional methods, Nature-based Solutions (NbS) are increasingly proposed, such as coastal wetlands. Coastal wetlands have many ecological benefits, but also aid flood protection, especially tidal marshes. Tidal marshes protect the dikes behind them through wave attenuation and reduce flood damage if the dike is breached. Meanwhile, flood risk assessment relies on dike breach modelling to estimate the breach discharges for inundation simulations. Yet, how these foreshores (e.g., tidal marsh) affect dike breach development is largely unknown. For this reason, we experimentally explore how different foreshores affect the dike breaching process. In this study we performed a series of breach tests with a 1.5-1.9 m high model sand-dike with and without a foreshore. We tested two types of foreshores, an erodible sand and low-erodible clay layer, acting as proxies for a sandy beach and unvegetated tidal marsh. Because dike breach flow closely resembles weir flow, the standard weir equation applies, which is also frequently used in breach discharge models. The observed foreshore effects are qualitatively evaluated using this weir equation. Depending on foreshore stability, we find that foreshores affect breach hydrodynamics which alters the weir shape, leading to reduced breach width growth and ultimately limits the specific discharge.
Coastal flood risk is expected to increase substantially in the near future. Main drivers are climate induced sea level rise, increased storm surge and land subsidence. Meanwhile, land subsidence compounds to increased extreme water levels as more land is susceptible to flooding. Without coastal defense or adaptation 50% more people are exposed to flooding than present day (Kireczi et al., 2020).
Coastal regions are currently primarily protected by hard (grey) flood defenses such as storm surge barriers, seawall, dikes and dunes. Periodically, strengthening of these grey structures is necessary to comply with current or updated safety standards. For dikes, conventional strengthening methods are crest heightening or (base) widening. However, these methods have structural and financial limits. Instead, more sustainable methods are explored in which nature also plays a larger role. These solutions are known as Nature based Solutions (NbS).
For flood protection, tidal marshes have gained great interest as a Nature based Solution in the past two decades. Tidal marshes provide a lot of ecosystem services (Barbier et al., 2011). One such service is flood protection, attributed to wave attenuation (Vuik et al, 2016). A secondary effect is flood impact reduction (Zhu et al., 2020) due to the high elevation of tidal marshes limiting the inflow to the breach. Secondly, the tidal marsh can act as a sill in front of the breach when water levels drop below the tidal marsh level.
To quantify the effect of tidal marshes on flood impact the breaching process in combination with a tidal marsh (or foreshore in general) needs to be understood. In this study we performed a large-scale physical dike experiment where we breached a dike seven times. Three tests are done without a sediment layer in front of the dike (no foreshore), two with a sandy layer (sandy beach) and two with a clay layer (tidal marsh without vegetation). From the experiments we gain insight into differences in the dike breaching process with and without an erodible sediment layer in front of the dike ...
Coastal regions are currently primarily protected by hard (grey) flood defenses such as storm surge barriers, seawall, dikes and dunes. Periodically, strengthening of these grey structures is necessary to comply with current or updated safety standards. For dikes, conventional strengthening methods are crest heightening or (base) widening. However, these methods have structural and financial limits. Instead, more sustainable methods are explored in which nature also plays a larger role. These solutions are known as Nature based Solutions (NbS).
For flood protection, tidal marshes have gained great interest as a Nature based Solution in the past two decades. Tidal marshes provide a lot of ecosystem services (Barbier et al., 2011). One such service is flood protection, attributed to wave attenuation (Vuik et al, 2016). A secondary effect is flood impact reduction (Zhu et al., 2020) due to the high elevation of tidal marshes limiting the inflow to the breach. Secondly, the tidal marsh can act as a sill in front of the breach when water levels drop below the tidal marsh level.
To quantify the effect of tidal marshes on flood impact the breaching process in combination with a tidal marsh (or foreshore in general) needs to be understood. In this study we performed a large-scale physical dike experiment where we breached a dike seven times. Three tests are done without a sediment layer in front of the dike (no foreshore), two with a sandy layer (sandy beach) and two with a clay layer (tidal marsh without vegetation). From the experiments we gain insight into differences in the dike breaching process with and without an erodible sediment layer in front of the dike ...
Coastal flood risk is expected to increase substantially in the near future. Main drivers are climate induced sea level rise, increased storm surge and land subsidence. Meanwhile, land subsidence compounds to increased extreme water levels as more land is susceptible to flooding. Without coastal defense or adaptation 50% more people are exposed to flooding than present day (Kireczi et al., 2020).
Coastal regions are currently primarily protected by hard (grey) flood defenses such as storm surge barriers, seawall, dikes and dunes. Periodically, strengthening of these grey structures is necessary to comply with current or updated safety standards. For dikes, conventional strengthening methods are crest heightening or (base) widening. However, these methods have structural and financial limits. Instead, more sustainable methods are explored in which nature also plays a larger role. These solutions are known as Nature based Solutions (NbS).
For flood protection, tidal marshes have gained great interest as a Nature based Solution in the past two decades. Tidal marshes provide a lot of ecosystem services (Barbier et al., 2011). One such service is flood protection, attributed to wave attenuation (Vuik et al, 2016). A secondary effect is flood impact reduction (Zhu et al., 2020) due to the high elevation of tidal marshes limiting the inflow to the breach. Secondly, the tidal marsh can act as a sill in front of the breach when water levels drop below the tidal marsh level.
To quantify the effect of tidal marshes on flood impact the breaching process in combination with a tidal marsh (or foreshore in general) needs to be understood. In this study we performed a large-scale physical dike experiment where we breached a dike seven times. Three tests are done without a sediment layer in front of the dike (no foreshore), two with a sandy layer (sandy beach) and two with a clay layer (tidal marsh without vegetation). From the experiments we gain insight into differences in the dike breaching process with and without an erodible sediment layer in front of the dike
Coastal regions are currently primarily protected by hard (grey) flood defenses such as storm surge barriers, seawall, dikes and dunes. Periodically, strengthening of these grey structures is necessary to comply with current or updated safety standards. For dikes, conventional strengthening methods are crest heightening or (base) widening. However, these methods have structural and financial limits. Instead, more sustainable methods are explored in which nature also plays a larger role. These solutions are known as Nature based Solutions (NbS).
For flood protection, tidal marshes have gained great interest as a Nature based Solution in the past two decades. Tidal marshes provide a lot of ecosystem services (Barbier et al., 2011). One such service is flood protection, attributed to wave attenuation (Vuik et al, 2016). A secondary effect is flood impact reduction (Zhu et al., 2020) due to the high elevation of tidal marshes limiting the inflow to the breach. Secondly, the tidal marsh can act as a sill in front of the breach when water levels drop below the tidal marsh level.
To quantify the effect of tidal marshes on flood impact the breaching process in combination with a tidal marsh (or foreshore in general) needs to be understood. In this study we performed a large-scale physical dike experiment where we breached a dike seven times. Three tests are done without a sediment layer in front of the dike (no foreshore), two with a sandy layer (sandy beach) and two with a clay layer (tidal marsh without vegetation). From the experiments we gain insight into differences in the dike breaching process with and without an erodible sediment layer in front of the dike
Coastal flood risk is expected to increase due to climate change and population
growth. Much of our coastlines is protected by “grey” infrastructure such as a dike.
Dike maintenance and strengthening requires ever increasing capital and space,
putting their economic viability in question. To combat this trend, more
sustainable alternatives are explored, also known as Nature based Solutions. A
promising option has shown to be tidal marshes. Tidal marshes are coastal
wetlands with high ecological and economic value. Also, they protect dikes
through wave attenuation and in case of a dike breach reduce its development.
However, the effectiveness of a tidal marsh on reducing dike breach development
rates highly depends on the stability of the tidal marsh itself. Not much is known
about the stability of a tidal marsh under dike breach conditions, which are
accompanied with flow velocities that can reach 4–5ms−1. In this study we tested
the vegetation response and erodibility of a mature tidal marsh, in-situ, under high
flow velocities ( > 0.5ms−1). Our results confirm that tidal marshes similar to the
one tested in this study are highly erosion resistant with low erodibility. More
research is necessary to confirm this for tidal marshes with different soil and
vegetation properties. For tidal marshes similar to what is tested thus far, erosion
under dike breach conditions is negligible and other erosion mechanisms such as
headcut erosion probably dominate the erosion process. ...
growth. Much of our coastlines is protected by “grey” infrastructure such as a dike.
Dike maintenance and strengthening requires ever increasing capital and space,
putting their economic viability in question. To combat this trend, more
sustainable alternatives are explored, also known as Nature based Solutions. A
promising option has shown to be tidal marshes. Tidal marshes are coastal
wetlands with high ecological and economic value. Also, they protect dikes
through wave attenuation and in case of a dike breach reduce its development.
However, the effectiveness of a tidal marsh on reducing dike breach development
rates highly depends on the stability of the tidal marsh itself. Not much is known
about the stability of a tidal marsh under dike breach conditions, which are
accompanied with flow velocities that can reach 4–5ms−1. In this study we tested
the vegetation response and erodibility of a mature tidal marsh, in-situ, under high
flow velocities ( > 0.5ms−1). Our results confirm that tidal marshes similar to the
one tested in this study are highly erosion resistant with low erodibility. More
research is necessary to confirm this for tidal marshes with different soil and
vegetation properties. For tidal marshes similar to what is tested thus far, erosion
under dike breach conditions is negligible and other erosion mechanisms such as
headcut erosion probably dominate the erosion process. ...
Coastal flood risk is expected to increase due to climate change and population
growth. Much of our coastlines is protected by “grey” infrastructure such as a dike.
Dike maintenance and strengthening requires ever increasing capital and space,
putting their economic viability in question. To combat this trend, more
sustainable alternatives are explored, also known as Nature based Solutions. A
promising option has shown to be tidal marshes. Tidal marshes are coastal
wetlands with high ecological and economic value. Also, they protect dikes
through wave attenuation and in case of a dike breach reduce its development.
However, the effectiveness of a tidal marsh on reducing dike breach development
rates highly depends on the stability of the tidal marsh itself. Not much is known
about the stability of a tidal marsh under dike breach conditions, which are
accompanied with flow velocities that can reach 4–5ms−1. In this study we tested
the vegetation response and erodibility of a mature tidal marsh, in-situ, under high
flow velocities ( > 0.5ms−1). Our results confirm that tidal marshes similar to the
one tested in this study are highly erosion resistant with low erodibility. More
research is necessary to confirm this for tidal marshes with different soil and
vegetation properties. For tidal marshes similar to what is tested thus far, erosion
under dike breach conditions is negligible and other erosion mechanisms such as
headcut erosion probably dominate the erosion process.
growth. Much of our coastlines is protected by “grey” infrastructure such as a dike.
Dike maintenance and strengthening requires ever increasing capital and space,
putting their economic viability in question. To combat this trend, more
sustainable alternatives are explored, also known as Nature based Solutions. A
promising option has shown to be tidal marshes. Tidal marshes are coastal
wetlands with high ecological and economic value. Also, they protect dikes
through wave attenuation and in case of a dike breach reduce its development.
However, the effectiveness of a tidal marsh on reducing dike breach development
rates highly depends on the stability of the tidal marsh itself. Not much is known
about the stability of a tidal marsh under dike breach conditions, which are
accompanied with flow velocities that can reach 4–5ms−1. In this study we tested
the vegetation response and erodibility of a mature tidal marsh, in-situ, under high
flow velocities ( > 0.5ms−1). Our results confirm that tidal marshes similar to the
one tested in this study are highly erosion resistant with low erodibility. More
research is necessary to confirm this for tidal marshes with different soil and
vegetation properties. For tidal marshes similar to what is tested thus far, erosion
under dike breach conditions is negligible and other erosion mechanisms such as
headcut erosion probably dominate the erosion process.
Tidal stream turbines are becoming an affordable option for harvesting sustainable energy in coastal areas. They can be retrofitted in barrages, providing an integral solution for flood protection and emission-free power generation, within environmental constraints. To optimize the turbine-barrage configuration with respect to these objectives, simulation tools are needed to predict the efficiency of the turbines as well as their impact on the adjacent tidal system. These tools should be based on an accurate representation of the underlying flow processes, which cover a wide range of spatial scales — from meters at the barrage and turbines to tenths of kilometers in the tidal basin. This article presents the development of such a tool by linking an analytical model for turbine fences in barrage gates to a regional flow model. The turbine model is validated with experimental data, and data from a thoroughly monitored tidal energy pilot project. Simulations reveal how clustering the turbines in small arrays can increase their efficiency, owing to array blockage effects, with only little effect on the tidal exchange. We also demonstrate the potential of using turbine fences to manipulate the tidal jet, issued from the barrage, with benefits for coastal — and wildlife protection in the basin. The presented research helps understanding how turbine fences in barrages can be configured with high energy yield and calculated impact to the environment.
...
Tidal stream turbines are becoming an affordable option for harvesting sustainable energy in coastal areas. They can be retrofitted in barrages, providing an integral solution for flood protection and emission-free power generation, within environmental constraints. To optimize the turbine-barrage configuration with respect to these objectives, simulation tools are needed to predict the efficiency of the turbines as well as their impact on the adjacent tidal system. These tools should be based on an accurate representation of the underlying flow processes, which cover a wide range of spatial scales — from meters at the barrage and turbines to tenths of kilometers in the tidal basin. This article presents the development of such a tool by linking an analytical model for turbine fences in barrage gates to a regional flow model. The turbine model is validated with experimental data, and data from a thoroughly monitored tidal energy pilot project. Simulations reveal how clustering the turbines in small arrays can increase their efficiency, owing to array blockage effects, with only little effect on the tidal exchange. We also demonstrate the potential of using turbine fences to manipulate the tidal jet, issued from the barrage, with benefits for coastal — and wildlife protection in the basin. The presented research helps understanding how turbine fences in barrages can be configured with high energy yield and calculated impact to the environment.
The performance of a weir-mounted tidal turbine
An experimental investigation
The tidal flow between bridge pillars and through open barriers is a promising source of ocean energy which can be exploited using tidal stream turbines, as proven recently by operational demonstration plants. The aim of this study is to clarify the consequences for the power output of tidal turbines when placing them in a hydraulic structure. To this end, experimental measurements of turbine power and wakes are performed, using a down-scaled turbine mounted at a submerged weir. The results are compared to an analytical model, validating its range of application for optimising turbine-weir geometries. The experimental data show that the power coefficient of the turbine can be increased by optimising the blockage of the channel and the distance between the turbine and the structure, which is related to the wake configuration. In this way, the power coefficient increased by 40% when the turbine was re-positioned from the upstream to the downstream end of the structure. The theoretical model could reproduce the measured power within 10% accuracy, proving its value as a rapid assessment tool. As such, this work advances the knowledge needed to meet targets on the transition towards renewable energy.
...
The tidal flow between bridge pillars and through open barriers is a promising source of ocean energy which can be exploited using tidal stream turbines, as proven recently by operational demonstration plants. The aim of this study is to clarify the consequences for the power output of tidal turbines when placing them in a hydraulic structure. To this end, experimental measurements of turbine power and wakes are performed, using a down-scaled turbine mounted at a submerged weir. The results are compared to an analytical model, validating its range of application for optimising turbine-weir geometries. The experimental data show that the power coefficient of the turbine can be increased by optimising the blockage of the channel and the distance between the turbine and the structure, which is related to the wake configuration. In this way, the power coefficient increased by 40% when the turbine was re-positioned from the upstream to the downstream end of the structure. The theoretical model could reproduce the measured power within 10% accuracy, proving its value as a rapid assessment tool. As such, this work advances the knowledge needed to meet targets on the transition towards renewable energy.
The ability to estimate the erosion rate along an underwater breach face is crucial to understand the evolution of breaching failure. To this end, breaching erosion models were developed and applied in numerical models. However, these erosion models have never been validated, owing to the scarcity of direct measurements of turbidity currents that accompany breaching. The aim of this study is to evaluate the existing breaching erosion models using direct measurements of recently performed laboratory experiments on breaching flow slides. We found out that the erosion model put forward by Mastbergen & Van Den Berg (2003) provides good agreement with the data and performs better than the one proposed by Van Rhee (2015). The latter tends to overestimate the erosion rate, particularly at steeper slopes.
...
The ability to estimate the erosion rate along an underwater breach face is crucial to understand the evolution of breaching failure. To this end, breaching erosion models were developed and applied in numerical models. However, these erosion models have never been validated, owing to the scarcity of direct measurements of turbidity currents that accompany breaching. The aim of this study is to evaluate the existing breaching erosion models using direct measurements of recently performed laboratory experiments on breaching flow slides. We found out that the erosion model put forward by Mastbergen & Van Den Berg (2003) provides good agreement with the data and performs better than the one proposed by Van Rhee (2015). The latter tends to overestimate the erosion rate, particularly at steeper slopes.
Recent analysis of equilibrium and quasi-equilibrium channel geometry in engineered (fixed-width) rivers has successfully shown that two temporal scales can be distinguished, with quasi-static (long-term) and dynamic (short-term) components. This distinction is based on the fact that channel slope cannot keep pace with short-term fluctuations of the controls. Here we exploit the distinction between the two temporal scales to model the transient (so time-dependent) phase of channel response, which is the phase wherein the channel approaches its new equilibrium. We show that: (a) besides channel slope, also the bed surface texture cannot keep pace with short-term fluctuations of the controls, and (b) mean transient channel response is determined by the probability distributions of the controls (e.g., flow duration curve rather than flow rate sequence). These findings allow us to set up a rapid numerical method that determines the mean transient channel response under stochastic controls. The method is based on distinguishing modes (i.e., sets of controls) and takes the probability density of each mode into account. At each time step, we compute the mode-specific flow, sediment transport rate, and corresponding change in bed level and surface texture. The net change within the time step is computed by weighting the mode-specific changes in bed level and surface texture with the probability density of each mode. The resulting mean transient channel response is a deterministic one, despite the controls being stochastic variables. We show that the proposed method provides a rapid alternative to Monte Carlo analysis regarding the mean time-dependent channel response.
...
Recent analysis of equilibrium and quasi-equilibrium channel geometry in engineered (fixed-width) rivers has successfully shown that two temporal scales can be distinguished, with quasi-static (long-term) and dynamic (short-term) components. This distinction is based on the fact that channel slope cannot keep pace with short-term fluctuations of the controls. Here we exploit the distinction between the two temporal scales to model the transient (so time-dependent) phase of channel response, which is the phase wherein the channel approaches its new equilibrium. We show that: (a) besides channel slope, also the bed surface texture cannot keep pace with short-term fluctuations of the controls, and (b) mean transient channel response is determined by the probability distributions of the controls (e.g., flow duration curve rather than flow rate sequence). These findings allow us to set up a rapid numerical method that determines the mean transient channel response under stochastic controls. The method is based on distinguishing modes (i.e., sets of controls) and takes the probability density of each mode into account. At each time step, we compute the mode-specific flow, sediment transport rate, and corresponding change in bed level and surface texture. The net change within the time step is computed by weighting the mode-specific changes in bed level and surface texture with the probability density of each mode. The resulting mean transient channel response is a deterministic one, despite the controls being stochastic variables. We show that the proposed method provides a rapid alternative to Monte Carlo analysis regarding the mean time-dependent channel response.
Conference paper
(2020)
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W.J. van Os, W.S.J. Uijttewaal, R.J. Labeur, A.L. Osorio, E. Mosselman, L.R. Lokin
Groynes have been replaced by longitudinal training walls in an 11-km long pilot project to optimize training of the river Waal in the Netherlands. These train ing walls improve navigability, reduce flood levels, create a sheltered second channel with more favourable ecological conditions, and decrease the erosive action on the river bed that is responsible for large-scale bed degradation. River managers wish to assess whether longitudinal training walls could have similar advantages along other parts of the Dutch Rhine branches (without excessive increase of maintenance costs). The required maintenance dredging depends on the amount of sediment entering the sheltered channel over an entrance sill situated at the upstream edge of the longitudinal training wall. Currently operational morphodynamic models cannot reliably compute this sedi ment flux. We present laboratory experiments to study the passage of bed sediment at different discharge distributions between the main and sheltered channel, and different degrees of submergence.
...
Groynes have been replaced by longitudinal training walls in an 11-km long pilot project to optimize training of the river Waal in the Netherlands. These train ing walls improve navigability, reduce flood levels, create a sheltered second channel with more favourable ecological conditions, and decrease the erosive action on the river bed that is responsible for large-scale bed degradation. River managers wish to assess whether longitudinal training walls could have similar advantages along other parts of the Dutch Rhine branches (without excessive increase of maintenance costs). The required maintenance dredging depends on the amount of sediment entering the sheltered channel over an entrance sill situated at the upstream edge of the longitudinal training wall. Currently operational morphodynamic models cannot reliably compute this sedi ment flux. We present laboratory experiments to study the passage of bed sediment at different discharge distributions between the main and sheltered channel, and different degrees of submergence.
The performance of a weir-mounted tidal turbine
Field observations and theoretical modelling
An economical way to harvest tidal energy is by integrating free stream turbines in coastal infrastructure. While numerous studies have investigated how turbines should be arranged in idealized geometries to optimize their performance, only a few have considered the influence of realistic bed features. This research investigates the influence of a hydraulic structure on the performance of a tidal turbine, using the combination of field monitoring of full scale turbines installed in a Dutch storm surge barrier - comprising a weir and pillars - and by developing a corresponding theoretical model. The observed production by the turbines was large compared to situations with an unconstrained flow for two reasons. Firstly, the flow contraction by the weir increased the mass flux through the rotor plane. Secondly, the turbine suppressed energy losses in the recirculation zone downstream of a weir. The proposed model provides a quantitative estimate of these effects and is validated against field data. The model can be used as a design tool or parametrization of turbines in a large scale shallow water model, providing performance estimates covering a range of turbine-weir configurations. The work contributes to efficiently exploiting tidal energy with turbines in coastal bridges or flood defenses.
...
An economical way to harvest tidal energy is by integrating free stream turbines in coastal infrastructure. While numerous studies have investigated how turbines should be arranged in idealized geometries to optimize their performance, only a few have considered the influence of realistic bed features. This research investigates the influence of a hydraulic structure on the performance of a tidal turbine, using the combination of field monitoring of full scale turbines installed in a Dutch storm surge barrier - comprising a weir and pillars - and by developing a corresponding theoretical model. The observed production by the turbines was large compared to situations with an unconstrained flow for two reasons. Firstly, the flow contraction by the weir increased the mass flux through the rotor plane. Secondly, the turbine suppressed energy losses in the recirculation zone downstream of a weir. The proposed model provides a quantitative estimate of these effects and is validated against field data. The model can be used as a design tool or parametrization of turbines in a large scale shallow water model, providing performance estimates covering a range of turbine-weir configurations. The work contributes to efficiently exploiting tidal energy with turbines in coastal bridges or flood defenses.
Conference paper
(2020)
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Jakob M. Maljaars, Robert Jan Labeur, Nathaniel A. Trask, Deborah L. Sulsky
A particle-mesh strategy is presented for scalar transport problems which provides diffusion-free advection, conserves mass locally (i.e. cellwise) and exhibits optimal convergence on arbitrary polyhedral meshes. This is achieved by expressing the convective field naturally located on the Lagrangian particles as a mesh quantity by formulating a dedicated particle-mesh projection based via a PDE-constrained optimization problem. Optimal convergence and local conservation are demonstrated for a benchmark test, and the application of the scheme to mass conservative density tracking is illustrated for the Rayleigh–Taylor instability.
...
A particle-mesh strategy is presented for scalar transport problems which provides diffusion-free advection, conserves mass locally (i.e. cellwise) and exhibits optimal convergence on arbitrary polyhedral meshes. This is achieved by expressing the convective field naturally located on the Lagrangian particles as a mesh quantity by formulating a dedicated particle-mesh projection based via a PDE-constrained optimization problem. Optimal convergence and local conservation are demonstrated for a benchmark test, and the application of the scheme to mass conservative density tracking is illustrated for the Rayleigh–Taylor instability.
Abstract
(2020)
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Annemarie van Os, Lieke Lokin, Erik Mosselman, Wim Uijttewaal, Robert Jan Labeur, Ana Luisa Osorio
Breaching flow slides result in a turbidity current running over and directly interacting with the eroding, submarine slope surface, thereby promoting further sediment erosion. The investigation and understanding of this current are crucial, as it is the main parameter influencing the failure evolution and fate of sediment during the breaching phenomenon. In contrast to previous numerical studies dealing with this specific type of turbidity currents, we present a 3D numerical model that simulates the flow structure and hydrodynamics of breaching-generated turbidity currents. The turbulent behavior in the model is captured by large eddy simulation (LES). We present a set of numerical simulations that reproduce particular, previously published experimental results. Through these simulations, we show the validity, applicability, and advantage of the proposed numerical model for the investigation of the flow characteristics. The principal characteristics of the turbidity current are reproduced well, apart from the layer thickness. We also propose a breaching erosion model and validate it using the same series of experimental data. Quite good agreement is observed between the experimental data and the computed erosion rates. The numerical results confirm that breaching-generated turbidity currents are self-accelerating and indicate that they evolve in a self-similar manner.
...
Breaching flow slides result in a turbidity current running over and directly interacting with the eroding, submarine slope surface, thereby promoting further sediment erosion. The investigation and understanding of this current are crucial, as it is the main parameter influencing the failure evolution and fate of sediment during the breaching phenomenon. In contrast to previous numerical studies dealing with this specific type of turbidity currents, we present a 3D numerical model that simulates the flow structure and hydrodynamics of breaching-generated turbidity currents. The turbulent behavior in the model is captured by large eddy simulation (LES). We present a set of numerical simulations that reproduce particular, previously published experimental results. Through these simulations, we show the validity, applicability, and advantage of the proposed numerical model for the investigation of the flow characteristics. The principal characteristics of the turbidity current are reproduced well, apart from the layer thickness. We also propose a breaching erosion model and validate it using the same series of experimental data. Quite good agreement is observed between the experimental data and the computed erosion rates. The numerical results confirm that breaching-generated turbidity currents are self-accelerating and indicate that they evolve in a self-similar manner.
Journal article
(2020)
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S.M.S. Alhaddad, R.J. Labeur, W.S.J. Uijttewaal
Breaching flow slides are accompanied by the generation of turbidity currents. Measurements of these currents are critical for understanding the interaction between the turbidity current and the slope surface, as well as for the validation of numerical models. However, there are insufficient data available detailing the velocity distribution or sediment concentration in these currents. This paper presents experimental results of unique large‐scale physical model experiments on breaching flow slides conducted at the water lab of Delft University of Technology, The Netherlands. The model tests were carried out in a tank with a subaqueous sandy slope steeper than the internal friction angle for that sand. We performed a series of novel experiments in which various non‐vertical initial slope angles were tested, providing the first quantitative data for such initial conditions. Measurements of flow thickness, velocities, and sediment concentrations are obtained, providing the spatial and temporal evolution of the turbidity currents and the resulting underwater slope morphology. The experimental results reveal that the breaching‐generated turbidity currents are self‐accelerating, and that they dominate the problem of breaching flow slides. We evaluated the theoretical expression for the erosion velocity in the case of pure beaching ‐ where the turbidity current has no influence on the erosion ‐ and it overestimated the observed erosion. The analysis of the failure evolution showed that the sand erosion rate increases due to the acceleration of the turbidity currents downslope, until a certain threshold, leading to the steepening of the breach face and thus the occurrence of surficial slides.
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Breaching flow slides are accompanied by the generation of turbidity currents. Measurements of these currents are critical for understanding the interaction between the turbidity current and the slope surface, as well as for the validation of numerical models. However, there are insufficient data available detailing the velocity distribution or sediment concentration in these currents. This paper presents experimental results of unique large‐scale physical model experiments on breaching flow slides conducted at the water lab of Delft University of Technology, The Netherlands. The model tests were carried out in a tank with a subaqueous sandy slope steeper than the internal friction angle for that sand. We performed a series of novel experiments in which various non‐vertical initial slope angles were tested, providing the first quantitative data for such initial conditions. Measurements of flow thickness, velocities, and sediment concentrations are obtained, providing the spatial and temporal evolution of the turbidity currents and the resulting underwater slope morphology. The experimental results reveal that the breaching‐generated turbidity currents are self‐accelerating, and that they dominate the problem of breaching flow slides. We evaluated the theoretical expression for the erosion velocity in the case of pure beaching ‐ where the turbidity current has no influence on the erosion ‐ and it overestimated the observed erosion. The analysis of the failure evolution showed that the sand erosion rate increases due to the acceleration of the turbidity currents downslope, until a certain threshold, leading to the steepening of the breach face and thus the occurrence of surficial slides.
Coastal infrastructure, such as bridges and storm surge barriers with weirs, provides an attractive location for harvesting renewable energy using tidal turbines. Often stone layers are applied downstream of coastal infrastructure to protect the sea bed from erosion. However, little is known about the potential effect of tidal energy extraction on the stability of this granular bed protection. This paper describes a study of the flow conditions influencing the stability of the bed protection downstream of a weir-mounted tidal turbine, using hydrodynamic data of an experimental test. The analysis indicates that the flow recirculation zone downstream of a weir may become shorter and flatter due to the presence of a horizontal-axis turbine. As a result, energetic turbulence eddies can transport more horizontal momentum towards the bed - hence the reason a heavier bed protection may be required for granular beds downstream of weirs when a turbine is installed. This information is essential when designing safe bed protections for coastal infrastructure with tidal turbines.
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Coastal infrastructure, such as bridges and storm surge barriers with weirs, provides an attractive location for harvesting renewable energy using tidal turbines. Often stone layers are applied downstream of coastal infrastructure to protect the sea bed from erosion. However, little is known about the potential effect of tidal energy extraction on the stability of this granular bed protection. This paper describes a study of the flow conditions influencing the stability of the bed protection downstream of a weir-mounted tidal turbine, using hydrodynamic data of an experimental test. The analysis indicates that the flow recirculation zone downstream of a weir may become shorter and flatter due to the presence of a horizontal-axis turbine. As a result, energetic turbulence eddies can transport more horizontal momentum towards the bed - hence the reason a heavier bed protection may be required for granular beds downstream of weirs when a turbine is installed. This information is essential when designing safe bed protections for coastal infrastructure with tidal turbines.
This paper starts with surveying the state-of-the-art knowledge of breaching flow slides, with an emphasis on the relevant fluid mechanics. The governing physical processes of breaching flow slides are explained. The paper highlights the important roles of the associated turbidity current and the frequent surficial slides in increasing the erosion rate of sediment. It also identifies the weaknesses of the current breaching erosion models. Then, the three-equation model of Parker et al. is utilised to describe the coupled processes of breaching and turbidity currents. For comparison’s sake, the existing breaching erosion models are considered: Breusers, Mastbergen and Van Den Berg, and Van Rhee. The sand erosion rate and hydrodynamics of the current vary substantially between the erosion models. Crucially, these erosion models do not account for the surficial slides, nor have they been validated due to the scarcity of data on the associated turbidity current. This paper motivates further experimental studies, including detailed flow measurements, to develop an advanced erosion model. This will improve the fidelity of numerical simulations.
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This paper starts with surveying the state-of-the-art knowledge of breaching flow slides, with an emphasis on the relevant fluid mechanics. The governing physical processes of breaching flow slides are explained. The paper highlights the important roles of the associated turbidity current and the frequent surficial slides in increasing the erosion rate of sediment. It also identifies the weaknesses of the current breaching erosion models. Then, the three-equation model of Parker et al. is utilised to describe the coupled processes of breaching and turbidity currents. For comparison’s sake, the existing breaching erosion models are considered: Breusers, Mastbergen and Van Den Berg, and Van Rhee. The sand erosion rate and hydrodynamics of the current vary substantially between the erosion models. Crucially, these erosion models do not account for the surficial slides, nor have they been validated due to the scarcity of data on the associated turbidity current. This paper motivates further experimental studies, including detailed flow measurements, to develop an advanced erosion model. This will improve the fidelity of numerical simulations.
Scour at the eastern scheldt storm surge barrier
Linking laboratory experiments to field observations
The Eastern Scheldt storm surge barrier is one of the primary flood defense structures in the Netherlands. Since its construction, large scour holes have developed downstream of the applied bed protection at both sides of the inlet that have grown much deeper than foreseen during the design phase. Flow velocity data collected at the location of the scour holes revealed a pronounced contraction of the tidal flow towards the deepest part of the scour hole, in combination with a suppression of vertical flow separation when horizontal velocity differences are present and large enough. To investigate these phenomena we set up a laboratory experiment. First results of this experiment show strong similarities between the flow observed in the field, and the flow observed in the laboratory. Therefore, this set-up can be used to further investigate the phenomena that are observed at the Eastern Scheldt and other similar configurations.
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The Eastern Scheldt storm surge barrier is one of the primary flood defense structures in the Netherlands. Since its construction, large scour holes have developed downstream of the applied bed protection at both sides of the inlet that have grown much deeper than foreseen during the design phase. Flow velocity data collected at the location of the scour holes revealed a pronounced contraction of the tidal flow towards the deepest part of the scour hole, in combination with a suppression of vertical flow separation when horizontal velocity differences are present and large enough. To investigate these phenomena we set up a laboratory experiment. First results of this experiment show strong similarities between the flow observed in the field, and the flow observed in the laboratory. Therefore, this set-up can be used to further investigate the phenomena that are observed at the Eastern Scheldt and other similar configurations.
Recent studies have revealed that breaching, rather than liquefaction, is the dominant failure process in underwater slopes of fine sand and the main driver of observed flow slides in nature. As a result, breaching is getting more attention from hydraulic and geotechnical researchers. Measurements of breaching-generated turbidity currents are substantial for understanding the interaction between the turbidity current and the slope surface, as well as for the validation of numerical models. However, these measurements are scarce in the literature. To this end, laboratory experiments are planned to be carried out in the water lab of Delft University of Technology, the Netherlands. This paper describes the special experimental setup that will be employed to obtain the required data.
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Recent studies have revealed that breaching, rather than liquefaction, is the dominant failure process in underwater slopes of fine sand and the main driver of observed flow slides in nature. As a result, breaching is getting more attention from hydraulic and geotechnical researchers. Measurements of breaching-generated turbidity currents are substantial for understanding the interaction between the turbidity current and the slope surface, as well as for the validation of numerical models. However, these measurements are scarce in the literature. To this end, laboratory experiments are planned to be carried out in the water lab of Delft University of Technology, the Netherlands. This paper describes the special experimental setup that will be employed to obtain the required data.
Coastal infrastructure, such as bridges and storm surge barriers with weirs, provides an attractive location for harvesting renewable energy using tidal turbines. Often stone layers are applied downstream of coastal infrastructure to protect the sea bed from erosion. However, little is known about the potential effect of tidal energy extraction on the stability of this granular bed protection. This paper describes a study of the flow conditions influencing the stability of the bed protection downstream of a weir-mounted tidal turbine, using hydrodynamic data of an experimental test. The analysis indicates that the flow recirculation zone downstream of a weir may become shorter and flatter due to the presence of a horizontal-axis turbine. As a result, energetic turbulence eddies can transport more horizontal momentum towards the bed – hence the reason a heavier bed protection may be required for granular beds downstream of weirs when a turbine is installed. This information is essential when designing safe bed protections for coastal infrastructure with tidal turbines.
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Coastal infrastructure, such as bridges and storm surge barriers with weirs, provides an attractive location for harvesting renewable energy using tidal turbines. Often stone layers are applied downstream of coastal infrastructure to protect the sea bed from erosion. However, little is known about the potential effect of tidal energy extraction on the stability of this granular bed protection. This paper describes a study of the flow conditions influencing the stability of the bed protection downstream of a weir-mounted tidal turbine, using hydrodynamic data of an experimental test. The analysis indicates that the flow recirculation zone downstream of a weir may become shorter and flatter due to the presence of a horizontal-axis turbine. As a result, energetic turbulence eddies can transport more horizontal momentum towards the bed – hence the reason a heavier bed protection may be required for granular beds downstream of weirs when a turbine is installed. This information is essential when designing safe bed protections for coastal infrastructure with tidal turbines.
Flow separation and its control have been the subject of intensive research for decades. Flow separation occurs when the boundary layer loses contact with the associated confining wall, which is usually caused by a pressure gradient acting against the local flow direction. Numerous strategies exist to control flow separation, and in this study we demonstrate experimentally that vertical flow separation over steep slopes in shallow free-surface flows may be suppressed by contracting the flow horizontally upstream of the slope. We found that, unexpectedly, introducing lateral non-uniformity in the upstream flow field could suppress vertical flow separation for steep slopes up to 1 in 2. This study reveals the possibility of two different flow states over steep slopes; (i) a vertically attached flow combined with horizontal convergence, and (ii) a vertically detached flow combined with horizontal divergence. A detailed analysis of the dynamics of the two different flow states is presented. Although a predictive relation determining the transition point between the two flow states was not found in the current study, the observed phenomena were shown to be strongly related to the magnitude of the lateral gradient at the upstream edge of the slope. The results demonstrate a significant influence of the vertical flow state - separated or attached - on the shear stress at the confining boundaries of the flow.
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Flow separation and its control have been the subject of intensive research for decades. Flow separation occurs when the boundary layer loses contact with the associated confining wall, which is usually caused by a pressure gradient acting against the local flow direction. Numerous strategies exist to control flow separation, and in this study we demonstrate experimentally that vertical flow separation over steep slopes in shallow free-surface flows may be suppressed by contracting the flow horizontally upstream of the slope. We found that, unexpectedly, introducing lateral non-uniformity in the upstream flow field could suppress vertical flow separation for steep slopes up to 1 in 2. This study reveals the possibility of two different flow states over steep slopes; (i) a vertically attached flow combined with horizontal convergence, and (ii) a vertically detached flow combined with horizontal divergence. A detailed analysis of the dynamics of the two different flow states is presented. Although a predictive relation determining the transition point between the two flow states was not found in the current study, the observed phenomena were shown to be strongly related to the magnitude of the lateral gradient at the upstream edge of the slope. The results demonstrate a significant influence of the vertical flow state - separated or attached - on the shear stress at the confining boundaries of the flow.