P. Hoekstra - Academia.edu (original) (raw)
Consulting & Advisory reports by P. Hoekstra
Netherlands commission for environmental assessment (NCEA), Oct 24, 2016
Het Rijksprojectbesluit Gaswinning onder de Waddenzee vanaf de locaties Moddergat, Lau-wersoog en... more Het Rijksprojectbesluit Gaswinning onder de Waddenzee vanaf de locaties Moddergat, Lau-wersoog en Vierhuizen (hierna het Rijksprojectbesluit) geeft de Nederlandse Aardolie Maat-schappij BV (NAM) de mogelijkheid om onder voorwaarden aardgas te produceren in het Waddenzeegebied uit de zes velden Moddergat, Nes, Lauwersoog C, Lauwersoog West, Lau-wersoog Oost en Vierhuizen Oost.
De belangrijkste voorwaarde is dat de bodemdaling door de gaswinning samen met de zee-spiegelstijging binnen de toegestane gebruiksruimte blijft.1 De andere voorwaarde is dat de (dynamische) natuur in en rondom de Waddenzee2 niet wordt aangetast door bodemdaling als gevolg van de gaswinning. Mocht dit wel het geval zijn dan wordt de gaswinning beperkt of gestopt. Dit is het zogenaamde ‘Hand aan de kraan’-principe. Om te bepalen of aan deze voorwaarden wordt voldaan, is in het Rijksprojectbesluit en de Natuurbeschermingswetver-gunningen (verder de Nb-wetvergunningen) bepaald dat de bodemdaling en de natuurwaar-den moeten worden gemonitord door de NAM. De NAM rapporteert jaarlijks over de monitoring aan de minister van Economische Zaken (EZ).
Papers by P. Hoekstra
Tidal Signatures in Modern and Ancient Sediments
ABSTRACT Report TR121-2: A9.1-A9.4
Journal of Southeast Asian Earth Sciences, 1988
Alaalraet-River outflow, sediment transport, depositional processes and facies, as well as delta ... more Alaalraet-River outflow, sediment transport, depositional processes and facies, as well as delta morphology of the riven Solo and Porong (Fast Java, Indonesia) have been studied in detail during the Indom~m-Duteh Snellius II expedition (1984-1985). The humid tropical climate of Java in combination with the mountainous relief and the presence of easily erodibie volcanic soils in relation to an intensive agriculture, results in a tremendous soil erosion and denudation rates are extremely high. Since river flow in the dry season is strongly reduced, sediment transport is mainly restricted to the wet season. Every wet season large volumes of sediment are carried towards the sea and the surplus of sediment in the coastal zone has resulted in the formation of two major river deltas. The Solo delta, a single-finger delta, is made up of one straight major channel with only few and small naturat crevasses. Prominent natural levees are present; a consequence of the fact that in the wet season maximum outflow of the fiver coincides with high water. The development of this delta is considered to be a direct response to the high outflow velocities of the river, the huge mudload in the wet season and the low current and wave action. The Porong delta is a half-circular, lobate delta with a multidistributary network of channels which resulted in a regular distribution of sediment. A small part of the delivered sediments in both deltas escapes to sea and is seized by offshore, monsoon-induced coastal currents. The monsoon-type climate has a significant influence on depositional processes by determining river regime and sediment transport, by modifying river outflow and generating a coastal drift of water and suspended sediments.
Ocean Dynamics, 2014
ABSTRACT The characteristics of ebb-tidal deltas are determined by the local hydrodynamics. The l... more ABSTRACT The characteristics of ebb-tidal deltas are determined by the local hydrodynamics. The latter depend, among others, on the geometry of the adjacent back-barrier basin. Therefore, interventions in the back-barrier basin can affect the geometry of ebb-tidal deltas. In this study, the effect of the length of the back-barrier basin on the sand volume and spatial symmetry of ebb-tidal deltas is quantified with the use of a numerical model. It is found that the length of the back-barrier basin affects the tidal prism, the amplitude and phase of the primary tide and its overtides, and the residual currents that, together, determine the sand volume of the ebb-tidal delta. In particular, it is found that no unique relationship exists between tidal prism and sand volume of an ebb-tidal delta. The spatial symmetry of ebb-tidal deltas is also found to be affected by the length of the back-barrier basin. This is because the basin length determines the phase difference between alongshore and cross-shore tidal currents. The numerical model results give a possible explanation for the changes that are observed in the geometry of the ebb-tidal deltas that are located seaward of the Texel Inlet and Vlie Inlet after the closure of the Zuiderzee.
Ocean Dynamics, 2004
The relation between tidal flow asymmetry and net transport of sediment in the semidiurnal regime... more The relation between tidal flow asymmetry and net transport of sediment in the semidiurnal regime has been extensively described. This study reveals that in the diurnal regime, the direction of long-term net bed-load transport and resulting morphologic changes is partly determined by the phase-angle relationship of O1, K1, and M2. Simple analytical formulations of time-averaged bed-load transport were derived which separate the relative contributions of tidal asymmetry from that of residual flow with tidal stirring. In this particular case, the Red River Delta in Vietnam, transports related to tidal asymmetry are larger than those induced by the monsoon currents, and are an order of magnitude larger than those associated with topographic residual flow around the delta. Tide-induced morphologic changes dominate at water depths between 10 and 25 m, of which the patterns of erosion and deposition overlap with observed bathymetric changes. Additional observed changes that occur in more shallow water cannot be explained by tidal asymmetry and are probably related to wave action and to deposition from the buoyant river plume.
Ocean & Coastal Management, 2012
Marine Geology, 1998
The systematic contributions of short waves, infragravity waves and mean flows to the cross-shore... more The systematic contributions of short waves, infragravity waves and mean flows to the cross-shore sediment transport were studied with direct measurements of instantaneous near-bed sediment concentrations and velocities as well as with an energetics-based sediment transport model using measured time series of near-bed cross-shore flow as input. The study was conducted at four cross-shore positions in the multiple bar system of Terschelling, Netherlands during three field campaigns, each with a five-week duration. The data were collected in 3-to 9-m water depth during a wide range of conditions, from low-energy non-breaking conditions to almost fully saturated breakers. The height over depth ratio H s = h was applied at all four measuring stations as a local conditional parameter. All model predictions were grouped into H s = h classes with a width of 0.02 to highlight the generality of the data. The energetics approach predicted that the suspended load dominated over the bedload at all four stations. Both the sediment flux measurements as well as the model predictions showed that the largest contributors to the gross suspended sediment transport were made by the short waves and the undertow, inducing an onshore and offshore transport, respectively. Their contributions to the gross suspended transport were about 40 to 50% under surf zone conditions. Bound infragravity waves were observed and predicted to result in an offshore directed transport that was of subordinate magnitude compared to the observed and estimated magnitudes by the short waves and the undertow. However, as these transports almost cancelled out because of their about equal magnitude but opposite sign, the suspended transport by bound infragravity waves may not be ignored and may, rather paradoxically, have a relatively large influence on the net suspended sediment transport. Hydrodynamical processes that do not seem to be of importance to the onshore and offshore sediment transport in 3-to 9-m water depth in the long run are mean flows under non-breaking conditions and free infragravity motions. A direct comparison between measured sediment fluxes and the model predictions suggests that simple energetics models are suitable for predicting cross-shore sediment transport in 3-to 9-m water depth.
Marine Geology, 1996
The relative distribution of individual grain size fractions across the cross-shore coastal profi... more The relative distribution of individual grain size fractions across the cross-shore coastal profile of Terschelling (North Sea) displays a constant shape when “average” conditions are considered. The shape of the distribution curve depends on the hydrodynamic processes acting in the littoral zone and, if the curve is contemplated in a normalized, relative form, it can be considered as a permanent
Journal of Sea Research, 2013
ABSTRACT The Mosselwad project studies the stability and opportunities for restoration of mussel ... more ABSTRACT The Mosselwad project studies the stability and opportunities for restoration of mussel beds in the Wadden Sea. In this context we seek to predict mussel bed stability with respect to hydrodynamic forcing. To make accurate predictions with models, field experiments are needed to determine relevant processes and to establish representative estimates for model parameters. These parameter values were determined by a six week campaign on a relatively young mussel bed in the Wadden Sea. During this period wave height, period, propagation velocity, dissipation and flow velocities were measured. From these measurements values for the total rate of energy dissipation and the rate of wave energy dissipation were estimated. Results show a large increase in measured bed shear stress over the mussel bed compared with the uncovered parts of the intertidal flat. This is caused by the large roughness of the mussels. The turbulent kinetic energy was high above the covered parts of the bed. From the dissipation rate of TKE values for the corresponding bed shear stress and roughness height were estimated. These estimates were subsequently applied to calibrate a wave model which was used to determine the spatial distribution of the wave forcing. Model results show that the bed shear stress decreases over the mussel bed but increases behind the bed. Furthermore, a model study on the situation before the mussel bed settled shows that a minimum in wave forcing coincides with the seaward edge of the bed. This suggests that the bed is currently located in the optimal location with respect to wave forcing.
Netherlands Journal of Sea Research, 1989
Netherlands Journal of Sea Research, 1989
... Perhaps the most striking conclusion of this tracer study is the very limited influence of bo... more ... Perhaps the most striking conclusion of this tracer study is the very limited influence of both the Solo and Brantas river systems on shelf seas and oceanic basins. Significant environmental effects remain restricted to the coastal zone of East Java. ...
Journal of Marine Systems, 2011
ABSTRACT The Bay of Banten provides an example of a semicircular shallow tidal embayment in a tro... more ABSTRACT The Bay of Banten provides an example of a semicircular shallow tidal embayment in a tropical ROFI (Region Of Freshwater Influence), subject to a mixed, mainly diurnal tidal regime and a monsoon-driven residual circulation. A partially inactivated delta shapes the east coast of the bay and constitutes a distributed source of freshwater during the wet season. Measurements of flow velocity, salinity and temperature were taken at two contrasting sites in the bay, to investigate stratification dynamics throughout a spring-neap cycle. Bulk Richardson numbers are shown to vary both at tidal and subtidal frequencies, and indicate that the water column occasionally remains stratified all through a tidal cycle. About half of the variance of the subtidal flow can be explained from wind forcing. The wind-driven flow varies between 1 and 5% of the wind speed, with a pronounced difference between the two locations understudy. Stratification was further analyzed by estimating the contributions of advection by the depth-mean flow, tidal straining, vertical advection and heating, to variation in potential energy anomaly (PEA). To do so, horizontal gradients in density and PEA were estimated from multi-linear regression analysis. The obtained results show that spatial gradients of density and PEA can reverse sign frequently in the absence of strong mixing, associated to pulses of freshwater discharge. The freshwater discharge, in turn, is distributed over small streams. The contribution of vertical mixing was found to be relatively small, and none of the remaining terms were clearly dominant. Estimates of the boundary layer thickness of clockwise and anticlockwise rotary components of the tidal flow show the impact of baroclinic processes on velocity profiles.
Journal of Geophysical Research, 2003
Tidal current asymmetry is usually associated to nonlinear tidal interactions in shallow water, g... more Tidal current asymmetry is usually associated to nonlinear tidal interactions in shallow water, generating compound tides and overtides. The present paper demonstrates that tidal asymmetry is not only caused by nonlinear tidal interaction, but is also generated by astronomical tides in absence of shallow-water effects. In particular, the K 1 , O 1 , and M 2 constituents give an asymmetrical periodic flow pattern, with the largest peak velocities persistently in the same direction. A theoretical explanation for this phenomenon is presented, and an inventory is made of other, subordinate constituents that may contribute to repetitive asymmetric flow patterns. Tidal current asymmetry may induce residual transport of sediment. Analytical expressions are derived which quantify the residual transport of sediment due to the K 1 , O 1 , and M 2 tides, as a function of the phases and amplitudes of these constituents, and the time lag between variation in suspended sediment transport and flow velocity. The residual transport induced by the K 1 , O 1 , and M 2 tides is compared to the residual transport due to the asymmetry associated with the M 2 and M 4 tides. The former mechanism is particularly relevant in diurnal, or mixed, mainly diurnal tidal regimes, where the K 1 and O 1 tides are dominant.
Netherlands commission for environmental assessment (NCEA), Oct 24, 2016
Het Rijksprojectbesluit Gaswinning onder de Waddenzee vanaf de locaties Moddergat, Lau-wersoog en... more Het Rijksprojectbesluit Gaswinning onder de Waddenzee vanaf de locaties Moddergat, Lau-wersoog en Vierhuizen (hierna het Rijksprojectbesluit) geeft de Nederlandse Aardolie Maat-schappij BV (NAM) de mogelijkheid om onder voorwaarden aardgas te produceren in het Waddenzeegebied uit de zes velden Moddergat, Nes, Lauwersoog C, Lauwersoog West, Lau-wersoog Oost en Vierhuizen Oost.
De belangrijkste voorwaarde is dat de bodemdaling door de gaswinning samen met de zee-spiegelstijging binnen de toegestane gebruiksruimte blijft.1 De andere voorwaarde is dat de (dynamische) natuur in en rondom de Waddenzee2 niet wordt aangetast door bodemdaling als gevolg van de gaswinning. Mocht dit wel het geval zijn dan wordt de gaswinning beperkt of gestopt. Dit is het zogenaamde ‘Hand aan de kraan’-principe. Om te bepalen of aan deze voorwaarden wordt voldaan, is in het Rijksprojectbesluit en de Natuurbeschermingswetver-gunningen (verder de Nb-wetvergunningen) bepaald dat de bodemdaling en de natuurwaar-den moeten worden gemonitord door de NAM. De NAM rapporteert jaarlijks over de monitoring aan de minister van Economische Zaken (EZ).
Tidal Signatures in Modern and Ancient Sediments
ABSTRACT Report TR121-2: A9.1-A9.4
Journal of Southeast Asian Earth Sciences, 1988
Alaalraet-River outflow, sediment transport, depositional processes and facies, as well as delta ... more Alaalraet-River outflow, sediment transport, depositional processes and facies, as well as delta morphology of the riven Solo and Porong (Fast Java, Indonesia) have been studied in detail during the Indom~m-Duteh Snellius II expedition (1984-1985). The humid tropical climate of Java in combination with the mountainous relief and the presence of easily erodibie volcanic soils in relation to an intensive agriculture, results in a tremendous soil erosion and denudation rates are extremely high. Since river flow in the dry season is strongly reduced, sediment transport is mainly restricted to the wet season. Every wet season large volumes of sediment are carried towards the sea and the surplus of sediment in the coastal zone has resulted in the formation of two major river deltas. The Solo delta, a single-finger delta, is made up of one straight major channel with only few and small naturat crevasses. Prominent natural levees are present; a consequence of the fact that in the wet season maximum outflow of the fiver coincides with high water. The development of this delta is considered to be a direct response to the high outflow velocities of the river, the huge mudload in the wet season and the low current and wave action. The Porong delta is a half-circular, lobate delta with a multidistributary network of channels which resulted in a regular distribution of sediment. A small part of the delivered sediments in both deltas escapes to sea and is seized by offshore, monsoon-induced coastal currents. The monsoon-type climate has a significant influence on depositional processes by determining river regime and sediment transport, by modifying river outflow and generating a coastal drift of water and suspended sediments.
Ocean Dynamics, 2014
ABSTRACT The characteristics of ebb-tidal deltas are determined by the local hydrodynamics. The l... more ABSTRACT The characteristics of ebb-tidal deltas are determined by the local hydrodynamics. The latter depend, among others, on the geometry of the adjacent back-barrier basin. Therefore, interventions in the back-barrier basin can affect the geometry of ebb-tidal deltas. In this study, the effect of the length of the back-barrier basin on the sand volume and spatial symmetry of ebb-tidal deltas is quantified with the use of a numerical model. It is found that the length of the back-barrier basin affects the tidal prism, the amplitude and phase of the primary tide and its overtides, and the residual currents that, together, determine the sand volume of the ebb-tidal delta. In particular, it is found that no unique relationship exists between tidal prism and sand volume of an ebb-tidal delta. The spatial symmetry of ebb-tidal deltas is also found to be affected by the length of the back-barrier basin. This is because the basin length determines the phase difference between alongshore and cross-shore tidal currents. The numerical model results give a possible explanation for the changes that are observed in the geometry of the ebb-tidal deltas that are located seaward of the Texel Inlet and Vlie Inlet after the closure of the Zuiderzee.
Ocean Dynamics, 2004
The relation between tidal flow asymmetry and net transport of sediment in the semidiurnal regime... more The relation between tidal flow asymmetry and net transport of sediment in the semidiurnal regime has been extensively described. This study reveals that in the diurnal regime, the direction of long-term net bed-load transport and resulting morphologic changes is partly determined by the phase-angle relationship of O1, K1, and M2. Simple analytical formulations of time-averaged bed-load transport were derived which separate the relative contributions of tidal asymmetry from that of residual flow with tidal stirring. In this particular case, the Red River Delta in Vietnam, transports related to tidal asymmetry are larger than those induced by the monsoon currents, and are an order of magnitude larger than those associated with topographic residual flow around the delta. Tide-induced morphologic changes dominate at water depths between 10 and 25 m, of which the patterns of erosion and deposition overlap with observed bathymetric changes. Additional observed changes that occur in more shallow water cannot be explained by tidal asymmetry and are probably related to wave action and to deposition from the buoyant river plume.
Ocean & Coastal Management, 2012
Marine Geology, 1998
The systematic contributions of short waves, infragravity waves and mean flows to the cross-shore... more The systematic contributions of short waves, infragravity waves and mean flows to the cross-shore sediment transport were studied with direct measurements of instantaneous near-bed sediment concentrations and velocities as well as with an energetics-based sediment transport model using measured time series of near-bed cross-shore flow as input. The study was conducted at four cross-shore positions in the multiple bar system of Terschelling, Netherlands during three field campaigns, each with a five-week duration. The data were collected in 3-to 9-m water depth during a wide range of conditions, from low-energy non-breaking conditions to almost fully saturated breakers. The height over depth ratio H s = h was applied at all four measuring stations as a local conditional parameter. All model predictions were grouped into H s = h classes with a width of 0.02 to highlight the generality of the data. The energetics approach predicted that the suspended load dominated over the bedload at all four stations. Both the sediment flux measurements as well as the model predictions showed that the largest contributors to the gross suspended sediment transport were made by the short waves and the undertow, inducing an onshore and offshore transport, respectively. Their contributions to the gross suspended transport were about 40 to 50% under surf zone conditions. Bound infragravity waves were observed and predicted to result in an offshore directed transport that was of subordinate magnitude compared to the observed and estimated magnitudes by the short waves and the undertow. However, as these transports almost cancelled out because of their about equal magnitude but opposite sign, the suspended transport by bound infragravity waves may not be ignored and may, rather paradoxically, have a relatively large influence on the net suspended sediment transport. Hydrodynamical processes that do not seem to be of importance to the onshore and offshore sediment transport in 3-to 9-m water depth in the long run are mean flows under non-breaking conditions and free infragravity motions. A direct comparison between measured sediment fluxes and the model predictions suggests that simple energetics models are suitable for predicting cross-shore sediment transport in 3-to 9-m water depth.
Marine Geology, 1996
The relative distribution of individual grain size fractions across the cross-shore coastal profi... more The relative distribution of individual grain size fractions across the cross-shore coastal profile of Terschelling (North Sea) displays a constant shape when “average” conditions are considered. The shape of the distribution curve depends on the hydrodynamic processes acting in the littoral zone and, if the curve is contemplated in a normalized, relative form, it can be considered as a permanent
Journal of Sea Research, 2013
ABSTRACT The Mosselwad project studies the stability and opportunities for restoration of mussel ... more ABSTRACT The Mosselwad project studies the stability and opportunities for restoration of mussel beds in the Wadden Sea. In this context we seek to predict mussel bed stability with respect to hydrodynamic forcing. To make accurate predictions with models, field experiments are needed to determine relevant processes and to establish representative estimates for model parameters. These parameter values were determined by a six week campaign on a relatively young mussel bed in the Wadden Sea. During this period wave height, period, propagation velocity, dissipation and flow velocities were measured. From these measurements values for the total rate of energy dissipation and the rate of wave energy dissipation were estimated. Results show a large increase in measured bed shear stress over the mussel bed compared with the uncovered parts of the intertidal flat. This is caused by the large roughness of the mussels. The turbulent kinetic energy was high above the covered parts of the bed. From the dissipation rate of TKE values for the corresponding bed shear stress and roughness height were estimated. These estimates were subsequently applied to calibrate a wave model which was used to determine the spatial distribution of the wave forcing. Model results show that the bed shear stress decreases over the mussel bed but increases behind the bed. Furthermore, a model study on the situation before the mussel bed settled shows that a minimum in wave forcing coincides with the seaward edge of the bed. This suggests that the bed is currently located in the optimal location with respect to wave forcing.
Netherlands Journal of Sea Research, 1989
Netherlands Journal of Sea Research, 1989
... Perhaps the most striking conclusion of this tracer study is the very limited influence of bo... more ... Perhaps the most striking conclusion of this tracer study is the very limited influence of both the Solo and Brantas river systems on shelf seas and oceanic basins. Significant environmental effects remain restricted to the coastal zone of East Java. ...
Journal of Marine Systems, 2011
ABSTRACT The Bay of Banten provides an example of a semicircular shallow tidal embayment in a tro... more ABSTRACT The Bay of Banten provides an example of a semicircular shallow tidal embayment in a tropical ROFI (Region Of Freshwater Influence), subject to a mixed, mainly diurnal tidal regime and a monsoon-driven residual circulation. A partially inactivated delta shapes the east coast of the bay and constitutes a distributed source of freshwater during the wet season. Measurements of flow velocity, salinity and temperature were taken at two contrasting sites in the bay, to investigate stratification dynamics throughout a spring-neap cycle. Bulk Richardson numbers are shown to vary both at tidal and subtidal frequencies, and indicate that the water column occasionally remains stratified all through a tidal cycle. About half of the variance of the subtidal flow can be explained from wind forcing. The wind-driven flow varies between 1 and 5% of the wind speed, with a pronounced difference between the two locations understudy. Stratification was further analyzed by estimating the contributions of advection by the depth-mean flow, tidal straining, vertical advection and heating, to variation in potential energy anomaly (PEA). To do so, horizontal gradients in density and PEA were estimated from multi-linear regression analysis. The obtained results show that spatial gradients of density and PEA can reverse sign frequently in the absence of strong mixing, associated to pulses of freshwater discharge. The freshwater discharge, in turn, is distributed over small streams. The contribution of vertical mixing was found to be relatively small, and none of the remaining terms were clearly dominant. Estimates of the boundary layer thickness of clockwise and anticlockwise rotary components of the tidal flow show the impact of baroclinic processes on velocity profiles.
Journal of Geophysical Research, 2003
Tidal current asymmetry is usually associated to nonlinear tidal interactions in shallow water, g... more Tidal current asymmetry is usually associated to nonlinear tidal interactions in shallow water, generating compound tides and overtides. The present paper demonstrates that tidal asymmetry is not only caused by nonlinear tidal interaction, but is also generated by astronomical tides in absence of shallow-water effects. In particular, the K 1 , O 1 , and M 2 constituents give an asymmetrical periodic flow pattern, with the largest peak velocities persistently in the same direction. A theoretical explanation for this phenomenon is presented, and an inventory is made of other, subordinate constituents that may contribute to repetitive asymmetric flow patterns. Tidal current asymmetry may induce residual transport of sediment. Analytical expressions are derived which quantify the residual transport of sediment due to the K 1 , O 1 , and M 2 tides, as a function of the phases and amplitudes of these constituents, and the time lag between variation in suspended sediment transport and flow velocity. The residual transport induced by the K 1 , O 1 , and M 2 tides is compared to the residual transport due to the asymmetry associated with the M 2 and M 4 tides. The former mechanism is particularly relevant in diurnal, or mixed, mainly diurnal tidal regimes, where the K 1 and O 1 tides are dominant.
Estuarine, Coastal and Shelf Science, 2006
The present paper comments on a recent publication by Woodworth et al. [Woodworth, P.L., Blackman... more The present paper comments on a recent publication by Woodworth et al. [Woodworth, P.L., Blackman, D.L., Pugh, D.T., Vassie J.M., 2005. On the role of diurnal tides in contributing to asymmetries in tidal probability distribution functions in areas of predominantly semi-diurnal tide. Estuarine, Coastal and Shelf Science 64 (2¿3), 235¿240.]. It is shown how tidal asymmetries associated with astronomical tides