T. Aagaard | University of Copenhagen (original) (raw)
Papers by T. Aagaard
Journal of Coastal Research, 2004
... 2. Six maps based on digitized vertical aerial photographs show shoreline changes at Skalling... more ... 2. Six maps based on digitized vertical aerial photographs show shoreline changes at Skalling ... The different morphological stages are also seen in Figure 3. ... (2009) Facies architecture and depositional processes of the Holocene-Recent accretionary forced regressive Skagen ...
Coastal Sediments '07, 2007
The aim of this study is to compare long-term (30 yr) bar migration characteristics at two field ... more The aim of this study is to compare long-term (30 yr) bar migration characteristics at two field sites on the Danish North Sea coast. One site displays systematically onshore migrating bars (Skallingen) while the bar migration at the other site (Vejers) is systematically offshore. Using cross-shore profile data and a sediment transport model, the paper discusses the mechanisms that cause divergent longshore bar behaviour at the two sites. The main reason for the difference appears to be larger undertow speeds at Vejers, caused by a steeper outer shoreface and a larger bar relief, leading to breaking wave reformation between bars.
This paper provides insight into the spectral signatures of wave runup (swash) from contrasting b... more This paper provides insight into the spectral signatures of wave runup (swash) from contrasting beach types; their nature and range and the morphodynamic controls exerted by beach morphology and wave exposure. An extensive field data set is used covering the full range of micro-tidal beach types (reflective, intermediate and dissipative), and includes beach gradients ranging from approximately 1:6 to 1:60 exposed to offshore significant wave heights of 0.5 m to 3.0 m. The data investigated here consists of 187 individual swash spectra. The ratio of swash energy in the short wave (f>0.05 Hz) to infragravity wave (f<0.05 Hz) frequency bands is found to be significantly different between the three beach types. Swash energy at short wave frequencies is dominant on reflective and intermediate beaches and swash at infragravity wave frequencies is dominant on dissipative beaches; consistent with previously reported spectral signatures for the surf zone on these beach types. The swash spectra were classified using an automated algorithm known as CLARA into five different classes. The ordered classes represent an evolution in the spectrum shape described by a frequency downshifting of the energy peak from the short-wave into the infragravity wave band and an increase in the infragravity swash energy level compared to a consistent short-wave swash energy level. A universally common feature of spectra from all beach-states was an 4 f energy roll-off in the short-wave frequency band. In contrast to the broadly uniform appearance of the short-wave frequency band across all the beach-states, the appearance of the infragravity wave frequency band was highly variable. A conceptual model for the evolution of swash spectra under increasing and decreasing incident wave energy level, with due consideration of beach morphology, is proposed.
ABSTRACT Earlier field measurements of sediment transport in bar troughs have indicated that onsh... more ABSTRACT Earlier field measurements of sediment transport in bar troughs have indicated that onshore transport across troughs is very small and that on barred beaches, sediment mass transfer from the bar to the beach occurs mainly through bar welding. Here, we revisit the issue using novel instrumentation that is capable of recording suspended sediment transport at high resolution down to about 0.8 cm above the seabed. It was found that under accretionary fairweather conditions, a bar eroded and a beach berm accreted but the net suspended sediment transport across the trough was very small and seaward directed. The observed berm accretion could also not be ascribed to bedload transport through migration of wave ripples. It is concluded that even under favourable conditions, onshore sediment transport across bar troughs appears to be limited.
Coastal Sediments '07, 2007
Marine Geology, 2005
The processes responsible for sediment transport and morphological change in the intertidal zone ... more The processes responsible for sediment transport and morphological change in the intertidal zone of natural beaches are not well known. Consequently, factors determining whether the intertidal beach erodes/accretes or remains stable in the face of significant wave impact, and the mechanisms behind the elusive concept of dequilibrium beach slopeT are poorly understood. A field experiment was designed to measure suspended sediment transport in the swash and inner surf zone through successive tidal cycles in the intertidal zone at Egmond Beach, The Netherlands, using a stack of fiberoptic backscatter sensors (UFOBS-7). The experimental period comprised three large storm events, including an extreme storm with offshore significant wave heights in excess of 7 m. Surveys spanning a N1 km long section of the beach as well as observations from an ARGUS video camera system showed that gently sloping shoreline salients (megacusps) were highly stable in position and displayed only minor slope adjustments through the storm period, whereas steeply sloping embayments were eroded significantly. The sediment transport measurements at a shoreline salient indicated that the subdued net beach response was because offshore sediment transport occurring under surf zone conditions at high tide was compensated by onshore sediment transport at low tide when swash zone processes dominated. Such temporally varying, tidally modulated sediment transport rates and directions are suggested to be able to maintain a quasi-equilibrium intertidal beach slope and the results indicate that a negative feedback existed between initial beach slope and net sediment transport patterns. D
Marine Geology, 2012
ABSTRACT New field experiments of sediment transport on the shoreface of a reflective beach (R = ... more ABSTRACT New field experiments of sediment transport on the shoreface of a reflective beach (R = 0.4–0.6) during pre-storm, storm and post-storm conditions are reported. Data were collected outside the breakpoint and include water surface elevations, cross-shore and long-shore flow velocity, suspended sediment concentrations, bedform dimensions and the morphological response. Instruments included two pulse-coherent ADPs, an ADV, multiple OBS and a pencil-beam sonar. The depth-averaged mean velocity was always offshore and increased by a factor five during the storm, and this increase is attributed to undertow being enhanced by strong backwash plumes from cusp bays. Estimates of cross-shore suspended sediment transport are landward during the pre- and post-storm phases, consistent with the morphological response. During the storm, the shoreface quickly appeared to reach equilibrium, despite energy levels being maintained, with sediment transport rates reducing rapidly after the initial phase of the storm. Streaming in the wave boundary layer was observed and strongest during the pre-storm phase, consistent with the observed shoreward transport. During the storm the near-bed streaming was too small to reverse the increased offshore current, consistent with the observed net offshore transport. The generally small orbital velocity skewness observed on this steep reflective beach suggests that wave reflection may reduce the overall skewness of the velocity, in turn contributing to the stability of reflective beach types.
Journal of Coastal Research, 2006
AAGAARD, T.; HUGHES, M.; MØLLER-SØRENSEN, R., and ANDERSEN, S., 2006. Hydrodynamics and sediment ... more AAGAARD, T.; HUGHES, M.; MØLLER-SØRENSEN, R., and ANDERSEN, S., 2006. Hydrodynamics and sediment fluxes across an onshore migrating intertidal bar. Journal of Coastal Research, 22(2), 247-259. West Palm Beach (Florida), ISSN 0749-0208.
Earth Surface Processes and Landforms, 2009
ABSTRACT Berm formation and morphological development of the beach face have been observed during... more ABSTRACT Berm formation and morphological development of the beach face have been observed during a neap-neap tidal cycle on the gently sloping and accreting beach at Vejers, Denmark. During the field campaign, an intertidal bar migrated onshore and stabilized as a berm on the foreshore. A new intertidal bar occurred on the lower beach face, migrated onshore on the rising tide and finally merged with the pre-existing berm. As the tide continued to rise, the new berm translated further onshore as an intertidal bar to the uppermost part of the foreshore. The sediment transport during the berm transition was onshore directed in the upper swash and offshore directed in the lower swash. This berm development can be described through both the neap-berm, ridge-and-runnel and berm-ridge development concepts proposed by Hine (Sedimentology 1979; 26: 333-351), and all three stages were observed during only three tidal cycles. The main factors controlling this fast transformation were the gentle slope of the cross-shore profile, rapid water level translation rates, substantial swash overtopping of the berm, and low infiltration rates. Despite the onshore migration of intertidal bars and berm formation, no net foreshore accretion took place during the field campaign. This was largely due to the formation of rip channels with strong rip currents cutting through the intertidal bars and the berm, which acted as a sediment drain in the profile.
Earth Surface Processes and Landforms, 2008
ABSTRACT The formation of an inner nearshore bar was observed during a high-energy event at the s... more ABSTRACT The formation of an inner nearshore bar was observed during a high-energy event at the sandy beach of Vejers, Denmark. The bar accreted in situ during surf zone conditions and the growth of the bar was associated with the development of a trough landward of the bar. Measurements of hydrodynamics and sediment fluxes were obtained from electromagnetic current meters and optical backscatter sensors. These process measurements showed that a divergence in sediment transport occurred at the location of the developing trough, and observed gradients in cross-shore net sediment flux were consistent with the morphological development. The main cause for the flux gradients were cross-shore gradients in offshore-directed mean current (undertow) speed which depended upon local relative wave height and local bed slope. Copyright © 2007 John Wiley & Sons, Ltd.
Continental Shelf Research, 2010
Long-term, net offshore bar migration is a common occurrence on many multiple-barred beaches. The... more Long-term, net offshore bar migration is a common occurrence on many multiple-barred beaches. The first stage of the process involves the generation of a longshore bar close to the shoreline that oscillates about a mean position for some time, followed by a stage of net offshore migration across the upper shoreface, and finally a stage of decaying bar form through loss of sediment volume at the outer boundary of the upper shoreface. The phenomenon has been previously documented in the Netherlands, the USA, the Canadian Great Lakes, and in New Zealand, but our present understanding of the morphodynamic processes and sediment transport pathways involved in bar decay is limited. In this paper, long-term, net offshore bar migration is investigated at Vejers Beach, located on the North Sea coast of Denmark where offshore bar migration rates are of the order of 45-55 m a À 1 . A wave height transformation model confirmed that the decay of the outer bar results in increased wave heights and undertow speeds at the more landward bar potentially causing this bar to speed up its offshore migration. The causes for outer bar decay were investigated through field measurements of sediment transport at the decaying bar and at a position further seaward on the lower shoreface. The measurements showed that a cross-shore transport convergence exists between the bar and the lower shoreface and that the loss of sediment involved in bar decay is associated with a longshore directed transport by non-surf zone processes. At Vejers, and possibly elsewhere, the net offshore migration of bars and the subsequent loss of sand during bar decay is an important part of the beach and shoreface sediment budget.
Continental Shelf Research, 1995
Continental Shelf Research, 2003
... Troels Aagaard Corresponding Author Contact Information , E-mail The Corresponding Author , a... more ... Troels Aagaard Corresponding Author Contact Information , E-mail The Corresponding Author , a and Karin R. Bryan b , 1. ... A few studies from enclosed seas or lakes have provided some indications of frequency selection ( [Aagaard (1990]; [Bauer and Greenwood (1990]). ...
G. Masselink, T. Aagaard and A. Kroon, 2011. Destruction of intertidal bar morphology during a su... more G. Masselink, T. Aagaard and A. Kroon, 2011. Destruction of intertidal bar morphology during a summer storm surge event: Example of positive morphodynamic feedback.
Journal of Coastal Research, 2004
... 2. Six maps based on digitized vertical aerial photographs show shoreline changes at Skalling... more ... 2. Six maps based on digitized vertical aerial photographs show shoreline changes at Skalling ... The different morphological stages are also seen in Figure 3. ... (2009) Facies architecture and depositional processes of the Holocene-Recent accretionary forced regressive Skagen ...
Coastal Sediments '07, 2007
The aim of this study is to compare long-term (30 yr) bar migration characteristics at two field ... more The aim of this study is to compare long-term (30 yr) bar migration characteristics at two field sites on the Danish North Sea coast. One site displays systematically onshore migrating bars (Skallingen) while the bar migration at the other site (Vejers) is systematically offshore. Using cross-shore profile data and a sediment transport model, the paper discusses the mechanisms that cause divergent longshore bar behaviour at the two sites. The main reason for the difference appears to be larger undertow speeds at Vejers, caused by a steeper outer shoreface and a larger bar relief, leading to breaking wave reformation between bars.
This paper provides insight into the spectral signatures of wave runup (swash) from contrasting b... more This paper provides insight into the spectral signatures of wave runup (swash) from contrasting beach types; their nature and range and the morphodynamic controls exerted by beach morphology and wave exposure. An extensive field data set is used covering the full range of micro-tidal beach types (reflective, intermediate and dissipative), and includes beach gradients ranging from approximately 1:6 to 1:60 exposed to offshore significant wave heights of 0.5 m to 3.0 m. The data investigated here consists of 187 individual swash spectra. The ratio of swash energy in the short wave (f>0.05 Hz) to infragravity wave (f<0.05 Hz) frequency bands is found to be significantly different between the three beach types. Swash energy at short wave frequencies is dominant on reflective and intermediate beaches and swash at infragravity wave frequencies is dominant on dissipative beaches; consistent with previously reported spectral signatures for the surf zone on these beach types. The swash spectra were classified using an automated algorithm known as CLARA into five different classes. The ordered classes represent an evolution in the spectrum shape described by a frequency downshifting of the energy peak from the short-wave into the infragravity wave band and an increase in the infragravity swash energy level compared to a consistent short-wave swash energy level. A universally common feature of spectra from all beach-states was an 4 f energy roll-off in the short-wave frequency band. In contrast to the broadly uniform appearance of the short-wave frequency band across all the beach-states, the appearance of the infragravity wave frequency band was highly variable. A conceptual model for the evolution of swash spectra under increasing and decreasing incident wave energy level, with due consideration of beach morphology, is proposed.
ABSTRACT Earlier field measurements of sediment transport in bar troughs have indicated that onsh... more ABSTRACT Earlier field measurements of sediment transport in bar troughs have indicated that onshore transport across troughs is very small and that on barred beaches, sediment mass transfer from the bar to the beach occurs mainly through bar welding. Here, we revisit the issue using novel instrumentation that is capable of recording suspended sediment transport at high resolution down to about 0.8 cm above the seabed. It was found that under accretionary fairweather conditions, a bar eroded and a beach berm accreted but the net suspended sediment transport across the trough was very small and seaward directed. The observed berm accretion could also not be ascribed to bedload transport through migration of wave ripples. It is concluded that even under favourable conditions, onshore sediment transport across bar troughs appears to be limited.
Coastal Sediments '07, 2007
Marine Geology, 2005
The processes responsible for sediment transport and morphological change in the intertidal zone ... more The processes responsible for sediment transport and morphological change in the intertidal zone of natural beaches are not well known. Consequently, factors determining whether the intertidal beach erodes/accretes or remains stable in the face of significant wave impact, and the mechanisms behind the elusive concept of dequilibrium beach slopeT are poorly understood. A field experiment was designed to measure suspended sediment transport in the swash and inner surf zone through successive tidal cycles in the intertidal zone at Egmond Beach, The Netherlands, using a stack of fiberoptic backscatter sensors (UFOBS-7). The experimental period comprised three large storm events, including an extreme storm with offshore significant wave heights in excess of 7 m. Surveys spanning a N1 km long section of the beach as well as observations from an ARGUS video camera system showed that gently sloping shoreline salients (megacusps) were highly stable in position and displayed only minor slope adjustments through the storm period, whereas steeply sloping embayments were eroded significantly. The sediment transport measurements at a shoreline salient indicated that the subdued net beach response was because offshore sediment transport occurring under surf zone conditions at high tide was compensated by onshore sediment transport at low tide when swash zone processes dominated. Such temporally varying, tidally modulated sediment transport rates and directions are suggested to be able to maintain a quasi-equilibrium intertidal beach slope and the results indicate that a negative feedback existed between initial beach slope and net sediment transport patterns. D
Marine Geology, 2012
ABSTRACT New field experiments of sediment transport on the shoreface of a reflective beach (R = ... more ABSTRACT New field experiments of sediment transport on the shoreface of a reflective beach (R = 0.4–0.6) during pre-storm, storm and post-storm conditions are reported. Data were collected outside the breakpoint and include water surface elevations, cross-shore and long-shore flow velocity, suspended sediment concentrations, bedform dimensions and the morphological response. Instruments included two pulse-coherent ADPs, an ADV, multiple OBS and a pencil-beam sonar. The depth-averaged mean velocity was always offshore and increased by a factor five during the storm, and this increase is attributed to undertow being enhanced by strong backwash plumes from cusp bays. Estimates of cross-shore suspended sediment transport are landward during the pre- and post-storm phases, consistent with the morphological response. During the storm, the shoreface quickly appeared to reach equilibrium, despite energy levels being maintained, with sediment transport rates reducing rapidly after the initial phase of the storm. Streaming in the wave boundary layer was observed and strongest during the pre-storm phase, consistent with the observed shoreward transport. During the storm the near-bed streaming was too small to reverse the increased offshore current, consistent with the observed net offshore transport. The generally small orbital velocity skewness observed on this steep reflective beach suggests that wave reflection may reduce the overall skewness of the velocity, in turn contributing to the stability of reflective beach types.
Journal of Coastal Research, 2006
AAGAARD, T.; HUGHES, M.; MØLLER-SØRENSEN, R., and ANDERSEN, S., 2006. Hydrodynamics and sediment ... more AAGAARD, T.; HUGHES, M.; MØLLER-SØRENSEN, R., and ANDERSEN, S., 2006. Hydrodynamics and sediment fluxes across an onshore migrating intertidal bar. Journal of Coastal Research, 22(2), 247-259. West Palm Beach (Florida), ISSN 0749-0208.
Earth Surface Processes and Landforms, 2009
ABSTRACT Berm formation and morphological development of the beach face have been observed during... more ABSTRACT Berm formation and morphological development of the beach face have been observed during a neap-neap tidal cycle on the gently sloping and accreting beach at Vejers, Denmark. During the field campaign, an intertidal bar migrated onshore and stabilized as a berm on the foreshore. A new intertidal bar occurred on the lower beach face, migrated onshore on the rising tide and finally merged with the pre-existing berm. As the tide continued to rise, the new berm translated further onshore as an intertidal bar to the uppermost part of the foreshore. The sediment transport during the berm transition was onshore directed in the upper swash and offshore directed in the lower swash. This berm development can be described through both the neap-berm, ridge-and-runnel and berm-ridge development concepts proposed by Hine (Sedimentology 1979; 26: 333-351), and all three stages were observed during only three tidal cycles. The main factors controlling this fast transformation were the gentle slope of the cross-shore profile, rapid water level translation rates, substantial swash overtopping of the berm, and low infiltration rates. Despite the onshore migration of intertidal bars and berm formation, no net foreshore accretion took place during the field campaign. This was largely due to the formation of rip channels with strong rip currents cutting through the intertidal bars and the berm, which acted as a sediment drain in the profile.
Earth Surface Processes and Landforms, 2008
ABSTRACT The formation of an inner nearshore bar was observed during a high-energy event at the s... more ABSTRACT The formation of an inner nearshore bar was observed during a high-energy event at the sandy beach of Vejers, Denmark. The bar accreted in situ during surf zone conditions and the growth of the bar was associated with the development of a trough landward of the bar. Measurements of hydrodynamics and sediment fluxes were obtained from electromagnetic current meters and optical backscatter sensors. These process measurements showed that a divergence in sediment transport occurred at the location of the developing trough, and observed gradients in cross-shore net sediment flux were consistent with the morphological development. The main cause for the flux gradients were cross-shore gradients in offshore-directed mean current (undertow) speed which depended upon local relative wave height and local bed slope. Copyright © 2007 John Wiley & Sons, Ltd.
Continental Shelf Research, 2010
Long-term, net offshore bar migration is a common occurrence on many multiple-barred beaches. The... more Long-term, net offshore bar migration is a common occurrence on many multiple-barred beaches. The first stage of the process involves the generation of a longshore bar close to the shoreline that oscillates about a mean position for some time, followed by a stage of net offshore migration across the upper shoreface, and finally a stage of decaying bar form through loss of sediment volume at the outer boundary of the upper shoreface. The phenomenon has been previously documented in the Netherlands, the USA, the Canadian Great Lakes, and in New Zealand, but our present understanding of the morphodynamic processes and sediment transport pathways involved in bar decay is limited. In this paper, long-term, net offshore bar migration is investigated at Vejers Beach, located on the North Sea coast of Denmark where offshore bar migration rates are of the order of 45-55 m a À 1 . A wave height transformation model confirmed that the decay of the outer bar results in increased wave heights and undertow speeds at the more landward bar potentially causing this bar to speed up its offshore migration. The causes for outer bar decay were investigated through field measurements of sediment transport at the decaying bar and at a position further seaward on the lower shoreface. The measurements showed that a cross-shore transport convergence exists between the bar and the lower shoreface and that the loss of sediment involved in bar decay is associated with a longshore directed transport by non-surf zone processes. At Vejers, and possibly elsewhere, the net offshore migration of bars and the subsequent loss of sand during bar decay is an important part of the beach and shoreface sediment budget.
Continental Shelf Research, 1995
Continental Shelf Research, 2003
... Troels Aagaard Corresponding Author Contact Information , E-mail The Corresponding Author , a... more ... Troels Aagaard Corresponding Author Contact Information , E-mail The Corresponding Author , a and Karin R. Bryan b , 1. ... A few studies from enclosed seas or lakes have provided some indications of frequency selection ( [Aagaard (1990]; [Bauer and Greenwood (1990]). ...
G. Masselink, T. Aagaard and A. Kroon, 2011. Destruction of intertidal bar morphology during a su... more G. Masselink, T. Aagaard and A. Kroon, 2011. Destruction of intertidal bar morphology during a summer storm surge event: Example of positive morphodynamic feedback.