Papers by Pushpa Dissanayake
Rocky coastal zones, including beaches constrained by rocky headlands, represent approximately 80... more Rocky coastal zones, including beaches constrained by rocky headlands, represent approximately 80% of the world's coastline. In such coastal environments, with complex topo-bathymetry, the offshore wave conditions can be very different compared to the conditions at the wave breaking point due to refraction and diffraction processes that play an important role in nearshore wave transformation. The obliquity of waves at the breaking point controls the direction and intensity of the longshore drift and the induced sediment transport. In addition, on macrotidal beaches, tides induce water level variations that strongly impact nearshore current circulation. The present study analyses longshore currents in the surf zone of a sandy beach in a complex coastal macrotidal environment: Vougot beach (France). Wave refraction is shown to invert the predicted longshore current (wave direction changed by almost 90°). In the surf zone, the current is first driven by the tidal current. The wave climate influences the magnitude: the westward longshore current reaches 0.6 m/s during an energetic event and is proportional to the significant wave height. Commonly used formula to predict wave-generated longshore currents failed to reproduce observations. Numerical modelling is in progress to test the influence of tides and waves on surf zone currents.
Marine Geology, 2018
Climate change (CC) is likely to affect the thousands of bar-built or barrier estuaries (here ref... more Climate change (CC) is likely to affect the thousands of bar-built or barrier estuaries (here referred to as Small tidal inlets-STIs) around the world. Any such CC impacts on the stability of STIs, which governs the dynamics of STIs as well as that of the inlet-adjacent coastline, can result in significant socioeconomic consequences due to the heavy human utilisation of these systems and their surrounds. This article demonstrates the application of a process based snapshot modelling approach, using the coastal morphodynamic model Delft3D, to 3 case study sites representing the 3 main STI types; Permanently open, locationally stable inlets (Type 1), Permanently open, alongshore migrating inlets (Type 2) and Seasonally/Intermittently open, locationally stable inlets (Type 3). The 3 case study sites (Negombo lagoon-Type 1, Kalutara lagoon-Type 2, and Maha Oya river-Type 3) are all located along the southwest coast of Sri Lanka. After successful hydrodynamic and morphodynamic model validation at the 3 case study sites, CC impact assessment are undertaken for a high end greenhouse gas emission scenario. Future CC modified wave and riverflow conditions are derived from a regional scale application of spectral wave models (WaveWatch III and SWAN) and catchment scale applications of a hydrologic model (CLSM) respectively, both of which are forced with IPCC Global Climate Model output dynamically downscaled to ~ 50 km resolution over the study area with the stretched grid Conformal Cubic Atmospheric Model CCAM. Results show that while all 3 case study STIs will experience significant CC driven variations in their level of stability, none of them will change Type by the year 2100. Specifically, the level of stability of the Type 1 inlet will decrease from 'Good' to 'Fair to poor' by 2100, while the level of (locational) stability of the Type 2 inlet will also decrease with a doubling of the annual migration distance. Conversely, the stability of the Type 3 inlet will increase, with the time till inlet closure increasing by ~75%. The main contributor to the overall CC effect on the stability of all 3 STIs is CC driven variations in wave conditions and resulting changes in longshore sediment transport, not Sea level rise as commonly believed.
Coastal Engineering, Dec 1, 2013
ABSTRACT Potential physical impacts of an anthropogenic effect on a tidal basin evolution are inv... more ABSTRACT Potential physical impacts of an anthropogenic effect on a tidal basin evolution are investigated applying the Delft3D model suite under both tidal and wave boundary forcings. Study area is based on a peninsula construction of the Ley Bay in the East Frisian Wadden Sea. Model simulation spans from 1975 to 1990 in two stages of which the second stage begins with the implemented peninsula on the 1984 predicted morphology. The model bed consists of initially distributed three sediment fractions. Sensitivity of the Ley Bay evolution is analysed under three different sediment transport formulations: 1) Van Rijn, 1993 (VR93), 2) Soulsby, 1997 (SVR) and 3) Van Rijn et al., 2004 (VR04). Offshore tides and waves are transformed up to the model boundaries via a nested modelling approach and a statistically derived highly schematised wave climate is adopted in the simulations. Predicted morphologies indicate lower agreement with the measured morphology due to including very sparse data. Despite this discrepancy, they reproduce the major changes in the Ley Bay caused by the peninsula construction while each formula results in a slightly different channel/shoal pattern. Predicted evolution under the SVR shows the strongest sediment exporting system and therefore the lowest agreement with the 1990 measured morphology. Both VR93 and VR04 formulas resulted in marginal exporting systems and more or less similar morphologies. In fact, only the VR04 prediction indicates a fair agreement with the 1990 data. Temporal evolution under the VR04 shows concentrated velocity patterns at the bay entrance and in the eastward bay channel resulting in the development of this channel and sedimentation in the southern part of the bay as found in the data.
Proceedings of ... Conference on Coastal Engineering, Oct 25, 2012
A numerical approach was undertaken to investigate an anthropogenic effect (i.e., construction of... more A numerical approach was undertaken to investigate an anthropogenic effect (i.e., construction of a peninsula in 1984) on the tidal basin evolution of the Ley Bay in the East Frisian Wadden Sea. Coupled hydro-morphodynamic model runs span from 1975 to 1990 applying both tidal and wave boundary forcings with a multiple-sediment bed. The offshore tidal force was transformed to the study area applying a cascade modeling approach while a highly schematised wave climate was adopted in this simulation. Two bathymetries (i.e., 1975 and 1990) were available for this study and therefore the peninsula was implemented based on the 1984 predicted bed. Resulting morphodynamic evolution indicates that the deeper areas (channels) are well reproduced by the model rather than the shallow areas (inter-tidal areas). In addition to the peninsula construction, the human interference by dredging and dumping appears to have influenced the Ley Bay evolution. Due to the lack of data availability, such information was not incorporated in this study. Comparisons of predicted and measured 1990 topographies in terms of qualitative and quantitative parameters show that the predicted bathymetry sufficiently resembles with the data. Application of a highly schematised wave climate affects on the bed evolution. Further investigation on the wave schematization could help to optimize the model prediction. Ongoing work uses this model setup in order to forecast the Ley Bay evolution under future sea level rise scenarios.
Coastal Engineering Proceedings
The Schleswig-Holstein (SH) Baltic Sea coast consists of different morphological features (e.g., ... more The Schleswig-Holstein (SH) Baltic Sea coast consists of different morphological features (e.g., bars, beaches, dunes, cliffs), which are continuously shaped by various along- and cross-shore sediment transport patterns driven by local currents generated from tides, seiches, winds and waves (Soomere et al., 2012). Understanding local scale sediment transport is of utmost importance to manage the scarce coastal sediments and to identify suitable mitigation strategies against extreme forcing scenarios (Hofstede and Hamann, 2022). This investigation was carried out at the exemplary cliff coast at Stohl, at which the glacial till cliff (maximum height about 20 m) is the source of nearshore sediments (Averes et al., 2021). The main objective of this study is to investigate the relative importance of different forcing conditions on local sediment transport pathways.
Presented at: Coastal Structures Conference 2019, Hannover, Germany, September 30th – October 2nd... more Presented at: Coastal Structures Conference 2019, Hannover, Germany, September 30th – October 2nd, 2019
Coastal Sediments 2019, 2019
Die Sedimente der Flachwasserzone von Seen unterliegen einer hohen Dynamik bestehend aus der Resu... more Die Sedimente der Flachwasserzone von Seen unterliegen einer hohen Dynamik bestehend aus der Resuspension (Remobilisierung) der Decksedimente durch Oberflächenwellen, dem Transport durch beckenweite Strömungen und der erneuten Deposition nach der Verfrachtung (DITTRICH & WESTRICH, 1988; HARFF et al., 1999; VAN RIJN, 1993). Oberflächenwellen als hydrodynamisch bedeutendster Prozess in der Flachwasserzone sind nicht nur in der Lage die abgelagerten Sedimente zu resuspendieren, sondern diese in Suspension zu halten und somit die Umlagerung durch Transportprozesse (Dispersion, Advektion) zu befördern (HOFMANN et al., 2011). Ort, Zeit und Intensität der Resuspension werden durch das Zusammenwirken von Oberflächenwellen mit den saisonalen Wasserspiegelschwankungen bestimmt (HOFMANN et al., 2008a). Der Transport und die spätere Deposition der Sedimente sind an die beckenweiten, aber auch an die lokalen und kleinskaligen Strömungsmuster und -geschwindigkeiten gekoppelt (ALLEN, 1985).
Journal of Marine Science and Engineering, 2022
Coastal storm erosion can lead to episodic morphological changes and hinterland flooding that req... more Coastal storm erosion can lead to episodic morphological changes and hinterland flooding that requires sustainable management. An accurate estimation of storm erosion can determine the success of hazard mitigation strategies. Two morphological models, Delft3D and XBeach, were applied separately to a stormy period with “Roller” and “No Roller” wave dynamics activated, to estimate erosion of the beach and dune system on the Sylt island. This is the first numerical impact assessment of roller dynamics on coastal erosion using the two models. The choice of model had more impact on the hydrodynamic and morphological predictions than the option to include or omit roller dynamics. Agreement between measured and simulated waves was higher in Delft3D (R2 > 0.90 and RMSE < 0.15 m) than XBeach. Storm erosion in both models had the highest sensitivity to the roller parameter Beta. Both models predicted a similar storm erosion pattern along the coast, albeit different magnitudes. It is fou...
Coastal Sediments 2019, 2019
Die Sedimente der Flachwasserzone von Seen unterliegen einer hohen Dynamik bestehend aus der Resu... more Die Sedimente der Flachwasserzone von Seen unterliegen einer hohen Dynamik bestehend aus der Resuspension (Remobilisierung) der Decksedimente durch Oberflächenwellen, dem Transport durch beckenweite Strömungen und der erneuten Deposition nach der Verfrachtung (DITTRICH & WESTRICH, 1988; HARFF et al., 1999; VAN RIJN, 1993). Oberflächenwellen als hydrodynamisch bedeutendster Prozess in der Flachwasserzone sind nicht nur in der Lage die abgelagerten Sedimente zu resuspendieren, sondern diese in Suspension zu halten und somit die Umlagerung durch Transportprozesse (Dispersion, Advektion) zu befördern (HOFMANN et al., 2011). Ort, Zeit und Intensität der Resuspension werden durch das Zusammenwirken von Oberflächenwellen mit den saisonalen Wasserspiegelschwankungen bestimmt (HOFMANN et al., 2008a). Der Transport und die spätere Deposition der Sedimente sind an die beckenweiten, aber auch an die lokalen und kleinskaligen Strömungsmuster und -geschwindigkeiten gekoppelt (ALLEN, 1985).
Inland Waters, 2019
Model skills of two 3D hydrodynamic models, ELCOM and Delft3D, were assessed by simulating intern... more Model skills of two 3D hydrodynamic models, ELCOM and Delft3D, were assessed by simulating internal seiches and surface currents and comparing the results with ଏeld data in Upper Lake Constance, Germany. Both models were set up using the same horizontal and vertical discretisation and forced by a horizontally resolved wind ଏeld. Model simulations were carried out during a stable stratiଏcation period in summer. Oscillations of the simulated and the measured isotherms in Lake Überlingen (north basin of Upper Lake Constance) indicated the existence of a Kelvin-type wave with a period of 84 h. The major wave pattern in period and amplitude was consistent between the simulations and the measurements, whereas discrepancies appeared in the phase match. The measured and the simulated vertical velocity structure in the middle of the main basin of Upper Lake Constance showed occurrence of a Poincaré-type wave with a 14 h period. Major oscillations of the velocity structure were captured by both models, and the agreement between the simulations and the measurements was higher in deep layers than in near-surface layers. Model prediction of the near-surface velocities as well as the surface circulation pattern was more accurate under high current velocities during strong winds than under weak winds. This study concluded that both ELCOM and Delft3D were signiଏcantly skilled at simulating internal wave motions and surface currents, and there was no signiଏcant diଏerence in reproducing the measured lake dynamics between ELCOM and Delft3D.
Inland Waters, 2019
Model skills of two 3D hydrodynamic models, ELCOM and Delft3D, were assessed by simulating intern... more Model skills of two 3D hydrodynamic models, ELCOM and Delft3D, were assessed by simulating internal seiches and surface currents and comparing the results with ଏeld data in Upper Lake Constance, Germany. Both models were set up using the same horizontal and vertical discretisation and forced by a horizontally resolved wind ଏeld. Model simulations were carried out during a stable stratiଏcation period in summer. Oscillations of the simulated and the measured isotherms in Lake Überlingen (north basin of Upper Lake Constance) indicated the existence of a Kelvin-type wave with a period of 84 h. The major wave pattern in period and amplitude was consistent between the simulations and the measurements, whereas discrepancies appeared in the phase match. The measured and the simulated vertical velocity structure in the middle of the main basin of Upper Lake Constance showed occurrence of a Poincaré-type wave with a 14 h period. Major oscillations of the velocity structure were captured by both models, and the agreement between the simulations and the measurements was higher in deep layers than in near-surface layers. Model prediction of the near-surface velocities as well as the surface circulation pattern was more accurate under high current velocities during strong winds than under weak winds. This study concluded that both ELCOM and Delft3D were signiଏcantly skilled at simulating internal wave motions and surface currents, and there was no signiଏcant diଏerence in reproducing the measured lake dynamics between ELCOM and Delft3D.
ABSTRACT The expected change of global warming will create secondary effects being an unknown cha... more ABSTRACT The expected change of global warming will create secondary effects being an unknown challenge for coastal protection: Accelerated sea-level rise, stronger storms create both higher set-ups of storm surges and stronger waves. Moreover, the adaption of intertidal areas in the coastal areas might be delayed providing larger water depths in front of coastal structures allowing again the occurrence of stronger waves. Necessarily the question, if and when how coastal areas could remain safe against the sea is of increasing importance. Alternatives to the presently exercised keeping the line of protection are discussed with reference to historical experience. INTRODUCTION The expected changes in global climate and the consequently resulting acceleration of sea-level rise require a thorough reevaluation of coastal protection strategies in many parts of the world.
Comment on nhess-2021-210 Pushpa Dissanayake (Referee) Referee comment on "The role of morphodyna... more Comment on nhess-2021-210 Pushpa Dissanayake (Referee) Referee comment on "The role of morphodynamics in predicting coastal flooding from storms on a dissipative beach with sea level rise conditions" by Jairo E. Cueto et al., Nat.
Journal of Marine Science and Engineering
Coastal systems are highly dynamic morphological environments due to erosion and sedimentation at... more Coastal systems are highly dynamic morphological environments due to erosion and sedimentation at different spatio-temporal scales as a result of natural forcing [...]
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Papers by Pushpa Dissanayake