Morphodynamic Evolution of Dredged Sandpits

2003

Abstract: Within the framework of the SANDPIT project a number of benchmarking datasets were defined. These tests are used to establish the efficacy of morphodynamic models in predicting the behavior of large-scale mining pits and so to contribute to improve coastal zone management. This paper discusses the results of a verification of WL | Delft Hydraulics’ Delft3D-model using field data collected by Rijkswaterstaat, in and around an offshore pit in the North Sea some 10 km off the Dutch coast near Hoek van Holland. The main focus was on assessing the quality of hydrodynamic and morphological model predictions made in depth-averaged and 3-dimensional mode. The comparison has shown that the model is able to give satisfactory predictions of the depth-averaged velocities inside and outside the pit. Moreover, the 3D model provided very accurate predictions of the longshore and cross-shore vertical velocity profiles. Although depth-averaged velocities predicted in 2DH and 3D mode were c...

Within the framework of the SANDPIT project a number of benchmarking datasets were defined. These tests are used to establish the efficacy of morphodynamic models in predicting the behavior of large-scale mining pits and so to contribute to improve coastal zone management. This paper discusses the results of a verification of WL | Delft Hydraulics' Delft3D-model using field data collected by Rijkswaterstaat, in and around an offshore pit in the North Sea some 10 km off the Dutch coast near Hoek van Holland. The main focus was on assessing the quality of hydrodynamic and morphological model predictions made in depth-averaged and 3-dimensional mode. The comparison has shown that the model is able to give satisfactory predictions of the depth-averaged velocities inside and outside the pit. Moreover, the 3D model provided very accurate predictions of the longshore and cross-shore vertical velocity profiles. Although depth-averaged velocities predicted in 2DH and 3D mode were comparable, the morphological predictions showed significant differences that are primarily ascribed to the complex 3-dimensional velocities that occur.

Coastal Engineering, 2014

Offshore sand and gravel extraction for aggregates and beach nourishment is an important economic activity and has been a common practice in various countries worldwide for many years. The evolution of a sandpit, in particular its migration and rate of replenishment, depends strongly on the type of sediments involved, and on the physical and hydrodynamic characteristics of the surrounding area. In order to fully assess the associated impacts on local ecosystems and on the neighboring coastline morphology it is essential to make accurate predictions of the excavation recovery times. For this purpose it is fundamental to investigate areas where there is an adequate observational control of the evolution of the sandpit, prior and after the excavation, to properly calibrate existing numerical models with observations and fully evaluate their prediction adequacy. The present work investigates the evolution of an offshore sandpit located off Vale do Lobo, Algarve, Portugal, within a time span of four years (2006-2010), based on 4 bathymetric surveys, prior to and after the dredging operations, complemented with the analysis of wave data and numerical modeling simulations. The bathymetric data were used to evaluate the morpho-sedimentary evolution and to calculate the sediment volume changes. The results show an infill of approximately 17% of the initial exploration pit in the first 4 years, with an overall smoothing of the initial excavation bottom topography. Observations combined with modeling results demonstrate that the pit evolution depends mainly on storm events, since it is essentially during these periods that there is a significant sediment movement at the site water depth. Based on (1) the predicted number of stormy days for the forthcoming years, assuming that (2) the yearly average of such events in the past 57 years is representative, and considering (3) a decrease of the sandpit recovery rate in time, predicted by models and observations, it was possible to estimate that the Vale do Lobo sandpit recovery period is of ca. 38 years for its full, or near full, replenishment.

Journal of Coastal Research, 2007

Coastal erosion and flooding are largely determined by extreme wave action and nearshore wave heights are controlled through energy dissipation on offshore sandbanks. East Anglia is the area of study here. The finite-element codes for tidal flows (TELEMAC-2D), for wave propagation (TOMAWAC) and for morphodynamics (SISYPHE), are used for this study. Preliminary results are presented to show the effect of waves, currents and sea level rise (SLR) on morphodynamics for a period of one year with an initial bathymetry of the present day. Offshore wave climate is driven by onshore wind data from 2002, as an example. Annual residual sediment fluxes are computed to show typical sediment pathways for the domain. Alongshore sediment transport due to waves, from the CERC formula, is qualitatively similar to that computed.

Journal of Coastal Research

This work is a contribution to the beach morphodynamic studies which are the essential key to improve the knowledge of the beach system evolution. This report wants to contribute with a study of the exposed Louro Beach (NW Iberian Peninsula). Several topographic surveys, grain size tracer trend experiments and wave and current numerical models (OLUCA-SP y COPLA-SP, IH-Cantabria) have been applied. Louro beach is an intermediate type (MASSELINK and SHORT, 1993 Conceptual Beach Model) belonging to a sand barrier-lagoon system. Three zones can be differentiated according to their morphologic and dynamic patterns. A conceptual model of beach morphodynamic behaviour has been developed in relation to different wave-approach directions. With NW waves, typical from summer conditions, the beach is dominated by a clockwise circulation inducing a southern longshore sediment transport trend. This is a low energetic situation favourable to beach accretion conditions. However, with SW waves, typi...

Environmental Fluid Mechanics, 2013

We present a numerical model based on the hydro-morphodynamical coupling to study coastal sandbar migration. In order to improve both nonlinear and dispersive wave processes in relatively shallow water, we developed a finite element model based on the Legendre polynomials and on the Extended Boussinesq model. This model reproduces the propagation of wave trains with a high degree of accuracy on a greater range of depths than the standard Boussinesq models. We also implemented the Total Variation Diminishing schemes to improve the quality of the computed hydrodynamic fields, especially in areas where sharp flow gradients occurred. The coupled morpho-hydrodynamical model is then used to simulate the migration of real sandbars observed at Rousty beach (Mediterranean French coast). For verification the model results are compared with field measurements obtained from a small-scale field campaign carried out over two years at Rousty beach, and the results of this comparison are thoroughly discussed and analyzed.

Estuarine, Coastal and Shelf Science, 2001

A linear stability analysis has been carried out to investigate the influence of parameters like tidal current, depth and Coriolis force on sand bank formation. The applied analytical model allowed the computation of the horizontal wave number corresponding to the fastest initial growth rate of seabed form perturbations. Sandbank wavelength and the angle between the crest of the sandbank and tidal current direction were also obtained. The experiments show an exponential decay of this angle for increasing values of tidal currents. At a particular depth, the sand bank wavelength grows almost linearly with the velocity. We compared these theoretical results with observations of sandbank features in several geographic areas (Persian Gulf, the Gulf of Korea, Gulf of California and the North Sea). The results show that all observed sand bank wavelengths and angles are in very close agreement with the calculated theoretical curves.

The nearshore environment is a very dynamic and complex system with interaction of waves, currents and sediments movement on the inner continental shelf. Some of the most common morphological features are the cross-shore sand-bars. The present work investigate the evolution of an offshore sandbar located off Anjos cove, Rio de Janeiro, within a time span of 55 years, based on 6 bathymetric surveys and the sediment distribution analysis. The bathymetric data were used to evaluate the morphodynamic evolution and to calculate the sediment volume changes. The morphology of studied sandbar area extents up to 2002 m, with a length of 1052 m and average depth between-1.0 to-2.0 m, and usually increase both volume and extension in the southwest direction. The difference in area and volume during the studied period was 310,851 m 2 and 1157,772 m 3 respectively. These results show that the cross-shore sandbar migrated preferable shoreward during moderate to severe storm conditions from the northeast quadrant. The northeast waves are responsible for the sediment transportation and deposition in the sandbar environment. These conditions explain the low rate, 1.72 m/years, of migration in the cross-shore sandbar studied. This rate is lower than usual rates reported to mic-rotidal environments. The morphological feature studied has the same direction northeast-southwest of the dunefields located in the region of Arraial do Cabo and Cabo Frio. Along the years, if the same deposition condition is preserved, it will be expected the formation of a barrier island in the sandbar area.

Environmental Problems in Coastal Regions VI, 2006

Ondarreta Beach is one of the most emblematic and visited beaches in the Basque Country. This beach is part of La Concha bay, with Santa Clara Island in the middle of the bay and a semi submerged dike and reef in front of it. The combination of this morphology together with the tidal range in the area (max. springs 4.6m) results in a very particular wave climate. The beach contains different strata of sand, natural rocks and the remains of artificial structures. Therefore, rocks sometimes appear at the surface, depending on the changes of the beach profile. This situation harms the normal use of the beach and can be a risk for the users. During recent years this process has increased and, especially in the summer, a big part of the intertidal profile of the West part of the beach shows a gravely and rocky profile. In 2003, the town council of San Sebastian entrusted AZTI Foundation with the study of this beach. For this purpose AZTI planned the data analysis and the morphodynamic study of the evolution of the beach in three different time scales, in order to clarify whether the recent problems at the beach are an increasing erosive process, or part of a natural cycle. The three different time scales of analysis of morphodynamic processes taking place in this beach are:-Short term scale: wave data measurement and analysis, modeling of wave, current and sediment transport.-Medium term scale: analysis of seasonal equilibrium shape and profile.-Long term scale: analysis of morphodynamic evolution of the whole system (the bay). This analysis has contributed to the management of the beach, implying artificial seasonal sand movements, trying to encompass the natural processes and shoreline protection with the recreational use of the beach.