Sandra Plecha

Establishing the wave climate influence on the morphodynamics of a coastal lagoon inlet

The morphologic changes in estuaries and coastal lagoons are very complex and constitute a challenging task in coastal research. The bathymetric changes result from the combined action of tides, waves, rivers discharge and wind stress in the area of interest. Additionally, an accurate knowledge of the sediment transport is essential to achieve a good morphological characterization. This work establishes the influence of the wave climate on the morphodynamics of the Ria de Aveiro lagoon inlet by analysing the numerical results of the morphodynamic modelling system MORSYS2D. The numerical simulations considered a realistic coupled forcing of tidal currents and waves. The computed sediment fluxes and bathymetric changes are analysed and compared with the erosion and accretion trends obtained from the numerical simulations forced only by tidal currents, in order to establish the wave climate influence. The final bathymetry and the corresponding changes are compared with bathymetric data collected through surveys. It is concluded that: (a) the morphodynamics of the study area is dominated by the wave regime in the lagoon inlet and nearshore areas, while in the inner areas is tidally dominated; and (b) the inclusion of the wave regime forcing constitutes an improvement in order to accurately reproduce the local morphodynamics.

Study of suspended sediment dynamics in a temperate coastal lagoon: Ria de Aveiro (Portugal)

ABSTRACT Plecha, S., Picado, A., Chambel-Leitão, P., Dias, J.M., Vaz, N., 2014. Study of suspended sediment dynamics in a temperate coastal lagoon: Ria de Aveiro (Portugal). In: Green, A.N. and Cooper, J.A.G. (eds.), Proceedings 13th International Coastal Symposium (Durban, South Africa), Journal of Coastal Research, Special Issue No. 70, pp. 604–609, ISSN 0749-0208. Suspended sediment concentrations are simulated at Ria de Aveiro, a lagoon located in a temperate climate area in the northwest of Portugal. The fine-grained suspended sediment concentration is analyzed using the numerical model MOHID (www.mohid.com) and spatial maps of instantaneous and maximum concentration and also temporal variability at specified locations are analyzed in order to characterize the influence of the tide and sea level conditions in the suspended sediment concentrations within the lagoon. The highest suspended sediment concentrations were found in upstream areas during ebb conditions due to the rivers proximity, while the minimum concentrations were observed near the inlet due to the presence of marine water poor in suspended sediments. When a 0.42 m sea-level rise (an estimate of conditions in 2100) is modelled, a decrease in suspended sediment concentration is observed for the overall study area, as result of an increase in the tidal prism.

Sediment Transport Modelling and Morphological Trends at a Tidal Inlet

Currently it is of great concern the morphological changes observed at distinct coastal systems. Frequently, drastic episodes of coastal erosion, threatening the houses built near the shore, and land loss are reported. At coastal lagoons and estuaries, increasing accretion can interfere with the water renewal and therefore with the local ecosystems. At inland harbors, the accretion of inlet and channels can restrict the navigation and consequently the harbor conditions and its management. The sediment transport, identified by patterns of erosion or accretion, can be evaluated by single formulae that compute the bedload and suspended load or by sediment transport models. In the literature are published formulae for bed-load transport of sediments in conditions characteristic of coastal waters, covering current alone, current plus symmetrical waves, current plus asymmetrical waves alone and integrated longshore transport. Due to the fact that sand transport models are often based on semi-empirical equilibrium transport formulae that relate sediment fluxes to physical properties, such as velocity, depth and grain size, it is crucial to perform sensitivity analysis of the formulae used. Pinto et al. (2006) compared four sediment transport formulations considering only the tidal current only: Ackers and White (1973); Engelund and Hansen (1967); van Rijn (1984a,b,c) and Karim and Kennedy (1990). The authors concluded that the van Rijn formula is the most sensitive to basic physical properties. Hence, it should only be used when physical properties are known with precision. The sediment transport modules, coupled with hydrodynamic and wave modules compose the morphodynamic models, resulting in very complex systems emergent in the last years. In the past 30 years, morphodynamic models have been developed (Nicholson et al., 1997). Among them the MIKE21, developed by the Danish Hydraulic Institute DHI (Warren and Bach, 1992) and DELFT3D, by WL|Delft Hydraulics DH (Roelvink and Banning, 1994), are the most popular. Most of the morphodynamic studies are generally related on engineering methods and techniques for coastal defense. The DELFT3D model was applied by several authors, such as Grunnet et al. (2004), Xie et al. (2009) and Tung et al. (2009), between others. Grunnet et al. (2004) applied DELF3D to