Coastal zone geomorphologic interactions: natural versus human-induced driving factors

J. Pinho1, H. Granja1,3, L. Bastos2,3, J. Gonçalves2,3, R. Henriques1, A. Bio3, J. Mendes1, A. Magalhães2

1 University of Minho 2

University of Porto 3 CIIMAR – Interdisciplinary Centre of Marine and Environmental Research

MORPHODYNAMICS OF TWO PORTUGUESE RIVER MOUTHS: DOURO AND CÁVADO

University of MinhoUniversity of Minho

AbstractSediment dynamics in the vicinity of river mouths reveal complex patterns due to the joined effect of river discharges, tides and waves. These phenomena underlie current mathematical and simulation models, which constitute powerful tools that can be used as a complement to monitoring field work for the characterization of morphodynamical patterns in these complex coastal environments.The present work discusses and synthesizes results from an integrated approach, involving monitoring field surveys and the application of mathematical models, to describe the main features of these two coastal inlets and adjacent costal zones, considering the 15 km from the Neiva River to Apúlia for the Cávado river and the 22 km from Crestuma dam to Porto for the Douro river. The influence of river and tidal characteristics are discussed for both cases.

MethodsSediment erosion and deposition studies must be based on a fully dynamic model description. In the absence of stratification, the RMA2/SED2D software packages (WES-HL, 1996, WES-HL, 2000) are excellent tools to predict the sand transport patterns. If vertical stratification prevails, a two-dimensional vertical modelling approach or a three-dimensional approach must be applied. The two-dimensional hydrodynamic models (Figs 3 and 4) in the horizontal plane (2DH) were implemented using the RMA2 software that is based on the finite element method (WES-HL, 1996). This model can be applied for situations where the water flow does not exhibit a significant vertical variation, as was mentioned before, and for situations that are not highly dependent on the waves action. It solves the depth-integrated equations of fluid mass and momentum conservation in two horizontal directions. The sediment transport model, SED2D (WES-HL, 2000) is applied to simulate the sediment transport in aquatic environments. This model can be applied to clay or sand bed sediments where flow velocities can be considered two-dimensional in the horizontal plane (i.e., velocity and direction can be satisfactorily represented as a depth-averaged velocity). It is useful for both deposition and erosion studies. The model treats two categories of sediment: noncohesive (sand) and cohesive (clay). Both clay and sand may be analyzed, but the model considers a single, effective grain size during each simulation. Therefore, a separate model run is required for each effective grain size. Fall velocity must be prescribed along with the water surface elevations, x-velocity, y-velocity, diffusion coefficients, bed density, critical shear stresses for erosion, erosion rate constants, and critical shear stress for deposition. There are four major computations, using a finite element based numerical method: convection-diffusion governing equation; bed shear stress calculation; bed source/sink term; and bed discretization.

Study areasThe Cávado estuary (Fig.1) inlet is situated near Esposende, Portugal and the Douro Estuary inlet (Fig. 2) about 50 km south of it, next to Porto. During the last years, inland beach migration, sand loss and cliff retreat have been recurrent in this North Portuguese, Atlantic coastal areas, which can be considered to be of the mixed energy and wave-dominated type. Local tides are mesotidal and semidiurnal, with a maximum equinoctial spring tide high-water level of about 3.9 m and 4.0 m and a minimum low-water level of 0.2 m and 0.1 m, for the Esposende and Porto regions, respectively. Mean spring tides are about 3.5 m.

Results River CávadoThe maximum current velocities occur at the river channel entrance. Depth average velocity at this location varies from 1.40 ms1 to 0.40 ms1 during flood tide and from 3.00 ms1 to 1.00 ms1 during ebb tide (Fig. 6). Results for sediment deposition after 12.5 hours and 25 hours are presented in Fig. 7. For the spring tides simulation, two erosion zones are in evidence: one located south of the Esposende breakwater and the other located at the right of the sandy spit.

Results River DouroThe hydrodynamics model results show that the currents in the river mouth are strongly influenced by the river flow regime and to a lesser extent by the tide action (Fig. 8). Before the construction of the breakwater, the maximum flow velocity occurs only near the sand spit, and now there is also a high flow velocities area in the constriction between the breakwater extremity and the north jetty. Concerning sediment transport (Fig. 9), erosion prevails in areas of maximum current velocities as was expected and sediments deposition continues to be prevalent in wither areas of the estuary and downstream the new jetty, in an area more distant from the coastline.

Figure 1. River Cávado inlet Figure 2. River Douro inletFigure 6. River Cávado hydrodynamic results

Figure 7. River Cávado sediment transport results

Figure 3. River Cávado numerical model

Figure 4. River Douro numerical model

2,3 m/s 2,3 m/s

6,0 m/s6,0 m/s

After breakwater construction

Before breakwater construction

Sediments deposition

After breakwater construction

Figure 8. River Douro hydrodynamic results

ReferencesGranja, H., Bastos, L., Pinho, J.L.S., Gonçalves, J., Henriques, R., Bio, A.,

Magalhães, A. (in press) Small Harbours Risks: Lowering for Fishery and Increasing Erosion. The Case of Portinho da Aguda (NW Portugal). Proceedings of Littoral 2010, 2123 Setembro 2010, Londres, Reino Unido.

WES-HL (1996) Users Guide To RMA2 Version 4.3. US Army Corps of Engineers - Waterways Experiment Station Hydraulics Laboratory, Vicksburg, USA

WES-HL (2000) Users Guide To SED2D Version 4.5. US Army Corps of Engineers - Waterways Experiment Station Hydraulics Laboratory, Vicksburg, USA

AcknowledgementThe authors gratefully acknowledge to Porto de Leixões, SA the bathymetric data used in the models construction.

Modelling is being complemented by different field monitoring techniques (Fig. 5) in order to validate and improve the overall analysis capabilities of the sediment transport complex patterns (Granja et al., in press).

Bathymetry

Situation after breakwater construction

Height (m)

Aerial photographs Topographical surveys Bathymetric profiles Sand samples and profiles Field observations

Figure 5. Monitoring techniques

Figure 9. River Douro sediment transport results

After breakwater construction

Before breakwater constructionAfter breakwater construction