Morpho-hydrodynamics of a highly altered tidally-dominated bay

Resumen

The morphology of many tidal inlets and estuaries is characterized by a complex pattern of channels and sandy shoals, both in space and time. This dynamic behaviour is caused by the interaction between water motion and sediment transport. Observations in many study areas clearly indicate that morphological behaviour is sensitive to changes in external conditions caused e.g. by sea level rise and/or human activities (Van der Spek, 1997; Schuttelaars and de Swart, 2000). In this Thesis, hydrodynamic and human activities influences on morphology are addressed in the Bay of Cádiz, an estuary located in the south–west of Spain. The Bay of Cádiz is a highly altered embayment in which socio-economical develpments and ecological interests conflict. Shipping requires the stability of the navigation channel, maintained with periodical dredgings. Further modifications on the morphology of this area are marshes occupation and the construction of a new bridge to favor the road connectivity between the city of Cádiz and mainland. These usually confronts with the natural status of the ecosystem and the activities derived from it. This work studies both through observations and numerical simulations the impact of some of these modifications on the morpho-hydrodynamics of the Bay of Cádiz. The Bay of Cádiz is a semidiurnal, mesotidal, and low-inflow estuary located in the SW of the Iberian Peninsula, facing towards the Gulf of Cádiz (Atlantic Ocean). The Bay of Cádiz is dynamically comprised of three areas with a total extension of 140km2: a deeper outer area connected to the open sea; a shallower inner area, which harbors most of the remaining marshes; and a central sector, the Puntales Channel, that connects the inner and outer areas, characterized by irregular boundaries due to urban developments and port infrastructures. This current overall morphology is the result of the natural evolution induced by past geological events, climatic agents, biological factors and human activities. Recently, since 2012, the morphology of the Bay of Cádiz has changed again as a result of human activities. “La Pepa” Bridge was completed in 09/24/2015. It is one of the longest (5km) and highest (69m over mean sea level) bridges of Europe, and crosses the Puntales Channel connecting the city of Cádiz with the Peninsula. A new container terminal (which increase 22.5% the port surface) is under construction at the Port of Cádiz; when it is done, 3.6
106m3 of sediment will be dredged to shift and deepen the current navigation channel. The impacts of these interventions on the water and sediment exchange in the whole estuarine area are assessed in this Thesis. The methodology followed to reach this objective is twofold. Firstly, a comprehensive field survey was carried out with a total of 10 instruments deployed at 7 stations from December 22, 2011 to April 18, 2012. Measurements of sea level, currents, water quality, and wind were used to evaluate water circulation and sediment transport within the bay and, in particular, the water exchange between the inner and outer basins on intratidal and subtidal/morphodynamics time scales. Secondly, the field observations and its analysis are supported and complemented by the results of two numerical models implemented, calibrated and validated within the Bay of Cádiz and inner shelf. Modeling scenarios are devised to assess the consequences of current and future interventions within the Bay, namely, the construction of the new port terminal, the deepening of the navigation channel, and the construction of the new bridge. The models are (1) the DELFT3D model, which simulates hydrodynamic flow and sediment transport in three dimensions, suitable for simulating the overall circulation of the Bay (Lin et al., 1998, 2002), and (2) the model developed by Carniello et al. (2005), which combines wind waves with tidal fluxes in a tidal basin. The Carniello model is more convenient in shallower, intertidal areas, and irregular basins (e.g. tidal creeks). Results indicate that, at intra-tidal time scale, the estuary is tidally driven and dynamically short. The water levels and currents are in quadrature. The amount of dissipated tidal energy destabilizes the water column, resulting in a weakly stratified system. This effect is more significant at the constriction, where, the change of the bottom slope apparently induces cyclonic rotation in the semidiurnal tidal ellipse. At the subtidal scale, barotropic subtidal flows were evaluated and analyzed. The circulation patterns of the inner (seasonally hypersaline) and outer portions operate almost independently. The observed residual water volume exchange between the inner and outer basins does not exceed 2
107m3. The residual exchange is largely wind-driven. For winds over 8m=s, the cross-correlation between the wind and residual volumes attains values 0.70, whereas lower cross-correlations are found for weaker winds. The capability of the bay to exchange water and transport sediment between the inner and outer bay deteriorates after the deepening of the navigation channel and the construction of the new port terminal and the new bridge. This may have an impact on the ecological status of the bay. The influence of the dredging and the new terminal are concentrated at the entrance to the central sector of the bay and close to the channel. The dredging would increase siltation in the shallower areas close to the new channel, which subsequently reduce the amount of sediment input into the basins. The bridge mostly affect the Puntales Channel and the inner bay. Due to the presence of the new bridge, the difference of tidal elevation between the inside and outside water levels may increase. The hydrodynamic changes and their effect on sediment erosion, deposition and transport may cause far-field geomorphological changes away from the dredge location, including the potential erosion of intertidal areas. The most changes in the erosion/deposition patterns are found in the area with strong bottom frictions and tidal asymmetries. At the scales analyzed in this work, tidal currents, the spring-neap cycles cause suspended sediment concentration to vary in time and space. Wind forcing is also found to be important. It is capable of entrain and transport sand and mud from the inner area, specially northward winds. The hydro- and morphodynamic changes show that the effects of future interventions are far from damped out, and it will take many decades before a new equilibrium will be reached.