Numerical modeling of the impact of the Ancão Inlet relocation (Ria Formosa, Portugal)

Abstract

This work describes the application of hydrodynamic (ELCIRC) and transport (VELA and VELApart) models to the Ria Formosa lagoon (Portugal) to study the impact of the relocation of the Ancão Inlet. Located in the south of Portugal, this lagoon is a mesotidal barrier island system that communicates with the sea through 6 inlets. The Old Ancão Inlet was artificially closed and the New Ancão Inlet was relocated into a westward position. This work investigates the hydrodynamic patterns and the potential pathways of tracers in Ria Formosa in two distinct configurations: before and after the Ancão Inlet relocation. The hydrodynamic model was successfully calibrated and validated against elevation, velocity and inlet discharges data, accurately reproducing the tidal propagation. The inlet relocation increases the magnitude of tidal currents, residual velocities and the tidal prism across the bar, suggesting a better stability. The tracers transport simulations suggest enhanced water exchanges through the Ancão Inlet and smaller residence times in the western part of Ria Formosa with the present configuration. Overall, it is concluded that the Ancão Inlet relocation had a positive contribution towards increasing the water renewal of the western part of the lagoon, thus decreasing its vulnerability to pollution.

Introduction

Coastal lagoons are shallow water bodies partially isolated from the adjacent sea by a sedimentary barrier and connected to it through one or several tidal inlets. These systems usually run parallel to the coastline, in contrast to estuaries that are normally perpendicular to the coast. About 13% of the world's coastline is occupied by wetlands, many of which are lagoons, but in Europe only 5.3% of the coastline (2690 km) falls into this category (Barnes, 1980, Newton and Mudge, 2003).

The Ria Formosa is a barrier lagoon on the southern coast of Portugal (Fig. 1). This lagoon exhibits an extremely complex geometry, with innumerable channels and straits and several inlets. The importance of the existence and persistence of tidal inlets in coastal systems is fundamental for water quality, navigability, and beach/barrier stability. The Ria Formosa is protected by a statute of Natural Reserve and is a place privileged for the reproduction and development of oceanic species and molluscs, with an important economic interest. It also constitutes a valuable regional resource for tourism, fisheries, aquaculture and salt extraction industries, as well as a natural habitat for various species of birds.

Water in inner areas of the lagoon has different temperature and salinity characteristics compared to the inflowing coastal water, in both winter and summer. These differences in temperature and salinity are detectable both at low water and at high water neaps. The tidal currents have their maximum values close to the inlets, decreasing towards the channels heads. Therefore, deterioration of water quality is more likely to occur in the inner areas of the lagoon. The results of this deterioration would negatively affect the tourism, fisheries and the amenity value of the system.

This study focuses on the Ancão Inlet located on the western part of Ria Formosa. This inlet experienced very rapid changes, mostly consisting of a South-Eastern migration at a mean rate of ∼67 m/year. In the nineties, the Ancão Inlet exhibited a very eastward position (past configuration), making it poorly efficient from a hydraulic viewpoint and causing water quality problems (Vila-Concejo et al., 2004). In order to overcome these problems, the local authorities decided to intervene in the system and the inlet was relocated in 1997 to a more westward position (present configuration). The fast and complex dynamics along with its environmental and economical importance has motivated several studies in the Ria Formosa, including: (1) sediment dynamic studies at ocean-exposed beaches and tidal inlets (Vila-Concejo et al., 2004); (2) water quality studies based on water samples (Baptista, 1993, Bebiano, 1995, Newton and Mudge, 2003); (3) numerical modeling studies of bacterial contamination and water circulation in Ria Formosa (Mendonça, 2001, Salles et al., 2005, Martins et al., 2006).

Numerical models are often used to evaluate the potential impact of human modifications on natural water bodies in order to mitigate their consequences (Liu et al., 2005). In this context, the main aim of this work is to study the impact of the relocation of the Ancão Inlet, by performing numerical simulations of Ria Formosa for the past and present configurations, which was not addressed in previous studies. It is also the purpose of this study to contribute to establish a methodology to analyse this kind of environmental issues, as well as to understand the effects of changing the tidal inlet characteristics at a coastal lagoon, namely on water circulation, inlet stability and water renovation.

This paper first describes the application of the hydrodynamic model ELCIRC (Zhang et al., 2004) to study water circulation within the Ria Formosa driven by tides. The simulation of tracers transport was then made using the models VELA (Oliveira et al., 2006c) and VELApart (Oliveira and Fortunato, 2002b), with the objective of studying the potential pathways of tracers disposed from the “Faro ETAR” (station of residual water treatment) (VELA) and to evaluate residence times (VELApart), for both bathymetries (past and present configurations). Finally, the impact of Ancão Inlet relocation in terms of inlet stability and water renewal is discussed.