Effect of the Bathymetric Changes on the Hydrodynamic and Residence Time in Obidos Lagoon (Portugal)

Obidos Lagoon is a small coastal system connected to the sea through a shallow, narrow and mobile inlet. The lagoon is characterized by large accretion rates and cyclic water quality problems due to large water renewal time scales. Over the years the inlet position often drifted from north-south and south-north. As a consequence, every time the inlet position approaches to the north or south shoreline the associated erosion threatens local houses and some, emergency interventions to protect the northern shoreline were needed in the past. In the 1990s Danish Hydraulic Institute (DHI) proposed some local solutions to improve the water fluxes between the lagoon and the ocean, and to create conditions to maintain the inlet in a central position. These interventions included the construction of a guiding wall to prevent the inlet migration to the north. However, in the last years an extreme migration of the inlet to south was observed, generating severe erosion problems and damage to the local houses. This paper investigates the effect of artificial morphological changes (e.g., dredging works) on tidal propagation and residence time (RT) in Obidos coastal lagoon. Hydrodynamic and RT Simulations were performed for three bathymetric configurations of the lagoon, representing the reference or present situation and two dredging scenarios. The results confirmed that tidal propagation depends strongly on the bathymetric configuration. Dredging the channels and relocating the inlet may increase the tidal prism by about 20% to 35% in relation to the reference scenario. The major differences are observed in the lower lagoon with an improving in velocity fields in the north and south channels and the reposition of the inlet to the central position. The tidal prism increasing promotes higher water renewals in the lower lagoon and contributes to improve the water quality in this area. In the upper lagoon the effects of the dredging, it are less visible and relevant changes in the water RT (of the order of 3 weeks) were not observed. The most relevant changes can be observed after dredging the Barrosa branch, with an increasing in the RT on the order of 50% compared to reference situation.