Impact of seasonal bathymetric changes and inlet morphology on the 3D water renewal and residence times of a small coastal stream

Abstract

The sustainable management of small, intermittent coastal systems is a challenging task due to the strong morphological dynamics of their inlets, which causes hydrodynamics, morphodynamics and water renewal to be highly variable, at both seasonal and shorter time scales. This strong variability makes these coastal systems very sensitive to pollution events because the consequences of these events may differ significantly depending on the bathymetry at the time they occur. Here, a 3D modeling-based study of the water renewal variability of a small coastal system, the Aljezur stream, is presented, targeting 1) the impact of different measured bathymetric conditions, different seasonal forcings and processes included in its water renovation, and 2) the confirmation of the small role of light in the decay of fecal coliforms on the stream. The analysis is conducted for 4 distinct scenarios, including two real settings, using a 3D baroclinic model and an associated particle model. Results confirm the major role of bathymetry on the residence times of the Aljezur stream, leading to 100% differences. Wind is shown to have a significant impact on the water renovation, as well as the instant of particle release within the tidal cycle. A low permanence of particles in the upper layer of the water column, defined through laboratory experiments using the stream water, confirmed the small importance of light in promoting fecal bacteria decay in this stream.