The effect of tidal regime and river flow on the hydrodynamics and salinity structure of the Lima Estuary: use of a numerical model to assist on estuary classification

Abstract

The purpose of this work is to characterize the hydrodynamics of the Lima estuary (Portugal), regarding its flow regime and salinity structure. The vertical structure was studied with field campaigns collecting data from vertical profiles at chosen spots of selected transects. A vertically integrated bi-dimensional hydrodynamic numerical model (SIMSYS2D) was set to run several simulations, namely tidal propagation upstream, horizontal structure of currents, comparison between currents for different stations and behaviour of hydrodynamics with varying river flow. The salt water input from the tide into the estuary was found to be varying between 3.7×106 m3 for a neap tide with a tidal range of about 1.07 m and 12.6×106 m3 for a spring tide with a tidal range of about 3.07 m. For a low river discharge (4 m3s-1) and a spring tide the estuary can be considered well mixed (0.007 flow ratio). For a high river discharge (100 m3s-1) and a neap tide the estuary can’t still be classified as highly stratified although the values are above the partially mixed scenario (0.58 flow ratio). The average tide and the average river discharge (50 m3s-1) results in a flow ratio of 0.12, which although within the partially mixed classification, is close to the well mixed range. For maximum river discharge of 200 m3s-1 and neap tides conditions, the ratio falls within the highly stratified classification, with a ratio of 1.05, although it must be considered that these river flows are anthropogenic in its origin, being only experienced with high turbinated dam flow.