Abstract: Details are given of the surface analysis method implementation to characterize the free surface and velocity influences of a shallow mesotidal estuary to changes in freshwater discharges, with particular application to the Suances Estuary (SE), located in north of Spain. The surface analysis method is based on the use of numerical models in order to assess water levels and current velocities in several model scenarios which represent the environmental forcing variability in the estuary. Thus, a two-dimensional depth-integrated model was used to evaluate water levels and current velocities in the innermost area of the SE, applying a mesh nesting strategy to achieve high resolution results in order to evaluate the pollutant transport in further studies. The hydrodynamic model was calibrated and validated against field data, collected during spring and neap tidal cycles, for water levels and current velocities. This tool allowed us to study a set of 45 model scenarios under fixed river flow and tidal amplitude conditions. These results were then fitted trough quadratic interpolation functions to create plots of response surfaces for the mentioned variables. From these surfaces, water levels and velocity distributions can be calculated for long-term periods at any estuarine location saving a remarkable quantity of computational cost and time. To validate the surface analysis method, the obtained results were tested against the outputs provided by the hydrodynamic modelling of the estuary for a 6-month period. The comparison showed a fairly good agreement with less computational cost than a typical numerical modelling, obtaining a computational demand decrease of 55%. Although, it is relevant to mention that the surface analysis method performs better in estimating velocities than water levels, we evaluated the river flow and astronomic tide influence on estuarine water levels and current velocities for a long-term period of 10 years.

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