Aim of the study:
Numerical modeling of estuaries is a complex task due to the effect of the tide and its interaction between the estuary outlet and the sea. Therefore, the following question arises: Does the length and shape of the geometric domain at the downstream boundary of an estuary (interaction lagoon-mar) affect its hydrodynamic modeling?

Material and methods:
Two-dimensional (2D) numerical simulations were performed using HEC-RAS V6.3.1. Input data included a high-resolution digital model, a bathymetric survey of the study area, tidal data, and a field measurement campaign. Five geometric domains were tested as downstream boundary conditions: rectangular; large semi-circular; medium semi-circular; small semi-circular; and a cross-section. Additionally, two types (versions) of the solution equations were tested: simplified, considering only the diffusion terms, and with the full equations.

Results and conclusions:
Two profiles were selected to analyze the results: longitudinal and transversal over the estuary outlet (lagoon-sea interconnection channel). The water surface levels, hydraulic slope, and velocities were compared. We found little significant difference in the offshore-dominated conditions but a considerable difference in the cross-sectional condition. Also, as expected, significant differences were found when using the full versus simplified equations.