Importance of geometric characteristics for salinity distribution in convergent estuaries

► We investigate the influence of geometric characteristics on salinity distribution using a theoretical salt-intrusion model. ► We present some innovations to the model considering the intra-tidal and sub-tidal variations of geometric profile. ► A series of field surveys for salinity are conducted to examine and validate the method. ► The validation shows good results in predicting spatial and temporal salinity distribution compared to the original model. ► The extended theory provides a key for the further investigation of salt intrusion for estuaries having complex geometry. We investigate changes in geometric characteristics and related processes of the longitudinal salinity distribution in an alluvial estuarine system. We focus on tide-dominated estuaries, which are usually characterized by a convergent shape, and analyze effects of intra-tidal and sub-tidal variations of the geometric characteristics on the salinity distribution. A theoretical salt intrusion model is applied to the Red River estuarine system (RRES), where the channel shape and size greatly vary as a function of tidal amplitudes. The model was originally derived for single-channel estuaries by considering the cross-sectional profile only at tidally averaged condition. This assumption makes the application of the model to the RRES a challenging task. In this paper, we split the estuary branch into multi-connected segments to describe the longitudinal variation of geometric characteristics. In addition, by prescribing geometric parameters to exponentially vary under different tidal conditions, we were able to estimate spatial and temporal changes in the salinity distribution. A series of field surveys for salinity in the dry season of 2006, 2008, and 2009 were used to examine and validate the method. We found that the modified model described salt intrusion with a good fit between the computed and measured salinity. This finding provides a key for the further investigation of salt intrusion for estuaries having complex geometry.