Influence of channel junction geometry on subtidal salt transport processes and salt intrusion

<p>In estuarine networks, channel junctions control the division and dispersion of salt between network branches. Whereas dominant width-averaged salt transport processes in single channels are relatively well understood in an idealized sense (e.g. Hansen and Rattray (1965)), the intrinsic three-dimensional geometry and bathymetry in channel junctions, which in urbanized regions are often heavily influenced by human engineering, complicates similar idealized analyses. Expanding our knowledge of salt transport processes around channel junctions is needed to understand salt distribution in estuarine networks and develop efficient one-dimensional salt intrusion models.</p> <p>As a first step in resolving salt transport processes around junctions, we construct a three dimensional subtidal idealized model for water motion and salinity in partially stratified estuaries. It provides an extremely fast and numerically accurate way of computing salinity distributions in general geometries and analyzing the dominant salt transport processes. The model extends the width-averaged approaches of Hansen & Rattray (1965) and MacCready (2004) to general 3D geometries. Following these authors, the vertical dimension is solved analytically. The solutions for the horizontal dimensions is extended to a numerical finite element method with flexible grid size. The resulting coupled system of nonlinear partial differential equations is solved iteratively. The idealized model is limited to well-mixed and partially stratified conditions and will be compared to high-complexity numerical models to test its validity.</p> <p>As a proof of concept using the newly derived model, we investigate the sensitivity of dominant salt transport processes and salt intrusion with respect to channel junction geometries, such as cross-sectional shapes and angles between the branches. Systematic exploration of these sensitivities is expected to lead to improved salt dispersion coefficients and, eventually, nodal point relations between junction branches.</p> <p><strong>References</strong></p> <p>Hansen, D. V., & Rattray, M. (1965). Gravitational circulation in straits and estuaries. <em>Journal of Marine Research</em>, <em>23</em>(2), 104&#8211;122.</p> <p>MacCready, P. (2004). Toward a unified theory of tidally-averaged estuarine salinity structure. <em>Estuaries</em>, <em>27</em>(4), 561&#8211;570. https://doi.org/10.1007/BF02907644</p>