engleski

One-dimensional numerical model for layered shallow water flow in highly stratified estuaries

A numerical model for simulating freshwater and seawater flow in highly stratified estuaries was developed and validated. The governing equations for one-dimensional, two-layer, and time-dependant shallow water flow were derived from mass and linear moment conservation laws. The resulting equations are written as a system of non-linear, hyperbolic, partial differential equations. To solve this system, a modified Q- scheme based on a finite volume method was used. Particular emphasis was placed on the definition of boundary conditions and numerical treatment of friction, entrainment and wet-dry transitions. To asses performance of the proposed model and examine the main physical processes, a field sampling campaign was conducted during 2014-15 in the Rječina River estuary. Entrainment rates were estimated by a two-layer box model, based on observed flow rates and salinities. The interfacial friction factor was determined by fitting the numerical results to observed salt- wedge profiles. For this purpose, a simpler arrested salt-wedge model based on a finite difference method was used. When the friction factor was calibrated the numerical results agreed favourably against field observations for both steady and variable flow conditions. The combination of field observations and numerical experiments allowed for a more detailed investigation of main physical processes in microtidal estuaries. The salt- wedge dynamics in the Rječina River estuary seems to depend mainly on the river flow rate, while the sea-levels and the channel geometry impose a second-order control. Furthermore, a very strong stratification was present under all observed flow conditions. Finally, none of the existing entrainment and interfacial friction laws proved to be adequate ; therefore, new equations were proposed.

Estuaries ; Salt-wedge ; Shallow water flow ; numerical modelling ; finite volume method ; Q-scheme ; stratification ; friction ; entrainment ; Rječina

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