Morphodynamic Estuarine Evolution Using a 2D Numerical, Process-based Approach

Introduction. The morphodynamics of estuaries is complex and subject to an interactive system of different spatial scales and time scales. Although some sort of equilibrium can be observed over decades (ie the location of intertidal flats), longer term processes might cause ongoing evolution. Focus of extensive modeling research has been on specific aspects like the characteristics of tidal hydrodynamics within embayments and their assumed impact on the morphology, 1D morphodynamic models and 2D morphodynamic models focusing on stability analysis and initial pattern formation. Aim The current research aims to investigate long-term estuarine evolution characteristics, including effects of pattern formation on the development of the longitudinal profile in a rectangular embayment. Special emphasis is put on evaluating the model results in terms of energy dissipation and validating the results against empirical relationships. Methodology Use is made of a 2D process-based numerical model (Delft3D), which is based on a relatively simple formulation using the shallow water equations, the Engelund-Hansen sediment transport formulation and bed slope effects. Use is made of an advanced morphological update scheme that allows for bed level updates every time step. Wetting and drying of intertidal flats is allowed by a water level criterion. This means that cells are extracted from hydrodynamic and morphodynamic calculations when the water level becomes lower than a certain threshold value (0.1m) and are re-activated when the water level in the surrounding cells exceeds this value again. A special procedure allows for bank erosion and erosion of (dry) intertidal flats. This is done by assigning the calculated erosion in a wet cell to the adjacent dry cell. Model configuration The focus of the research is on different model configurations, ie, a 2.5 km wide basin of respectively 20 km, 80 km and 400 km long. The model domain is extended seaward 20 km so that the seaward boundary is not disturbed by morphodynamic processes and a harmonic water level boundary is prescribed with an amplitude of 1.75 m and a period of 12 hours. The landward boundary and the banks of the embayment are defined by a zero flux (no water flow, no sediment transport) boundary. The sediment is uniform of size (0.24 mm). Grid size is uniform over the domain with a size of 100 by 200 m. The computational time step is 2 minutes and the morphological acceleration factor is 400. Results Model results show the formation of a channel-shoal system within the first 100 years. The pattern becomes less dynamic on longer timescales of millennia. Empirical equilibrium relations (like between the tidal prism and cross sectional area) are fulfilled and the basin wide energy dissipation decreases at decreasingly lower rate.