Bed composition generation for morphodynamic modeling: case study of San Pablo Bay in California, USA

Abstract

Applications of process-based morphodynamic models are often constrained by limited availability of data on bed composition, which may have a considerable impact on the modeled morphodynamic development. One may even distinguish a period of "morphodynamic spin-up" in which the model generates the bed level according to some ill-defined initial bed composition rather than describing the realistic behavior of the system. The present paper proposes a methodology to generate bed composition of multiple sand and/or mud fractions that can act as the initial condition for the process-based numerical model Delft3D. The bed composition generation (BCG) run does not include bed level changes, but does permit the redistribution of multiple sediment fractions over the modeled domain. The model applies the concept of an active layer that may differ in sediment composition above an underlayer with fixed composition. In the case of a BCG run, the bed level is kept constant, whereas the bed composition can change. The approach is applied to San Pablo Bay in California, USA. Model results show that the BCG run reallocates sand and mud fractions over the model domain. Initially, a major sediment reallocation takes place, but development rates decrease in the longer term. Runs that take the outcome of a BCG run as a starting point lead to more gradual morphodynamic development. Sensitivity analysis shows the impact of variations in the morphological factor, the active layer thickness, and wind waves. An important but difficult to characterize criterion for a successful application of a BCG run is that it should not lead to a bed composition that fixes the bed so that it dominates the "natural" morphodynamic development of the system. Future research will focus on a decadal morphodynamic hindcast and comparison with measured bathymetries in San Pablo Bay so that the proposed methodology can be tested and optimized.

Cohesive and mixed sediment in the Regional Ocean Modeling System (ROMS v3.6) implemented in the Coupled Ocean–Atmosphere–Wave–Sediment Transport Modeling System (COAWST r1234)

“…Many have a form similar to the Ariathurai and Arulanandan (1978) equation used in ROMS (Warner et al, 2008), which relates erosional flux E (kg m −2 s −1 ) to the normalized excess shear stress as E = E 0 (1 − φ) [(τ sf − τ c ) /τ sf ] when τ sf > τ c , where E 0 (kg m −2 s −1 ) is an empirical rate constant, φ (m 3 m −3 ) is sediment porosity, τ sf (Pa) is the skin-friction component of the bottom shear stress, and τ c (Pa) is the critical shear stress for erosion. The erosion of cohesive sediment in some models (for example Delft3D; van der Wegen et al, 2011;Caldwell and Edmonds, 2014) uses a similar formulation subject to a user-specified critical shear stress for erosion. It is recognized that τ c may increase with depth in sediment, and erosion rate formulae have been proposed that incorporate depth-dependent profiles for E 0 and/or τ c (Whitehouse et al, 2000;Mehta, 2014).…”

“…Biodiffusion may alter stratigraphy, and there are many 1DV models that treat the diffusive mass flux of sediment and reactive constituents in the bed, mostly motivated by water quality and geochemical concerns (e.g., Boudreau, 1997;DiToro, 2001;Winterwerp and van Kesteren, 2004). Several regional-scale circulation and sediment transport models treat sediment stratigraphy, including ECOMSED (HydroQual, Inc., 2004), ROMS/CSTMS (Warner et al, 2008), Delft3D (van der Wegen et al, 2011), FVCOM, TELEMAC/SISYPHE (Villaret et al, 2011;Tassi and Villaret, 2014), and MARS3D (Le Mengual et al, 2017) and some have unpublished treatments for cohesive processes. Sanford (2008) pioneered an approach in which the critical shear stress for each bed layer was nudged toward an assumed equilibrium value, and the critical stress for erosion of the surface layer alternately became smaller or larger in response to deposition and erosion.…”

Lagoon Sediment Dynamics: A Coupled Model to Study a Medium-Term Silting of Tidal Channels

“…Calibration parameters are fundamental together with boundary conditions and an appropriate combination of hydrodynamic forces [26,27]; usually a great effort is spent on researching a reliable reconstruction of the initial bed composition [8,28], but the same is not carried out for the variability of threshold bed shear stress for erosion, often assumed constant both in space and time [6,29], contrary to experimental evidence [30][31][32].…”

Discontinuous hindcast simulations of estuarine bathymetric change: A case study from Suisun Bay, California

“…In general, sediment distributions coarsened (strong fraction dominant) with time. This method is similar to that employed by Van der Wegen et al (2010) who found that allowing for a spin-up of bed composition led to more representative geomorphic evolution.…”