Turbulent and numerical mixing in a salt wedge estuary: Dependence on grid resolution, bottom roughness, and turbulence closure

Abstract

The Connecticut River is a tidal salt wedge estuary, where advection of sharp salinity gradients through channel constrictions and over steeply sloping bathymetry leads to spatially heterogeneous stratification and mixing. A 3‐D unstructured grid finite‐volume hydrodynamic model (FVCOM) was evaluated against shipboard and moored observations, and mixing by both the turbulent closure and numerical diffusion were calculated. Excessive numerical mixing in regions with strong velocities, sharp salinity gradients, and steep bathymetry reduced model skill for salinity. Model calibration was improved by optimizing both the bottom roughness (z0), based on comparison with the barotropic tidal propagation, and the mixing threshold in the turbulence closure (steady state Richardson number, Rist), based on comparison with salinity. Whereas a large body of evidence supports a value of Rist ∼ 0.25, model skill for salinity improved with Rist ∼ 0.1. With Rist = 0.25, numerical mixing contributed about 1/2 the total mixing, while with Rist = 0.10 it accounted for ∼2/3, but salinity structure was more accurately reproduced. The combined contributions of numerical and turbulent mixing were quantitatively consistent with high‐resolution measurements of turbulent mixing. A coarser grid had increased numerical mixing, requiring further reductions in turbulent mixing and greater bed friction to optimize skill. The optimal Rist for the fine grid case was closer to 0.25 than for the coarse grid, suggesting that additional grid refinement might correspond with Rist approaching the theoretical limit. Numerical mixing is rarely assessed in realistic models, but comparisons with high‐resolution observations in this study suggest it is an important factor.

Wave Generation, Dissipation, and Disequilibrium in an Embayment With Complex Bathymetry

“…In this study, a spatially uniform bottom roughness of z 0 = 0.1 cm is applied, similar to the values used in previous estuarine circulation modeling studies (Lin et al, ; McSweeney et al, ; Warner et al, ). The z 0 for our model is slightly lower than used for a previous modeling study of the Delaware Estuary that used a coarser resolution grid (McSweeney et al, ), which is consistent with a study of the Connecticut River estuary that found that z 0 decreased as model resolution increased (Ralston et al, ).…”

Correlation of Remotely Sensed Surface Reflectance With Forcing Variables in Six Different Estuaries

“…In this study, values were insensitive to river discharge in the CTR and CRE (Figures b and c). values in the CTR were found to be between the mouth and 9 km upstream, consistent with the finding by Ralston et al () based on salinity observations. values were found in the upper estuary within the DR and GE, and they were significantly influenced by the river discharge ( = 42.0% and 49.0%; Figures d and e).…”

Salt wedge dynamics lead to enhanced sediment trapping within side embayments in high‐energy estuaries

“…Grid resolution is greatest in the estuary region from the mouth of the Connecticut up to Hamburg Cove, with typical horizontal grid spacing of 15–20 m. The vertical grid was terrain following with 30 uniformly spaced sigma layers. The model has been extensively evaluated against time series of water level, velocity, salinity, and suspended sediment concentration and found to be largely consistent with observations [ Ralston et al ., ]. A detailed comparison of model results against high spatial and temporal resolution field surveys found that a fine grid resolution was necessary to mitigate numerical mixing and simulate the strong stratification and sharp horizontal salinity gradients.…”

“…Modern fine-grained sedimentation rates within the deepest sections of Hamburg Cove of 4.2 cm/yr [Woodruff et al, 2013] are roughly 16 times the average rate of sea level rise since 1940 (2.55 mm/yr) [NOAA, 2016]. However, it has been unclear whether this sediment is introduced primarily during moderate-to-high river discharge events, when suspended sediment concentrations in the river are greatest and the cove is Journal of Geophysical Research: Oceans 10.1002/2016JC012595 fresh [Woodruff et al, 2013], or during low river discharge when the salinity intrusion extends to the mouth of the cove and estuarine-induced, upriver transport is enhanced [Ralston et al, 2017].…”

“…Tidal velocities in the estuary approach 1 m/s [Ralston et al, 2017]. During moderate-to-high discharge, the Connecticut estuary is a tidal salt wedge, with a stratified, frontal salinity intrusion propagating tidally 5-10 km from the mouth [Ralston et al, 2017]. Under low discharge conditions, the estuary tends to be more partially mixed and the salinity intrusion can extend 15 km or more from the river mouth [Horne and Patton, 1989].…”