Shear and turbulence production across subtidal channels

Abstract

In intertidal regions with subtidal channels, effects of bathymetry on overlying flow vary greatly with tidal stage. Around low water when mudflats and marsh are exposed, flow is constrained to channels, but when water depths are greater, tidal forcing may not necessarily be aligned with meandering channel axes. Flow across the channel can generate strong shear and turbulence at the elevation of the channel banks and can significantly increase turbulent energy in the middle of the water column. Field observations in a mudflat channel of San Francisco Bay indicate that cross-channel shear regularly occurs there early in ebb tides. With increased freshwater flow, baroclinic forcing can enhance shear by decoupling flow between dense water flooding in the channel and fresher water ebbing above the channel banks. A water column numerical model with k-ε turbulence closure is modified to represent the cross-channel shear production. Numerical results with uniform density indicate that turbulence production increases with the angle between the barotropic tidal forcing and the channel axis. When a longitudinal salinity gradient is imposed, cross-channel shear production contributes to breakdown of periodic stratification. Turbulence produced at the channel banks locally exceeds dissipation, and the excess energy is either lost to buoyancy or diffuses vertically to lower energy regions near the surface and near the bed. The balance among shear production, buoyancy production, and diffusion of turbulence depends on the flow angle and the strength of the longitudinal salinity gradient.

Near‐bed shear stress, turbulence production and dissipation in a shallow and narrow tidal channel

“…The turbulence production is often larger than the turbulence dissipation throughout the tidal cycle, which suggests that advection and vertical diffusion are not negligible in this environment. The turbulence production and dissipation estimates are both an order of magnitude larger than found in previous studies (Rippeth et al ., , ; Ralston and Stacey, ; Wiles et al ., ; Razaz and Kawanisi, ; Korotenko et al ., ). The proximity of the measurements to the bed where more shear exists may explain this difference.…”

“…Tides in the region are mixed semidiurnal with a maximum tidal range at the study site of 1.4 m. The study site was located within narrow channels at approximately 39.088°N and -75.437°W. Three instrument stations were set up in Ralston and Stacey (2006) 0.25-2.2 1 × 10 À 4 À 2 × 10 À 4 0.18 2 0.5 Razaz and Kawanisi (2012) 1.5 À 4.5 1.6 × 10 À 4 0.5 4 0.65 Korotenko et al (2013) 15 À 20 2 × 10 À 4 À 5 × 10 À 4 3 × 10 À 5 2 7 1.5 À 4 × 10 À 5 Rippeth et al (2002) 13 À 17 1 × 10 À 3 1 3 À 5 1.2 Wiles et al (2006) 19 À 25 1 × 10 À 5 À 5 × 10 À 5 2.5 × 10 À 6 À 1.5 × 10 À 5 3 À 14 6 1 Rippeth et al (2001) 29 À 35 1 × 10 À 5 0 À 10 5 0.7 *h is water depth, z is the measurement elevation above the bed, H is the tidal range and u max is the maximum streamwise velocity. Figure 1.…”

“…High levels of turbulence production are found near the bed and decrease strongly upward (Rippeth et al ., ; Wiles et al ., ; Razaz and Kawanisi, ; Korotenko et al ., ). High turbulence production is associated with peak flow conditions, with weaker turbulence close to slack tide (Rippeth et al ., , ; Ralston and Stacey, ; Wiles et al ., ), indicating that turbulence production is proportional to the mean current speed as might be expected. However, in some cases where the velocities were more rapid during flood flow, Reynolds stress and turbulence production were larger during ebb flow (Razaz and Kawanisi, ), which may be due to flow acceleration (Rippeth et al ., ).…”

“…Studies that have investigated turbulence production and dissipation simultaneously found that turbulence dissipation is smaller than turbulence production, within an order of magnitude (Wiles et al, 2006;Korotenko et al, 2013). Field experiments to determine turbulence production and dissipation in tidal channels often were carried out in channels with water depths exceeding 10 m (Rippeth et al, 2001(Rippeth et al, , 2002Wiles et al, 2006;Korotenko et al, 2013), though several experiments have been conducted in smaller channels, with water depths between 2 and 5 m (Ralston and Stacey, 2006;Razaz and Kawanisi, 2012). The maximum turbulence production in the larger channels was between 1 × 10 À 5 and 1 × 10 À 3 m 2 /s 3 , while the turbulence dissipation was between 2.5 × 10 À 6 and 4 × 10 À 5 m 2 /s 3 (Table I).…”

Currents in a small channel on a sandy tidal flat

“…In a small channel in the salt marshes and mudflats of San Francisco Bay, observations of currents were made along 3 cross-channel transects for two few-day (Ralston and Stacey 2005b) and two 2-week periods (Ralston and Stacey, 2006). Owing to the curvature of the channel, the larger-scale flow changed from mostly along-channel to across-channel between the 3 transects, allowing investigation of flows with different orientations to the channel axis.…”