Variable diffusion boundary layer and diffusion flux at sediment-water interface in response to dynamic forcing over an intertidal mudflat

Abstract

The diffusion boundary layer (DBL) significantly limits the exchange between sediment and overlying water and therefore becomes a bottleneck of diffusive vertical flux at the sediment-water interface (SWI). Variable DBL thickness and diffusion flux in response to dynamic forcing may influence replenishment of nutrients and secondary pollution in coastal waters. In situ measurements of velocity in the bottom boundary layer (BBL) and oxygen concentration in the DBL were made over an intertidal mudflat, using an acoustic Doppler current and mini profiler. A linear distributed zone in the oxygen profile, the profile slope discontinuity and variance of concentration can be used to derive accurate DBL thickness. Diffusion fluxes calculated from the water column and sediment are identical, and their bias is less than 6%. A numerical model PROFILE is used to simulate the in situ dissolved oxygen profile, and layered dissolved oxygen consumption rates in the sediment are calculated. The DBL thickness (0.10-0.35 mm) and diffusion flux (15.4-53.6 mmol m) vary with a factor of 3.5 during a tidal period. Over an intertidal mudflat, DBL thickness is controlled by flow speed U in the BBL, according to δ DBL =1686.1DU −1 +0.1 (D is the molecular diffusion coefficient).That is, the DBL thickness δ DBL increases with decreasing flow speed U. Changes of diffusion flux at the SWI are caused by variations in the water above the sediment and the turbulent mixing intensity. The diffusion flux is positively related to the turbulent dissipation rate, friction velocity and turbulent energy. Under the influence of dynamics in the BBL, DBL thickness and flux vary significantly. diffusion boundary layer, diffusion flux, dynamic forcing, mini profiler, dissolved oxygen, intertidal mudflat Citation:Wang J N, Zhao L, Wei H. Variable diffusion boundary layer and diffusion flux at sediment-water interface in response to dynamic forcing over an intertidal mudflat. Chin Sci Bull, 2012Bull, , 57: 15681577, doi: 10.1007 The diffusion boundary layer (DBL) is a thin (less than 1 mm) film of water covering the sediment. When solutes and particles are transported from the bottom boundary layer (BBL) to the DBL, molecular diffusion instead of turbulent eddy diffusion dominates the vertical transport [1][2][3]. The DBL limits exchange between the sediment and overlying water, and therefore becomes a bottleneck of diffusive vertical flux at the sediment-water interface (SWI). theoretically revealed that dynamic forcing in the BBL has a direct effect on DBL thickness and diffusion flux at the SWI. Under the influence of dynamics in the BBL, DBL thickness and flux vary significantly. Understanding how the DBL and diffusion transport responds to dynamic forcing is therefore crucial for accurately quantifying diffusion flux, and for estimating the replenishment of nutrients and secondary pollution in coastal waters. Modeling also requires numerical parameterization of diffusion flux as a function of BBL dynamics. The diffusive vertical flux of dissolved oxy...

Diffusive boundary layer influenced by bottom boundary hydrodynamics in tidal flows

“…Lorke et al [2003] and Bryant et al [2010a] found correlation among seiche-induced turbulence, d DBL and diffusive flux of dissolved oxygen in lake water. Wang et al [2012] found correlation between d DBL and the BBL flow speed in an intertidal mudflat. Glud et al [2009] studied the variations of DBL along parallel transects in the deep ocean.…”

“…Through synthesizing data reported in Wang et al . [] and incorporating new data with different dynamic conditions, we derived a scaling of L B based on the law of wall, using U m , the kinematic viscosity ( ν ), the molecular diffusivity ( D ), and the bottom roughness length ( z 0 ). The results show that this scaling well fits the data from the two sites with distinctly different dynamic conditions.…”

“…In the coastal tidal flow during this study, there were no evident phase lags observed between U m and ε m at the same height; thus, a simple scaling of δ DBL with U m was used by Wang et al . []. Through synthesizing data reported in Wang et al .…”

“…Measurements in the intertidal mudflat were preliminarily analyzed by Wang et al . []. In this study, new observations in Bohai Bay are added and a more in‐depth analysis of both datasets is carried out.…”

“…Wang et al . [] found correlation between δ DBL and the BBL flow speed in an intertidal mudflat. Glud et al .…”