Impacts of Currents and Waves on Bottom Drag Coefficient in the East China Shelf Seas

Abstract

High-frequency measurements of waves, currents, and turbulence were made using the bottom-mounted tripod equipped with the Nortek 6-MHz acoustic Doppler velocimetry at eight mooring stations in the East China Shelf Seas. The observational data are analyzed to estimate the bottom drag coefficient and also the contributions from currents and waves. Variations of bottom drag coefficient caused by currents show no obvious relationship with water depth. Generally, the current-induced drag coefficient decreases with the strengthening currents, and the wave-induced drag coefficient increases with the enhancing waves. Synthesis analyses of the observational data reveal a scale relationship between the current-induced drag coefficient and the turbulent Reynolds number and that between the wave-induced drag coefficient and the bottom wave orbital velocity. The analyses show no significant impacts of waves on turbulent Reynolds stress, and the total bottom drag coefficient is the sum of that due to currents and waves. The empirical formula for bottom drag coefficient derived from this study may help to improve the simulation of tides, storm surges, and sediment transport in coastal seas.Plain Language Summary Ocean floor exerts a frictional force on ocean currents, and this force is particularly important in coastal oceans. This force is difficult to measure directly and is usually parameterized using the near-bottom currents and a bottom drag coefficient. In this study, we derive estimates of bottom drag coefficient from observed currents and turbulence at eight mooring stations in the East China Shelf Seas. The estimated bottom drag coefficient is not a constant. Empirical relationships are established to link the impacts of currents and waves on bottom drag coefficient to separate dynamic parameters, that is, the turbulent Reynolds number and the bottom wave orbital velocity. The total bottom drag coefficient can be taken as the sum of the two components. The empirical formula for bottom drag coefficient derived from this study may help to improve the simulation of tides, storm surges, and sediment transport in coastal seas.

Estimation of Bottom Friction Coefficient in Multi‐Constituent Tidal Models Using the Adjoint Method: Temporal Variations and Spatial Distributions

“… (a) Average values of the estimated BFCs in Exp1 (red line with circles) change as a function of corresponding water depth with 5 m interval, and the observed BFCs versus water depth in Fan et al. (2019) (blue line with asterisks). (b) Same as (a) but for those when the water depth is less than 500 m. Along the bottom, the magenta line, cyan line, and green line denote periods with decreasing BFCs, increasing BFCs and constant BFCs, respectively.…”

“…Taylor (1920) introduced the quadratic bottom friction formulation as a function of the bottom friction coefficient (BFC) and velocity and estimated the bulk value for BFCs. The value of BFC is critical for accurate simulations of tides, storm surges, circulation, and sediment transport (Fan et al., 2019; Sana & Tanaka, 1997), which are key research fields of physical oceanography and provide essential information for the design and planning of coastal ocean engineering (Chen et al., 2007; Lee & Jeng, 2002). However, the inherent uncertainty in setting the values of BFC severely affects relevant studies and applications (Ludwick, 1975).…”

“…The BFC is typically tuned according to the difference between the modeled and observed data (Fringer et al., 2019), but the estimation of the BFC by trial and error is tedious and impractical (Khatibi et al., 1997; Siripatana et al., 2018), especially for spatially and temporally varying BFCs. In addition, the regression relationships between the BFC and the influencing factors, obtained from the in situ observations, cannot be used directly and need to be further tuned before applying them to other regions because the regression coefficients are related to the local conditions (Fan et al., 2019).…”

The Impact of Wind on Flow and Sediment Transport over Intertidal Flats

“…We expect that the results provided by this study are especially relevant at locations where the dominant wind direction and the main tidal flow direction are aligned with the coastline (such as in estuaries and lagoons, but less so for exposed, broad muddy coastlines). in the range 0.001-0.004, e.g., [16,41,[45][46][47][48][49][50]. A value of 0.002 has been selected, based on which the roughness length z 0 has been estimated along the full measurement period, as a function of water depth (Equation (A3)).…”

“…The estimation of the roughness height z 0 is not straightforward, as it depends on the skin friction (sediment diameter), on the presence of bed forms (e.g., current and wave induced ripples), on the hydrodynamics, and among other factors, on sediment stratification, e.g., [16,45,46]. Values of the current related friction coefficient c f c available in literature suggest a variability of the parameter…”