skip to main content

Title: Sources of Drag in Estuarine Meanders: Momentum Redistribution, Bottom Stress Enhancement, and Bend-Scale Form Drag
Abstract Curvature can create secondary circulation and flow separation in tidal channels, and both have important consequences for the along-channel momentum budget. The North River is a sinuous estuary where drag is observed to be higher than expected, and a numerical model is used to investigate the influence of curvature-induced processes on the momentum distribution and drag. The hydrodynamic drag is greatly increased in channel bends compared to that for straight channel flows. Drag coefficients are calculated using several approaches to identify the different factors contributing to the drag increase. Flow separation creates low-pressure recirculation zones on the lee side of the bends and results in form drag. Form drag is the dominant source of the increase in total drag during flood tides and is less of a factor during ebb tides. During both floods and ebbs, curvature-induced secondary circulation transports higher-momentum fluid to the lower water column through vertical and lateral advection. Consequently, the streamwise velocity profile deviates from the classic log profile and vertical shear becomes more concentrated near the bed. This redistribution by the lateral circulation causes an overall increase in bottom friction and contributes to the increased drag. Additionally, spatial variations in the depth-averaged velocity field due to the curvature-induced flow are nonlinearly correlated with the bathymetric structure, leading to increased bottom friction. In addition to affecting the tidal flow, the redistributed momentum and altered bottom shear stress have clear implications for channel morphodynamics.  more » « less
Award ID(s):
2123002 1634481
Author(s) / Creator(s):
; ;
Date Published:
Journal Name:
Journal of Physical Oceanography
Page Range / eLocation ID:
1629 to 1650
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. Abstract

    Flow separation has been observed and studied in sinuous laboratory channels and natural meanders, but the effects of flow separation on along‐channel drag are not well understood. Motivated by observations of large drag coefficients from a shallow, sinuous estuary, we built idealized numerical models representative of that system. We found that flow separation in tidal channels with curvature can create form drag that increases the total drag to more than twice that from bottom friction alone. In the momentum budget, the pressure gradient is balanced by the combined effects of bottom friction and form drag, which is calculated directly. The effective increase in total drag coefficient depends on two geometric parameters: dimensionless water depth and bend sharpness, quantified as the bend radius of curvature to channel width ratio. We introduce a theoretical boundary layer separation model to explain this parameter dependence and to predict flow separation and the increased drag. The drag coefficient can increase by a factor of 2–7 in “sharp” and “deep” sinuous channels where flow separation is most likely. Flow separation also enhances energy dissipation due to increased velocities in bends, resulting in greater loss of tidal energy and weakened stratification. Flow separation and the associated drag increase are expected to be more common in meanders of tidal channels than rivers where point bars that inhibit flow separation are more commonly found. The increased drag due to flow separation reduces tidal amplitude and affects velocity phasing along the estuary and could result in morphological feedbacks.

    more » « less
  2. Abstract

    Observations from a tidal estuary show that tidal intrusion fronts occur regularly during flood tides near topographic features including constrictions and bends. A realistic model is used to study the generation of these fronts and their influence on stratification and mixing in the estuary. At the constriction, flow separation occurs on both sides of the jet flow downstream of the narrow opening, leading to sharp lateral salinity gradients and baroclinic secondary circulation. A tidal intrusion front, with a V-shaped convergence zone on the surface, is generated by the interaction between secondary circulation and the jet flow. Stratification is created at the front due to the straining of lateral salinity gradients by secondary circulation. Though stratification is expected to suppress turbulence, strong turbulent mixing is found near the surface front. The intense mixing is attributed to enhanced vertical shear due to both frontal baroclinicity and the twisting of lateral shear by secondary circulation. In the bend, flow separation occurs along the inner bank, resulting in lateral salinity gradients, secondary circulation, frontogenesis, and enhanced mixing near the front. In contrast to the V-shaped front at the constriction, an oblique linear surface convergence front occurs in the bend, which resembles a one-sided tidal intrusion front. Moreover, in addition to baroclinicity, channel curvature also affects secondary circulation, frontogenesis, and mixing in the bend. Overall in the estuary, the near-surface mixing associated with tidal intrusion fronts during flood tides is similar in magnitude to bottom boundary layer mixing that occurs primarily during ebbs.

    more » « less
  3. Abstract

    Idealized numerical simulations were conducted to investigate the influence of channel curvature on estuarine stratification and mixing. Stratification is decreased and tidal energy dissipation is increased in sinuous estuaries compared to straight channel estuaries. We applied a vertical salinity variance budget to quantify the influence of straining and mixing on stratification. Secondary circulation due to the channel curvature is found to affect stratification in sinuous channels through both lateral straining and enhanced vertical mixing. Alternating negative and positive lateral straining occur in meanders upstream and downstream of the bend apex, respectively, corresponding to the normal and reversed secondary circulation with curvature. The vertical mixing is locally enhanced in curved channels with the maximum mixing located upstream of the bend apex. Bend-scale bottom salinity fronts are generated near the inner bank upstream of the bend apex as a result of interaction between the secondary flow and stratification. Shear mixing at bottom fronts, instead of overturning mixing by the secondary circulation, provides the dominant mechanism for destruction of stratification. Channel curvature can also lead to increased drag, and using a Simpson number with this increased drag coefficient can relate the decrease in stratification with curvature to the broader estuarine parameter space.

    more » « less
  4. Abstract

    In field observations from a sinuous estuary, the drag coefficientbased on the momentum balance was in the range of, much greater than expected from bottom friction alone.also varied at tidal and seasonal timescales.was greater during flood tides than ebbs, most notably during spring tides. The ebb tidewas negatively correlated with river discharge, while the flood tideshowed no dependence on discharge. The large values ofare explained by form drag from flow separation at sharp channel bends. Greater water depths during flood tides corresponded with increased values of, consistent with the expected depth dependence for flow separation, as flow separation becomes stronger in deeper water. Additionally, the strength of the adverse pressure gradient downstream of the bend apex, which is indicative of flow separation, correlated withduring flood tides. Whilegenerally increased with water depth,decreased for the highest water levels that corresponded with overbank flow. The decrease inmay be due to the inhibition of flow separation with flow over the vegetated marsh. The dependence ofduring ebbs on discharge corresponds with the inhibition of flow separation by a favoring baroclinic pressure gradient that is locally generated at the bend apex due to curvature‐induced secondary circulation. This effect increases with stratification, which increases with discharge. Additional factors may contribute to the high drag, including secondary circulation, multiple scales of bedforms, and shallow shoals, but the observations suggest that flow separation is the primary source.

    more » « less
  5. Abstract

    The impact of a floating oyster aquaculture farm on the intratidal dynamics of a low inflow estuary was investigated using field observations and an idealized numerical model. Measurements of current velocities, temperature, electrical conductivity, and velocity shear were collected around a floating oyster farm during two semidiurnal tidal cycles in a curved portion of the estuary. During flood, farm‐induced friction enhanced the lateral straining of velocity shears, which induced vertical mixing near the surface and extended the farm's frictional footprint. The streamwise flow reduction near the farm limited the development of lateral circulation. During ebb, flows largely bypassed the farm, which resulted in a weaker streamwise flow reduction through the farm and allowed the lateral circulation to develop across the estuary. To capture the farm effects in numerical simulations, a bulk drag coefficient for the farm was calculated as 8.4 × 10−3 ± 9.1 × 10−4, while the drag for a single cage ranged from 0.58 to 0.92. An idealized simulation in the Regional Ocean Modeling System (ROMS) demonstrated that the limited development of lateral circulation during flood was due to farm‐imposed friction rather than the combined influence of channel‐shoal morphology and the natural channel bend. These results showed that even though a farm may encompass a small portion of the estuary, it can affect the momentum and mixing outside of the immediate farm area. Therefore, it is important to consider the hydrodynamic responses of farms in estimates of carrying capacity and siting decisions.

    more » « less