The bottom boundary layer of the bay stem plains environment of lower Chesapeake bay

Wright, L. D.; Boon, John D.; Xu, Jingping; Kim, S.C.

doi:10.1016/s0272-7714(05)80054-x

Cited by 43 publications

(11 citation statements)

References 23 publications

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“…In most experimental ecosystem enclosures, flow at the bottom is too irregular to use standard boundary layer techniques for determining bottom shear stress (e.g. profiles of average flow velocity fit to a theoretical log profile; Grant & Madsen 3 986, Wright et al 1992), but such techniques may be used in benthic flumes (Grant et al 1982.…”

Section: W E L L -M I X E D M E S O C O S M S T R a T I F I E D M E Smentioning

confidence: 99%

Turbulent mixing in experimental ecosystem studies

Sanford¹

1997

Mar. Ecol. Prog. Ser.

122

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ABSTRACT. Turbulent mixing is an integral aspect of aquatic ecosystems Turbulence affects ecosystem features ranglng from phytoplankton blooms at large scales through microscale interactions in the plankton. Enclosed experimental ecosystems, if they are to mimic the function of natural ecosystems, also must mimic natural turbulence and its effects. Large-scale velocity gradients and unstable buoyancy fluxes generate turbulent mixing in nature, most often at the surface and bottom boundaries and in the pycnocline. Large eddy sizes are controlled by the mixed layer depth, boundary layer thickness, or overturning length in the pycnocl~ne. Turbulent energy cascades through smaller and smaller eddy scales until it can be dissipated by molecular viscosity at the smallest scales. In contrast, artificial apparatuses frequently are used to generate turbulent m~x i n g in the interior of experimental ecosystem enclosures Large eddy sizes are controlled by the size of the generation apparatus, and they usually are much smaller than in nature Mismatched large eddy length scales and differences in turbulence generation mechanisms are responsible for the difficulties in mimicking natural turbulent mixing in experimental enclosures. The 2 most important turbulence parameters to consider in experimental ecosystem research are overall mixing time, T,,,, and turbulence dissipation rate, E. If the levels, spatial dlstribut~ons, and temporal variability of T,,, and E can be matched between an enclosure and the natural system it IS to model, then potential mixing artifacts can be minimized. An important additional consideration is that benthic ecosystems depend on the time-averaged boundary layer flow a s much as the turbulence. Existing designs for m i x~n g experimental ecosystems are capable of reasonably representing some aspects of natural turbulent mixing Paddle and grid stirring are the best available techniques for water column mixing, and flumes are best for benthic turbulence. There is no design at present that represents both environments adequately. More work also is needed on mixing of flexiblewall in situ enclosures. A more serious problem, however, is that turbulent mixing in experimental ecos)~stem studies too often is ignored, inadequately characterized, or unreported. Several methods are available for reasonable characterlzation of mixing in enclosures without sophisticated technology, and the technology for direct velocity measurements is becoming more accessible. Experimental ecosystem researchers should make a concerted effort to implement, characterize, and report on turbulent mixing In t h e~r enclosures.

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Section: W E L L -M I X E D M E S O C O S M S T R a T I F I E D M E Smentioning

confidence: 99%

Turbulent mixing in experimental ecosystem studies

Sanford¹

1997

Mar. Ecol. Prog. Ser.

122

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“…Because of their extreme sediment concentrations, fluid muds have the capacity to transport material across bathymetric gradients at greater magnitudes than other advective processes [e.g., Ross and Mehta , ; Ogston et al , ; Traykovski et al , ], with a component in the downslope (typically offshore) direction. The importance of fluid muds for sediment transport has been recognized in estuarine [e.g., Nichols and Biggs , ] and shelf environments near mouths of large rivers [e.g., Wright et al , ; Kineke et al , ]. The mixing of fresh, sediment‐laden river water and salty ocean water can form aggregate particles and generate convergent estuarine flow, developing a near‐bed layer of very high suspended‐sediment concentration (SSC) that can move seaward under its self‐weight.…”

Section: Introductionmentioning

confidence: 99%

In situ observations of wave‐supported fluid‐mud generation and deposition on an active continental margin

Hale

Ogston

2015

JGR Earth Surface

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Wave‐supported fluid muds (WSFM) are a type of gravity flow that can rapidly transport sediment across continental margins. They occur when wave‐induced bed stress maintains suspended‐sediment concentrations (SSC) >10 g L−1, and sediment‐induced stratification near the top of the wave boundary layer limits upward diffusion of sediment. Observations from near‐bed instrumentation are used to evaluate the conditions under which WSFMs form on the continental shelf offshore of the Waipaoa River, NZ. An event in July 2010 featured >130 h of energetic ocean conditions, and water discharge >1900 m3 s−1. A calibrated acoustic backscatter sensor at the midshelf measured near‐bed SSC >50 g L−1, with a strong lutocline occurring >15 cm above the predicted wave‐current boundary layer, resulting in ~5 cm deposition. A velocity anomaly occurred during this time, with offshore‐directed currents faster at 1 m above bed (mab) than at 3.5 mab. Using these observations, we empirically solve a simple buoyancy‐drag force balance to estimate the gravity‐driven velocity of the WSFM, which is always <0.03 m s−1. Extending the force balance across a shelf transect suggests that WSFM‐carried sediment can reach the shelf edge in 50–240 h. Spatial and temporal patterns of deposition predicted by the gradient of modeled sediment flux correlate well with seabed observations on the Waipaoa shelf reported in Walsh et al. (2014). This study highlights the importance of WSFMs for cross‐shelf sediment transport, despite relatively slow gravity‐driven velocities and the infrequency with which they occur.

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“…Indeed, bottom friction is hard to measure. Wright [1989] and Wright et al [1987Wright et al [ , 1992 found that large spatial and temporal variations in the bottom roughness result in large variations in the bed shear stress and the hydraulic roughness. There is also a strong indication that the biogenic roughness dominates the flow-induced roughness in the Bay stem.…”

Section: Introductionmentioning

confidence: 99%

Estimate of bottom and surface stress during a spring‐neap tide cycle by dynamical assimilation of tide gauge observations in the Chesapeake Bay

Spitz¹,

Klinck²

1998

J. Geophys. Res.

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Abstract.Dynamical assimilation of surface elevation from tide gauges is investigated to estimate the bottom drag coefficient and surface stress as a first step in improving modeled tidal and wind-driven circulation in the Chesapeake Bay. A two-dimensional shallow water model and an adjoint variational method with a limited memory quasi-Newton optimization algorithm are used to achieve this goal. Assimilation of tide gauge observations from 10 permanent stations in the Bay and use of a two-dimensional model adequately estimate the bottom drag coefficient, wind stress, and surface elevation at the Bay mouth. Subsequent use of these estimates in the circulation model considerably improves the modeled surface elevation in the entire Bay. Assimilation of predicted tidal elevations yields a drag coefficient, defined in the hydraulic way, varying between 2.5 x 10 -4 and 3.1 x 10 -3.The bottom drag coefficient displays a periodicity corresponding to the spring-neap tide cycle with a maximum value during neap tide and a minimum value during spring tide. From assimilation of actual tide gauge observations, it is found that the fortnightly modulation is altered during frontal passage. Furthermore, the response of the sea surface to the wind forcing is found to be more impbrtant in the lower Bay than in the upper Bay, where the barometric pressure effect seems to be more important.

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The bottom boundary layer of the bay stem plains environment of lower Chesapeake bay

Cited by 43 publications

References 23 publications

Turbulent mixing in experimental ecosystem studies

Turbulent mixing in experimental ecosystem studies

In situ observations of wave‐supported fluid‐mud generation and deposition on an active continental margin

Estimate of bottom and surface stress during a spring‐neap tide cycle by dynamical assimilation of tide gauge observations in the Chesapeake Bay

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