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Papers by Stephen McLean

Journal of Hydraulic Engineering, Feb 1, 2004

The current-wave boundary layer.

Water Resources Research, 2006

We address some unsolved methodological issues in modeling of natural rough-bed flows by critical... more We address some unsolved methodological issues in modeling of natural rough-bed flows by critically examining existing approaches that parameterize rough-bed flows. These often utilize loosely defined variables. Here we suggest that double-averaging (in time and in a volume occupying a thin slab in a plane parallel to the mean bed) provides a rigorous, straightforward alternative that can aid in the parameterization process. We further present two examples: two-dimensional bed form and gravel bed flows. We argue that the double-averaging approach based on momentum equations should serve as a better methodological basis for modeling, phenomenological developments, and parameterizations. These equations explicitly include drag terms and form-induced momentum fluxes due to spatial heterogeneity of the time-averaged flow in the near-bed region. They also provide a solid basis for better definitions of basic flow variables including the shear stress partitioning into turbulent and form-induced momentum fluxes, skin friction, and pressure (form) drag. We show that for a range of rough-bed flows the vertical distribution of the double-averaged velocity consists of two distinct regions: (1) a linear region below roughness tops, and (2) a logarithmic region above them.

North Sea Dynamics, 1983

A knowledge of the transport of sediment in a shallow, high energy environment such as the North ... more A knowledge of the transport of sediment in a shallow, high energy environment such as the North Sea is important to the general understanding of such a system. The shape of the basin itself is largely a product of the interaction between the currents and the sediments. The engineers are interested in maintaining harbors and predicting morphological adjustments caused by man-made alterations of currents. The environmentally conscious are concerned about pollution and water quality which both are often strongly linked to the sediments. Sediment transport involves a complex interaction between the flow and the bottom. Ultimately an accurate knowledge of the forces acting on the bottom (i.e., the boundary shear stress) is necessary in order to predict the initiation of sediment movement and subsequent transport rates. In the North Sea this is complicated by the unsteady nature of the flow. This arises from both storms and tides. In order to better understand such a system, it was decided to begin by investigating the boundary layer in regions of active transport where the flow was dominated by tidal effects.

River, Coastal and Estuarine Morphodynamics, 2001

Page 27. 2 Sediment Entrainment and Transport in Complex Flows Jonathan M. Nelson1, Mark W. Schme... more Page 27. 2 Sediment Entrainment and Transport in Complex Flows Jonathan M. Nelson1, Mark W. Schmeeckle2, Ronald L. Shreve3, Stephen R. McLean4 1 US Geological Survey, National Research Program, Denver, Colorado ...

Marine Geology, 1981

Sand ribbons typically are shallow-water sedimentary features characterized as thin, elongate ban... more Sand ribbons typically are shallow-water sedimentary features characterized as thin, elongate bands of sand overlying a.coarser lag sediment which is exposed between them. The ribbons are oriented along the direction of the dominant current, and are formed by a secondary circulation pattern that includes a small component of the boundary shear stress directed toward the center of the sand ribbons. A perturbation expansion, coupled with flume experiments, indicates that this circulation pattern is linked to differences in the bottom roughness between the sand ribbons and the adjacent coarser strips. The nonuniform turbulence field arising from this roughness distribution drives a secondary helical flow. The expansion indicates that there is a preferred ratio of sand ribbon spacing to water depth (-4:1) for unstratified channel flow conditions. The secondary flow pattern forces the downstream flow to readjust so that the boundary shear stress over the smooth sand strips is considerably smaller than the mean stress. This serves to retard the movement of sand through such a system.

Journal of Hydraulic Engineering, 1999

Journal of Geophysical Research, 1979

To assist in elucidating the turbulent structure of flow over regular sets of bed forms, data pro... more To assist in elucidating the turbulent structure of flow over regular sets of bed forms, data procured with small mechanical current meters above 2 m high 96 m long sand waves in the Columbia River are analyzed. The high-frequency resolution of the flow sensors and the half hour records that were collected with them are shown to be sufficient to

Bottom Turbulence, Proceedings of The 8th International Liege Colloquium on Ocean Hydrodynamics, 1977

Abstract An accurate knowledge of flow in the immediate vicinity of the sea bed is important in m... more Abstract An accurate knowledge of flow in the immediate vicinity of the sea bed is important in marine geological, benthic ecological, geochemical, and sediment transport studies. However in many cases, the velocity field is complicated by the presence of ripples and dunes on the sea bed and suspended sediment-induced stratification in the flow. Recently techniques for handling these factors were developed by the authors, but they were applied only to a situation where the sea bed was comprised of a single size and specific gravity class. In this paper these techniques are extended to the case where the suspended material is characterized by an ensemble of settling velocities and critical shear velocities. The results are applied first to a flat sea bed and then to spatially averaged flow over a wavy boundary such as might be produced by natural bed forms. These calculations indicate that the sediment transport process can have a significant effect on flow near the sea bed and that proper account must be taken of the settling velocity distribution comprising the suspended sediment concentration field.

such as ripples and dunes represent bottom features that exist at in least quasi- equilibrium wit... more such as ripples and dunes represent bottom features that exist at in least quasi- equilibrium with the overlying flow. Because of flow separation that is typically associated with these features, the flow over them is highly complex. Sediment transport is highly dependent on the nature of the overlying turbulence field. The turbulence is characteristic of neither wakes nor of classic boundary layers and its structure is spatially variable. Topography-induced acceleration plays a critical role in the nature of the turbulence field. Measurements of mean flow and turbulence are presented here for separated flow over a range of bed slopes simulating bedforms of different steepness ratios. Steep slopes are found to significantly reduce the overlying turbulence. Future experiments to assess the effects of slope on sediment flux are described.

Coastal Engineering 2000, 2001

An experimental study is carried out in the large wave channel in Hannover (GWK / FZK) under the ... more An experimental study is carried out in the large wave channel in Hannover (GWK / FZK) under the name SISTEX99 (= Small‐scale International Sediment Transport Experiment 1999) with the aim to investigate the sediment dynamics taking place near the bed in controlled non‐ ...

Physics of Fluids, 2006

Experiments utilizing two-dimensional fixed dune profiles and varying flow depth (dune regime flo... more Experiments utilizing two-dimensional fixed dune profiles and varying flow depth (dune regime flows) highlight the equilibrium (self-similar) nature of the near-bed boundary layer over developing dunes with flow separation in the dune lee. The negligible variation in roughness layer ( ...

In an effort to improve the understanding of the nature of flows over three-dimensional ripples a... more In an effort to improve the understanding of the nature of flows over three-dimensional ripples and dune shapes on erodible beds, two complete sets of detailed flow measurements over fixed, artifical, sinuous-crested 3d dunes have been recently finished in the UCSB Ocean Engineering Laboratory's tilting-recirculating flume. Early results showed a relative increase in total boundary shear stress felt by the fluid due to the 3d dunes when compared with the 2d dunes for flows of the same mean depths and discharges. More detailed results from the completed 3d dune measurements are now presented and compared with the 2d dunes. In particular, both the turbulent kinetic energy and the Reynolds stresses over the 3d dunes are significantly lower than over the 2d dunes. To explain how an overall increase in total boundary shear stress could still result, estimates are made of the form drag from integrating the vertical component of the momentum equations, and of the skin friction from lo...

Marine Geology, 1985

... Mar. Geol., 42: 49--74. McLean, SR, 1983. Turbulence and sediment transport in measurements i... more ... Mar. Geol., 42: 49--74. McLean, SR, 1983. Turbulence and sediment transport in measurements in a North Sea tidal intel (The Jade). ... McLean, 1983S.R. McLean, Turbulence and sediment transport in measurements in a North Sea tidal intel (The Jade). ...

Journal of Geophysical Research, 1977

An understanding of the mechanics of nonuniform flow is important in a variety of ecological and ... more An understanding of the mechanics of nonuniform flow is important in a variety of ecological and geophysical fluid mechanical problems. Moreover, the ability to predict local boundary shear stress on an uneven bed is essential in erosion and sediment transport problems. In order to elucidate the important fluid mechanical phenomena active over natural quasi-two-dimensional bed forms a series of detailed flow measurements were made above 60-to 100-m-long 1-to 3-m-high dunes in the Columbia River. In this paper, velocity profiles obtained by averaging these flow data along lines of constant distance above the riverbed are examined, and it is shown that they can be constructed from well-known uniform flow result• used in conjunction with a hypothesis about the structure of internal boundary layers. This approach permits skin friction as well as total boundary shear stress, averaged over one wavelength of the bed form, to be determined from spatially averaged velocity profiles and, conversely, provides a mechanism whereby zero-order velocity profiles can be constructed for two-dimensional nonuniform channel flows. Corrections for changes in the bottom roughness parameter caused by bed load transport and for flow stratification induced by suspended load transport are derived and applied in order to make the results consistent with the measured spatially averaged shear stress field.

Journal of Geophysical Research, 1992

Governing equations are derived for calculating suspended sediment transport for noncohesive sedi... more Governing equations are derived for calculating suspended sediment transport for noncohesive sediments, and the results are presented in form of graphs of profiles of velocity, suspended sediment concentration, and other parameters of sediment transport. It is shown that calculations of suspended sediment transport (especially for lower transport states) should use the top of the bed load layer as its lower bound, in order to avoid overestimation of the concentration field due to inaccurate extrapolations of the concentration profiles.

Journal of Geophysical Research, 1994

Turbulence measurements from the flow over two-dimensional fixed dune shapes are presented along ... more Turbulence measurements from the flow over two-dimensional fixed dune shapes are presented along with analysis and discussion of the ramifications of the observations for transport of sediment as bed load over bed forms. The spatial structure of the local transport rate determines the shape and stability of bed forms such as tipples and dunes, and the transport of sediment is a highly nonlinear process that is profoundly affected by the statistics of the temporal fluctuations in the near-bed flow field. The measurements presented herein show strong spatial evolution of the joint probability distribution of the streamwise and bednormal fluctuating velocity components. Unlike measurements in uniform boundary layers, these distributions and the higher moments of the velocity do not scale with the local shear velocity,indicating that it is probably inappropriate to use the shear stress to characterize the sediment flux. This conclusion is supported by observations of sediment flux over a dune. Introduction Ripples and dunes play a critical role in the complex interrelation between flow, sediment flux, and bed morphology in many natural flows over erodible beds. In spite of the importance of these features, our ability to predict their occurrence, their sizes and shapes, and their effect on the flow is limited. This is primarily a result of our incomplete understanding of (1) the flow adjustments caused by bed forms, (2) the response of sediment to local fluid forcing, and (3) the subsequent evolution of the bed forms. These processes are interrelated, and none can be investigated completely independently of the others: the local rate of sediment transport is dependent on the near-bed flow field, which is controlled by the shape of the bottom topography, which in turn is altered if there is a local convergence or divergence of sediment flux. In addition, sediment motion acts to modify the flow field by enhancing roughness in the case of bed load and by damping turbulence in the case of suspended-sediment load. Furthermore, the shape of the bottom can alter the sediment load characteristics by inducing gravitational effects associated with bed slope. This complex, highly nonlinear interdependence often leads to responses of the system that are

Journal of Geophysical Research: Oceans, 1999

Vertical profiles of streamwise velocity measured over bed forms are commonly used to deduce boun... more Vertical profiles of streamwise velocity measured over bed forms are commonly used to deduce boundary shear stress for the purpose of estimating sediment transport. These profiles may be derived locally or from some sort of spatial average. Arguments for using the latter procedure are based on the assumption that spatial averaging of the momentum equation effectively removes local accelerations from the problem. Using analogies based on steady, uniform flows, it has been argued that the spatially averaged velocity profiles are approximately logarithmic and can be used to infer values of boundary shear stress. This technique of using logarithmic profiles is investigated using detailed laboratory measurements of flow structure and boundary shear stress over fixed twodimensional bed forms. Spatial averages over the length of the bed form of mean velocity measurements at constant distances from the mean bed elevation yield vertical profiles that are highly logarithmic even though the effect of the bottom •opography is observed throughout the water column. However, logarithmic fits of these averaged profiles do not yield accurate estimates of the measured total boundary shear stress. 1. Introduction When acted upon by a flow that is of sufficient strength to initiate general motion of the uppermost particles, erodible beds are typically unstable. Asymmetrical bed forms such as ripples or dunes generally result when the mode of transport of these particles is primarily as bed load, wl•ere the grains slide, roll, or hop along the bed. Such bed features cause the ove, rlying flow to separate, creating a flow that is characterized by complex interactions between the flow field, the topography, and the sediment itself. The complex interdependence among flow, bed geometry, and sediment transport limits the ability of investigators to make accurate predictions of boundary shear stress, sediment-transport rates, and changes in bottom topography. Such predictions are of critical importance in certain aspects of environmental science and engineering such as design, maintenance, or restoration of channels and harbors. An important quantity of interest regarding flow over bed forms is the total drag. In rivers and flood channels, flow resistance is a key factor in predicting the stage as a function of discharge. In the marine environment, drag coefficients depend strongly on the nature of bed forms that may be present. The total drag exerted by flow over a boundary that is characterized by forms such as ripples and dunes is greater than if the boundary were flat. The increase in drag arises because of the pressure distribution over the bed forms (form drag). The pressure is lowest over the crest and separation region downstream of it and rises rapidly to its highest value in the vicinity of the reattachment region near the base of the upslope. Form drag often accounts for the majority of the total drag; therefore accurate estimates of this quantity are critical. Because the length scale that characterizes the variation in