Pressure fluctuations within subsurface gravel bed caused by turbulent open-channel flow

Detert, Martin; Jirka, G. H.; Jehle, Markus; Klar, Michael; Jähne, Bernd; Köhler, Hans-Jürgen; Wenka, Thomas

doi:10.1201/b16998-90

Cited by 9 publications

(10 citation statements)

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“…The σ ≈ τ 0 dependence was mainly demonstrated for impermeable (continuous) smooth and rough beds. More recently, Detert et al (2004) reported similar results with σ ≈ (3-3.5)τ 0 for hydraulically rough flow over a multi-layered and homogeneous stratum of spherical particles of diameter 10 mm. Smart & Habersack (2007) showed that this dependence is also observed in gravel-bed rivers, and through their field data reported that σ ≈ 3τ 0 .…”

Section: Pressure Forces On Sediment Particles In Turbulent Open-chansupporting

confidence: 62%

“…It was found that σ D and σ L were 2-2.6 and 2.5-3.4 times τ 0 , respectively. While this link between bed shear stress and pressure fluctuations was first derived for smooth beds (Kraichnan 1956), the present results along with those reported by Detert et al (2004) and Smart & Habersack (2007) suggest a technique for estimating local boundary shear stress if instantaneous near-bed pressure measurements can be recorded for flows over rough granular beds. However, the exact flow mechanisms responsible for this link between τ 0 and σ p over rough beds remain to be identified and tested.…”

Section: Discussionmentioning

confidence: 54%

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Pressure forces on sediment particles in turbulent open-channel flow: a laboratory study

2014

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An experimental investigation into the fluctuating pressure acting on sediment particles on the bed of an open-channel flow was carried out in a large laboratory flume for a range of flow depths and bed slopes. The pressure measurements were made using 23 spherical particles instrumented with differential pressure sensors. These measurements were complemented with simultaneous measurements of the velocity field using high-resolution stereoscopic particle image velocimetry. The pressure statistics show that the standard deviations of the drag and lift fluctuations vary from 2.0 to 2.6 and from 2.5 to 3.4 times the wall shear stress, respectively, and are dependent on relative submergence and flow Reynolds number. The skewness is positive for the drag fluctuations and negative for the lift fluctuations. The kurtosis values of both drag and lift fluctuations increase with particle submergence. The two-particle correlation between drag and lift fluctuations is found to be relatively weak compared to the two-point drag-drag and lift-lift correlations. The pressure cross-correlations between particles separated in the longitudinal direction exhibit maxima at certain time delays corresponding to the convection velocities varying from 0.64 to 0.72 times the bulk flow velocity, being very close to the near-bed eddy convection velocities. The temporal autocorrelation of drag fluctuations decays much faster than that for the lift fluctuations; as a result, the temporal scales of lift fluctuations are 3-6 times that of drag fluctuations. The spatial and temporal scales of both drag and lift fluctuations show dependence on flow depth and bed slope. The spectral behaviour of both drag and lift fluctuations is also assessed. A f −11/3 slope is observed for the spectra of the drag fluctuations over the majority of the frequency range, whereas the lift spectra suggest two scaling ranges, following a f −11/3 slope at high frequencies and f −5/3 behaviour at lower frequencies.

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Section: Pressure Forces On Sediment Particles In Turbulent Open-chansupporting

confidence: 62%

Section: Discussionmentioning

confidence: 54%

Pressure forces on sediment particles in turbulent open-channel flow: a laboratory study

2014

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“…Since wood causes shedding vortices and enlarges turbulence intensity in the surface water [ Mutz , 2003], instream wood is likely to increase turbulent dispersion. However, turbulent pressure variations and dispersion in a porous bed are limited to approximately two to three times the roughness height of a porous bed [ Detert et al , 2004; Dittrich and Träbing , 1999]. Assuming that roughness height is the d 90 of the grain size distribution [ Schneider , 1998], the sediment layer that may have been affected by the exchange mechanism in the experiments was 1.7 to 2.3 mm deep.…”

Section: Discussionmentioning

confidence: 99%

Effect of instream wood on vertical water flux in low‐energy sand bed flume experiments

Mutz

Kalbus

Meinecke

2007

Water Resources Research

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[1] In streams, interaction of current with bed forms and solid objects can produce vertical water flux across the streambed. Instream wood is a major obstacle to flow and alters the bed topography of natural sand bed streams. In the present experimental study in large circulating flumes simulating sand bed streams with low constant slope, we compared systems with and without natural quantities of wood. The introduction of wood resulted in the production of irregular bed forms. As a result, flow resistance tripled, and vertical water flux across the streambed increased by a factor of 1.8 to 2.5. The mixing depth was spatially variable and seemed unrelated to the local bed form. After the addition of wood, surface water mixed deeper into the sediments, and the total sediment pore water volume involved in mixing increased by factors of 1.2 to 1.5. The practice of increasing instream wood should be a valuable tool for hyporheic rehabilitation of degraded sand bed streams.

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“…Hydrodynamically induced flow can also be generated by boundary layer turbulence in open channel flow [e.g., Blois et al ., ], especially in gravels where pore water velocities near the bed interface can reach magnitudes in the order of 0.2 m s −1 [ Nagaoka and Ohgaki , ]. These flow velocities are generated by coherent flow structures in the turbulent boundary layer, as well as flow separation induced by bed‐topography, and may be expected to generate significant pressure gradients across the bed and into the pores beneath [ Detert et al ., ]. These flows result in intense turbulent fluctuations in the hyporheic flow within the pore spaces in the Brinkman layer (defined as the transition layer between the fully turbulent flow and the deeper groundwater Darcian flow, Goharzadeh et al .…”

Section: Introductionmentioning

confidence: 99%

A numerical investigation into the importance of bed permeability on determining flow structures over river dunes

Sinha

Hardy

Blois

et al. 2017

Water Resources Research

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Although permeable sediments dominate the majority of natural environments past work concerning bed form dynamics has considered the bed to be impermeable, and has generally neglected flow between the hyporheic zone and boundary layer. Herein, we present results detailing numerically modeled flow which allow the effects of bed permeability on bed form dynamics to be assessed. Simulation of an isolated impermeable bed form over a permeable bed shows that flow is forced into the bed upstream of the dune and returns to the boundary layer at the leeside, in the form of returning jets that generate horseshoe‐shaped vortices. The returning flow significantly influences the leeside flow, modifying the separation zone, lifting the shear layer adjoining the separation zone away from the bed. Simulation of a permeable dune on a permeable bed reveals even greater modifications as the flow through the dune negates the formation of any flow separation in the leeside. With two dunes placed in series the flow over the downstream dune is influenced by the developing boundary layer on the leeside of the upstream dune. For the permeable bed case, the upwelling flow lifts the separated flow from the bed, modifies the shear layer through the coalescence with vortices generated, and causes the shear layer to undulate rather than be parallel to the bed. These results demonstrate the significant effect that bed permeability has on the flow over bed forms that may be critical in affecting the flux of water and nutrients.

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Pressure fluctuations within subsurface gravel bed caused by turbulent open-channel flow

Cited by 9 publications

References 0 publications

Pressure forces on sediment particles in turbulent open-channel flow: a laboratory study

Pressure forces on sediment particles in turbulent open-channel flow: a laboratory study

Effect of instream wood on vertical water flux in low‐energy sand bed flume experiments

A numerical investigation into the importance of bed permeability on determining flow structures over river dunes

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