Cooper, J

Ockelford, A

Powell, P

Rice, S

University of Liverpool Repository

See	discussions,	stats,	and	author	profiles	for	this	publication	at:	http://www.researchgate.net/publication/258775799Effect	of	Bed	Porosity	on	Momentum	Exchangein	Gravel-Bed	RiversCONFERENCE	PAPER	·	APRIL	2013DOI:	10.13140/2.1.1547.8084READS304	AUTHORS,	INCLUDING:James	CooperUniversity	of	Liverpool47	PUBLICATIONS			93	CITATIONS			SEE	PROFILEStephen	Philip	RiceLoughborough	University85	PUBLICATIONS			1,636	CITATIONS			SEE	PROFILEMark	PowellUniversity	of	Leicester46	PUBLICATIONS			928	CITATIONS			SEE	PROFILEAvailable	from:	James	CooperRetrieved	on:	19	November	2015Geophysical Research AbstractsVol. 15, EGU2013-5536, 2013EGU General Assembly 2013© Author(s) 2013. CC Attribution 3.0 License.Effect of Bed Porosity on Momentum Exchange in Gravel-Bed RiversAnnie Ockelford (1), James Cooper (2), Stephen Rice (1), and Mark Powell (3)(1) Loughborough University, Department of Geography, Loughborough, United Kingdom, (2) University of Liverpool,Department of Geography and Planning, School of Environmental Sciences, Liverpool, United Kingdom, (3) University ofLeicester, Department of Geography, Leicester, United KingdomMomentum exchange between a rough, porous gravel substrate and the overlying turbulent flow is a key controlof fine sediment ingress, pollutant exchange, spawning success and hyporheic flows. The surface topography andporosity of the substrate are known to be important controls on the near-bed hydraulics and momentum transfermechanisms. However, previous attempts to examine the effect of bed porosity on sediment-water exchangemechanisms have either been undertaken over idealised roughness elements, or over water-worked gravel bedswhere it has not been possible to isolate the influence of bed porosity from that of surface topography.In the present study, a series of flume experiments have been undertaken that examined the near-bed flowdynamics over porous and non-porous gravel beds that exhibited the same surface topography. Using differentgrain-size distributions and sediment feed rates, a number of water-worked, porous beds with differing surfacetopography were created within the flume. A novel casting technique was used to produce non-porous facsimiles ofeach of the porous beds that faithfully replicated the topography of the original surfaces. The near-bed hydraulicsover these beds were measured using Particle Image Velocimetry and used to deduce the temporal and spatialcharacteristics of the flow. Using these data, we compared the Reynolds and form-induced stress contributions tothe momentum budget for the porous and non-porous beds. The experiments provided an evaluation of the linkbetween bed porosity and fluid momentum transfer across sorting and bed structure gradients, thereby overcominga major limitation in existing understanding of sediment-water exchange mechanisms.Early results from this work reveal (1) porous and non-porous beds differ in the relative role of Reynoldsstress, and thus turbulent fluid exchange, in momentum transfer; (2) within the roughness layer, the total fluidstress is distributed differently in the vertical; and (3) the effects of bed porosity vary with surface topography andcan be related to the arrangement of surface grains. These advances underpin the development of more realisticcharacterisations of sediment-water exchange mechanisms, which are vital for the continued development of finesediment deposition and pollutant transfer models.

Effect of bed porosity on momentum exchange in gravel-bed rivers

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