556 UNIDIRECTIONAL AQUEOUS FLOW ticle-Driven Subaqueous Gravity Processes; Deposition from Suspension; Fluxes and Budgets Further Reading Figure 14 Flow acceleration above the raised limb of a burrow creates a lower pressure at B than at A, inducing flow within the tube (after Vogel S (1994) Life in Moving Fluids Chichester: Prince ton University Press) The Porosity of the Bed Surface Many studies of unidirectional flows have assumed that the bed is impermeable and that the subsurface flows exert little influence on the overlying boundary layer However, this assumption is invalid, and the flow within porous beds, which comprise the surface of most sedimentary environments, can significantly alter the nature of the overlying boundary layer This is especially true in the presence of bed morphology, which may generate differential velocities and fluid pressures around the topography Such flow is seen, for instance, out of relict burrows that have a raised rim (Figure 14), where flow through the burrow and towards the area of raised topography results from lower fluid pressure associated with increased velocities at this raised opening However, flow within porous beds may also lead to modification of the near-bed velocity profiles, and it has been suggested that the presence of a porous bed can decrease nearbed velocities and cause the velocity profile to deviate from a logarithmic form, with turbulence penetrating the top part of the porous bed See Also Sedimentary Environments: Alluvial Fans, Alluvial Sediments and Settings; Deserts; Storms and Storm Deposits Sedimentary Processes: Erosional Sedimentary Structures; Depositional Sedimentary Structures; Par- Ashworth PJ, Bennett SJ, Best JL, and McLelland SJ (1996) Coherent Flow Structures in Open Channels Chichester: John Wiley and Sons Baas JH and Best JL (2002) Turbulence modulation in clay rich sediment laden flows and some implications for sediment deposition Journal of Sedimentary Research 72: 336 340 Bridge JS (2003) Rivers and Floodplains Oxford: Blackwell Publishing Buffin Belanger T and Roy AG (1998) Effects of a pebble cluster on the turbulent structure of a depth limited flow in a gravel bed river Geomorphology 25: 249 267 Cantwell B, Coles D, and Dimotakis P (1978) Structure and entrainment in the plane of symmetry of a turbulent spot Journal of Fluid Mechanics 87: 641 672 Elghobashi S (1994) On predicting particle laden turbulent flows Applied Scientific Research 52: 309 329 Leeder MR (2000) Sedimentology and Sedimentary Basins: From Turbulence to Tectonics Oxford: Blackwell Publishing Lo´ pez F and Garcı´a MH (1999) Wall similarity in turbulent open channel flow Journal of Hydraulic Engineering 125: 789 796 Middleton GV and Southard JB (1984) Mechanics of Sedi ment Movement SEPM Short Course Notes Tulsa: Society of Economic Palaeontologists and Mineralogists Nelson JM, McLean SR, and Wolfe SR (1993) Mean flow and turbulence fields over two dimensional bedforms Water Resources Research 29: 3935 3953 Nezu I and Nagkagawa H (1993) Turbulence in Open Channel Flows Balkema: International Association for Hydraulic Research Pope SB (2000) Turbulent Flows Cambridge: Cambridge University Press Robinson SK (1991) Coherent motions in the turbulent boundary layer Annual Review of Fluid Mechanics 23: 601 639 Van Rijn LC (1990) Principles of Fluid Flow and Surface Waves in Rivers, Estuaries, Seas and Oceans Oldemarkt: Aqua Publications Vogel S (1994) Life in Moving Fluids Chichester: Princeton University Press Williams J (1996) Turbulent flow in rivers In: Carling PA and Dawson M (eds.) Advances in Fluvial Dynamics and Stratigraphy, pp 67 125 Chichester: Wiley and Sons