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1B Clastic Sediments Lecture 28 BEDFORMS IN COHESIONLESS SUBSTRATE Structure of bedforms Formative conditions Unidirectional and Oscillating flows NH

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BURSTS AND SWEEPS Flow streaks in wall region. Spacing of streaks, depends on flow properties: Re * = u * / = 100 Re * is boundary Reynolds no. u * = √ 0 / is shear velocity. Burst-sweep process is main creator of turbulence. Inrush of high-velocity sweeps may locally exceed threshold of sediment motion.

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RIPPLE INITIATION Ripples form when random points of high boundary shear stress (sweeps) cause formation of a pile of grains. Pile of height v causes flow disturbance ~100 v long downstream, similar to the separation zone behind a ripple. D > 0.7 mm: grains disrupt viscous sublayer and discrete flow disturbances no longer occur. Ripples do not form, bed is plane. Bedform wavelength ~100 v

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FLOW OVER BEDFORM Ripples and dunes formed under uni- directional flow have shallow upstream or stoss faces, dominated by rolling grains, and steep downstream or lee slopes, dominated by grain avalanching.

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BEDFORM MIGRATION AND SEDIMENT FLUX Downstream flux of sediment due to bedform migration: where U B is speed of bedform, H is height of bedform, is porosity of bed material.

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SEDIMENT FALLOUT Climbing ripples Angle of climb and preservation of stoss and lee side are determined by balance of downstream translation and vertical build up.

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BEDFORMS PLANFORM AND INTERNAL STRUCTURE Planar cross stratification Trough cross stratification Basic bedform: crescent

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FLOW OVER BEDFORM Ripples and dunes formed under uni- directional flow have shallow upstream or stoss faces, dominated by rolling grains, and steep downstream or lee slopes, dominated by grain avalanching. Dunes: ~ 2 h dune height = h/3 to h/D where h is flow depth, and D is grain size. Ripples:height < 4 cm

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BEDFORMS UNDER SHEAR FLOW On flat bed, resistance to flow is due to boundary roughness (~ grain size): skin friction 0 ~ U a Developing bedforms become main roughness element: form drag With increasing flow velocity :1) bedforms grow, shear stress up. 2) dunes wash out, replaced by flat bed: shear stress down. 3) standing waves and antidunes form: shear stress up. Shear stress bad indicator of state of bed; use flow velocity. Flat bed Antidunes

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CONTROLS ON BEDFORM: Assumptions: steady and uniform flow, equilibrium bedforms, mean grain size describes bed material. Variables: Grain size D[L] Density of grains s [ML -3 ] Density of fluid f [ML -3 ] Viscosity of fluid [ML -1 T -1 ] Gravitational acceleration g [LT -2 ] Flow depth h[L] Flow velocity U[LT -1 ]

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BEDFORMS STABILITY FIELDS Flow depth: 0.25 – 0.40 m

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BEDFORMS STABILITY FIELDS Absence of ripples in course sand: Lack of viscous sublayer over hydraulically rough boundary. Upper plane bed in fine grains: Due to high sediment concentration damping turbulence.

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FLOW REGIMES Lower flow regime Upper flow regime Hydraulic jump Gravity works to flatten a rough flow: Froude number is dimensionless product expressing balance of inertial and gravitational forces Fr 1: supercritical flow

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BEDFORMS STABILITY FIELDS; FLOW REGIMES Super critical Upper Sub critical Lower flow regime

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CONTROLS ON BEDFORM: DIMENSIONAL ANALYSIS Assumptions: steady and uniform flow, equilibrium bedforms, mean grain size describes bed material. Variables: Grain size D[L] Density of grains s [ML -3 ]exclude Density of fluid f [ML -3 ]repeat Viscosity of fluid [ML -1 T -1 ]repeat Gravitational acceleration g [LT -2 ]repeat Flow depth h[L] Flow velocity U[LT -1 ] Dimensionless Products: Experimental set up: Water, quartz sand, variable temperature. constant in temp

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BEDFORMS STABILITY FIELDS Bedform stability can be represented in 3D plot of standardized flow velocity, flow depth and grain size. Sections through this cube can be viewed. h 10

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BEDFORMS UNDER OSCILATORY WAVES Controls on bedform:Flow velocity Sediment grain size Wave period Form Index: L/H

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BEDFORMS UNDER OSCILATORY WAVES

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