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Lecture 11

Lecture 11.pdf

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Wilfrid Laurier University
James Hamilton

Fluvial Processes April 10, 2014 1:29 PM • Introduction to Stream Flow • Channel processes ○ Stream boundary conditions ○ Sediment  Entrainment, transport  Load ○ Trenhaile Chapter 10 Introduction to Stream Flow • Stream Velocity ○ In general, velocitiesare higher where:  Channel bed and banks are 'smooth'  Channel gradients are steep  Channel width to depth ratio is low ○ May be measured directly ○ Estimated using a flow formula such as the:  Chezy Equation,  Darcy-WeisbachEquation, or the  Manning Equation ○ ○ Values of n, more friction= higher n • How do channel width and depth influence velocity? ○ Consider the hydraulic radius ® • Hydraulic Radius (R) ○ What is the impact of R if we hold width constant but increase depth?  If depth = 1m and width= 10m □ R is 10m2/12m □ R= 0.83  If depth= 3m and width= 10m  R is 30m2/ 16m  R= 1.9 ○ If we increase R, what happens to velocity? ○ How does R vary between channels, is there a difference between wide and shallow channels and narrow deep channels? ○ Width= 10m, depth =0.5m  R= (10m2)/11m=0.45m ○ Width= 5m, depth= 2m  R= (10m2)/9m=1.1m ○ Wide and shallow channel:  R is relatively small ○ Deep and narrow channel:  R is relatively large • Examine the impact of R on stream velocity using the manning equation • Examine the impact of R on stream velocity using the manning equation • Manning Equation ○ • In a channel: ○ Width, depth, gradient and bed materials will change, thus velocity and flow conditions will vary Channel Processes • Stream Boundary Conditions ○ Laminar and Turbulent Flow ○ ○ Laminar Flow  Water molecules movein parallel paths that slide over one another with little mixing ○ Turbulent Flow  Water molecules movealong highly variable paths with considerable mixing ○ Reynolds Number (Re):  Used to define the transition from laminar to turbulent flow: □ Re= (vR)/v  Where □ v= velocity □ R= is length (distance) □ v(second v)= Kinematic Viscosity (measure of the resistance of a fluid to deform) When Re<500the flow is laminar   For turbulent flow, the Re is higher ○ Froude Number (Fr): Nature of the Turbulence  Fr= ((v)/(gd)^0.5) □ Where v is velocity,d is depth, g is accelerationdue to gravity (m/s2)  When Fr<1: flow is more tranquil  When Fr>1: flow is more chaotic and erosive Chanel Processes: Boundary Layer • Velocity profiles for smooth turbulent (a) and roug
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