454 lecture 6

FLUVIAL PROCESSES

Water flowing downhill has kinetic & potential energy due to the

pull of gravity. This energy can be expended in

friction against bed & banks

carrying sediment

internal friction (eddies, turbulence, viscosity)

Velocity reflects the balance between energy causing flow &

energy consumed by resistance to flow

Flow equations describe how energy is distributed in a flow:

eg. Reynolds number, Re

Re = (vRρ)/ μ = driving/resisting

ρ is density, μ is molecular viscosity, R = area/wetted perimeter

Re < 500 laminar Re > 750 turbulent

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Boundary roughness

Internal resistance

Sediment movement

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Flow resistance equations relate velocity to flow parameters:

eg. Chezy v = C √ R SC is constant of proportionality relating to resistance factors

S is slope

eg. Manning v = (1.49/n) R2/3 S1/2

n is Manning roughness coefficient

External resistance caused by

particle size channel irregularities

bedforms structures

vegetation

Internal resistance comes from the amount of sediment being

carried by the water: increases in sediment decrease resistance

as turbulence effects decrease because mixing within the fluid

decreases

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Entrainment: processes that initiate motion of particles from

bed & banks

(amount entrained depends on erosive power of flow relative to

resistance (due to size & packing) of particles)

depositiontransport

erosion

particle size

me

an v

elo

city

Hjulström

Curve

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Competence: size of largest particle stream can entrain under

given hydraulic conditions

Capacity: maximum quantity of sediment transported

Load: actual quantity

A threshold is reached when forces promoting & resisting

entrainment meet; initial movement can be specified either by

critical velocity or critical shear stress – either way, the process

is difficult to model/predict because of stochastic conditions –

special spatial or temporal conditions at the time of entrainment

are unmeasurable due to their transiency

Sediment entrainment can be generalized in the manner of the

Hjulström Curve

Supply-limited or capacity-limited systems

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Wash load – material in solution or suspension

Suspended load – sediment in suspension

Bed material load – rolling, sliding, saltating in contact with the

streambed

DH-48 &samplebottle

Helley-Smith

Downstream, clasts are smaller,

rounder, better-sorted due to

abrasion (mechanical)

sorting (selective entrainment &

differential transport)

454 lecture 6

Channel Pattern & Shape

Most common classification is straight

meandering

braided

Straight vs meandering depends on sinuosity

P = (stream length)/(valley length)

P > 1.5 meandering

Straight/meandering vs braided depends on division of river

into more than one channel – arbitrary divisions, which can

change with stage

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Straight

• least common

• have alternate bars with pools & riffles

• spacing analogous to meanders

• thalweg (deepest part of channel) meanders

Meandering

• most common river form

• flow has strong lateral components (helical flow)

• river shifts by eroding on outside of bends & depositing on inside

• meander wavelength related to discharge & width

Meandering channels tend to be narrow & deep, and carry fine

sediments

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pool bar

riffle

thalweg

straight channels

BB’

braided channels

B B’

x-section

plan view

λ

meandering

channels

454 lecture 6

northern California

North Fork Poudre River, CO

N. St. Vrain Creek, CO

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North Park,CO

Wood River, Alaska

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Braided channels

• single trunk channel divided into networks of branches with

growth & stabilization of intervening islands

• steep & shallow, with rapid shifts (relative to other channel

types)

Braided channels occur where there are

erodible banks

large volumes of bedload

rapid & frequent discharge variations

Braid bars can be

transverse: perpendicular to current, grow downstream,

have cross-beds

point: on channel margins, beside banks

longitudinal: planar beds, grow downstream

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Bars develop as

• local channel condition allows deposition

• as particles move across reach, they are deposited on the

lower end, where depth increases & velocity decreases

• flow is deflected around widening bar – banks are eroded

• bar emerges as island flanked by two channels

Combined effects of discharge & sediment set stability range

for 3 channel patterns, which shift among each other as these

controls change

454 lecture 6

1969 flood boulder train, Jordan River

central AZ Death Valley, CA

coastal PeruArid-region braided rivers

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Matanuska River, Alaska

Jasper National Park, British Columbia

Annapurna region, Nepal

Glacial braided rivers

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Mt. St. Helensarea, WA, 1997

Aggradation above tributary junction, Khumbu, Nepal

upper Amazon basin, Ecuador

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Adjustments in the Fluvial System

discharge

sediment load all mutually adjust &

slope tend toward equilibrium

channel shape

Equilibrium – all of the interacting variables are mutually

adjusted to each other, & to prevailing external

conditions so that, in the absence of perturbations,

the system is stable

Most systems only tend toward equilibrium because different

variables respond differently to change due to lag times &

thresholds

454 lecture 6

Leopold & Maddock (1953) hydraulic geometry:

statistical relation between discharge & other variables

w = a Qb

d = c Qf

v = k Qm

a c k = 1 = b + f + m

w, d, v vary in response to Q

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Sequence of bedforms related to increasing flow intensity,& corresponding values of Darcy-Weisbach friction factorin flume experiments (after Simons & Richardson, 1966)

ripples0.052 < ff < 0.13

dunes &superimposedripples

dunes0.042 < ff < 0.16

transition

plane bed0.02 < ff < 0.03

antidunes; standing waves0.02 < ff < 0.036

chute & pool0.07 < ff < 0.09

antidunes;breaking waves0.07 < ff < 0.08

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ripples, Fall River, CO

Dirty Devil River,Utah

sand ripples migrating over gravel, MercedRiver, Yosemite

454 lecture 6

point bar stratigraphyGeegully Creek, Australia