streams chapter6

8/3/2019 STREAMS Chapter6

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Stream Behavior Is Predictable

Streams seek a state of dynamic equilibrium

Equilibrium is a function of the flow and

sediment

Equilibrium is naturally associated with a main

channel and a flood-prone area

Effective (bankfull) discharge forms the

main channel Streams meander in a predictable manner


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Stream Stability

Natural stream channel stability is

achieved by allowing the river to develop a

stable dimension, pattern and profile such

that channel features are maintained andthe stream system neither aggrades nor

degrades (Leopold)

Correctly engineered rigid channels can be

part of a stable stream system.


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River Dimensions


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Velocity Distribution In A Channel

Depth-averaged velocity is above

the bed at about 0.4 times the depth


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Deep Overbank Flow


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Features Of A River


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Salt Creek (Hocking County Ohio)


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Flow in Streams

Effective Discharge

Shear Stresses

Sediment Transport

Bed Load Movement Land Use and Land Use Change

Open Channel Hydraulics

Resistance Equations

Compound Channel Pattern & Profile


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Effective Discharge The effective, dominant, and bankfull discharge are

often considered as synonymous.

The bankfull discharge is considered to be thechannel-forming or effective discharge (Leopold,

1994).

The bed load fraction of the total sediment load ismost influential in channel forming processes and

effective discharge (Emmett and Wolman, 2001).

The effective discharge typically occurs less than ahand full of times annually


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Bankfull Depth Versus Discharge


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Loramie Creek, Ohio

0

10000

20000

30000

40000

50000

0 500 1000 1500 2000 2500 3000

discharge rate

s

edimentdischarge(tonsx10

geomorphic

work

0

50

100

150

200

250

daysofoccurren

ce

A: sediment function (x 20) C: geomorphic work B: days

A:Sediment Transport Rate

B: Days Occurring C: Geomorphic Work

Effective Discharge


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Be d Load of Pre-Dev elopment and No C ontrol

of Post-dev elopme nt

0

1

2

3

4

5

6

0.1 1 10 100

Recurrence Interval (yr)

B

edLoad(m

3yr-1)

pre-development

no control

Effective Discharge


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Bed Material is Predictable

Bed MaterialBed Material


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Shear Stresses on the Bed and Bank

Mean bed material size is a function of the shear

stresses produced by the effective (critical) discharge


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Mean Particle vs. Tractive Force at Incipient Motion

0.1

1

10

100

1000

0.01 0.1 1 10 100

Tractive Force, T (kg/m2)

MeanDiameterBed

Material,(mm)

Upper Limit

Low Limit


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Andys 1 x 1 = 1 Rule

A 1 ft flow depth and a 1 % bed slope can

move 1 inch diameter bed material

A 4 ft flow depth and a 0.5 % bed slope can

move 2 inch diameter bed material(d50 approximately 2inch)


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Shear Stress on Banks

On straight banks they are 0.7 - 0.8

the mean bed shear stress

On bends they are 2-4 times the mean

bed shear when the ratio of the radius

of curvature to width is less than 5


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Bed Material Particle Sizes

Pebble Count, Small alluvial stream in the Midwest USA

0%

20%

40%

60%

80%

100%

0.01 0.10 1.00 10.00 100.00 1000.00

Particle Size (mm)

Percen

tFiner

Than

Cumulative Percent Percent Item

Based on a Wolman Pebble Count


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Sediment Transport

Wash Load

Suspended Load

Bed Load

Types

Methods Shear Stress

Power

Parametric


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Meanders


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Meander Length vs Channel Width


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One meander length is equal to

10-14 Bankfull Widths


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Meander Geometry


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Meandering adjusts the slope

for the best stability.

Slope = rise / run

Starts Here

Ends Here

Straighter, Steeper,

Faster

Meandering


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Flood Hydraulics within a Meander

Sellin and Willetts,Floodplain Processes, Walling, 1996


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Streamway Concept


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1989

1951

1966

1980

1997

Drainage Area

30 sq. mi.

Streamway

120 * DA 0.43 = 518 ft

Salt CreekVinton County, Ohio


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Channel Features: Profile (Slope)


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Pools and RifflesRiffle - Pools Features

(Spaced at 5-7 Bankfull Widths)


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Stream Changes Are Predictable


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Channel Evolution

Pre-Development

Downcutting

Widening

Restored Floodplain


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Bank Erosion Hilliard, Ohio


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Loss of Stream Equilibrium

Lost Floodplain AccessLost Floodplain Access

Channel

Resizing

Channel

Resizing

Loss of Buffering from SmallLoss of Buffering from Small

Streams and FloodplainsStreams and Floodplains


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Thank You!

streams chapter6

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