Bedform Alluvial (2)

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Bedform & Flow Regim

- A bedform is a feature that develops at the interfaand a moveable bed, the result of bed material be

by fluid flow.

- Flow regime relates bedforms in alluvial channels

velocity. The classification also shows the relations

between flow velocity and the mode of sediment tr

the concentration of sediment being transported a

phase relation between the bed and the water (wa

surface).



Flow regime and its relationship to bed forms and other charac

(Lewis, 1984)



Froude number & Flow regime

• F <1 tranquil flow (lower flow regime)

• F = 1 critical flow conditions (flow regime transition)

• F> 1 rapid flow (upper flow regime)

In general, the basic shape of the flow regime, sediment is classified into• A. Regime low flow

• B. Regime transition flow

• C. Regime high flow



A. REGIME LOW

(Froude number <0.4 to 1 with ramps transition)

Consist of :

• Flat bed

• Ripples

• Dunes



Flat bed, is a sediment transport wdeformation and the movement detatossing and turning. And the magnitu

the shear of stress is exactly above frocritical.





DUNES, ALL SIZES OF SEDIMENT AND THE SH

OF STRESS INCREASES TO THE FRONT SIDE

FRONT SIDE IS SLOPE SLIGHTLY, THE BACK SIS STEEPER. EROSION CAN OCCUR ON ALL S

OF THE UPPER REACHES, AND DEPOSITION

OCCURS AT THE BOTTOM OF THE DOWNSTR

SIDE.





C REGIME

HIGH FLOW

(Froude Number> 0.4 to 1, a relatively small flow resistan

and large sediment’s transport)

Consist of :

• Plane bed

• Antidunes

• Chute and Pools



PLANE BED, HAS A FLOWRATE GRADUALLYRISING, SEDIMENT

TRANSPORT HAS A FLATHEIGHT. THE MOVEMENT

OF THE GRAIN ISROLLING OR SLIDINGAND CHANGES AT A

PARTICULAR PLACE. AFINE MATERIAL OCCURS

SALTASI.





Chute and Pools, occurs in the slope, velocity andsediment discharge which are relatively large. Thebasic form is a hill - a large sediment hill. The stateof the flow in chute is supercritical or subcritical.



Bedform Forecast

Description:

• s = specific weight of bed material

• y = height of bedform at x along the river

• t = time

• qs = sediment flow in a weight unity wide and time



The first limitation showed a decrease in the rate of

the base, and the second limit sediment transport s

change in the change of the distance x along the rivout that both these limits gradually always opposite

when the base is formed

positive

negative



The cross-sectional shape at time t and t + dt from

bedform that moves downstream.

In the upper part of the lower forms of the basicsituation which is a function of time, so

negative

positive

EXNER (1925) ASSUMED THAT



EXNER (1925), ASSUMED THAT:QS = AO. UO

AO = CONSTANT

UO = FLOW VELOCITY NEAR THE BASE

BY ENTERING AO AND UO INTO THE SEDIMEQUALITION CONTINUITY BEFORE, THEN O:







The concept of "lag

distance" was firstproposed by Kenne

(1963) and is the moimportant factor



Fr and kdSeen in Fr2, great

/ kd) tanh kd, and t

Curve Fr2 = (1 / kdgive upper limit for

and dunes Fr. Tran

area between Fr =

= 0.844



Relationship between the wavelength L

antidunes with average speed U as follow

Known as the equation for the wave spe

in the water



Conclution :

Passed antidunes surface waves break wh

position is steep. Comparison between wave h

wavelength is between 0.13 and 0.16 and betw

retrieved value is 0.14 for the water wave steep

the time began to break



alluvial roughnessIn an alluvial channel, the various regimes of bed formare the results of complicated interactions between the

overlying flow and the mobile bed sediments.

Why???



Because… The physics of bed form is complicated because tflow boundary is not fixed but changes dynamica

according to the sediment characteristics, channshape and flow strength, among other factors. Tvariable bed forms modify the flow resistance atherefore the stage-discharge relationship of tchannel conveyance. The mobile bed resistandepends on many interrelated factors including the sor grain resistance and form drag or bed fo

resistance. The former is dependent on the depth flow and grain size at the boundary surface while tlatter is the resistance associated with the edformations and secondary circulations set up by tflow over the bed form.



• ρ= fluid density

• g= gravitational acceleration• R= hydraulic radius related to bed

• S= energy slope

τ0=ρgRS





Divide hydraulic roughness to 2parts: R’ and R’’

Which is R=R’+R’’



Th d t t pgR



They assumed a constant pgRon both sides of equation (2)andintroduced an alternativeapproach based on the directsummation of two energyslopes. Equation (2) becomes

S=S’+S”

Th G hi b E l d H



The Graphic by Engelund-Hansen



Lovera-Kennedy 1969) and Alam-

Kennedy 1969)

Restrictions for Lovera-Kennedy dan AlKennedy:

• Base sediment characteristic =D50

• Limited analysis for gravel and water

• Gravity reaction=0 and there is no freewaves

The analysis result is: λ ’=f(Re=(U.R)/v.R/• v= kinematic viscosity of fluid

• R= hydraulic radius

• λ ’=friction factor Darcy Weisbach

Reference:



Reference: • Priyantoro, Dwi. 1987,Teknik Pengangkutan

Sedimen, Himpunan Mahasiswa Pengairan FT-UB,Malang

• Engelund, F., and E. Hansen (1967), A monographon sediment transport in alluvial streams, report,

Tech. Univ. of Denmark, Copenhagen.• Einstein, H. A., and N. L. Barbarossa (1952), River

channel roughness, Trans. Am. Soc. Civ. Eng., 117,1121 – 1146.

• Kennedy, J. F. (1963), The mechanics of dunes andantidunnes in erodiblebed channels, J. Fluid Mech.,16(4), 521 – 544.

• Chien, N., and Z. Wan (1999), Mechanics ofSediment Transport, Am. Soc. of Civ. Eng., Reston,Va



THANK YOU

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Bedform Alluvial (2)

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