SCOUR IN LONG CONTRACTIONS

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SCOUR IN LONG CONTRACTIONS
Rajkumar V. Raikar Doctoral Research
Fellow Department of Civil Engineering Indian
Institute of Technology Kharagpur INDIA
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INTRODUCTION
Channel contraction - Reduction in width of
waterway of river or channel Purpose - To
reduce length of structure to minimize cost
Examples - Bridges, barrages, weirs,
cross-drainage works, cofferdams and end
dump channel contractions used for the
maintenance of the riverbanks
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Classification of channel contractions - Long
contractions for L/b1 gt 1 (Komura, 1966) L/b1 gt
2 (Webby, 1984) Present study - L/b1 ? 1 L
length of contracted zone b1 approaching
channel width Effect - Increase in flow
velocity Increase in bed shear stress
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Classification of channel contractions - Long
contractions for L/b1 gt 1 (Komura, 1966) L/b1 gt
2 (Webby, 1984) Present study - L/b1 ? 1 L
length of contracted zone b1 approaching
channel width Effect - Increase in flow
velocity Increase in bed shear stress
Channel contraction scour
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LITERATURE
Straub (1934) - pioneer in long contraction
- proposed one-dimensional theory Ashida
(1963), Laursen (1963), Komura (1966), Gill
(1981) and Webby (1984) - extended and modified
Lim (1993) - empirical equation of
equilibrium scour depth in long contractions
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SCOPE

• Earlier investigation on sand-beds
• Estimation of scour depth in the gravel-beds
  within long contractions unexplored
• Mathematical models developed for determination
  of scour depth inadequate

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OBJECTIVES

• Present investigation emphasizes on
• Experimental study of flow field within channel
  contractions
• Parametric investigation on scour depth within
  long contractions through experimental study
• Development of mathematical model for
  computation of scour depth within long
  contractions
• Determination of empirical equation for maximum
  equilibrium scour depth within long contractions

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EXPERIMENTATION
Flume details tilting flume (up to 1.7 ), 0.6 m
wide, 0.7 m deep and 12 m long Contraction
models made of perspex sheets, Opening ratio,
b2/b1 0.7, 0.6, 0.5 and 0.4 Length of
contracted zone 1 m
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Sediments used uniform sediments, ?g lt
1.4 median diameters (d50) Sand - 0.81 mm,
1.86 mm and 2.54 mm Gravel - 4.1 mm, 5.53 mm,
7.15 mm, 10.25 mm and 14.25 mm Approach flow
velocity 0.9 lt U1/Uc lt 0.98 U1 average
approaching flow velocity Uc critical
velocity for sediments
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Study of flow field within channel contractions

• Using acoustic Doppler velocimeter (ADV)

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Schematic view of a long rectangular channel
contraction
b2 contracted width of channel h1
approaching flow depth h2 flow depth in
contracted zone ds equilibrium scour depth
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Normalized velocity vectors along the centerline
for the channel opening ratios (a) 0.7, (b) 0.6,
(c) 0.5, and (d) 0.4
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Parametric study
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2
ds/b1 d50/b1 Fo densimetric
Froude number, U1/(?gd50)0.5 h1/b1
channel opening ratio, b2/b1 ?g
non-uniformity coefficient of bed sediment
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• Development of mathematical model
• Clear-water scour energy and continuity
  equations
• Live-bed scour energy, continuity and sediment
  continuity equations

Clear-water scour model Energy and continuity
equations applied between sections 1 and 2 for
flow situation at equilibrium condition
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U1 approaching flow velocity U2 flow
velocity in contracted zone hf head loss
between sections 1 and 2 negligible for
contractions with gradual transition (Graf
2003)
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Determination of scour depth with sidewall
correction For the equilibrium scour depth ds to
reach in a long contraction, the flow velocity U2
? critical velocity Uc for sediments The flow
velocity
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uc critical shear velocity for sediments
obtained from the Shields diagram fb friction
factor associated with the bed evaluated by the
Colebrook-White equation
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ks equivalent roughness height ( 2d50) Ab
flow area associated with the bed Pb wetted
perimeter associated with the bed ( b2) Rb
flow Reynolds number associated with the bed
4Ab/(?Pb)
Vanonis (1975) method of sidewall correction
applied
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A total flow area ( h2b2) Pw wetted
perimeter associated with the wall ( 2h2)
For clear-water scour, the continuity equation,
Eq. (5)
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Eqs. (6) - (9) solved numerically to obtain
, h2, Rb and fb Equilibrium scour depth
ds obtained from Eq. (4)
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The comparison of nondimensional equilibrium
scour depths ( ds/h1) computed from Eq.
(10) with experimental data is shown
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Determination of scour depth without side wall
correction Following semi-logarithmic average
velocity equation used along with Eq. (9) to find
and h2
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Equilibrium scour depth ds obtained by Eq.
(10) Comparison of nondimensional equilibrium
scour depths ( ds/h1), for this case, with
experimental data shown
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• Live-bed scour model
• Equilibrium scour depth ds attained
• sediment supplied by the approaching flow into
  the contracted zone equaling sediment transported
  out of the contracted zone
• At equilibrium
• - sediment continuity equation between sections 1
  and 2

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? bed-load transport of sediments, estimated by
Engelund and Fredsøe (1976) equation
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u shear velocity
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At section 1, shear velocity u1
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Substituting u2 in Eq. (5)
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Eqs. (12), (14) and (15) solved numerically to
obtain U2 and h2 Equilibrium scour depth ds
determined from Eq. (4) as
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The comparison of nondimensional equilibrium
scour depths computed using model with live-bed
scour data shown
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Empirical equation for maximum equilibrium scour
depth From Eq. (1) using dimensional analysis,
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ds/h1 F1e excess flow Froude number,
U1e /(?gh1)0.5 U1e excess velocity of flow,
U1 d50/h1
From experimental data, with regression analysis
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Comparison between experimental data and computed
values of equilibrium scour depths using
empirical equation
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CONCLUSIONS

• The velocity vectors reveal that velocity
  increases towards the zone of maximum scour depth
  and then decreases
• Scour depth increases with increase in sediment
  size for gravels. Curves of scour depth versus
  sediment size have considerable sag at transition
  of sand and gravel
• Scour depth gradually reduces with increase in
  densimetric Froude number for larger opening
  ratios. However, for small opening ratios, trend
  is opposite
• Scour depth increases with increase in
  approaching flow depth at lower flow depths, but
  it becomes unaffected by approaching flow depth
  at higher flow depths

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• Scour depth increases with decrease in
  contracted width of channel
• Nonuniform sediments reduce scour depth to a
  great extent due to formation of armor-layer
  within scour hole
• Scour depths computed using the models are in
  excellent agreement with the experimental data
• Characteristic parameters affecting maximum
  equilibrium nondimensional scour depth, have been
  excess approaching flow Froude number, sediment
  size - approaching flow depth ratio, and channel
  opening ratio

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Thank you for your attention