SCOUR IN LONG CONTRACTIONS - PowerPoint PPT Presentation

1 / 34
About This Presentation
Title:

SCOUR IN LONG CONTRACTIONS

Description:

Department of Civil Engineering. Indian Institute of Technology. Kharagpur. INDIA. INTRODUCTION ... Determination of scour depth with sidewall correction ... – PowerPoint PPT presentation

Number of Views:125
Avg rating:5.0/5.0
Slides: 35
Provided by: rajkumar2
Category:

less

Transcript and Presenter's Notes

Title: SCOUR IN LONG CONTRACTIONS


1
SCOUR IN LONG CONTRACTIONS
Rajkumar V. Raikar Doctoral Research
Fellow Department of Civil Engineering Indian
Institute of Technology Kharagpur INDIA
2
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
3
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
4
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
5
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
6
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

7
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

8
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
9
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
10
(No Transcript)
11
Study of flow field within channel contractions
  • Using acoustic Doppler velocimeter (ADV)

12
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
13
(No Transcript)
14
(No Transcript)
15
Normalized velocity vectors along the centerline
for the channel opening ratios (a) 0.7, (b) 0.6,
(c) 0.5, and (d) 0.4
16
Parametric study
1
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
17
(No Transcript)
18
(No Transcript)
19
3
20
  • 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
4
5
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)
21
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
6
uc critical shear velocity for sediments
obtained from the Shields diagram fb friction
factor associated with the bed evaluated by the
Colebrook-White equation
7
22
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
8
A total flow area ( h2b2) Pw wetted
perimeter associated with the wall ( 2h2)
For clear-water scour, the continuity equation,
Eq. (5)
9
23
Eqs. (6) - (9) solved numerically to obtain
, h2, Rb and fb Equilibrium scour depth
ds obtained from Eq. (4)
10
The comparison of nondimensional equilibrium
scour depths ( ds/h1) computed from Eq.
(10) with experimental data is shown
24
(No Transcript)
25
Determination of scour depth without side wall
correction Following semi-logarithmic average
velocity equation used along with Eq. (9) to find
and h2
11
Equilibrium scour depth ds obtained by Eq.
(10) Comparison of nondimensional equilibrium
scour depths ( ds/h1), for this case, with
experimental data shown
26
(No Transcript)
27
  • 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

12
? bed-load transport of sediments, estimated by
Engelund and Fredsøe (1976) equation
13
u shear velocity
28
At section 1, shear velocity u1
14
Substituting u2 in Eq. (5)
15
Eqs. (12), (14) and (15) solved numerically to
obtain U2 and h2 Equilibrium scour depth ds
determined from Eq. (4) as
16
The comparison of nondimensional equilibrium
scour depths computed using model with live-bed
scour data shown
29
(No Transcript)
30
Empirical equation for maximum equilibrium scour
depth From Eq. (1) using dimensional analysis,
17
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
18
31
Comparison between experimental data and computed
values of equilibrium scour depths using
empirical equation
32
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

33
  • 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

34
Thank you for your attention
Write a Comment
User Comments (0)
About PowerShow.com