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9 th ISE 2012, Vienna
REDUCTION OF LOCAL SCOUR BY SHAPE OF SINGLE SPUR DIKE IN
RIVERBEND
ALIREZA MASJEDI Department of Agriculture, Ahvaz Branch, Islamic
Azad University, Ahvaz, IRAN
ELAHEH PEYMANI FOROUSHANI Researcher of Islamic Azad
University,Ahvaz, IRAN
AbstractA series of experiments were conducted in which the the
scour hole associated with model spur dike was
measured in a 180 degree laboratory flume bend under clear-water
overtopping ows. In this study, the localscour were conducted for
three different shapes of oblong, rectangulat chamfered of straight
spur dikes at the
bend with various Froude number. The main goals of the
experiments were to evaluate the effect of the threedifferent
shapes of straight spur dikes on the volume of scour and potential
aquatic habitat and on minimizingerosion adjacent to the
streambanks. The experiments showed that of the three different
shapes of straight spurdikes tested, the least erosion of the
around in the near bank region was associated with the spur dikes
withoblong shape, while the greatest volume of the scour hole was
associated with the rectangular shape. So it wasobserved that, as
Froude number increases, the scour increases.Key words: Straight
spur dike, 180 degree flume bend, Local scour, Froude number
1 INTRODUCTION
A spur dike may be de ned as a structure extending outward from
the bank of a stream for the purpose ofde ecting the current away
from the bank to protect it from erosion. In addition to bank
protection, spur dikeshave also been used to enhance aquatic
habitat by creating stable pools in unstable streams.
Shields et al. [6] documented signi cant increases in sh
numbers, size, biomass, and number of species inan incised stream
following modi cation of spur dikes to enlarge scour holes and
increase the percentage of
pools in the reach. Designers of bank stabilization structures
should, where possible, select spur geometry whichstabilizes the
bank and provides the largest scour volume subject to cost
constraints.
The scour occurring at a spur dike is divided into three
categories, general scour, constriction scour and localscour. Local
scour results directly from the impact of the spur dike on the
local flow pattern.
The flow past the spur dike may be divided into three zones: a
main flow zone from the head of the spurdike to the opposite side
of the channel, a wake zone behind the spur dike and a mixing zone
in-between them(Fig. 1).
Figure 1. Flow around spur dike
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Estimation of the length, width and depth of scour in the
vicinity of spur dikes has been the main concern of
engineers for years. Therefore, knowledge of the anticipated
maximum scour hole for a given discharge is asignificant criterion
for the proper design of a spur dike foundation.
Oliveto and Hager [5] studied the temporal evolution of
clear-water pier and abutment scour and found thatthe principal
parameter influencing the scour process is the densimetric particle
Froude number so suggested anlogarithmic formula. Ghodsian and
Vaghefi [4] studied scour and flow field in a scour hole around a
T-shapespur dike in a 90 degree bend. The effects of the length of
the spur dike, the wing length of the spur dike andFroude number on
the scour and flow field around a T-shape spur dike in a 90 degree
bend were investigated inthis study. The main results of this
experimental study are: At the upstream of the spur dike, a main
vortex withanti-clock wise direction is formed in the zone of the
spur dike. At section 77.5 degree of the bend a vortexhaving a
clock wise direction is formed between the spur dike wing and the
channel wall. The maximum value ofthe longitudinal velocity
component at section 65 degree of the bend is close to the outer
wall of the channel andnear the water surface. By increasing Froude
number the maximum scour depth and the volume of scour
holeincreases. The dimensions of the scour hole increase as a
result of increase in the length of the spur dike. Theamount of
scour at the upstream of spur dike is much more as compare to that
at the downstream of spur dike.
Masjedi et al. [7] studied on the time development of local
scour at a spur dike in a 180 degree flume bend.
Tests were conducted using one spur dike with 110 mm length in
position of 60 degree under four flowconditions. In this study, the
time development of the local scour around the spur dike plates was
studied. Theeffects of various flow intensities (u */u*c) on the
temporal development of scour depth at the spur dike were
alsostudied. The time development of the scour hole around the
model spur dike installed was compared with similarstudies on spur
dikes. The results of the model study indicated that the maximum
depth of scour is highlydependent on the experimental duration. It
was observed that, as flow intensities (u */u*c) increases, the
scourincreases. Measuring time and depth of scouring based on
experimental observation, an empirical relation isdeveloped with
high regression coefficient 97%.
Masjedi, et al. [8] studied investigation of effect of wing
shape on reduction of local scour at a T-shape spurdike in a 180
degree flume bend are presented. The study was conducted using in a
180 degree laboratory flume
bend. Experiments were conducted for three wing shapes of
T-shape spur dikes at the bend with various Froude
number. In this study, the time development of the local scour
around the T-shape spur dike plates was studied.The results of the
model study indicated that the maximum depth of scour is highly
dependent on theexperimental duration. It was observed that, as
Froude number increases, the scour increases. All Froudenumbers,
oblong wing at location of 60 degree results maximum reduction in
scour depth. Measuring depth ofscouring based on experimental
observation, an empirical relation is developed with high
regression coefficient95%. As it can be seen from the forgoing
paragraphs, fast majority of researches on scour at spur dike
areconducted at a straight flume. In practice there are many
examples where the spur dike the flume bend. In such acase the flow
patterns which are mostly the cause of scour would not be the same
as the case of straight flumeand therefore it is the principal
objective of this study is to carry out experimental tests on the
effect of straightspur dike shape on local scour at spur dike in
location of 45 and 60 degree in a 180 degree flume bend.
2 MATERIALS AND METHOD
The experiment reported herein was conducted in a recirculation
flume, with central angle of 180degree, centralradius of Rc=2.8m
and width of B=60cm. Relative curvature of bend was Rc/B=4.7 which
defines it as a mild
bend. Straight entrance flume with the length of 9.1 m was
connected to the 180 degree bend flume. This bendedflume is
connected to another straight flume with the length of 5.5m. The
test area of the flume is made up of analuminum bottom and
Plexiglas sidewalls along one side for most of its length to
facilitate visual observations.At the end of this flume a
controlling gate was designed to adjust the water surface height at
the desired levels(2).
Figure 2. Experimental setup (Plan)
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In this study to maintain the clear water condition without
formation of ripple, uniform sediment withmedian size of d 50 = 1.3
mm, and geometric standard deviation of g ~ 1.4 were used [2] was
used with athickness of 0.2 m and covered the total length of
channel. The spur dikes were made of Plexiglas in plan andlocated
at section 45 and 60 degree in the bend. The spur dikes were of 2
cm thick and 50 cm high.
The experiments was carried out using one length for spur dike
(i.e. L = 20% of the channel width) [3] andthree different shapes
of oblong, rectangulat chamfered and rectangular were used (Fig.
3).
Figure 3. A spur dike straight with different shapes
Four Froude numbers of 0.15, 0.18, 0.2 and 0.23 were applied in
order to investigate the effect of flowconditions on the scouring.
All the experimental tests where conducted under the same flow
depth and in twolocations of 45 and 60 degree in 180 degree flume
bend. A 60 degree triangular weir was used at the upstreamsection
of the flume for flow measuring. Equilibrium scour occurs when the
scour depth does not changeappreciably with time. In clear-water
scour, scour depth is approached asymptotically with time and may
take aninfinite amount of time for the equilibrium scour hole to
develop, while in live-bed scour the scour developsrapidly and then
fluctuates in response to the passage of bed forms. In this study,
a long time experiment wasconducted at a Froude number of 0.23 and
location of 60 degree for a spur dike of rectangular. Resulta
showsapproximately 94% of scouring occurs during the first 5 hours.
Therefore in all remaining of our experimentaltests, duration of 5
hours was selected for each test.
3 RESULTS AND DISCUSSION
3.1 Effect of Froude number at local scour The variation of
Froude number on the dimensional local scour (ds/y) was also
considered. Figure 4 shows thedimensional local scour around the
spur dike have been tested in this study. Four different Froude
numbers 0.15,0.18, 0.2 and 0.23 were applied in order to
investigate the effect of flow conditions on the scouring. The
spurdike was placed at two location of 45 and 60 degree at 180
degree flume bend. From this figure it is obviousthat, all shapes,
it can be seen that the higher the Froude numbers the deeper the
scour depth. The reason for thisis related to the fact that at
higher Froude number there is a greater acceleration of the flow
within the vicinity ofthe spur dike, and thus the intensity of the
downflow and the vortex is greater [7,8,4 and 1].
0.00
0.20
0.40
0.60
0.80
1.00
0.15 0.18 0.20 0.23
FR
d s
/ y
Spur dike with shape of oblong
0.00
0.20
0.40
0.60
0.80
1.00
0.15 0.18 0.2 0.23
FR
d s
/ y
Spur dike with shape of rectangular chamfered
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0.00
0.20
0.40
0.60
0.80
1.00
0.15 0.18 0.2 0.23
FR
d s
/ y
Spur dike with shape of rectangular
Figure 4. Dimensional local scour for different Froude
numbers
3-2 Effects of various shapes of spur dike at local scour
Figure 5 shows dimensional local scour (ds/y) around the spur
dike for three different shapes were used atlocations 45 and 60
degree in flume bend. As it can be seen from Figure 6, all Froude
numbers, at shape of
oblong results maximum reduction in scour depth and give a
maximum reduction in scour depth. The mainreason of such finding is
that minimum value of vortex at wing shape of oblong [7 and 4].
0.00
0.20
0.40
0.60
0.80
1.00
0.15 0.18 0.2 0.23
FR
d s
/ y
Location 45 degree
Figure 5. Dimensional local scour for different shapes of spur
dike
ACKNOWLEDGEAuthor thankfully acknowledges the financial support
provided by Islamic Azad University Ahvaz Branch
REFERENCES
[1]-Coleman, S.E., Lauchlan, C.S. and Melville, B.W. 2003.
Clear-water scour development at bridge abutments, Journal of
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and Sastry G. L. N. 1995. Clear water scour at circular piers: a
model. Journal of
Hydraulic Engineering, Vol. 121, No.12, pp. 869876.
[3]-Donat, M .1995. Bionengineering techniques for streambanj
restoration: A review of Central European practices, watershed
restoration project report No.2, University of British
Colombia,Austria.on bed scour in a 90o channel bend. International
Journal of Sediment Research, Vol. 21, No. 4, pp.
321-328.[4]-Ghodsian, M. and Vaghefi, M.2009. Experimental study on
scour and flow field in a scour hole around a T-shape spur dike in
a 90 bend, International Journal of Sediment Research , Vol. 24,
No. 2, pp. 145158.[5]-Oliveto, G. and Hager, W.H. 2002. Temporal
evaluation of clear-water pier and abutment scour. Journal of
Hydraulic Engineering, ASCE. 128(9): 811-820.[6]-Shields, F. D.,
Jr., Cooper, C. M., and Knight, S. S. 1995. Experiment in stream
restoration. J. Eng., ASCE ,121 (6), 494502.[7]-Masjedi, A., Shafai
Bejestan, M. and Moradi, A. 2010a. Experimental study on the time
development oflocal scour at a spur dike in a 180 degree flume
bend. Journal of Food, Agriculture & Environment, Vol.8
(2):904-907.[8]-Masjedi, A., Shafai Bejestan, M and Rahnavard, P.
2010b. Reduction of local scour at single T-shape spurdike with
wing shape in a 180 degree flume bend. World Applied Sciences
Journal 8 (9): 1122-1128.
0.00
0.20
0.40
0.60
0.80
1.00
0.15 0.18 0.2 0.23
FR
d s
/ y
Location 60 degree
