Measurement of long shore current before and after permeable groin structure installation is difficult especially in hydraulic model of small water depth. The problems arise that no instrument is dedicated to measure such current at very small depth and long shore current varies quite significantly from the location of breaking wave to the\ud
maximum run up position. Measuring instruments that can be used to measure the long shore currents on a laboratory scale is very limited. Measurement of long shore current using floating objects is a method to overcome the limitations of measuring instruments of longs shore current. Floating objects arranged width surf zone and then was dropped by observing the position of the wave. Then the movement of floating objects was recorded by using a video camera. The magnitude of long shore current velocity obtained from the observation of movement distance of floating objects per\ud
time unit. The results of measurements of long shore current by floating objects was verified by the results of calculations using the equations of long shore current (equation Longuet-Higgins, 1970). Verification of the measurement results with the results of calculations indicate relatively similar results, so the floating objects method can be recommended for the measurement of long shore current

Nizam

Triatmadja, R.

Umar, H.

Yuwono, N.

Hasanuddin University Repository

 409 
 
 
Proceedings of the 7th International Conference on Asian and Pacific Coasts  
(APAC 2013) Bali, Indonesia, September 24-26, 2013 
 
 
 
MEASUREMENT OF LONG SHORE CURRENT AFTER PERMEABLE GROIN BY 
FLOATING OBJECT 
 
H. Umar 
1
, N. Yuwono 
2
, R. Triatmadja 
2
 and Nizam 
2
 
 
 
ABSTRACT: Measurement of long shore current before and after permeable groin structure installation is difficult 
especially in hydraulic model of small water depth. The problems arise that no instrument is dedicated to measure such 
current at very small depth and long shore current varies quite significantly from the location of breaking wave to the 
maximum run up position. Measuring instruments that can be used to measure the long shore currents on a laboratory 
scale is very limited. Measurement of long shore current using floating objects is a method to overcome the limitations 
of measuring instruments of longs shore current. Floating objects arranged width surf zone and then was dropped by 
observing the position of the wave. Then the movement of floating objects was recorded by using a video camera. The 
magnitude of long shore current velocity obtained from the observation of  movement distance of floating objects per 
time unit. The results of measurements of long shore current by floating objects was verified by the results of 
calculations using the equations of long shore current (equation Longuet-Higgins, 1970). Verification of the 
measurement results with the results of calculations indicate relatively similar results, so the floating objects method can 
be recommended for the measurement of long shore current. 
 
Keywords: Floating object, longshore current, measurements  
 
 
                                                 
1 Coastal Engineeering Department, Hasanuddin University, Jl. Perintis Kemerdekaan km. 10, Makassar, 90245, INDONESIA 
2 Civil Engineering Department, Gadjah Mada University,Jl. Grafika No.2, Yogyakarta, INDONESIA 
INTRODUCTION 
Longshore current occurs when the wave comes at an 
angle to the shoreline. The closer to the shoreline slope 
is getting steeper wave will come along with the reduced 
depth and the wave will eventually break. Breaking 
wave at an angle to the shoreline which then led to flow 
along the coast. Longshore current reaches a maximum 
at the breaker line position and will decrease until it 
reaches the maximum run-up position. 
Small depth of water at the laboratory scale and 
variation of flow velocity significantly in areas surfzone 
currents along the coast of the measurement result to be 
difficult. Current measurement methods along the coast 
with floating object (buoy balls) was developed to 
overcome the problems of the current limitations of 
measuring tools along the coast on a laboratory scale. In 
this paper will discuss the current measurement method. 
 
PERMEABLE GROIN 
Permeable groins have a porous structure that still 
allows the current through the structure so the transport 
of sediment to the downdrift groin can still happen. The 
magnitude of longshore current and longshore sediment 
transport through permeable groins is controlled by the 
size of the gap in the structure of the groin. Permeable 
groins structure has also been extensively used 
especially permeable pile groin structures. 
One example of the use of permeable pile groins 
discussed in Raudkivi (1996), namely permeable groins 
on the Baltic Sea coast. Permeable pile groin structure 
can reduce longshore current and longshore sediment 
transport while still providing the supply of sediment to 
the downdrift groin. 
Permeable groins can be form of piles align with a 
certain distance in the direction perpendicular to the 
shoreline. Figure 1, shows one example of permeable 
pile groins on the Baltic coast, German (Raudkivi, 
1996). 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig. 1 Permeable groin Baltic Sea, Jerman (Raudkivi, 
1996) 
 
 
 
 
 
 
Measurement Of Long Shore Current After Permeable Groin By Floating Object 
 
410 
 
THEORY OF LONGSHORE CURRENT 
 
Longshore Current Without Permeable Groin 
Longuet-Higgins (1970) developed the long shore 
current theory by using radiation stress concept. Current 
is assumed as two dimensions (with no vertical 
variation), steady and uniform along the y, with the 
momentum equation of x direction drawn as follows: 
 
     0)( 




 byty
x
v
hv
x
                 (1) 
 
where vt is mixed coefficient (eddy coefficient),
 
v
 
is 
average velocity of the long shore current,
 
y
 
is shear 
stress due to wave,
 
by  is average bottom shear stress. 
When shear strength due to turbulence is ignored, the 
shear stress due to wave 
y  can only be balanced with 
base friction of 
by . Thus, it will results in: 
     
byy  
               (2) 
 
Where shear stress due to wave can be written as follow,  
 
      α β
m
ρ uyτ sintan
2
4
5
                  (3) 
 
and bottom shear stress can be written as follow,  
 
      v
m
ρ u
f
C
by
 
2

                     (4) 
 
where Cf is bottom friction coefficient, um is maximum 
orbital velocity of wave ))(
2
( ghum

  . 
Substituting Equation (3) and (4) into Equation (2), 
     
bmmf uvuC 

sintan
4
52 2
                  (5) 
 
then the equation of long shore current without the 
influence of lateral mixing as follows (Longuet-Higgins, 
1970), 
 
      sintan
16
5
b
b
f h
g
H
C
v                   (6) 
 
where um is maximum orbital velocity of wave (m/s), tan 
β is beach slope, b is angle of breaking wave, Hb is 
breaking wave height, hb is breaking wave depth (m).  
 
If the turbulence is not ignored so that the completion 
of the long shore current velocity equation using a 
settlement with the lateral mixing the long shore current 
velocity profile will not form a triangle with the sudden 
change in the position of the breaker line (Figure 1). 
Completion of the lateral mixing will smooth and reduce 
the long shore current velocity at breaker position. 
On the completion with influence of turbulence, 
Longuet-Higgins (1970) using a length mixed model of 
Prandtl for mixed coefficient (vt) in Equation (6) is as 
follows. 
    ghxNvt        (7) 
 
with dimensionless constants N (0 < N < 0.016).  
 
Besides of mixed coefficient parameter (vt) in the 
completion of with a lateral mixing is also used non-
dimensional parameters as follows, 
 
     
bx
x
X        
 
     
bv
v
V   and     
 
     
fC
Nm
P


                   (8) 
 
Then the differential equation is obtained as follows, 
 
      0,1         02
1
2
5
2/3XVX
X
V
X
X
P 











                 (9) 
 
The boundary conditions used were V near zero at 
the breaker line position and infinite position, as well as 
the value of V and 
X
V

  be continuous wave breaking 
position. Equation (9) is then integrated and obtained the 
following results (Longuet-Higgins, 1970). 
 
 10   ,
    X1          ,
1
1
2
2


 XAXXB
XB
p
pV                 (10) 
 
With 
 
     A
pp
p
BA
pp
p
B
21
1
2
21
2
1
1
dan  ,
1





               (11) 
 
And 
 
     
)
1
16
9
(
4
3
;
)
2
5
1(
1
1
p
p
P
A 


               (12) 
 
If P = 
2
5
 then equation (10) will be, 
   







1X0       ,ln
7
5
49
10
X1               ,
49
10 2/5
XXX
X
V                 (13) 
 
With the completion of the equation (10), the long shore 
current velocity profile will become continue as shown 
in Figure 1. 
 
 
H. Umar et al. 
 
411 
 
 
 
Fig. 2 Long shore current velocity profile without lateral 
mixing and with lateral mixing  
 
Longshore Current With Permeable Groin 
Development of longshore current equations with 
permeable groin based on the direction of longshore 
momentum equation without the influence of lateral 
mixing, Longuet-Higgins, 1970), with the basic 
assumption that the friction increased by the existence of 
permeable pile groins structure,  
 
   
groinbyy
                           (14) 
 
where 
groinby

 
is shear stress due to permeable groin 
barrier that is a function of the mean shear stress (shear 
stress between the water and the bottom) ( by ) added 
the average shear stress between water and piles of 
groins  ( g ), 
 
   gbygroinby
                  (15) 
 
Shear stress between water and piles of groins ( g ) 
is considered equal to the drag force of permeable groins 
(Fd) per unit area constraining groin area. 
 
   vudh
dL
N
C mtr
tg
dg 







 
2
1
                     (16) 
 
Total shear stress equation after the barrier of permeable 
pile groins can be written as follows,  
    
vudh
dL
N
CvuC mtr
tg
dmfgroinby 













 



2
1
4
2
  
    (17) 
 
Longshore current velocity equation through the 
permeable pile groins developed from the momentum 
equation y direction (parallel to the shoreline) after the 
groins, there is a balance between the shear stress due to 
waves (y) and the average shear stress due the 
permeable pile groin (
groinby
 ), 
 
groinbyy
                         (18) 
 
where 
groinby
  is the average shear stress due the 
permeable pile groin as a function of the average shear 
stress (shear stress between the water and the bottom) 
( by ) added the average shear stress between the 
water and the pile of groins ( g ), 
If Equation (3) and Equation (17) is substituted into (18), 
the longshore current velocity equation through 
permeable groins structure can be written as follows, 






















t
rd
f
b
b
b
groin
d
phC
C
h
g
H
v




2
4
4
2
)sin(tan
8
5
              (19) 
 
where  p is density of the groin (%), hr is the average 
depth of the submerged groin piles (m), dt is diameter of 
pile (m), Cd is drag coefficient. 
 
EKSPERIMEN 
Measurements of long shore currents on a laboratory 
scale facing constraints on gauge to be used. To obtain a 
better measurement then developed a method of 
measuring long shore currents using the floating object. 
The method of floating objects will measures average 
long shore current velocity by using a float balls. 
Floating object will throw using thrower as shown in 
Figure 3, taking into account the phase of the wave that 
happened, by putting measuring instrument of wave 
height near the thrower.   
 
 
  
Fig.  3 Thrower of floating objects 
Then the movement of the floating object was 
observed using a video camera, and the position of the 
floating object movements were analyzed using software 
With lateral mixing Without lateral mixing 
Wave probe 
 
Measurement Of Long Shore Current After Permeable Groin By Floating Object 
 
412 
 
that is made of the programming language Visual Basic 
6 to obtain the position x, y of a floating object that can 
later be determined magnitude of the average long shore 
current. 
Balls are used as a floating object is small balls with 
a diameter of 3.4 cm (1.7 cm radius), so the volume was 
20.6 cm3, a mass of 2.3 g. The ball that is used as a 
floating object must have a density close to the density 
of water so the ball movement can truly represent the 
movement currents. So the float balls used modified to 
have a density close to the density of water is 1 
gram/cm3. To obtain a density close to the density of 
water (1 gram/cm3) the float balls must have the same 
amount of mass or volume of a ball is close to the 
amount of 20.6 grams, while the mass of the empty ball 
only by 2.3 gr. Therefore float balls filled with water 
until the weight reached or approached the 20.6 gr. Once 
the float balls weight the same or close to the density of 
water, and then the float balls given a different color for 
ease in digitization of ball position x and y (Figure4). 
 
 
Fig. 4 Floating balls 
The process of digitization movement float balls 
accomplished using the track object (Triatmadja, 2011) 
using the programming language Visual Basic 6. Speed 
of video camera used is 30 frames per second. Before the 
movement of the float balls frame pieces in the program 
if the first track object motion video recording float balls 
being broken into photo frames using the program Ulead 
VideoStudio 11. In processing the movement of the float 
balls position in the track program object is used as 
photo frames number 1 or wave period equal to T 
seconds the movement of the ball. From the processing 
of data obtained track objects program x and y position 
of the float balls over timescales of one wave period (T 
seconds).  
Then the velocity of the movement of each float ball 
to the x direction (the direction of long shore) can be 
determined using the following equation, 
time
 framelast   the toframefirst   thefrom distance
velocity   
Data x and y position of float balls obtained from 
track object program is position in pixel units, so that the 
necessary unit conversions into centimeters. In analysis 
of current velocity data, determination unit conversion 
pixel to centimeter done in the following way. 
1. Measure the distance between the actual position of 
the ball 1 and ball 10 in a row at the start of data 
collection, which is equal to 40 cm. Then measured 
the actual distance in pixels by using the program 
track objects, and obtained the actual distance in 
pixels is 80.8 pixels. 
 
2. Having obtained the actual distance in centimeters 
and pixels, it can be determined the amount of the 
conversion unit distance from pixels to centimeters 
as follows (Table 1).  
Table 1. Converting pixel units to centimeterst 
 
Distance (cm) Distance (pixel) 
40 80.8 
1 2.02 
 
 
RESULT AND DISCUSSION 
 
Longshore Current Velocity Profile 
Long shore current velocity profile by ignoring the 
turbulence (without lateral mixing) using the equation 


sintan
16
5
b
b
f h
g
H
C
v   gives an overview of long 
shore current velocity profiles in the form of a triangle 
with a sudden change of velocity in a position breaker 
line (wave breaking point). Meanwhile, if the turbulence 
(with a lateral mixing) put as in Equation (10) is  10   ,
    X1          ,
1
1
2
2


 XAXXB
XB
p
pV , then the long shore 
current velocity profile will be more smooth and the 
velocity along the shore at a point in the breaker line will 
be reduced. Data of  measurement and velocity profiles 
shown in Figure 5. Figure 5 (b) shows the velocity 
profile of the current without lateral mixing triangles and 
current velocity profile with the influence of continuous 
curved lateral mixing. The figure also shows the position 
of the points along the flow velocity measurements using 
a float relatively close to shore curves flow velocity 
profile with the influence of lateral mixing. Current 
velocity data plotted in the graph is the mean flow 
velocity data from the measurement result using floats. 
At position X = 0 flow velocity measurements do not 
give the same rate to zero (v  0), it is due to float in that 
position still experiencing movement due to thrust run up 
and run down the wave (influenced by wave up rush).  
 
 
H. Umar et al. 
 
413 
 
 
 
(a) 
 
 
 
 
 
 
 
 
 
 
 
(b) 
 
 
Fig.  5  (a) The results of measurement and calculation of 
long shore current velocity on each float in the surf zone 
on condition with permeable groin. (b) Long shore 
current velocity profile on the condition with permeable 
groins 
Verification Result of Experiment and Theory  
The data experimental results and theoretical 
calculations can be seen in Table 2 and Table 3 and 
Figure 8, which shows the relationship breaking wave 
height (Hb) with a velocity of longshore current (v).  
Table 2 The results of measurements of the average long 
shore current velocity  
 
Kerapatan tiang Kecepatan arus, (v)  
(cm/det) 
Hb = 5.16 
cm 
Hb = 3.52 
cm 
Hb = 3.08 
cm 
54% (d = 1.5 cm) 12.95 9.55 7.84 
51% (d = 1.25 cm) 13.88 10.90 8.94 
47% (d = 1.5 cm) 11.27 9.13 8.53 
43% (d = 1.25 cm) 12.30 10.44 9.52 
 
 
Table 3. The result of calculation of long shore current 
velocity (Equation (19)) 
 
Cf Cd N Hb hb sin 
αb 
(v) 
      (cm) (cm)   (cm/det) 
0.04 0.01 23 1.8 2.31 0.071 3.87 
0.04 0.01 23 1.96 2.51 0.085 4.88 
0.04 0.01 23 2.44 3.13 0.092 5.88 
0.04 0.01 23 2.48 3.18 0.105 6.80 
0.04 0.01 23 3.08 3.95 0.107 7.65 
0.04 0.01 23 3.12 4 0.122 8.84 
0.04 0.01 23 3.52 4.51 0.135 10.40 
0.04 0.01 23 4.76 6.10 0.151 13.50 
0.04 0.01 23 5.16 6.62 0.165 15.30 
 
 
 
Figure 6. The relationship between breaking wave height 
(Hb) and average long shore current velocity (v), 
verification of the experimental results with a theoretical 
calculations (Equation 19). 
 
Figure 6 shows the suitability of the data long shore 
current velocity of the measurement results with the 
results of calculations using the equations Longuet-
Higgins (1970), where the data long shore current 
velocity (v) the of wave height (Hb) of the same 
theoretical line approach Longuet-Higgins equation 
(1970) with a coefficient of friction (Cf) of 0.04. 
Parameter of breaking wave height (Hb) is directly 
proportional to the flow velocity parameter (v), the larger 
the of wave height that occurs, the greater the current 
generated.  
 
CONCLUSION 
Verify the long shore current velocity equation (v) 
and experimental data showed a relatively suitable, using 
the coefficient of friction (Cf) = 0.04. So the method of 
current measurement using float balls can be 
recommended to measure the magnitude of the average 
long shore current through permeable groin.  
 
Theory 
Data 
 
Without lateral 
mixing (Theory) 
Data 
With lateral mixing 
(Longuet-Higgins) 
Data 
Data (permeable groin) 
 
Measurement Of Long Shore Current After Permeable Groin By Floating Object 
 
414 
 
ACKNOWLEDGEMENTS 
The authors would like to express their sincere 
gratitude to the Hydraulics Laboratory, Department of 
Civil Engineering and Environmental Engineering, 
Gadjah Mada University. Appreciation also extended to 
the entire academic staffs of Department of Coastal 
Engineering, Hasanuddin University. 
 
REFERENCES 
Dean dan Dalrymple. (2002), ”Coastal Processes with 
Engineering Applications”, Cambridge University 
Press, USA. 
Dominic, et.all. (2004), ”Coastal Engineering, 
Processess”, Theory and Design Practice, Spon 
Press, London dan New York. 
Horikawa. (1978), ”Coastal Engineering-An Introduction 
to Ocean Engineering”,  University of Tokyo Press. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Horikawa. (1988), ”Nearshore Dynamics and Coastal 
Processes”, University of Tokyo Press. 
Velden. (1983), “Coastal Engineering, Faculty of Civil 
Engineering”, TU Delft,  
Longuet-Higgins, (1970), Longshore Currents 
Generated by Obliquely Incident Sea Waves,1, 
Journal of Geophysical Research, Vol.75, No. 33, 
20 November 1970. 
Longuet-Higgins, (1970),  Longshore Currents 
Generated by Obliquely Incident Sea Waves,2, 
Journal of Geophysical Research, Vol.75, No. 33, 
20 November 1970. 
Longuet-Higgins and Stewart, (1964), Radiation stresses 
in water waves; a physical discussion, with 
applications, Deep Sea Research, Vol. 11, pp 529 to 
562: Pergamon Press Ltd. Printed in Great Britain. 
 
 


Measurement of long shore current after permeable groin by floating object

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Measurement of long shore current after permeable groin by floating object

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