332 THEORETICAL APPROACH OF LONG SHORE CURRENT REDUCTION COEFFICIENT THROUGH PERMEABLE GROIN Hasdinar Umar 1 *, Nur Yuwono 2 , Radianta Triatmadja 2 , and Nizam 2 1 Coastal Engineering Department, Universitas Hasanuddin 2 Civil and Environmental Engineering Department, Universitas Gajah Mada *Email: [email protected]; Phone: +62812-2760-8658 Abstract Permeable groin is a coastal protection structure that is flexible and effective to control the longshore current and hence longshore sediment transport. The ability to reduce both longshore currents and longshore sediment transport to a desired extent can provide more control to avoid unwanted sudden changes of shoreline in particular area. The average longshore current velocity (v) through permeable groins can be developed theoretically based on Longuet-Higgins, 1970 equation by incorporating structure parameters of permeable groin namely the distance between the piles (p), diameter of the pile (d t ), and number of the groin (n). Reduction coefficient (C r ) may be determined by comparison between longshore current through permeable groin (<v> groin ) and longshore current without permeable groin (<v> without groin ). The results showed that reduction coefficient (Cr) may be determined by , where the reduction coefficient (C r ) describes the magnitude of longshore currents reduction passing through the permeable groin. Keywords: permeable groin, longshore current, current reduction coefficient INTRODUCTION General Background Dynamic process in the form of wave forces, currents, wind, the movement of sediment and so on as well as land use in coastal areas cause many problems such as erosion, sedimentation, and siltation of the river mouth deflection, pollution and salt water intrusion. To overcome those problems, the first step that must be done is to find the cause of the erosion and sedimentation, so we can determine how to handle it. Erosion and sedimentation are usually overcome by building coastal protection structures; one of it is the permeable groins. Permeable groin is a coastal protection structure that is flexible and effective to control the longshore current and hence
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332
THEORETICAL APPROACH OF LONG SHORE CURRENT REDUCTION COEFFICIENT
THROUGH PERMEABLE GROIN
Hasdinar Umar1*, Nur Yuwono2, Radianta Triatmadja2, and Nizam2
1Coastal Engineering Department, Universitas Hasanuddin2 Civil and Environmental Engineering Department, Universitas Gajah Mada
Permeable groin is a coastal protection structure that is flexible and effective to control the longshore current and hence longshore sediment transport. The ability to reduce both longshore currents and longshore sediment transport to a desired extent can provide more control to avoid unwanted sudden changes of shoreline in particular area.
The average longshore current velocity (v) through permeable groins can be developed theoretically based on Longuet-Higgins, 1970 equation by incorporating structure parameters of permeable groin namely the distance between the piles (p), diameter of the pile (dt), and number of the groin (n). Reduction coefficient (Cr) may be determined by comparison between longshore current through permeable groin (<v>groin) and longshore current without permeable groin (<v>without groin).
The results showed that reduction coefficient (Cr) may be determined by , where the reduction coefficient (Cr) describes the magnitude
of longshore currents reduction passing through the permeable groin.
Keywords: permeable groin, longshore current, current reduction coefficient
INTRODUCTION
General Background
Dynamic process in the form of wave forces, currents, wind, the movement of sediment and so on as well as land use in coastal areas cause many problems such as erosion, sedimentation, and siltation of the river mouth deflection, pollution and salt water intrusion. To overcome those problems, the first step that must be done is to find the cause of the erosion and sedimentation, so we can determine how to handle it. Erosion and sedimentation are usually overcome by building coastal protection structures; one of it is the permeable groins. Permeable groin is a coastal protection structure that is flexible and effective to control the longshore current and hence
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longshore sediment transport. The ability to reduce both longshore currents and longshore sediment transport to a desired extent can provide more control to avoid unwanted sudden changes of shoreline in particular area.
Figure 1. Permeable groin at Teluk Penyu Coast, Cilacap
Design of permeable groins, especially pile groin requires basic theory in order to obtain results that support an effective design which is the reduction coefficient (Cr) longshore current after the permeable groins.
Permeable Groin
Permeable groins have a porous structure that still allows the current through the structure so that the transport of sediment to the down drift groin could still occurred. Longshore current and longshore sediment transport basically is controlled by density of pile groin (p). Several researches related to permeable groin were done by Hasdinar, et.al, (2011); Abdellah and Balah, (2001); Raudkivi, (1996); which were Hasdinar, et.al, (2011) studied the longshore current after permeable groin. Abdellah and Balah, (2001) has conducted research on the application of permeable pile groins as a coastal protection at Northwestern tourist beach, Egypt. Raudkivi, (1996) studied the application of permeable pile groin at 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 down drift groin.
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occurred. Longshore current and longshore sediment transport basically is
controlled by density of pile groin (p). Several researches related to permeable
groin were done by Hasdinar, et.al, 2011; Abdellah and Balah, 2001; Raudkivi,
1996; which were Hasdinar, et.al, 2011 studied the longshore current after
permeable groin. Abdellah and Balah, 2001 has conducted research on the
application of permeable pile groins as a coastal protection at Northwestern tourist
beach, Egypt. Raudkivi, 1996 studied the application of permeable pile groin at
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 down drift groin.
Figure 2. Scheme of beach profile with permeable pile groins and current velocity distribution without groins and with the groin (Raudkivi, 1996)
Methodology of Study
Method used in this research was analytical study regarding the formula of
longshore current reduction coefficient through permeable groin. To derivate the
reduction coefficient, two parameters were used. First, the longshore current
without permeable groin and second the longshore current with permeable groin.
Longshore current formula without permeable groin was derivate from Longuet
Higgins, 1970, which were used two parameters, bottom shear stress and shear
stress due to wave. Longshore current velocity with permeable groins was derived
without groin
with groin
Cur
rent
vel
ocity
Permeable pile groin
TerraceTrough Bar
Figure 2. Scheme of beach profile with permeable pile groins and current velocity distribution without groins and with the groin (Raudkivi, 1996)
Methodology of Study
Method used in this research was analytical study regarding the formula of longshore current reduction coefficient through permeable groin. To derivate the reduction coefficient, two parameters were used. First, the longshore current without permeable groin and second the longshore current with permeable groin. Longshore current formula without permeable groin was derivate from Longuet Higgins, (1970), which were used two parameters, bottom shear stress and shear stress due to wave. Longshore current velocity with permeable groins was derived by assuming that the shear stress will increase as a result of shear stress due to groin piles. So that the longshore current velocity after a permeable groins will be using three parameters: the shear stress due to waves, bottom shear stress and shear stress due to piles groin.
RESULTS AND DISCUSSION
Longshore Current without Permeable Groin
Longshore current equation of Longuet-Higgins, (1970) is the longshore current caused by the difference in radiation stress, where Longuet-Higgins and Stewart, 1964 has reviewed the theory of radiation stress which excised current momentum generated by the wave. In the development of longshore current equation, Longuet-Higgins, (1970) using four (4) basic review,
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Longshore current analysis without permeable groin developed by Longuet-Higgins, (1970) using the assumption that the current is two-dimensional (no variation in the vertical direction), steady and uniform in the y direction, so that the momentum equation in the direction along the shore (longshore direction) using the equation,
wherevt : mixed coefficient (eddy coefficient), ‹v› : longshore current velocity, τy : shear stress due to waves,‹τ by› : bottom shear stress.
Completion of the longshore current equation simplified by ignoring the shear stress due to turbulence so that the second term in Equation (5) can be neglected by the wave, making the shear stress due to waves (τy ) is only offset by the average value of shear stress ‹τby›.
If Equation (3) and (4) are substituted into Equation (6), then the longshore current equation of the conditions unhindered groin (Longuet-Higgins, 1970) as follows,
If we assume that the shallow-water theory is applied, then the average of longshore current equation without the influence of lateral mixing can be written as,
where Cf : bottom friction coefficient,Hb : wave height (m)hb : breaking wave depth (m), tan β : beach slope, ab : breaking wave angle.
Hasdinar, 2012 used a no sinusoidal wave assumption of the orbital velocity which derived from mean orbital velocity equation written in Dean and Dalrymple, (1984).
where um : orbital maximum velocity (m/det), tan β : beach slope, ab : breaking waves angle,g : ratio between breaking wave height (Hb) with a depth of a breaking
wave (h b), hb : depth of a breaking wave (m).
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Figure 3. Longshore current without groin
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 (Equation (10), Longuet-Higgins, 1970), with the basic assumption that the friction increased by the existence of permeable pile groins structure,
Shear stress between water and piles of groins ( ) is considered equal to the drag force of permeable groins (Fd) per unit area constraining groin area.
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Total shear stress equation after the barrier of permeable pile groins can be written as follows,
....................... (17)
Longshore current velocity equation through the permeable pile groins developed from the momentum equation y direction (parallel to the shoreline) after the groins, as shown in Figure 4, which illustrates that there is a balance between the shear stress due to waves (ty) and the average shear stress due the permeable pile groin groin (<tby >groin).
αb
Surf zone
Shoreline
Breaker line
ty
y
x
(v)
Figure 4. Sketch of longshore current parameters after a permeable groin
ty = <tby >groin ............................................................................................... (18)
where <tby >groin 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) (<tby >) added the average shear stress between the water and the pile of groins (<tg >),
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If Equation (3) and Equation (20) is substituted into (18), the longshore current velocity equation through permeable groins structure can be written as follows,
where p : density of the groin (%), hr : the average depth of the submerged groin piles (m), dt : diameter of pile (m). Cd : drag coefficient
Figure 5. Longshore current with permeable groin
Reduction Coefficient
Once known the magnitude of the average longshore current velocity after the groins then reduction coefficient can be determined. Longshore current velocity reduction coefficient (Cr) is the ratio between the longshore current velocity after groins ((v)groin) with longshore current velocity without groins ((v)without groin)).
If the beach is used permeable pile groins as shoreline protection, then the magnitude of the longshore current coefficient reduction will be influenced by two roughness parameters, bottom roughness coefficient (Cf) and drag coefficient (Cd). Based on Equation (13) and (21), reduction coefficient (Cr) equation can be written as follows,
Relationship between the reduction coefficient (Cr) and groin density (p) is shown in Figure 6.
Figure 6. The relationship between the reduction coefficient (Cr) and groin density (p)
CONCLUSION AND RECOMMENDATION
Based on longshore current reduction coefficient, which is analytically calculated after the permeable groin (Equation (23) and (24)), the followings are the conclusions:
5. Longshore current reduction coefficient is influenced by two roughness parameters, bottom roughness coefficient (Cf) and drag coefficient (Cd). Besides the reduction coefficient is also influenced by groin structure parameters as density of groin (p) and diameter of pile (dt)
6. Finally a relationship of reduction coefficient (Cr) with a groin density (p) may be constructed and is expected to be useful for designing a permeable groin that is more adaptable to the requirements of shore protection or reduction of longshore current and longshore sediment transport.
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.
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REFERENCES
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Dominic Reeve, Andrew Chadwick and Christopher Fleming, 2004, Coastal Engineering, Processes, Theory and Design Practice, pp.56-66, Spoon Press, New York.
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Elfiky, dkk, 2003, Pile Breakwater as Semi-Transparent Measure for Sea Defence, Coastal Engineering, Elsevier, Science Direct.
Hasdinar, et.al, 2011, The Influence of Permeable Groin on Longshore Current, Proceedings of International Conference on Water Related Risk Management, Jakarta, Indonesia, pp. 215-222
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