Source: ICHE Conference Archive - https://mdi-de.baw.de/icheArchiv

Barzali, Mohsen

Dehghani, Amir Ahmad

Fazloula, Ramin

Z.Ahmadi, Mirkhalegh

English

Hydraulic Engineering Repository

Conference Paper, Published VersionBarzali, Mohsen; Fazloula, Ramin; Dehghani, Amir Ahmad; Z.Ahmadi,MirkhaleghExperimental Investigation of Local Scouring around aSeries of L-Head GroynesZur Verfügung gestellt in Kooperation mit/Provided in Cooperation with:Kuratorium für Forschung im Küsteningenieurwesen (KFKI)Verfügbar unter/Available at: https://hdl.handle.net/20.500.11970/110066Vorgeschlagene Zitierweise/Suggested citation:Barzali, Mohsen; Fazloula, Ramin; Dehghani, Amir Ahmad; Z.Ahmadi, Mirkhalegh (2008):Experimental Investigation of Local Scouring around a Series of L-Head Groynes. In: Wang,Sam S. Y. (Hg.): ICHE 2008. Proceedings of the 8th International Conference on Hydro-Science and Engineering, September 9-12, 2008, Nagoya, Japan. Nagoya: NagoyaHydraulic Research Institute for River Basin Management.Standardnutzungsbedingungen/Terms of Use:Die Dokumente in HENRY stehen unter der Creative Commons Lizenz CC BY 4.0, sofern keine abweichendenNutzungsbedingungen getroffen wurden. 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In some cases a different, more restrictive license may apply; if applicable the terms ofthe restrictive license will be binding.EXPERIMENTAL INVESTIGATION OF LOCAL SCOURING AROUND A SERIES OF L-HEAD GROYNES  Barzali Mohsen1, Fazloula Ramin2, Dehghani Amir Ahmad3 and Z.Ahmadi Mirkhalegh 4  1 Msc Student, Department of Water Engineering, Sari University of Agricultural Sciences & Natural Resources 9th Km of Sea Road, Khazar Sq., Sari, Iran, e-mail: mbarzali@yahoo.com 2 Assistant Professor, Department of Water Engineering, Sari University of Agricultural Sciences & Natural Resources, 9th Km of Sea Road, Khazar Sq., Sari, Iran, e-mail: fazloula@umz.ac.ir 3 Assistant Professor, Department of Water Engineering, Gorgan University of Agricultural Sciences & Natural Resources, Pardis, Basig Sq., Gorgan, Iran, e-mail: a.dehghani@gau. ac.ir 4 Professor, Department of Water Engineering, Sari University of Agricultural Sciences & Natural Resources, 9th Km of Sea Road, Khazar Sq., Sari, Iran, e-mail: mzahmadi@yahoo.com   ABSTRACT   The groynes are structures which widely used for protecting the outer part of the bend and river plan stabilization projects. One of the most important problems for designers is the estimation of maximum local scour depth in vicinity of groyne. Due to constriction of flow area, the local velocity increased and the scouring around groyne is formed. Looking at literature shows that most of study has been conducted on straight groyne and a little information exists on local scouring around L-head groyne. Seri of L-head groynes consists of parallel and perpendicular wall to flow direction. In this study, the  effect distance between groynes on maximum depth of scour around L-head groynes was compared. Also the effect of size distribution and various discharge on scouring was investigated. By using dimension analysis, the dimensionless parameters were obtained and a general relation was developed by applying regression analysis.   The results show that variation distance between groynes not have important effect on  the maximum depth of scour around L-head groynes .Also relative depth scoure by increase kinematic froude number will decrease relative depth scoure. Also with increase distance between groynes increase depth scoure between groyne.   Keywords: Series of L-head groyne, Local scouring, clear-water and densimetric Froude.   1. INTRODUCTION  Groynes are structures constructed at an angle to the flow in order to deflect the flowing water away from critical zones. They serve to maintain a desirable channel for the purpose of flood control, improved navigation and erosion control. Various types of groynes can be distinguished according to their appearance in plan view, Examples are straight groynes, T-head, L-head, hockey shaped, and inverted hockey shaped groynes (Fig. 1). The simplest case of groynes layout is a single straight groynes in a straight reach of rivers. The groynes confines a certain part of the river cross section and affects appreciably the kinematics structure of the flow in its vicinity. Mean velocity and specific discharge increase due to the constriction. The increase in the mean velocity leads to a rise in their gradients and more intensive generation of macro-turbulent that brings scouring around this structure. Usually the groynes are built in group because of extending the protected zone.             Figure 1 Classification of groynes according to their appearance in plan.  The stability of these groynes against the river flow depends on stability of first groynes.  Most of the previous investigations on scour at abutments and groynes are concerned with prediction of the equilibrium or design scour depth on straight groynes.  Lacey (1936) gives a formula for determination of maximum depth of scour at a groyne. Based mainly on the field observation, Inglis (1949) compared the maximum depth of scour with those obtained from Lacey’s equation. He recommended certain coefficients to be applied to Lacey’s regime depth to find the maximum scour depth and also analyzed the influence of size of material on the maximum scour depth. Ahmad (1953) conducted investigations on the behaviour of groynes using sand of 0.37 mm and 0.7 mm mean diameter and drew some valuable conclusions regarding the effect of various parameters on the maximum scour. He studied the effect of different discharge intensity, sand grade, flow concentration and angle of the groynes to flow on the scour depth and scour pattern around a groynes. Andru and Blench (1957), after analyzing various laboratory and field data on the scour at obstructions, concluded that the depth of local scour depends on discharge intensity and the size of bed material. Izzard and Bradley (1957) feel that the sediment size and gradation affects only the rate of development of scour hole and not the maximum scour hole. Garde (1961) conducted experiments with four sizes of groynes four sizes of sand, According to Garde, Froude number is a significant parameter for scour around obstructions and also the size of sediment has an influence both on the rate of scour and the maximum scour depth. Gill (1972) conducted experiments using a flume 0.76 m wide and two sizes of sand. He concluded that depth of equilibrium scour is affected both by size of bed material and depth of uniform flow upstream of the spur location. Zaghloul (1983) conducted experiments to study the effects of upstream flow conditions, sediment characteristics, and groynes’s geometry on the maximum scour depth and scour pattern around a groynes. Ettema (2004) conducted a series of flume experiments conducted to determine scale effects in small-scale models of flow around a single groyne placed in a channel whose bed is fixed and flat. Hashemi Najafi et al. (2008) conducted the experiments on local scouring around single L- head groyne. The results show that the maximum depth of scour around L-head groynes with upstream parallel wall is less than straight groynes and the optimal angle of parallel wall respect to perpendicular wall in L-head groynes is α=110°  (for L-head in the downstream direction) and α=60° (for L-head in the upstream direction). Looking at literature shows that a few studies existed on the scouring around L-head groynes. In this study, the experiments were conducted to consider the effect of sediment size on maximum depth of scour around a series of L-head groynes.   2. EXPERIMENTS    Dimensional analyses             Straight    T-head       L-head                          Flow Direction   The parameters affecting the local scour at groynes can be grouped as follows: (1) Parameters relating to the geometry of channel: width of channel upstream of groyne location (B1), distance groynes(a) (2) Parameters relating to the geometry of groynes: length of groynes (L), length of parallel wall (Lt), the angle between parallel and perpendicular wall(θ). (3) Parameters relating to the bed sediment: median size (d50), submerged specifiect weight(γ ′ ), critical velocity for incipient motion of sediment particles (Uc). (4) Parameters relating to the fluid properties: density (ρ), dynamic viscosity (µ), gravitational acceleration (g). (5) Parameters relating to the approaching flow condition: mean flow velocity (U), flow depth (h), maximum scour depth (hs).water specified weight (γ ), So:  hs=f1(µ,ρ,θ,,B1,h,U,Uc,d50,g,L,Lt,γ ′ ,γ ,a)     (1)  By use of ρ, h and U as repetitive parameters and using the Buckingham π-theorem, dimensionless parameters were found as:  ))1(*Re,,(502dGgUFrLafhhss−==     (2)   Figure 2 The geometric parameters of L-head groynes with upstream and downstream parallel wall.    Experimental Arrangement   The experiments were conducted on straight rectangular channel in hydraulic laboratory of Gorgan University of Agricultural Science and Natural Resources in Iran. The channel was 10.8 m long, 0.6 m wide and 0.6 m deep and the bed slope of channel was adjusted to 0.001. The bed material consists of uniform sediment by median size of 1.05, 1.7 and 2.75 mm respectively. The bed level was measured by digital point gauge with 0.01 mm accuracy. The groyne was watertight, non absorbed and vertical. The depth of flow was adjusted with a tailgate. A 25 cm layer of uniform sand was placed on the bed and covered the 2 meter length of the channel. The groyne was installed at a section of 5.5 m from the inlet. The sand bed surface was properly levelled by a plate attached to the instrument carriage. The flume was first filled with water slowly and pump was started to reach a desired value of flow rate and then the tailgate adjusted to reach a suitable flow depth. The experiments were conducted under clear-water conditions with various hydraulic conditions and continued until relative equilibrium condition was occurred, i.e. when the maximum scour depth is approximately time invariant. The primarily experiments were conducted until 44 hours for estimation of primarily equilibrium time. During the run of experiments the maximum depth of scour was measured by time (Fig. 3). It was found that 95 percent of maximum scour depth was occurred after 23 hours.  After finishing the experiment, the water was drained slowly and bed topography was measured with digital point gauge.     Figure 3 The variation of maximum depth of scour against time.    3. RESULTS AND DISCUSSION   The variation of relative maximum scour depth ( shh, where sh is the maximum depth of scour and h is the upstream flow depth) against Densimetric Froude number is shown in Figure 2. The results show that by increasing the50*( 1)sUFrg G d=−(whereU is approach mean velocity), the relative maximum scour depth decreases. Figure 3 shows the variation of relative maximum scour depth against relative distance between groynes. The results show that by increasing the distance between groynes, the relative maximum scour depth in vicinity of first groyne decreases but the scour depth between second and third groynes increased.  1.41.451.51.551.61.650.5 1.5 2.5 3.5a/Lhs/h00.20.40.60.811.21.41.61.81.2 1.4 1.6 1.8 2 2.2 2.4 2.6Fr*hs/hCoarse sedimentMedium  sedimentFine sediment Figure 4 Variation of relative maximum scour  Figure 5 variation of relative maximum scour depth against densimetric froude number.        against relative distance between groynes.  Figure 4 also shows the variation of bed changes around a series of groynes. It is found that the maximum depth of scour occurs in vicinity of first groyne.            Figure 6 The bed contours for discharge 40 lit/s and 1al= .   4. CONCLUSIONS    The following results can be concluded from this experimental study: - By increasing the distance between groynes, the relative maximum scour depth in vicinity of first groyne decreases but the scour depth between second and third groynes increased. - It is found that the maximum depth of scour occurs in vicinity of first groyne.   REFERENCES  Ahmad, M. (1951), Spacing and projection of spurs for bank protection, Civil Engineering Flow Direction and Public Work Review, London, U. K. March 172-174, April 256-258. Ahmad, M. (1953), Experiments on design and behavior of groynes. Proceedings Minnesota International Hydraulics Convention, International Association of Hydraulic Research, Minneapolis, Minnesota. Ettema, R. ,and Muste, M. (2004),Scale Effects in Flume Experiments on Flow around a Spur               Dike in Flatbed Channel.” Journal of Hydraulics Engineering Vol. 130,No. 7, p.635. Garde, R. J. ,and Subramanya, K. ,and Nambudripad, K. D. (1961), Study of scour around groynes, ASCE Journal of the Hydraulics Division, 87: 23-37. Gill, M. A. (1972),Erosion of sand beds around groynes, Journal of the Hydraulics Division, 98: 1587-1602. Hashemi Najafi, F., Ayyoubzade, M and Dehghani, A. A. (2008), Experimental investigation of scouring around L-head groynes under clear water condition, Journal of Agricultural Science & Natural Resources, Gorgan University, 15(1), pp.1-10.  Izzard and Bradley (1957), Field verification of model tests on flow through highway bridges and culverts, Proceedings, 7th Hydraulic Conference, Iowa. Kuhnle, R. A. ,and Alonso, C.V. ,and Shields, F.D. (1999),Geometry of scour holes associated with 90-degree groynes, Journal of Hydraulics Engineering, Vol. 125 No. 9, p.972. Liu, M. K. ,and Chang, F. M. ,and Skinner, M. M. (1961), Effect of bridge construction on scour and backwater, Report No. CER60-HKL22, Department of Civil Engineering, Colorado State University, Fort Collins, Colorado. Melville, B.W. (1992),Local Scour at Bridge Abutments, Journal of Hydraulic Engineering, Vol. 118, No. 4, p.615. Saneie, M. (2006), Experimental Study on River Closure Effect on Groynes Scouring, 7th International River Engineering Conference, Ahwaz. In Persian. Zaghloul N. A. (1983), Local scour around spur-dikes.” Journal of Hydrology, 60 (1983) pp.123-140.  

Experimental Investigation of Local Scouring around a Series of L-Head Groynes

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Experimental Investigation of Local Scouring around a Series of L-Head Groynes

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