Laboratory Study on Comparison of the Scour Depth and Scour Length of Groundsill with the Opening and Groundsill without the Opening

Transcription

1 Journal of the Civil Engineering Forum Vol. 2 No. 1 (January 2016) Laboratory Stuy on Comparison of the Scour Depth an Scour Length of Grounsill with the Opening an Grounsill without the Opening Ani Hairani Department of Civil Engineering, Universitas Muhammaiyah Yogyakarta, Bantul, INDONESIA anihairani@umy.ac.i Djoko Legono Department of Civil an Environmental Engineering, Universitas Gajah Maa, Yogyakarta, INDONESIA jokolegono@ugm.ac.i ABSTRACT River be control structure what so calle grounsill or bottom sill is built for controlling a river be to remain stable against egraation. Unlike other river obstacles, the presence of grounsill might cause seiment retention upstream of the structure; hence it reuces the supply of seiment to the ownstream part of the river. At some extent, the above situation might create unexpecte negative impact, not only cease the seiment migration but also isable fish migration, as well as the utilization of river for navigation. This paper presents the hyraulic investigation on various moels of grounsill, i.e. grounsill without the opening an grounsill with a certain type of the opening. Series of laboratory experiments were conucte on an open channel flow of 0.75m with an longituinal slope of 0.05%. There were two types of grounsill, i.e. grounsill without the opening with 0.75m with an 0.05m height, an grounsill with the opening of 0.39 m with an m height of crest at the opening an 0.05m at the wings. Various flow rates were then introuce, necessary ata were taken, an the hyraulic phenomena were stuie. The results showe that grounsill without the opening prouce non-imensional (relative to the channel with) scour epth of 0.036, an scour length of Grounsill with the opening prouce scour epth of an near the ownstream en of the wing section an the opening respectively. The scour length of the grounsill with the opening is an near the ownstream en of the wing section an the opening section respectively. Moreover, it can be note that the presence of the scour epth an scour length of the grounsill with the opening was generally much smaller rather than that of grounsill without the opening. The above results give the evience that grounsill with the opening is much frienlier an also more suitable for the environment nees. Keywors: control structure, environmentally soun, grounsill, hyraulic phenomenon. 1 INTRODUCTION The river is water resource that has great benefit for life. Not only as natural rainage, but also use for rinking water, the habitat of water biotas, an transportation (Sosroarsono, 1985). Due to these important roles, environmental preservation an restoration are highly reuire to keep it being well functione. In general, the purposes of preservation an restoration are to prevent floo, seimentation, an to maintain the river stream from incriminating horizontal or vertical alignment changes. One of the flow stabilization work is by constructing grounsill which belongs to seiment control structure to maintain the river be elevation from egraation process. River as natural rainage has stream an be slope characteristics which may easily change. Construction of the river structure, an human activities that may cause flow or be slope alteration give impact to the changes of its length an be slope. Changes in the flow or be slope inuce the changes of the flow tractive force, which then isturbs the river be balance. It changes until a new euilibrium conition has met. The grounsill presence is expecte to maintain the river be elevation on a certain height, yet it also can bring negative effects incluing the ecreasing of seiment supply from the upstream, an the egraation in the river be on the ownstream of the grounsill. At the ry season, when the water elevation is very low, an so o the seiment supply, the existence of grounsill is consiere to be less environment-frienly. Therefore, grounsill shoul be esigne in such conition that negative impacts are minimize. It can be achieve by making an opening system on the grounsill (Novriska, 2000). The aforementione opening is expecte to still be use for boat traffic, also the possibility of water biota that migrates along the river flow. With the opening in the mile part of the grounsill, the balance between supply an withrawal of the seiment on the river upstream is possible. Furthermore, stuies on the hyraulic of grounsill structure type are neee to stuy the scour pattern after the grounsill existe, with variations of ischarge an shape of the grounsill, an to solve the complexity of the hyraulic characteristics on the grounsill by 39

2 Vol. 2 No. 1 (January 2016) Journal of the Civil Engineering Forum comparing the result between the approache theory an laboratory investigation. 2 LITERATURE REVIEW AND THEORETICAL BASIS Erosion an eposition process on the river are generally happene ue to changes in the flow pattern. It is cause by the existence of obstacles in the river flow in the mean of river construction, such as brige pillar an abutment, river curb, grounsill, water gate, an so forth. Accoring to Elsebaie (2013), scouring may be efine as the removal of material aroun or near structures locate in flowing water. Such phenomenon may, in turn, cause the lowering of the riverbe level by water erosions such that there is a tenency to expose the founations of structures. The lowering of the river be level as cause by the local scour can progressively unermine the founation the structure leaing to failure. The complete protection against scour is usually too expensive, generally, the maximum scour epth has to be estimate to consier the further protection esign against the risk of the failure (Khwairakpam an Mazumar, 2009). River construction such as water gate is eeme to be able to change the geometry of the stream flow an the flow pattern which leas to local scouring on the ownstream (Raukivi, 1967). This process takes place alongsie the river for several kilometers an for a long time (De Vries, 1975 recite in Hoffman 1977). Different type of scouring as given by Raukivi an Ettema (1982) in (1990) are as follows: a) General scour in the river channel oes not relate to the presence of the hyraulic structure on the river. b) Local scour is cause by the narrowing of river channel; the flow becomes centralize. c) Local scour in the vicinity of the structure is affecte by the local flow pattern. These three scouring events may occur simultaneously, yet in ifferent locations. The scouring type 2 an 3 are classifie into clean water scour an live be scour. Clean water scour occurs when the river be materials in the upstream of the hyraulic structure are in euilibrium conition (no material transporte). Live be scours is associate with be material transport. Accoring to Laursen (1952) recite in Legono (1990), the natural scouring is characterize as follow: a) The size of scouring is eual to the ifference of the amount of seiment materials conveye to the scouring area an the materials coming out of the scouring area. b) The amount of scouring is ecrease if the wette area in the scouring area is increase. c) There is such circumstance in which the scouring amount is the maximum an being asymptotic with time. The euation of maximum local scour in the ownstream threshol by Lacey is as follows: s 1,35 f 1, f (1) (2) where s is scour epth, calculate from the original be level on the ownstream (m), 50 represents average particle size or meian grain iameter, is flooe ischarge per river with unit (m 3 /s), an f is silt factor (the eroibility of the be material). Consiering the uncertainty value of silt factor in Euation (2) so that it is necessary to valiate the scour value with the moel test. With the increase of 50, the silt factor is increase as well, while the scour epth is ecrease. In practice, it is often foun that the river be grain size is generally not uniform. This nonuniformity leas to a physical phenomenon in which the scour epth value in the river be with non-uniform grain size is lower than the river be with uniform grain size. The reliability of Euation (1) is also influence by the flow ischarge per river with unit (). In the reality, is never uniform since the river flow generally has more than two imensions. In terms of practice, the recommene way to estimate the scour epth is by investigating the subgrae stratigraphy, or the ensity value, an so on. Some researchers mentione that the lowest level of the latest scour coul be iscovere by the ifference of ensity on river be channel. However, it shoul be note that those all are on the range of uncertainty. Lacey state that the average flow epth is influence by the flow ischarge an the silt factor, which was written into the following euation: 1 / 3 Q m 0.47 f (3) Where m is average flow epth (m), Q is flow ischarge (m 3 /secon), an f is silt factor which is liste in Table 1. To estimate the scour epth, the m value on the Euation (3) nees to be multiplie with a coefficient as the function of river shapes (see in Table 2). The Euation (3) is erive from Euation (1) so that the 40

3 Journal of the Civil Engineering Forum Vol. 2 No. 1 (January 2016) regime theory still ominates the preiction result of scour epth. Table 1. Silt Factor values for various grain (Neill, 1975) Grain iameter (mm) Silt Factor value (f) Fbo (feet/s) 50 from river be material (feet) Table 2. Multiplying factor accoring to river shapes (Neill, 1975) River shape Multiplying factor Straight 1.25 Meium curve 1.50 Strong curve 1.75 Suen curve 2.00 En of pillar 2.00 Along the pillar 2.25 En of crib 2.75 The general instructions that can be use for estimating the scour epth are not available since the information on scour epth recore in the fiel is still limite. It is recommene to conuct scour epth estimation by recoring the real event on the fiel. Laboratory experiment on investigating movable be moel shows phenomenon which is close to the real conition. Accoring to Blench (in Neill, 1975), if there is no available instruction, scour epth estimation coul also be approache with these following proceures: a) Estimate the flow ischarge f (ft 3 /secon) which is multiplication between flow velocity an average flow epth. It shoul be note it use the assumption of a wie profile river. b) Calculate the regime epth fo in feet consiering no seiment supply accoring to Euation (4): fo F 2 f bo 1/ 3 (4) where F bo is zero be factor accoring to Blench (see Figure 1). c) Estimate the maximum scour epth, which is multiplication between z x fo, whereas z is a factor accoring to Table 3. Figure 1. F bo value as 50 function (Neill, 1975). Table 3. Factor of z accoring to river shapes (Neill, 1975) River shape Near the groin Flow is near the curving toe 2.25 Flow is aligne with the toe LABORATORY EXPERIMENT Consiering the complexity of scouring phenomenon, the unerstaning of scouring behavior in the ownstream of grounsill was stuie through a couple series of laboratory experiments. Several preparations mae in part of the investigation relate to the preparation of the open channel moel, grounsill moel, also other materials/euipment. 3.1 Physical Moel The moel has close circuit flow as shown in Figure 2, with a longituinal slope of an channel with of 0.75 m. It use san materials with specific physical parameters as be river materials. On the channel moel, a flow ischarge measurement facility (Thomson type) was built. It was place in the first part of the open channel. Close to the flow ischarge measurement, ownstream sie, stilling basin was place; while on the ownstream sie, water level elevation controller was place Figure 2. Sketch of Laboratory Experiment. z 8 9 Description: 1.Reservoir, 2. Pump, 3. Water conveyor to Thomson tank, 4. Thomson tank type, 5. Fore Bay, 41

4 Vol. 2 No. 1 (January 2016) Journal of the Civil Engineering Forum 6. En sill tank, 7. Channel, 8. Deposit trap tank, 9. Tailrace 3.2 Grounsill Moel There were two grounsill moels those have been stuie, i.e. grounsill without the opening an grounsill with the opening. Figure 3 an 4 show the sketch of Grounsill Type I (grounsill without opening) an Grounsill Type II (grounsill with the opening) respectively. Measurement of the epth of scouring was carrie out by the use of the tape measuring euipment. The term of environmental frienly was efine as it allows water passing through the grounsill in low ischarge conition. Surface water level 0.03m 3.3 Material an Euipment The grain size istribution of san material use as moveable be materials was shown in Figure 6. Another euipment was also use, such as the Thomson ischarge controller, pump, as well as some tools for recoring the experiment results while running, such as stop watch, point gauge, ruler, an so forth. % Finer by weight Original be level 0.12m 0.05m s Grain size (mm) Figure 3. Grounsill Type I (without opening). Original be level 0.12m 0.75m Surface water level 0.03m 0.005m 0.045m Opening (0.39m) Wing Wing 0.18m 0.75m 0.18m Figure 4. Grounsill Type II (opening type). 0.05m Series of grounsill comprising of grounsill without the opening an grounsill with the opening was introuce in every run of various ischarges. The grounsill moel was place on the channel section that was relatively straight an was serially mounte with a certain istance (Figure 5). s Figure 6. Distribution curve on river be material grain size To estimate the effect of non-uniform material, a coefficient factor was use to theoretically evaluate the scour epth euation. Several literatures mentione that other than 50, the 84.1, an 15.9 coul be use for the non-uniformity grain levels (Vanoni, 1975). For practical reason, the 84.1 value coul be consiere eual to 84 value, while the 15.9 value coul be consiere eual to 16. Kinori mentione in Vanoni (1975) that the non-uniformity level of grain sizes coul be state in the Geometric Stanar Deviation (σg) an Graation Coefficient (G) as follows: g 84,1 15,9 1/ 2 G 1/ 2 84, ,9 (5) (6) The σ g an G values for the seiment characteristics which were use in the laboratory experiment accoring to the above euation are consecutively an Experiment Running The proceures for executing the laboratory experiment are as follows: Figure 5. Grounsill mounting on channel moel a) The grounsill was place in series. b) The channel was watere in a certain amount of ischarge an reuire time was recore, in 42

5 Journal of the Civil Engineering Forum Vol. 2 No. 1 (January 2016) which for it to reach the balance on be until no scouring happene again. c) The observation was one on the scour epth near the ownstream of the grounsill, as well as for the grounsill length an the scouring pattern. ) Measuring the river be elevation on a certain interval. e) Analysis an interpretation of the research result. Retrieval of the scour epth ata when the streaming occurre (on a certain amount of ischarge) was conucte for several times, until the conition where the scouring epth was no longer increase, so calle the euilibrium conition. 3.5 Experiment Result The laboratory experiment was conucte in the conition of scouring phenomenon with clear water scouring, in which the threshol height from the river be of 0.5 cm i not cause seiment movement from the grounsill threshol, both the Grounsill Type I an Grounsill Type II. To provie the unerstaning of the investigation results, all parameters are presente in non-imensional value. The imensionless ischarge was the ratio of measure flow ischarge an reference ischarge (-reference), while imensionless scour epth an scour length were the ratio of the epth an scour length an river with. The reference ischarge (-reference) was the lowest ischarge from the running, which was about 4.69 x 10-5 m 3 /secon or l/min. The laboratory experiment was conucte with movable be, with mean grain iameter ( 50) of 0.83 mm. Figure 7 shows that both in the Grounsill Type I an Grounsill Type II, maximum scour epth i not occur, even with large ischarge. The higher flow ischarge, the more severe scouring, yet optimum conition coul be reache on certain ischarge in which the increase of flow ischarge no longer cause the increase of scour epth. In such conition, the scour epth was ecrease in the vertical irection, yet increase in a longituinal irection until reaching a point where the scouring is stoppe (euilibrium). The be materials were only spinning aroun in the scour area, an coul not be entraine. When /-reference was less than 7, the maximum scours epth occurre at Grounsill Type I (without opening). At /-reference >7, the maximum scours epth occurre in the Grounsill Type II, while at the wing section, it generally was the smallest one. These phenomena inicate that in a certain limit, the esign of the Grounsill Type II nees to consier the hyraulic stability, particularly in the opening. Figure 7. The relationship between the relative unit ischarge (/-reference) an the relative scour epth (s/channel with). Scour epth was increase faster at the beginning of scouring. After a while, the increase rate of the scour epth was ecrease until reaching an euilibrium conition. Figure 8, Figure 9, an Figure 10 show the increase rate of the scour epth within time on varie flow ischarge for Grounsill Type I, Type II-opening type, an Type II-wing, consecutively. Generally, the increase rate of the scour epth was asymptotic against time. / _reference t (minute) Grounsill Type-I Grounsill Type-II Grounsill Type-II (wing) /_reference = 1.0 /_reference = /_reference = /_reference = /_reference = Figure 8. Time versus non-imensional scour epth (Grounsill Type I). 43

6 Vol. 2 No. 1 (January 2016) Journal of the Civil Engineering Forum s/lebar_sungai L/(channel with) Grounsill Type-I Grounsill Type-II (opening) Grounsill Type-II (wing) t (minute) /_reference /_referensi= = /_reference /_referensi= = /_reference /_referensi= = 4.1 /_reference /_referensi= = 7.6 /_reference /_referensi=15.6 = Figure 9. Time versus non-imensional scour epth (Grounsill Type II-Opening type). s/(lebar_sungai) t (minute) /_referensi= /_reference = /_referensi=15.6 /_reference = Figure 10. Time versus non-imensional scour epth (Grounsill Type II-Wing). The largest scour length on certain ischarge generally happene in the Grounsill Type I on its opening type, followe by Grounsill Type I an then the wing section of the Grounsill Type II. On certain ischarge (/-reference value < 10), there was no scouring occurre on the ownstream sie of the wing section of the Grounsill Type II (see Figure 11). The scour length also coul be influence by the water level at the ownstream of the grounsill. If the water level was relatively high, the scour length was smaller than low water level conition. In practice, the water level at the ownstream may vary accoring to the presence of grounsill. / / Figure 11. The relationship between the relative unit ischarge (/-reference) an the relative scour length (L/channel with). 4 ANALYSIS AND DISCUSSION To assess the avantages of Grounsill Type II compare to the Grounsill Type I, the analysis an iscussion (hyraulic) of both grounsill types are escribe as follows. The analysis comprises the relation between ischarge an scour epth, the relation between time an scour epth, an the relation between ischarge an the scour length. 4.1 Relation between ischarge an scour epth There was a iscrepancy on scour epth between experiment results an theoretical analysis accoring to Euation (1). Euation (2) can be solve by consiering seiment size parameters ( 50), ischarge per unit with (), cross section characteristic (geometry an n-manning roughness coefficient), with value as follows, a) 50 (from Figure 6) = 0.83 mm b) (ischarge per unit with), accoring to ones applie in the laboratory c) n-manning = ) river with = 0.75 m e) f = silt factor value, calculate with Euation (2) = = /(_reference) The results of comparison between scouring analyze theoretically an laboratory investigation for Grounsill Type I an Grounsill Type II were shown in Figure 12 an Figure 13, respectively. The scour epth value in the case of Grounsill Type I yiele from theoretical approach was initially lower than experiment results, particularly when the /-reference was less than 52. While when the /-reference was larger than 52, the contrary happene. In the case of Grounsill Type II, at low ischarge, the water woul pass through the opening; therefore no scouring occurre at the ownstream of the wing. The scour epth between theoretical approach an laboratory investigation inicate less conformity. This is 44

7 Journal of the Civil Engineering Forum Vol. 2 No. 1 (January 2016) unerstanable since it was ifficult to moel symmetric flow streamline. Thus, the flow ischarge per unit with coul not eually istribute, or on the other wor, there was a concentrate stream. s(/(channel with) Figure 12. Scour epth versus flow ischarge in the case of Grounsill Type I. s/lebar_sungai Euation (1) Euation (4) Experiment /(_reference) /(_reference) /_referensi Persamaan Euation (1) (1) - Wing - Sayap Persamaan Euation (2)(1) - Wing - Bukaan Persamaan Euation (3)(4) - Wing - Sayap Persamaan Euation (4) (4) - Wing - Bukaan Eksperimen Experiment - Sayap Wing Eksperimen Experiment - Bukaan Opening Figure 13. Scour epth versus flow ischarge for Grounsill Type II. 4.2 Relation between time an scour epth The scour epth was increase as a function of time. The increase rate of the epth was very rapi on the early time an graually became constant. The flow velocity was still high enough to entrain the be materials, then when the rag force was no longer strong enough ue to low velocity, scouring on the be was stoppe. Changes in the flow ischarge affecte the changes of scour epth by time. This process woul increase graually, until at a conition in which the increase of space followe by the ecrease of flow velocity coul no longer excee the critical shear stress. 4.3 Relation between ischarge an scour epth After running was conucte several times with 5 ischarge variations for each grounsill types, it showe that the local scour epth occurre in the Grounsill Type I was relatively eeper an shorter compare to the opening Grounsill Type II, particularly at the opening section. The scour pattern occurre on the Grounsill Type I was istribute along the threshol since the flow velocity relatively istribute evenly along the threshol. For Grounsill Type II, the scour epth occurre (with the same ischarge) on the wing section was relatively smaller than in the case of Grounsill Type I, an so o scour length. It was ue to the higher flow velocity on the Grounsill Type II as the effect of concentrate on the opening. From the observation, it is foun that the scouring pattern at near ownstream of the grounsill was basically much narrower on grounsill with the opening. This may lea to the conition that the channel bank at near grounsill with opening is generally more stable. The inconsistency between the scour epth obtaine theoretically an experiment results were cause by two main points, i.e.: a) The non-ieal flow behavior in which the flow streamline was not completely parallel or in the same irection with the main flow. b) The seiment characteristic i not completely have a uniform size, as well as the grain which was not fully roun. Slightly ifferent with Euation (2), the silt factor for the grain with iameter 0.83 mm (accoring to the linear interpolation shown in Table 1) was 1.598, while accoring to the Euation (2), it was The ifference on the silt factor create a ifference on the scour epth was consiere insignificant, only about 2-5%. The ifference on scour epth between experiment results an theoretical analysis was varie in the range of to 91.17%, an to % for Grounsill Type I, respectively, accoring to Euation (1) an (4) (see Figure 12). Meanwhile, the ifference on scour epth for the Grounsill Type II from experiment an theoretical analysis was ifficult to conclue. However, in general, it coul be state that the scour epth euation of Blench was closer to the experiment analysis result (see Figure 13). Therefore, in terms of unavailable empirical stuy, the scour epth preiction accoring to Blench euation is more recommene than the Lacey euation. 45

8 Vol. 2 No. 1 (January 2016) Journal of the Civil Engineering Forum 5 CONCLUSIONS The following conclusions wrap up the research results: a) The relative maximum scours epth occurre in the case of Grounsill Type I was 0.036, with the relative ischarge of The relative maximum scours epth occurre on Grounsill Type II was of at the opening, an at the wing section, with the relative ischarge of b) The relative maximum scours length occurre on Grounsill Type I was 0.253, with the relative ischarge of The relative maximum scours length occurre on Grounsill Type II was at the opening, an at the wing, with the relative ischarge of c) In general, it can be seen that the scour length in the case of Grounsill Type I is longer than on the Grounsill Type II (particularly on the opening) ue to concentrate stream (narrowing on the channel) affecte by the presence of the opening. ) In terms of the environment, the scour epth occurre on Grounsill Type II was worthwhile since longer an eeper scour on the opening woul create flow streamline on the river be so that it allows the be seiment transporte to the ownstream in low flow ischarge. 6 RECOMMENDATIONS The following recommenations are consiere important for further stuies: a) It is necessary to stuy the effect of the scour epth against the variation of opening elevation, a variation of opening number, or variation of the characteristic of the river be material. b) The hyraulic stability of the grounsill structure mae of ifferent construction materials, such as massive construction, rock gabion is reuire. c) This laboratory stuy is still a ualitative fining to highlight the better performance of the grounsill with the opening rather than that of grounsill without the opening. Further experiments on the similar stuy shoul be conucte taking into account various real river courses an flow conitions. REFERENCES Hoffmans, G. J. C. M. & Verheij, H. J., Scour Manual. Rotteram: A.A. Balkema. Elsebaie, I.H., 2009, An Experimental Stuy of Local Scour Aroun Circular Brige Pier in San Soil, International Journal of Civil & Environmental Engineering IJCEE-IJENS, Vol: 13 No: 01. Khwairakpam, P. an Mazumar, A., 2009, Local Scour Aroun Hyraulic Structures, International Journal of Recent Trens in Engineering, Vol. 1, No. 6, May Neill, C., Guie to Brige Hyraulics. Toronto: University of Toronto Press. Novriska, R., Aplikasi Bangunan Pengenali Dasar (Grounsill) Berwawasan Lingkungan [Implementation of Environmentally Soun River Be Control Structure (Grounsill)], Yogyakarta: Universitas Gajah Maa. Raukivi, A. J., Loose Bounary Hyraulics, Lonon: Pergamon Press Lt. Sosroarsono, S., Perbaikan an Pengaturan Sungai [River Restoration an Management]. Jakarta: Pt. Pranya Paramita. Vanoni, V. A., Seimentation Engineering. American Society of Civil Engineering. 46