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Monday, 26 March 2018

“EFFECTIVNESS OF RCC PORCUPINE SYSTEN IN PROTECTING RIVERBANK USING A 2D FLUME MODEL STUDY”


Ms. Utkarsha A.  Dubey
Mrs. V. S. Limaye
Mrs. A. V. Deshpande
M. E. (Hydraulics) 2nd Year,
Associate Professor,
Assistant Professor
Dept. of Civil Engg.
Dept. of Civil Engg.
Dept. of Civil Engg.
Sinhgad College of Engineering, Pune
Sinhgad College of Engineering, Pune
Sinhgad College of Engineering, Pune
55dubeyutkarsha@gmail.com
vslimaye.scoe@sinhgad.edu
Avdeshpande.scoe@sinhgad.edu

ABSTRACT:
The effect of porcupines on the reduction in velocity of flow near the banks and their ability to induce sedimentation has been investigated experimentally by various fraternities. An attempt has been made to access the pattern of deposition caused by various configurations of Porcupine field, the alignment of porcupine units, relationship between the Manning’s roughness coefficient (‘n’ value) and the quantum of porcupine units, etc.in an two dimensional Flume model at CWPRS, Pune for the present study. The main objective of this study is to propose a suitable methodology in terms of the placement, quantum, and orientation of porcupine units to increase the resistance in the erosion prone reaches of a channel. The basic principle of porcupine placing near the river bank is to offer resistance to flow thereby reducing velocity and inducing sedimentation near the erosion prone banks. This will in turn build the river banks resulting in shifting of channel course away from the banks. The main advantage of RCC porcupine units are its flexibility and also will be always in defense, if the river banks are attacked by the channel migration in future.
Various attempts have been made to overcome excessive erosion by constructing river training works. Porcupine systems are one of the novel techniques which have been adopted as a cost-effective method of river training. The effect of porcupines on the velocity of flow and their ability to capture sediment has been investigated experimentally. Also, an attempt has been made to logically study the pattern of deposition caused by various configurations of Porcupine field and hence to propose a preliminary design methodology.


Key Words: RCC Porcupine systems; river training; 2 D flume; cost-effective; riverbank protection.




I. INTRODUCTION

Rivers in alluvial plans are highly variable in their behavior and it is often unpredictable. A stream, which is quite trouble free during low flow, may attain a threatening condition during high stages. It may develop unforeseen meander, break through embankment, attack town, and important structures, outflank bridges and in general may create havoc. Therefore, whenever any hydraulic structure is built across an alluvial stream, adequate measures in form of  river-training works must be taken to establish the river course along a cretin alignment with a predetermined cross-section, all there works which are constructed to train the river are known as river-training works.
River training’ refers to the structural measures which are taken to improve a river and its banks. River training is an important component in the prevention and mitigation of floods and general flood control, as well as in other activities such as ensuring safe passage of a flood under hydraulic structure. For flash flood mitigation, the main aim is to control the water discharge regime in the watercourse by limiting its dynamic energy, thereby controlling the morphological evolution of the watercourse (Colombo et al. 2002). River training measures also manages sediment transportation and thus minimize bed and bank erosion. Many river training structures are implemented in combination with bioengineering techniques to lessen the negative effects on environment and landscape. There are a number of types of river training structure. The selection and design of the most appropriate structure depends largely on the site conditions.
River training is necessary in those reaches of the river where the river encounters excessive erosion of bed or banks. Braiding, meandering, breaching of embankments, damages of hydraulic structures, roads and railways, etc. are some of the consequences of an untrained river. Various river training measures which are commonly deployed include spurs/groynes (permeable and impermeable), submerged vanes, bank pitching, guide vanes, bundling etc. Details about planning, layout, design, and maintenance of permeable and impermeable type spurs are covered in IRC: 89 (1997) & IS: 8408(1994). Porcupine Systems have also been installed in big rivers in India like Brahmaputra and Ganga and have yielded fairly good results. After Repeated failures of earthen spurs upstream and downstream of the Farakka Barrage on the Ganga River (India), Central Water Commission (CWC) Used RCC To protect the erosion of left bank. It is reported that the porcupines were very effective and helped in siltation of the bank. Aamir and Sharma (2015) have developed a rational design methodology for riverbank protection using RCC Porcupines.    
             RCC Porcupines have also been deployed at Majuli Island, Assam, India and they have been found quite effective in reducing the intensity of the river Brahmaputra. A Porcupine is a unit of the system which comprises six members of RCC which are jointed together with the help of or on nuts and bolts to form a tetrahedral frame. Each member is 2-4 m in length, depending upon the requirements. At the time of concreting of members, holes are kept in the RCC poles for the bolts. Generally, RCC poles of 3 m length are used having a cross section of 15 cm × 15 cm. reinforcement is given using 4 Nos. of MS bars of 6 mm diameter, with stirrups at 15cm c/c. larger porcupines may also be used with greater cross section and heavier reinforcement as per the requirement. Bolts are normally 12-15 mm dimeter. Check nuts are to be provided for better grip. Washers are required at both ends for better grip with the RCC members. RCC porcupines should be connected together by wire rope and properly placed on the ground to avoid any disturbance caused by the intensity of flow. Figure 1 shows a three dimensional sketch of a typical RCC Porcupine unit.

          Porcupines are a form of permeable structure designed to reduce flow and trap sediment. They have pole-like projections in all directions, resembling a porcupine with its quills sticking into the air. They are used as flood control structures, and for river bank and bed protection. Porcupines can be used in a line forming a spur into a river, as silting aprons for larger spurs, and in a longitudinal line along an embankment. Originally such devices were made of timber or bamboo, but these have a limited lifespan. The use of wooden and bamboo porcupines combined with vegetation to form a green wall is described in the chapter on bioengineering. This section describes porcupines made of concrete.
          There are two kinds of concrete porcupine in common use: reinforced cement and pre-stressed cement (PSC). Quality control of RCC struts is difficult because each strut cannot be tested separately, although a rebound hammer can be used to test the uniformity of strength throughout. PSC porcupines are better in terms of size, shape, strength, concrete mix, and steel used.
          Porcupines can be constructed in two shapes, tetrahedral and prismatic. The following are their main uses.

Porcupine design
     Tetrahedral porcupines
                The most common shape is tetrahedral. The porcupine is formed by assembling six concrete struts of the same length in a tetrahedral pattern. Individual struts are bolted together, projecting beyond the joint. The bolts are passed through holes made at the appropriate point using cheap polythene tubes during casting. The size of a porcupine is denoted by the length of the individual struts, for example a 2 m porcupine or 3 m porcupine. The struts of 2–3 m porcupines have a cross-section of 10 x 10 cm. The individual struts are at 60 degrees to each other, thus a 2 m porcupine is about 1.7 m high and a 3 m porcupine 2.6 m high. The most common sizes in use are 2, 2.5, and 3 m. Mounted or long boom cranes are necessary to handle anything larger.

Fig. 1 Tetrahedral Porcupine

      Prismatic porcupines
                Prismatic porcupines are made with nine concrete struts joined in the form of a prism. Two end triangles are ford first and then joined together with three struts placed at the vertices. Struts are bolted together as for tetrahedrons.

Fig. 2 Prismatic Porcupine


II.             AIM:


  1. Assign variation in manning sand n value (Roughness coefficient) with the end of porcupine system.
  2. To study the flow condition with the different setup of porcupine system with orientation.
  3. Qualitative analyses sediment factors with introduction of porcupine system.

III.             OBJECTIVES:
The objectives of the study are:

  1. To reduced the momentum of flow to encourage sediment riverbank.
  2. To assize the effectiveness of RCC porcupines with the different orientation and density of porcupine.

IV.        LITERATURE SURVEY

The various papers referred for this area - Mohammad Aamir and Nayan Sharma (11 March 2015) - presented paper on River band protection with Porcupine systems development of rational design methodology. On this paper, the effect of porcupines on the velocity of flow and their ability to capture sediment has been investigated experimentally. Also, an attempt has been made to logically study the pattern of deposition caused by various configurations of Porcupine field layout and hence to propose a preliminary design methodology. Result show that there is a considerable reduction in the flow velocity resulting in the deposition of sediment with porcupines offering resistance to flow.
Presence of porcupines can cause considerable reduction in the flow velocity, which enhance further with increase in the number of units pleased in a series. A second tier of porcupines can be pleased over the other to improve their performance in the cases of high submergence. Densely configured porcupine field can capture more sediment that a sparse field. Sediment capture capacity also increases with increase in sediment concentration of flow. New suitable design indices have been developed in this study which could be used to form the basis for the preliminary design methodology and thus improve on the present day conjectural approach. Preliminary design template is developed which can provide the designer with the range of values of PFDI for different sediment concentration and PFSI, to achieve the desired objective of erosion control, moderate reclaim and heavy reclaim in the reach. Verification of the proposed design with field applications is recommended.)
Ashok Kharya and Piyush Kumar (10-14 April 2012) - presented paper on RCC Porcupines an effective river bank protection measure-A case study of protection of Majuli Island. On, the paper gives an overview of the Permeable structures in the form of RCC porcupines for bank protection measures and their successful application at Majuli Island.
Siltation measures in the form of R.C.C. porcupines have been found quite effective in reducing intensity of erosion of the river Brahmaputra at Majuli Island, Assam. The anti-erosion works for “Protection of Majuli Island” are still in progress. However, from the observations made by the panel of  Experts in 2011, it can be Concluded that the works in the form of RCC, porcupines which are cheap in cost, easy to construct, sustainable and without significant adverse effects in upstream or downstream or on opposite bank are a proven tool for checking erosion of river banks.

V.             METHODOLOGY

[1]     Experimental Setup:
                A 16 m length, 1.2 m width, and 1.5 m depth flume would be used at CWPRS, Pune. A depth of 0.5 m thick Khaswa sand having mean diameter of 0.32 mm (d50) would be placed in side the flume with certain initial bed slope. A divider using GI sheet would be inserted exactly at the centre of the flume from upstream of flume up to the flume from upstream of flume up to the end of glass portion as shown in figure Fig. 3 channel section


                A 2 feet standing wave flume would be used at up stream of flume for discharge measurement. The porcupine unit would be fabricated using 8 mm GI wire (photo…). Odd type propeller current mater, depth rod, digital pointer gauge would be used for extracting different hydraulic parameters.

[2]     Experimental procedure:
                Initially a discharge (based on the flume set up/ capacity of the flume) would be fed into the flume after measuring it through the 2’ standing wave flume. A constant depth of flow would be maintained in the flume. The flume would be run for about 1.5 to 2 hours until the mobile bed of the flume is stabilized and attain equilibrium. There after, the velocity, depth, bed slope etc. would be measured in the two different channels. Based on these parameters the manning’s “n” value (roughness coefficient) and discharge in the two bifurcated channels would be computed. The above experiments are for the existing or without porcupine unit conditions.
Similar experiments are erred out by placing a row of porcupine units at one of the bifurcated channels. All other procedures/ measurements are done as explained above. Like wise the different hydraulic parameters are extracted by providing additrmal rows of porcupines. The comparisons shall be made in the parameters obtained from both the bifurcated channel. This would establish a relationship of manning’s “n” value u/s the number of porcupines rows. This would help in determining the quantum of resistances to be offered in the channel to absorb the momentum of flow/ energy so that the adjust channel could be aestivated on higher degree. Apart from these experiments other alterative such as placing the porcupine units in a staggered way, changes in the shape of porcupine unit etc. could be during the course of experiments.



VI.           REFERENCES

[1]                  Detailed Project Report “Protection of Majuli Island from flood and erosion (Immediate measures)”;Brahmaputra Board (2003)
[2]                  Detailed Project Report “Protection of Majuli Island from flood and erosion”; Brahmaputra Board (2003)
[3]                 Master Plan of Majuli Island; Brahmaputra Board (2004)
[4]                 Kakean, s.p., and Keshri, K.N. (2012). Handbook for flood protection, anti erosion & river training work, central water commission, Govt. of India, New Delhi.
[5]                 Aamir, M., and Sharma, N. (2014). Sediment traps efficiency of Porcupine systems for river protection. 19th International Conference on Hydraulics, Water Resources, Coastal an Environmental Engg. MANIT, Bhopal, India.
[6]                 Akhtar, M.P., Sharma, N., and Ojha, C.S.P. (2011).”Braiding process and bank erosion in the Brahmaputra River.”
[7]                 Kharya, A., and Kumar, P. (2012).”RCC porcupines an effective bank protection measure – a case study of protection of Majuli Island.” In India Water Week, New Delhi, India.


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