Draft for comments only
DOC.WRD 22(352)
June 2005
BUREAU OF INDIAN STANDARDS
Draft Indian Standard
GUIDE LINES FOR PLANNING AND DESIGN OF PERMEABLE SPURS
IN ALLUVIAL RIVERS
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Not to be reproduced without the permission Last date for receiving comments:
of BIS or used as a STANDARD 2005-09-30
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FOREWORD
(Formal clauses of the national foreword will be added later)
Rivers are seldom straight. Depending on discharge, slope and sediments, some stretches are meandering with alternate bends while some stretches are braided with main and branches within the same flood plain. Unstable rivers, not in regime, are also found to change their courses. Usually a river is found to erode the outer bank and sediments get deposited on inner bank. In brained channels, however, rivers erode both bed and banks at high stage and the eroded materials are deposited on the bed or banks during low stage producing a number of channels within the flood plain with bed bars, dunes, antidunes etc.It is essential to train a river for protecting its banks to avoid excessive meandering, to prevent shifting its course, maintain navigability, etc.
Protection to the river banks is normally given by construction of stone revetments, impermeable spurs / bed bars etc. The costs of these methods are very high. Therefore, generally bank protection is restricted only to the important reaches. Use of permeable structures instead of stone structures is a cost effective alternative for the bank protection works.
Use of permeable screens / spurs for anti-erosion work is traditional and hasbeen commonly followed in BrahmputraBasin and other rivers in the country. In case of sediment laden streams, it helps to induce siltation along the bank resulting in shifting of river channel away from the eroded banks. These methods are easy for fabrication on the nearby ground close to the site, need no especially skilled labour and can be constructed with speed. As only locally available material is used, permeable spurs have been found very handy in bank protection in areas where good quality stones are costly and / or not available.
Essentially, only a dampening action on the velocity of flow is achieved by a permeable structure, distinguished from the deflecting or repelling action of an impermeable structure. The sediment transporting capacity of a flow is highly sensitive to the velocity. The theoretical considerations have shown that the weight rate of bed load transport is proportional to the sixth power of velocity. Therefore, the dampening of velocity results in deposition of coarser particles in the downstream direction.
These guide lines are prepared keeping in view the functional requirements and is based on the traditional methods in vogue for the construction and laying of permeable screens and spurs and the practical experience gained while applying the methods.
For the purpose of deciding whether a particular requirement of this standard is complied with, the final value, observed or calculated expressing the result of a test or analysis, should be rounded off in accordance with IS 2:1960 `Rules for rounding off numerical values (revised)'. The number of significant places retained in the rounded off value should be the same as that of the specified value in this standard.
DOC.WRD 22(352)
June 2005
Draft Indian Standard
GUIDE LINES FOR PLANNING AND DESIGN OF PERMEABLE SPURS
IN ALLUVIAL RIVERS
1.SCOPE
1.1These guidelines deal with the object, design details and layout of different types of permeable structures for river training and bank protection works for the rivers in alluvium.
2.OBJECT
2.1Permeable structures are a cost effective alternative to the bank protection works with impermeable stone spurs, making the projects more economical.
3. TERMINOLOGY
3.1The terminology followed for the design and layout of stone spurs (IS 8408-1992) is applicable for this document also. The additional terms used in this document are as below.
a. Structures - Different types of permeable structures made in the river channel to achieve the desired river training and bank protection works, viz, spurs, screens etc.
b. Elements - The permeable structures are made up of different types of smaller units called as elements. Many elements are arranged in specific pattern and joined together to form a permeable structure.
c. Material - The engineering material used for making the elements and the permeable structures, viz, bamboos, ballies, wire ropes, GI wires, etc.
4. PERMEABLE STRUCTURES
4.1Introduction -
Permeable screens, spurs and dampeners are the main types of permeable structures in vogue. Prima facie, the purpose, overall behaviour and layout of the above mentioned structures can be compared to those of submersible bund, spur and bank revetment respectively.
The permeable structures can be used either independently or with a support of other impermeable stone structures or river training and bank protection measures. Depending up on the purpose to serve, the permeable structures are constructed in transverse or parallel to the direction of flow.
4.2Functions of permeable structures
Permeable structures serve one or more of the following functions.
Training the river along a desired course.
Reducing the intensity of flow at the point of river attack.
Creating a slack flow to induce siltation in the vicinity of the permeable structures and in the downstream reach.
d.Providing protection to the bank by reducing dampening the velocity of flow along the bank.
e. Dissipating energy downstream of spurs through production of turbulence.
4.3Classification of permeable structures
The permeable structures can be classified as follows
a.According to function served, namely, diverting and dampening, sedimenting.
b.According to the method and material of construction, namely, bally, bamboo, tree and willow structures.
c.According to the conditions encountered, namely, submerged and non-submerged.
d.According to the type of structure provided, namely, spur type, screen type or dampeners (revetment) type.
4.4Structural elements
Different types of elements are used for making permeable structures. The dimensions specified for the material are according to the sizes readily and commercially available in the market. Therefore, variations in the dimensions, depending up on those available in the market can be made in the design.
a.Porcupines - Porcupines are made up of bamboos / ballies, have cubical shaped box at the central portion with their legs extending in all directions. The overall size is 2 m to 3 m. The central box is filled with stones for stability of individual unit during floods (Fig 1).
b.Cribs - This is a pyramid type of structure made up of bamboos / ballies with a box at the bottom for holding stones for stability during floods. Size of the box is generally square in shape of size 2 m to 2.5 m at the bottom. Total height of the structure is 3 m to 4 m (Fig 2).
c.Bally frames - Permeable bally structures are made up of main skeleton of large bamboos or ballies. Cross ballies are used for stability of the structure (Fig 3)
d.Tree branches - Branches of trees or trees of short height are hanged from a wire rope, duly weighted with stones and are aligned as a spur projecting into the river. The wire rope is duly anchored on the bank and in the river bed (Fig 4).
e.Willow frames - Willows (also called as Tarza or Shirkanda) is a type of bush, found in many parts of the country like Punjab, Bengal. The willow has sufficient rigidity and strength to withstand the pressure of flow and is resistant to decomposition and disintegration. Instead of willows, ordinary brushwood is also used. Frames are made up of ballies and the fillings are made up of willows. The panels are hanged from the wire rope and duly weighted with stones at the bottom to hold them submerged and vertical during floods (Fig 5).
4.5Construction materials :
4.5.1Following are the important points kept in mind while selecting the material for the construction.
a.Locally available material like bamboos, ballies, brushwood, willows, bricks etc is mainly used for the construction of permeable structures. GI wire, GI wire mesh, wire ropes, nails etc are the other important but commercially available material used for the structures.
b.The main criteria for the selection of suitability of the material are (i) the cost and (ii) easy / local availability. Other aspects are of secondary importance.
c.The dimensions of the material have been specified according to those normally used in the field. However, these could be changed according to the type and size of the elements and the design of the structure.
4.5.2Specifications for the important material used for the construction are as follows.
a.Bamboos - Standard commercially available bamboos of girth 20 cm to 30 cm are used for the porcupines and cribs. Normally, the larger girth of 25 cm to 30 cm is used for the main members, whereas, the smaller girth of 20 cm to 25 cm is used for bracings. If suitable sizes are not available, use of bamboos of girth smaller than the specified can be made. It may not significantly affect the overall result. Splicing and extension of length is not generally done in case of bamboos.
b.Ballies - Standard commercially available ballies of girth 15 cm to 25 cm are used for the bally structures. Normally, the larger girth of 20 cm to 25 cm is used for the main members, whereas, the smaller of 15 cm to 20 cm is used for bracings. If suitable sizes are not available, use of ballies of girth smaller than the specified can be made. It may not significantly affect the overall result.
Ballies can be spliced and extended if longer lengths are needed. As the strength of the ballies is very important aspect in the bally structure, care has to be taken while splicing. Provision of cross bracings at about 2 m interval and additional bracings close to the spliced joint is also necessary.
c.Nails - Standard commercially available nails of length 100 mm to 150 mm are used for the porcupines and cribs. Double nailing at critical joints and additional bracings show better results. Use of bolt-nuts has been tried but has not shown significant improvement compared to the additional cost involved.
d.GI wire - 4 to 5 strands of 4 mm GI wire should be used for interconnecting porcupines and cribs and anchor them to the ground. Alternatively, 12 mm 3-4 strands wire ropes should be used for the purpose.
Use of nylon ropes instead of GI wires has been tried. The nylon ropes are susceptible to disintegration in presence of Ultra Violet (UV) light. Therefore, care has to be taken to fabricate and lay the nylon crates fully protected from sunlight. The nylon ropes exposed to sunlight have been observed crumpled to power within a period of 6 months. Therefore, the protection from UV lights is of utmost important.
A new variety of nylon ropes resistant to UV light have also been developed. However, it has not been tested in bank protection works.
e.Anchors - Bally anchors are driven into the ground up to a depth of 2 m. Concrete anchors have an anchor rod of size 32-36 mm, well embedded in concrete cube. Wire crates anchors are of size 1.5 m X 1.5 m X 1.5 m made up of thick wires and filled with stones.
f.Stones - Stones in crates are used as counter weights, for nominal protection works, for anchor block made up of crate, etc. The life of bamboo structures is normally limited to one or two years only. Therefore, stones placed as counter weights in the central cube of porcupines / bottom tray of cribs / crates need not necessarily be of high quality.
g.Bricks - Bricks can be used instead of stones as counter weights, for nominal protection works, for anchor block made up of crate, etc. Over burnt bricks can be more resistant to abrasion than the ordinary bricks. Large bricks of size 35 cm X 25 cm X 12 cm weighing between 15 kg to 18 kg have been successfully manufactured for use in permeable structures.
5. DESIGN OF PERMEABLE STRUCTURES
5.1The concept
5.1.1Dampening of velocities is achieved by the use of permeable structures. If the flow is sediment laden, siltation is induced in the slack flow region and the channel is shifted away from the protected reach.
5.1.2If the flow is not carrying sufficient sediments, only dampening of velocities can result. However, sedimentation between the permeable structures and corresponding shifting of channel may not be observed.
5.1.3Only partial obstruction to the flow not exceeding 33 percent only is envisaged in the design. Higher obstruction causes more diversion of flow resulting in undesired scour around the structures, particularly at the nose portion. Additional protection to the nose and flanks (sides) is required to avoid such scour. Therefore, obstruction more than 33 percent is avoided.
5.1.4Different aspects of river training and bank protection works followed for the permeable structures are similar to those followed for impermeable stone structures. For example, the criteria for location, orientation, length and number as followed for impermeable stone spurs are followed for permeable spurs also. Some limitations are however imposed due to inherent weakness of the structural material and elements.
5.1.5Percentage of obstruction caused by (i) the individual element (ii) by the individual permeable structures and (iii) the group of permeable structures together are the important considerations to be kept in view during the designs. These considerations affect the other design aspects like stability of the structures, the number of elements to be provided in a structure, additional protection measures, if any, to be provided to the permeable structures, etc.
5.1.6Selection of the type of the elements and structures for the protection works also depends up on (i) the size and strength of the locally available material and (ii) the type of material commercially available.
5.2Design data -
5.2.1Following are the important hydraulic data required for the design.
a. Hydrology of the river, nature of floods (flashy or sustained), duration of floods, etc
b. Design discharge (peak flood discharge of 25 years return period) with corresponding HFL, bank level and maximum velocity of the flow attacking the bank or the channel under consideration.
c. Bankful discharge, corresponding average and maximum velocity of flow and maximum depth of flow.
d. Lowest discharge, corresponding water level, depth of flow during lean period at the site.
e. Soil properties of the bank, characteristics of the formation of the bank, particularly the existence of stiff clay layer resistant to erosion, its location and thickness with reference to the bed level, bank level and low water level.
f. River configuration in the plan including the tortuisity and obliquity of channel, percentage of discharge shared by the channel compared to the total river discharge, etc.
g. Sediment load in the river and in the channel under consideration at different stages of flood.
h. Emergency involved due to the eroding bank.
i.Cross sections of the river indicating the eroded bank, waterway, HFL, LBL etc., corresponding to design flood of 25 year return period.
Most of the above items can be estimated during site inspections and can be refined using the observed field data.
5.3Selection for alternative types -
5.3.1Permeable structures commonly used are the screens, spurs and dampeners. The structural elements commonly used are the porcupines, cribs, bally frames, tree branches and willows. A suitable combination of the structure and the elements is made for the design of protection works. Following points are kept in view while selecting the alternative types and their combination.
a.The porcupines, cribs and ballies are multipurpose elements used for all types of permeable structures.
b.The material like trees, bushes and willows are used for construction of spurs and dampeners, particularly during flood emergencies.
c.Permeable structures are usually designed as submersible. However, bally structures are generally designed as non-submersible.
d.The dampeners behave similar to revetments.
e.Permeable screens are used to close the secondary channels in a multi-channel river.
5.4Depth of flow -
a.In case of shallow water flows and up to a maximum depth of 3m to 4m, porcupines are used for both spurs and screens. For maximum depths of flow between 3 m and 5m to 6m, cribs are preferred. For the depths beyond these limits, bally spurs are preferred.
b.Spurs or dampeners made up of tree branches or willow mattresses are found effective up to a maximum depth of flow of 4 m to 6m, for greater than 6m, wooden pile or bamboo, spurs may be used.
c.Submergence up to 50 % above the structure is normally acceptable for porcupines. A slightly lower value of 20% submergence above the structure is acceptable for cribs. For tree and willow spurs it is normally limited to 5 % to 10 % only.
d.If the characteristics of bank material are favourable, the above limits can be exceeded considerably. Porcupine spurs and cribs have effectively protected depths two to two and a half times to those specified above.
5.5Layout in plan -
5.5.1Spurs -
a.The porcupine or crib elements are laid abutting to each other in a row. Walking space can be provided between the rows for inspection and repairs.
b.Each permeable spur is made up of 3 to 4 rows of porcupines or 4 to 6 rows of cribs. More rows are laid for higher velocity, deeper flows and flood waves of longer duration, inaccessible reaches difficult for maintenance (Fig 6).
c.On a straight reach, permeable spurs are spaced at 3 to 5 times its length. On a curved channel, depending up on the obliquity of flow, the spurs are spaced at 2 to 4 times the length.
d.Projection of the spurs into the river channel is normally 15 to 20 percent of waterway. However, more projection can be allowed depending up on the velocity and depth of flow.
e.In case of porcupine spurs, additional elements are sometimes provided at the head and width of the head is doubled. However, experience has shown no specific gain or betterment in the performance of the spurs.
f.In order to resist the tendency of outflanking, additional porcupines or cribs are provided along the sloping bank upstream and downstream of the spurs.
g.At least three spurs are provided for a specific reach to be protected. A single permeable spur is generally not found effective.
h.The standard practice of providing one or two additional spurs upstream and downstream of the eroding reach, alignment pointing towards upstream with reference to the flow, etc has to be followed for the permeable spurs also.