1 | IRC:SP:82-2008 i GUIDELINES FOR DESIGN Nl OF CAUSEWAYS AND SUBMERSIBLE BRIDGES INDIAN ROADS CONGRESS 2008IRC:SP:82-2008 First Published : November, 2008 Reprinted : June, 2009, (All Rights Reserved. No part of this publication shall be reproduced, translated or tranmitted in any form or by any means without the permission of Indian Roads Congress) Printed at India Ofiset Press, A-1, Mayapuri, New Delhi-110064 (500 copies)IRC:SP:82-2008 First Published : November, 2008 Reprinted June, 2009 (AIl Rights Reserved. No part of this publication shall be reproduced, translated or tranmitted in any form or by any means without the permission of Indian Roads Congress) Printed at India Offset Press, A-1, Mayapuri, New Delhi-110064 (500 copies) er aye en IRC:SP:82-2008 CONTENTS Page Personnel of the Bridges Specifications & Standards Committee @ Introduction 1 Scope 3 General Features 4 Hydrology and Hydraulics 15 Waterway and Afilux 60 Scour and Foundations n Design 96 Approaches, Protection Work and Appurtenances 130 References 146ee eee 23 IRC:SP:82-2008 PERSONNEL OF THE BRIDGES SPECIFICATIONS AND Sharan, G. (Convenor) Lal, Char (Memb Sinha, V.K Secretary) Agrawal, K.N. Alimehandani, C.R. Banerjee, A.K. Banerjee, T.B. Basa, Ashok Bandyopadhyay, De TK Bongirwar, P.L Chakraborty, 8.S. Chakrabarti, S.P. Dhodapkar, A.N. Gupta, R.K. Ghoshal, A. Indoria, R.P. Joglekar, S.G. Kand, Dr. CV. Kanhere, D.K. Koshi, Nina Kumar, Prafulla Kumar, Vijay Kumar, Dr. Ram STANDARDS COMMITTEE (As on 29,3,2008) Director General (Road Development), Ministry of Shipping, Road Transport & Highways, New Delhi Chief Engineer (B) (S&R), Ministry of Shipping, Road Transport & Highways, New Delhi Secretary General, Indian Roads Congress MEMBERS DG(W) (Retd.), CPWD, C-33, Chandra Nagar, Ghaziabad Chairman & Managing Director, STUP Consultants Ltd., Mumbai Member (Tech.) (Retd.), NHAI B-210, Second Floor, Chitranjan Park, New Delhi Chief Engineer (Retd.), Ministry of Shipping, Road Transport and Highways, B-830, Green Avenue, Indira Puram, Ghaziabad Director (Tech.) B. Engineers & Builders Ltd., Bhubaneswar Joint Director General, Institute for Steel Dev. and Growth, (INSDAG) Ispat Niketan Kolkata Advisor, L&T, B/1102, Pailiputra Co-op. Housing Society Ltd Four Bunglow Signal, Mumbai Managing Director, Consulting Engg. Services (I) Pvt. Ltd., 57, Nehru Place, New Delhi Chief Engineer (Retd.), MOST Consultant, Span Consultants (P) Ltd. 92C, Gurudwara Road, Madangir, New Delhi Chief Engineer, Ministry of Shipping, Road Transport and Highways, New Delhi Executive Director (B&S) Bridges & Structures Directt., Room No. 213, Annexe II, Research Design & Standards Orgn., Manak Nagar, Lucknow Director and Vice-President, STUP Consultants Ltd P-11, Darga Road, Park Circus, Kolkata Chief Engineer, Ministry of Shipping, Road Transport and Highways, New Delhi Director (Engg. Core), STUP Consultants Ltd., Plot No. 22A, Sector 19C, Palm Beach Road, Vashi, Navi Mumbai CE (Retd.), MP, PWD, Consultant, E-2/136, Mahavir Nagar, Bhopal Chief Engineer (Retd.) (NH), Block No. A-8, Building No. 12, Haji Ali Govt. Officers Qtrs. Mahalaxmi, Mumbai DG(RD) & Addi. Secy. (Retd.), MOST, H-54, Residency Greens, Green Woods City, Sector 46, Gurgaon (Haryana) DG(RD) & AS (Retd.), MORT&H, D-86, Sector 56, NOIDA hief (Retd.), UPPWD, E-002, Krishna Apra Residency, Sector 61, NOIDA (UP) Scientist-G, Central Road Research Instt, Delhi Mathura Road, New Delhi @24, 25. 26. 21. 28. 29. 30, 31. 32. 33, 34. 39. 36, 37. 38, 82-2008 Manjure, PY. Mukherjee, MK. Narain, A.D. Ninan, RS. Puri, S.K. Rajagopalan, Dr. N. Sharma, R.S. Sinha, N.K. Sinha, $ Tandon, Prof. Mahesh Tamhankar, Dr. M.G, Velayutham, V. Vijay, PB. Director & Head (Civil Engg.) Addl. Director General President, IRC Director General (Road Development) Secretary General Bhasin, PC Reddi, S.A. Raina, Dr. V.K. Rao, Dr. T:N. Subba Director, Freyssinet Prestressed, Concrete Co. Ltd., Mumbai Chief Engineer, (Retd.) Ministry of Shiping, Road Transport & Highways, 40/182, Chitaranjan Park, New Delhi DG (RD) & Adal, Secretary (Retd.), MOST, B-186, Sector 26, NOIDA Chief Engineer (Retd.), Ministry of Shipping, Road Transport & Highways, New Delhi Chief General Manager, National Highways Authority of India, Plot No. G-5 & 6, Sector 10, Dwarka, New Delhi Chief Technical Advisor, L&T-RAMBOLL Consulting Engineers Ltd, 339-340, Anna Salai, Nandanam, Chennai Past Secretary General, IRC, C-478 Second Floor, Vikas Puri, New Delhi DG(RD) & SS (Retd.) MORT&H, G-1365, Ground Floor, Chitaranjan Park, New Delhi Addl. Chief Transportation Officer, CIDCO Ltd. CIDCO Bhavan, 3rd Floor, CBD Belapur, Navi Mumbai Managing Director Tandon Consultants (P) Ltd., New Delhi Emeritus Scientist BH-1/44, Kendriya Vihar Kharghar, Sector-11, ‘Navi Mumbai DG(RD) & SS (Retd) MOSRT&H, Flat No. 4, Nalanda Appartment, D Block, Vikaspuri, New Delhi DG(W) (Retd.), CPWD, A-39/B, DDA Flats, Munirka, New Delhi Bureau of Indian Standards, Manak Bhavan, New Delhi Directorate General Border Roads, Seema Sadak Bhawan, Naraina, New Delhi Ex-officio Members (HLL. Mina), Secretary to the Govt. of Rajasthan PWD, Jaipur (G. Sharan), Ministry of Shipping, Road Transport and Highways, New Delhi (W.K, Sinha), Indian Roads Congress, New Delhi Corresponding Members ADG (B), MOST (Retd.) 324, Mandakini Enclave, New Delhi 72, Zenia Abad, Little Gibbs Road, Malabar Hill, Mumbai Flat No. 26, Building No. 1110 Road No. 3223, Mahooz Manama-332, Bahrain (Middle East) Chairman, Construma Consultancy (P) Ltd., Mumbai Gi)IRC:SP:82-2008 GUIDELINES FOR DESIGN OF CAUSEWAYS AND SUBMERSIBLE BRIDGES 1, INTRODUCTION 1.1. ‘The Guidelines for Design of Causeways and Submersible Bridges had been under {he consideration of the earlier General Design Features Committee since the year 2004. Later on (his Committee was merged with the General Design Features (Bridges and Grade Separated Structures Committee (B-1) at the time of reconstitution in January, 2006. The General Design Features Committee in its meeting held on 15! May, 2004 had constituted a Sub-group consisting of Shri P.L. Bongitwar, Dr. C.V. Kand, $/Shri D.K. Rastogi, M.V.B. Rao and Late Shri N.K. ‘atcl, Thereafter, the draft as prepared by the Sub-group and Shri 8.K. Kaiastha was considered by the reconstituted General Design Features (Bridges and Grade Separated Structures Committee, 1) in anumber of meetings and finalized it in its meeting held on 12 October, 2006 subject to incorporation of certain comments by its Convenor, Shri Prafulla Kumar. The personnel of B-1 Committee is given below Kumar, Prafulla a Convenor Indoria, R.P. Co-Convenor Rustagi, 8.K. . Member-Secretary Members Alimchandani, C.R. Kumar, Vij Arora, H.C Kumar, Kamlesh Agarwal, K.N Kurian, Jose Bagish, Dr. B.P. Naryan, Deepak Basa, Ashok Reddi, S.A. Bhowmick, Alok Ramakrishnan, R. Bongirwar, PLL. Rastogi, D.K. Chandak, PR. Reddy, Dr. TS Jangde, K Roy, B.C. Kand, Dr. C.V. Rep. of RDSO, Lucknow (S.C. Gupta) Kumar, Ashok Rep. of MSRDC, Mumbai (S.M. Sabnis) Corresponding Members ‘Tandon, Prof. M.C. Taunk, GS. Mukherjee, M.K. Ex-officio Members President, IRC DG(RD), MOSRT&H (Mina, HLL.) (Sharan, G) Secretary General, IRC (Sinha, V.K.)IRC:SP:82-2008 1.2. Thereafter, the draft guidelines for Design of Causeways and Submersible Bridges were considered by the Bridges Specifications and Standards Committee (BSS) in its meeting held on 3 November, 2007. The Committee formed a Sub-group comprising Shri Chaman Lal, CE(B) S&R, MOSRT&H, Shri M.V.B.Rao, Dr. C.V. Kand and Shri Sharad Varshney, Addl. Director (Technical), IRC for technical enhancement of the document. The Sub-group met thrice on 9.1.2008, |.2.2008 and 23.5.2008 and put up the draft document again to Bridges Specifications & Standards Commitee. 1.3, ‘The yaluable suggestions offered by the members of General Design Features (Bridges and Grade Separated Structures) Committee (B-1) and Bridges Specifications & Standards Committee are duly incorporated. 1.4. The draft document was approved by the Bridges Specifications and Standards Committee in its meeting held on 29.3.2008, and the Executive Committee in its meeting held on 11.4.2008 and authorized Secretary General, IRC to place the same before Council. The document was approved by the IRC Council in its 185" meeting held on 11.4.2008, at Aizwal (Mizoram) for printing subject to incorporation of some comments offered by the Council members— IRC:SP:82-2008 2. SCOPE this document contains guidelines for planning and design of submersible structures like {o1us, dips, causeways and submersible bridges on various categories of roads viz. State Highways, Major District Roads, Other District Roads and Village Roads in the country.IRC:SP:82-2008 3. GENERAL FEATURES 3.1. Definitions The following definitions shall be applicable for the purpose of these Guidelines 3.1.1. Bridge Bridge is a structure having a total length of above 6 m between the inner faces of the dirt walls for carrying traffic or other moving loads over a depression or obstruction such as channel, road or railway. These are classified as minor and major bridges as per classification given below (2) MinorBridge : A minor bridge is a bridge having a total length of upto 60 m. A minor bridge upto a total length of 30 m is sometimes classified as a small bridge. (b) Major Bridge: A major bridge is a bridge having a total length of above 60m 3.1.2. High level bridge A high level bridge is a bridge which carries the roadway above the highest flood level of the channel. 3.1.3. Submersible bridge A submersible bridge is a bridge designed to be overtopped during floods 3.1.4. Causeway A causeway isa paved submersible structure with or without openings (vents) which allows flood to pass through and/or over it. 5. Ford A ford is an unpaved shallow portion in a river or stream bed which can be used as a crossing during dry weather/normal flow. 3.1.6. Culvert A culvert is a cross-drainage structure having a total length of 6 m or less between the inner faces of the dirt walls or extreme ventway boundaries measured at right angles there to. 3.1.7, Channel Achannel means a natural or artificial watercourse. 3.1.8, Afflux Itis the rise in the flood level of the channel immediately on the upstream of a bridge as a result of obstruction to natural flow caused by the construction of the bridge and its approaches.I IRC:SP:82-2008 3.1.9. Highest Flood Level (HPL) Highest flood level is the level of the highest flood ever recorded or the calculated level for the design discharge, whichever is higher. 3.1.10, Ordinary Flood level (OFL) Ordinary flood level is the level of flood expected to occur every year. It can be determined by averaging the highest flood levels of seven consecutive years. Level (LWL) Low water level is the level of the water surface attained generally in the dry season. Itcan also be determined by averaging the low water levels recorded in seven consecutive years 3.1.12. Design Flood Level (DFL) ILis the highest flood level for which the structure must be designed. It comesponds to level of highest flood of 50 years or 100 years return period (whichever is chosen for design) or the Niwhout known flood level if the same happens to be higher. 4.1.13. Defined Cross-section ILis the undisturbed natural cross-section of river which does not exhibit signs of erosion or ilting of bed, 4.1.14, Protected Bed Level (PBL) Itis the level at which the bed surface is protected against erosion due to flow of water 3,2, ‘Types of Submersible Structures Bai: ‘ords Fords are unpaved structures and are suitable only for roads having very low volume of \wuftic, These are the simplest form of crossings where the stream is wide and shallow, velocity of flowing water is low and bed surface is relatively firm. In case the bed surface is not firm enough and not capable of carrying the vehicular traffic, the bed can be strengthened and made more even with buried stones just below the bed surface. If {he slones are likely to be carried away in flow, this is prevented by construction of barriers made of suitable size of boulders or wooden piles. Boulders ( neither too large which may result in of bed nor too small likely to be carried away by flow) are placed across the river bed at «lownstream side of the ford to filter the flow of water and retain small size particles of bed material Jike sand, gravels ete. resulting ina more even surface for vehicular traffic, Fig. 3.1 showsa typical ction of such type of ford.IRC: SP:82-2008 BOULDERS: \ PLUNGE POOL: (8) FORD WITH DOWNSTREAM BOULDERS tim ROCK FILLED GABION (B) FORD WITH DOWNSTREAM GABION Guibe GUIDE POSTS 100mm DIAMETER LOGS 2m LONG AT 600mm ¢/e (C) FORD WITH TIMBER POSTS Fig. 3.1. Typical Details of Fords3.2.2. Causeways There are mainly three types of causeways: (a) Flush causeway In this type of causeway which is also called paved dip or road dam, the top level of road is kept same as that of bed level of the channel. It is suitable where the crossing remains dry for most of part of year i.e. the stream is not perennial. Flush caus are not suitable for crossing the streams with steep bed slopes causing high velocity the causeway covers the full width of the channel Fig. 3.2. even in low floods. MARKER POSTS _/ MAX. FLOOD a7 _ WALL BURIED —#~ MARKER POSTS / BOLLARDS cur OFF WALL BURIED CCARRIAGEWAY 2. ‘Typical Features of Paved dip/Flush Causeway (b) Vented causeway ‘A causeway provided with vents to permit normal flow of the stream to pass under the causeway is known as vented causeway. Vented causeways are classified as low vented causeways and high vented causeways. Low vented causeway Low vented causeways are provided to cross quasi-perennial streams having sandy beds in areas with annual rainfall less than 1000 mm and where the carriageway of aIRC:SP:82-2008 flush causeway would be liable to get slushy due to post monsoon flow in the stream, The height is: the bed of the watercourse. Inexceptional cases, the height may be 1.50 m above the bedlevel. Small size of vents inthe form of hume pipes, short span slabs/R.C.C. Box cells are provided in the width of stream The sill level of vents is kept about 150 mm — 300 mm below the average bed level of the stream, (i) High vented causeway High vented causeway is provided when a road crosses astream having one or more of the following characteristics: ©" Sizeable catchment area with annual rainfall more than 1000 mm Gi) Depth of post monsoon flow is more than 900 mm Gi) Flow is perennial but not large G) Banks are low necessitating construction of high embankment in the stream bed from considerations of the free board in non-submersible portion ae well sx geometric standards of approach roads The height ofthe causeway above the bed is generally kept between 1.5 mto 3.0m and larger size of vents comprising of hume pipes or simply supported/continuowe R.C.C. slab Sere eerie Over a series of short masonry piers or series of arches or boxes with individual spans less than 3 m are provided, 3.2.3, Submersible bridge Rubmersible bridge is nomally sub-classified as high submersible bridge or low submersible bridge depending upon deck level with reference to OFL. The deck level of high submersible bridge is fixed with reference to OF FL and vertical clearance, and as such the structure serves as under higher floods with permissible number and duration ofinterruptions. This type of bridge is Suitable for streams having large variation between HEL and OFL site deck level of ow submersible bridge is fixed above the OFL. so as to ensure thatthe interruptions caused to traffic remain within permissible limits, 3.3, Selection of Type of Submersible Bridge/Causeway 3.3.1. General— IRC:SP:82-2008 4.3.2, Considerations in the selection of type of submersible structures Selection of type of submersible structures (i \nlematia depends on: e, ford or causeway or submersible bridge) (0) Requirements of user authority and availability of funds (b) Category, importance of road and traffic intensity (©) Population to be served (d)_ Nature of stream ie, flashy/perennial/seasonal etc. and velocity of water during floods (e) Duration, magnitude of floods and interruption to traffic (1) Spread and depth of water during floods and post monsoon period (y) Extent of catchment area Maa. teria for avoiding/selection of submersible structures In the absence of any directives/guidelines by the user authority, the following criteria may (we {hllowed for selection of suitable type of submersible structures including causeways on different sleuorion of roads. (1) ‘These should be avoided on National Highways (2) These may not be considered for adoption in the following situations: (i) Roads of economic importance, roads linking important towns or industrial areas or areas with population more than 10,000 where alternative all weather route with reasonable length of detour is not available (ii) On roads which are likely to be upgraded or included, from future traffic considerations, in the National Highway network (ii) Ifthe length of a high level bridge at such crossings would be less than 30 m except where construction of high level structure is not economically viable (iv) Maximum mean velocity of stream during floods is more than 6 m/sec (v)_ Ifthe cost of submersible bridge with its approaches is estimated to be more than approximately 70% of the cost of high level bridge with its approaches, near about the same site (Vi). Iffirm banks are available and approaches are in cutting or height of embankment for submersible portion of approaches is more than 2m (vii) Where there are faults in the river bed (viii) If after completion of the submersible structures, the number of interruptions in ‘year caused to traffic and duration of the interruptions are likely to exceed the suggested values given in Table 3.1 below. 9IRC:SP:82-2008 Table 3.1: Permissible Number and Duration of Interruptions 'S. | Category of Roads Maximum No. of Duration of interruption in No permissible interruptions | hours at a time in a year 1. | State Highways, MDRs., 6 2-6 h duration, less than 2h roads linking important not to be considered and more towns, industrial esta than 6 h not acceptable 2. |O.D.Rs,Village Roads | 6 6-12 h duration, less than 6 h not to be considered and more than 12 h not acceptable 3.3.4, Fords Fords (i.e. unpaved causeway), though the cheapest type of crossing, should be avoided as far as possible and its adoption should be limited to sites where stream is wide, shallow with depth of water not more than 200 mm, velocity of flow is low (less than 2 m/sec), bed is firm, volume of traffic is low and the water is not likely to become muddy due to the traffic, endangering the aquatic life in the watercourse or the environment, 3.3.5. Causeways Causeways for crossing a wide watercourse with low banks and having not too large but perennial flow should be proposed with caution, These should be proposed on rural and less important link roads, not likely to generate much traffic in near future due to situations like dead end, low habitation and difficult terrain conditions. The causeways may be proposed on streams of flashy nature with high frequency of short duration floods or at sites where construction of submersible bridge is not economically viable. 3.3.6. Submersible bridges ‘These can be provided in all situations other than those mentioned in paras 3.3.4 and 3.3.5 above where pro’ viable ion of submersible structures is technically feasible and economically 3.4. Geometric Standards 3.4.1. General (a) A road conforming to sound geometric standards results in economical operation of vehicles and ensures safety. Geometric standards for approach roads to a submersible bridge or causeway depends on the classification of road (i.e. State Highway (SH) or Major District Road (MDR) or Rural Road (RR) which include Other District Road (ODR) and Village Road (VR), location (i.e. in urban or non-urban area), terrair plain or rolling or mountainous or steep), length of crossing and requirements of the user authority (i.e. local, State Govt. ete...[_ ee IRC SP:82-2008 (b) ‘The geometric standards in general should conform to relevant IRC Publications (ie. IRC:5, IRC:38, IRC:52, IRC:73, IRC:86, IRC:SP:20, IRC:SP:23 and IRC:SP:48). (c) There is no specific separate guideline in the IRC codes regarding geometric design standards for submersible structures including immediate approaches except in which stipulates that vented causeways/submersible bridges shall provide for st two lanes of traffic (7.5 m wide carriageway) unless one lane of traffic m wide carriageway) is specially permitted in the design, However, the provision for single lane width is likely to be revised and has been increased in these guidelines Refer Table 3.2. 3.4.2. Width of cross drainage structures Cross-drainage structures are difficult to widen at a later date. As such, road width should ho selected carefully at the planning stage itself. In case a road is likely to be upgraded in the s desirable to adopt higher roadway width. {oreseeable future, it Minimum carriageway width of submersible structures, measured at right angles to the \onpltuulinal center line of the structure, between the inner faces of discontinuous kerbs/safety kerbs \\\erever provided or between the guideposts/stones (without kerbs), should be as given in Fable 3.2 Table 3.2 imum Width of Carriageway for Submersible Structures ry of road Minimum Width of Carriageway*(m) ] jain & Rolling Terrain Mountainous and Steep Terrain | Two lanes 18 | Single lane | 68 55 | ag Ett fest ol | } | | J Note: * Minimum width of carriageway should be suitably increased as per IRC:73 in case of structures located on curves. In case footpaths are provided, the width of footpaths should not be less than 1.5 m each. The width of discontinuous safety kerbs, if provided should not be less than 600 mm. Overall width between the outer faces of discontinuous kerbs/safety kerbs wherever provided or guideposts/stones/railings (without kerbs) of the structures with length upto 30m hould preferably be a little more to match with the roadway width of immediate approaches Table 3.4.3. Geometries of approach roads (i) Alignment of the road generally governs the site ofa submersible structure if the length of crossing is less than 60 m. However, if the length of the crossing is more than 60 m,the suitability of the site for the submersible structure and the geometric de ign of immediate approaches both should be considered together. In case the length of crossings more than 300m, the most suitable site fr the bridge should be the governing criteria, (i) The approaches on ether side ofa straight submersible bridge should have a minimum Straight length of 30 m and should be suitably inereased, where necessary, to provide for the minimum sight distance for a vehicular speed of 35 km/h, Gi) Horizontal curves in immediate approach roads fora length of about 100 m on either side of a submersibleistructure or ays should be avoided. If horizontal curves have to be provided in the approaches, the same should be located beyond the straight Portion on either side and the minimum radius of curvature, the super-elevation and transition length should be provided in accordance with relevant stipulations contained in IRC:38. Radi of horizontal curves in case of immediate approach roads however should, not be less than 60 m in case of plain and rolling terrain and 30 m in ease of hilly terrain from road user safety consideration, 3.4.4. Design speed From consideration of safety of road users, lower design speed than that recommended in IRC:73 should be adopted for the immediate approaches to a submersible bridge or causeway, {he informatory boards installed on approaches should indicate permissible speed of 35 lagyh case of plain and rolling terrain and 20 knv/h incase of mountainous and steep terrain inrespective of any higher speed adopted in the design of the road. 3.4.5. Roadway width Wi ith of roadway should be as shown in Table 3.3. Table 3.3: Width of Roadway (m) _ Z Road Classification Plain & Rolling Terrain Mountainous & | | Steep Terrain ** I 2 1. | State Highways | | i) single lane 12.0% 6.25" j ii) two lanes, 12.0 88 | 2 | Major District Roads | | i) single lane | 9.0 6.25" | | ii) to lanes 9.0 8.8 | Rural Roads i) single lane 7508 6.0 ii) two lanes 9.0 78 |IRC:SP:82-2008 If the possibility of sign of fen | © Vor single lane State Highways, width of roadway might be reduced to 9 n rgth of \sloning, the carriageway to two lanes is conside eming * Tho roadway widths in mountainous and steep terrain, (nial width 0.6 m) and side drains (usual width 0.6 m) oudway width for rural roads in plain and rollers terrain also ma Jhore Waftie intensity is less than 100 motor vehicles per day and traffic is not likely to increase \imum, Jo fo situations like dead end, low habitation and difficult terrain conditions. Lubject to heavy snow fall, where regular snow clearance is done over long periods to yen above are exclusive of parapets be reduced to 6.0 min case rovide {Onroad 1) the road open to traffic, the roadway width may be increased by 1.5 m, Thv ronday widths for Rural Roads are on the basis ofa single lane carriageway of 3.75 m. Ji; hard rock stretches, or unstable locations where excessive cutting might lead to slope failure, either ‘ ies Ih of roadway may be reduced by 0.8 m on two-lane roads and 0.4 m in other cases. Dm horizontal curves, the roadway width should be increased corresponding to the extra widening raight carriageway for curvature mand tained M46, € ber/erossfall wever nse of he camber/crossfall on straight sections of immediate approaches and on submersible {\j/0s should be unidirectional towards the downstream and as recommended in Table 3.4 Jojwelinyt on type of surface of pavement Table 3.4: Pavement Camber/Crossfall ed in a ae : Bee way. wrtnee Type -etional Cross fall (%) Vb in aay ctive How all categories of roads | High Type bituminous surfacing or cement 2.0 | {hin biwminous surfacing for approaches 2 Ivehistone set pavement 3.0 | Jesolo oF approach ads Tikely tobe submerged dering floods shouldbe paved to se cos al So, Superelevation 7 A\yporelevation to be provided on horizontal curves is calculated from the following formula indicated in Table 3. | yhoo! (0 the maximum values \yporelevation in m per m = (Design speed in km/h J? (225 x radius of curve in m) Table 3.5: Maximum Permissible Superelevation 7] 1 Plain/rolling terrain and snow bound hill roads 2 Hill roads not affected by snowIRGSP:82-2008 3.4.8. Gradients Asa general tule, values of ruling gradients specified in IRC:73 should be adopted. However, in case of immediate approaches to submersible structures, catrying substantial slow traffic, flatter efadients than ruling values should be preferred. Nevertheless, gtadients in iminediate approaches unless, otherwise pettnitted by user authority, should not exceed 5.0% (1 in 20) irrespec' ature of terrain.IRC:SP:82. 4, HYDROLOGY AND HYDRAULICS er, : Ml. Hydrology ter nes ALL, General of the design of an efficient and economical hydraulic structure, knowledge of hydrology sil the characteristics of the Stream/River are of paramount importance. A brie! {ven it Appendix 4.1. Inmost cases hydrological record of the stream particularly data regarding {uy Not be available. A rational estimation of design flood discharge for the specified return ‘oul loads to economical design of bridge foundations for submersible bridges, The failures of Jiwiille structures are very expensive as in most cases, the indirect costs are many times larger (we direct cost of bridge replacement. Some hydraulic structures especially bridges have failed ‘ihe past mainly due to inadequate assessment of HFL/ Design flood discharge and rarely due to We nal fhilures. Due attention to the determination of hydrology of the structure needs to be {rvational approach can lead to loss and destruction of the structure due to floods higher Wi the design floods, about hydrology 1.1.2, Determination of design discharge lischarge for which the waterway of most of the bridge including submersible (io be designed should be based on the flood discharge corresponding to highest observed |, iespective of the return period of that flood or the flood of 50 years” return period except in the case of important bridges when return period may be taken as ign discharge can be determined by the following methods (1) Empirical Methods (2) Slope Area Method () Rational Method (4) Unit Hydrograph Method 4.1.2.1, Empirical methods Huied on studies conducted, some empirical formulae for specific regions have been \ived, The empirical formulae for flood discharge sug Q = CA® gested are in the form: Gal) Max. flood discharge in m/s Catchment Area in sq. km An Empirical Constant, depending upon nature and location of catchment n A ConstantIRC:SP:82-2008 The most commonly adopted empirical formulae and recommended for use are: (i) Dicken’s formula based on data of rivers in Central India, (ii) Ryve’s formula based on Rivers in South India and (iii) Inglis formula based on West Indian rivers in the old Bombay Province. Details of these emperical formulae are given in Appendix 4.2 The empirical formule should, however, be used with due caution as given below (i) ‘These were developed for particular region and for small catchments and, therefore. have obvious limitations, The value of *C’ at the best is valid only for the region for which it has been determined, as each basin has its own characteristics affecting run- off (ii) These involve only one known variable factor viz. area of the catchment and therefore a large number of remaining factors that affect the run-off such as shape, slope, permeability of catchments ete, are to be accounted for in selecting an appropriate value of the coefficient *C’. ii) A correct value of ‘C’ can only be derived fora given region from an extensive analytical study of the measured flood discharge vis-a-vis characteristics of the basin, The value of *C* will therefore be valid only for the region for which it has been determined, as each basin has its own characteristics affecting run-off. A new designer should use these formulae only under the guidance of an experienced designer or expert 4.1.2.2. Slope ~area method In this method the maximum water level reached in a historic flood is estimated on the evidence of local witnesses, which may include identification of flood marks on structures or trees close to the bridge site. The discharge is then calculated by: Q=A Where, Q= discharge in m'/s and A= wetted area in m? V= velocity of flow in m/ (42) ec which can be calculated by the Manning’s formul V=1/n R% Where, R = hydraulic mean depth, $ =the energy slope which may be taken as equal to bed slope and n ~rugosity coefficient. ‘The details ofthe method are given in Appendix This method has also considerable room for error due to: (The variability of bed profile slope ete. during floods from those measured during survey.IRC:SP:82 2008 (ii) ‘The computation of stream velocity is dependent upon a subjective selection of an Empirical Coefficient of rugosity for different conditions of bed out of the v values recommended by Manning. ious 4.1.2.3. Rational method The rational method for flood discharge takes into account the intensity, distribution and \\yation of rainfall as well as the characteristics of the catchment area, such as shape, slope, ability and initial wetness of the catchment. The rational formula is as follows: Q =A, (4.4) Whore Qo Maximum flood discharge in m* /s A Catehment area in hectare I Max. intensity of rainfall in erv/h Function depending upon characteristics of the catchment in producing peak run- offand given by a 0.056 FP = (4.5) Where, ‘f° is the area correction factor, “t,” is the time of concentration in hours and *P” is poreubility coefficient of the catchment depending on the soil cover conditions and slope of Wehmenters, The details about Rational Method are given in Appendix 4.4. The formulae may Jworally be adopted for catchment areas upto 500 sq. km and upto 2000 sq. km in exceptional 4.1.24, Unit hydrograph method (i) Unit Hydrograph: The unit hydrograph or unit graph is defined as the hydrograph of {orm run-offat a given point in the river, resulting from an isolated rainfall of unit duration (normally taken as 6 h to 12h) occurring uniformly over the catchment and producing unit run-off. The unit run-offadopted is | cm depth over the catchment (ii) A Committee of Engineers appointed by Govt. of India recommended a rational methodology based on use of design storms and unit hydrographs for estimating design floods for different zones/sub-zones of India. A list of these zones and sub-zones is iuiven in ‘Annexure A’ of Appendix 4.5. The report as prepared jointly by CW RDSO (Railways), MoSRT&HI and IMD have been published by CWC, Govt. of India, These reports give methodology through a set of charts and graphs for quick estimation of design flood of 25, 50 or 100 years of retum periods for ungauged catchments,IRC:SP:82-2008 Gil) Unit hydrographs are prepared either by computation from direct run-off hydrograph for gauged streams or are synthetically prepared from catchment characteristics for ungauged catchments and then used for finding design flood of desired return period The detailed procedure for constructing Synthetic unit hydrograph and how to obtain design flood from storm of corresponding return period is illustrated in an example given in Appendix 4.5. iv) The unit hydrograph method can give fairly precise results for drainage areas upto 5000 sq. km. Variation in assumptions made for larger areas (>5000 sq. km) in the method are usually too great to be ignored. 4.1.2.5. Fixing design discharge Flood discharge can be estimated by three or more different methods and the values obtained should be compared. The highest of these values should be adopted as the design discharge provided it does not exceed the next highest discharge by more than 50%, Ifit does, restrict it to that limit, 4.1.3. Discharge through a submersible bridge The total discharge in the stream after the construction of a submer found by the method suggested by Johnson Victor as given sible bridge can be Total discharge Q= Q, + Q, + Q. (4.6) i H +h)? -h, 3? and Q=A,x 2 ¢, v2g Cth hs (46.1) 2. H C, ¥2g. Vth, (4.6.2) Q, =A, x C, ¥2g. Vth, vs (4.6.3) Where. Q, = Discharge between afflux upstream water level and down stream water level and A, is its area of flow Q, = Discharge between downstream water level and deck level A, = Area of flow between downstream water level and deck level Q. = Discharge through vents and, is the area of vents C,,C, & C, are coefficients of discharge Ho = AffluxP:82-2008 h, = Head due to velocity of approach. C, = 0.625 for equation (4.6.1) 9 for equations (4.6.2) and (4.6.3) (Refer Fig, 4.1 for various parameters of flow.) 4.1.4. In cases where the cross-section of the stream has wide spill zones of shallow \epth, the discharge through causeway or low level submersible bridge can also be found by \ddling the calculated discharge of the three parts viz. (a) Discharge through vents of area AI (b) Flow over the causeway/submersible bridge proper through area A2 and (c) Flow over hallow triangular compartments of area A3 on either side of the main stream at the crossing. (See Fig 4.2). AFFLUXED HL wu ho HEAFFLUX ag LSOFRT LeveL TEE st Lr cRoss— ws AFFLUXED HEL : TOP oF SUB BOTTOM OF SLAB AVERAGE Bet LONGITUDINAL SECTION Fig. 4.1. Total Discharge at a Submersible Bridge 19IRC:SP:82-2008 sl Waa 500: Tee Beith Fig. 4.2 Ty al Vented Causeway 4.2. Forces due to Water 4. Hydro Static Foree Force of stationary water on a solid surface is called the hydrostatic force. It includes force due to the afflux head and the force of buoyancy. A body submerged in water experiences an upward force due to water pressure and this force is called ‘Buoyancy’. It must be considered for stability of structure if there is possibility where while considering combination of forces, stability ofthe structure is to be affected. [tis recommended that while checking for minimum pressure on foundation, the maximum uplift pressure at high water level should be considered. Further, while checking for maximum pressure the minimum uplift pressure at the low water level should be taken into account. In case of submersible bridges, full buoyancy effect on the superstructure also needs to be considered, 4.2.2. Hydrodynamic force of water current 4.2.2.1, Water current forces on foundation above scour level and on substructure Water current causes hydrodynamic force on the submerged part of a body. These forces onamember can be calculated by the following formula as given in Clause 213 of IRC:6. P= 52KV? (47) Where, P Intensity of pressure due to water current in kg/m? 20IR P:82-2008 V The velocity of the current at the point where the pressure intensity is being calculated in meter per second and k A constant having the following values for different shapes of members as given in Table 4.1. Table 4.1: Shapes of Bridges Piers & Value of K JES OF K SHAPES OF MEMBERS IN PLAN — (ano FOR SUPERSTRUCTURE) pre (AR OR SEMICIRCULAR ENDS E TRANGULAR (THE ANGLE B59 THE FACES BEING 30 0% yay TRANGULAR (THE ANGLE INCLUDED BETWEEH 0.50 10 0.70 << — > THE FACES @EING MORE THAN 30 a OIGREES BUT LESS THAN 60 DEGREES) ‘TRIANGULAR (THE ANGLE INCLUDED BETWEEN 0,70 To 090 ‘THE FACES BEING 60 TO 90 DEGREES OF LESS 045 EQUILATERAL ARCS OF CIRCLES 050 INTERSECTING AT 90 DEGREES The maximum velocity at the top surface of flow shall be assumed to be V2 times the maximum mean velocity of the current, Square of velocity at a height X from the point of deepest se our=Ut= 2V?X i Where V is the maximum mean velocity. (4.8) The value of V? in the equation (4.8) is assumed to vary linearly from zero at the point of deepest scour to the square of the maximum velocity at the free surface of water (Fig. 4.3). 2IRC:SP:82-2008 FREE SURFACE— OF WATER ol ] | i POINT OF — | DEEPEST ScOUR | | | Fig. 4.3 4.2.2.2. Water current forces on superstructure @ Gi Gi) The importance of water current forces on the superstructure is significant due to the extent of obstruction offered by the bridge superstructure and its location, Since the submerged area of superstructure exposed to water current forces is sufficiently large and the velocity of current at its level is also high, the stresses on foundations due to ‘water current forces acting on the submerged superstructure are quite pronounced. Flowing water produces two types of forces on a submerged or partially submerged superstructure viz. the drag force and the lift force. These are characterized by two factors ie. the drag force co-efficient (C,) and coefficient of lift (C,). Both drag force and lift force depend largely on the shape of the body and several other factors and these can be best determined by conducting hydraulic model studies, as explained in Appendix 4, ‘The results of model studies conducted so far do not conclusively recommend any generalized values of co-efficient of drag (C,) and co-efficient of lift (C,). However, presently the following method is adopted for calculation of drag force and uplift pressure on superstructures, in cases where it is not feasible or economically viable to conduct hydraulic model studies (a) The expression P= 52 KV? as given in para 4.2.2.1 be adopted with value of K as 1.5 for drag force. (b) The expression p= wh may be adopted for calculating uplift pressure, Where ‘w’ is the unit weight of water and ‘h’ is the uplift head under the deck and can be estimated as h= thickness of slab + wearing coat and afflux after deducting the head loss due to increase in velocity through vents, The head loss is given by the expression (V,?- V?)/2g, Where V, is the velocity through vents and V is velocity of approach. 2IRC:SP:82-2008 Appendix 4.1 A BRIEF ON HYDROLOGY Hydrology deals with depletion and replacement of our water resources. The basic knowledge of this scietice is muist for Civil Engineer, particularly the one who is engaged in design planning and construction of hydraulic structures such as Bridges. The Hydrologic Cycle: Most of the earth’s water sources such as rivers, lakes, oceans and underground sources, etc. get their supply from the rains, while the rain water in itself is the evaporation from these sources. Water is lost to the atmosphere as vapour from the earth, which is then precipitated back in the form of rain, show, hail, dew, sleet or frost, ete. This evaporation and precipitation continues forever and thereby, a balance is maintained between the two. This process is knowti as Hydrologic Cycle. Itcah be represented graphically, as shown in Fig. 4.4 Precipitation i.e. x oe ‘Snow, Hail, Sleet etc.) fa As we if Transpiration ~ from o> Representing Hydrologic Cycle Evaporation i Fig. 4.4 Run-off Run-off and surface run-off are two different items and should not be confused. Run- off includes all water flowing in the stream at any given section, and therefore it can also be named as ‘Discharge of the Stream’ while surface run-off includes only the water that reaches the stream channel without first percolating down to the water table. Run-off consists of the following (Fig. 4.5). (Direct precipitation over the surface of the stream and its distributaries, this is very small and ignored, (ii) Surface run-off consisting of true surface run-off and sub-surface run-off. 23IRC:SP:82-2008 Tee $.R.O. = sub-Surface R.O, Ground Water Flow Fig. 4.5 (ii) Ground water flow. (iv) True surface run-off :- Water that flows directly over the ground surface to the stream, (¥). Sub-surface run-off: Water that infiltrate the soil, moves laterally and before joining water table it joins the river channel and this quantity of water is known ‘As sub-surface run-off. It behaves nearly like a surface run-off and not like a ground water flow, because it reaches stream so quickly that itis difficult to differentiate from true surface run-off. For this reason Sub-Surface Run-offis always treated as surface run-off. Hence, Run-off= Surface Run-off + Ground Water flow. ‘The ground water is often times, long delayed before it reaches the stream. Itis to be farther noted that Ground water flow is important for “Minimum flow’ in the stream while surface run-offis important for the ‘Maximum flow’ of the stream. Run-off depends upon (a) Characteristics of drainage basin and (b) Characteristics of rainfall precipitation which further depends on following factors: (i) Characteristics of Drainage basin depend upon (i Size, (i) Shape (Fan or fern), (iii) Elevation of water shed. Besides these three important characteristics ofthe drainage basin, the arrangement of the stream channels formed by nature within the basin, the type of soil, the type of vegetation cover are various other factors influencing the run-off. Gi) Characteristics of Rainfall Precipitation depend upon (3) Slope of Channel, (iy Shape in plan (Layout) (ii) Nature of bed (iv) Sub-soil storage characteristics of the bed and banks (v) Status of flow at commencement of precipitation. IN. The Rainfall precipitations are of following types @ Cyclonic Precipitation - Cyclonic precipitation is of two types: Tropical and Entra Tropical. The tropical cyclones originate in the open ocean and are primary source of monsoon rainfall in the country. Extra tropical eyclonie precipitation is responsible for most of the winter rains in North- Western India 24RS ee ESSE SESE See CSE eee EEC ee Cee eee eer eee etd IRC:SP:82-2008 Gi) Convection Precipitation Convection precipitation generally occurs in tropics in the form of showers of high intensity and short duration, (ii) Orographic Precipitation :- It is most important precipitation and is responsible for most of the rains in India. Orographic precipitation is caused by air masses which strike some topographic barriers like mountains and can not move forward, hence rise up causing condensation and precipitation, and greatest amount of precipitation falls on wind ward side. A striking example of such natural barriers is in southern slopes of the hills of Meghalaya. The rainfall is dependent on various factors and combination which are numerous such as: (a) Duration (b) Quantum (c) Intensity (d) Direction of storm (e) Special distribution of rain over the catchments (f) Temperature and Humidity (g) Velocity and duration of wind 1V. Point/Station Rainfall Point rainfall, also known as station rainfall refers to the rainfall data of a station. Depending upon the need, data can be listed as daily, weekly, monthly, seasonal or annual values for various periods. In practice, however, hydrological analysis requires a knowledge of the rainfall over an area, such as over a catchment. To convert the point rainfall values at various stations into an average value over a catchment, the following three methods are in use: (a) Arithmetic-mean method, (b) — Thiessen-polygon method, and (c) _ Isohyetal method. (a) Arithmetic-Mean Method When the rainfall measured at various stations in a catchment show little variation, the erage precipitation over the catchment area is taken as the arithmetic mean of the station values, thus ifP,, P,,....... P, are the rainfall values in a given period in n stations within a catchment, then the value of the mean precipitation P over the catchment by the arithmetic-mean method is P,+P,+ 1 - = = P, n n ‘This method is explained in Fig. 4.6(a). (b) Thiessen-Mean Method In this method the rainfall recorded at each station is given a weightage on the basis of an area closest to the station. The procedure of determining the weighting area is as follows: Consider catchment area shown in Fig. 4.6 (b) containing eleven raingauge stations, of which five lie shown and positions of the outside the catchment but in its neighbourhood. The catchment area isIRC:SP:82-2008 oss / 182) fr 2382 \ 2 \\ saet.9212.0044:5002.984500 og ws 459) - (208 500/ 1.85 Observed reat Percent recip, toto (em) (sq km) oreo oss ? 1 1.48 120 19 1.92 109 18 269 120 19 134 20 3 298 92 5 5.09 82 3 1.50 76 2 626 “T00° Averoge = 2.84 cm (b) Thiessen method (em) (sq km) (sq km) A 22h oY Ee eee ae 66, 2 aka, feo — 1 595, 193 495, = mmn-offinm’s, Ais catchment area in sq. km and C isa constant, having values as C = 6.8 for areas within 25 km off the coast = 8.5 for areas between 25 km & 160 km off the coast 10.0 for limited areas near the hills, Gi) Inglis Formula. Col. Inglis, who was working in old Bombay Presidency, after study of the run-off.and floods in the region, evolved a formula Gg» (4.2.3) VA+10 Where, Q Run-off in m/s and A Catchment area in sq. km. 28IRC:SP: 82-2008 IRC:SP:82-2008 Appendix 4.3 EMPIRICAL FOR 5 SMULAEFOR CALCIhy sLOPE-AREA METHOD ) Dickens? Formula: Qecax reliable data regarding the highest level of discharge at or Where, Ming velocity. Itis generally easy to obtain highest level of one hie oldest inhabitants in the area or by observing old flood Cc _ HMOMFInm,s, Ais catchment"? MOleet site. The determination of flood discharge ean = ment| e\ermining discharge in open channels. Site data to be =n ere ae A Where the annual rainfal} Where annual rainfall i n = 22n Western Gha ™m Ghats, fat the site of the probable scoured bed line Ryve’s F, yVe's Formula rmula; of river Q= Cas J Water in the stream noted by observations during floods or Where, Q = mum u-OtTin m5, A is catchment af lion ofthe river should be taken one at the proposed site ea f downstream, distances being as given in Table 4.2 below. = 68 for areas with 5 ee ae OF th cing of Cross-Section on Streams Distance apart for Cross-Section. Inglis Form ula: 100 m (Seale not less than 1m to 10 m or (1/1000) “Ol. Inglis, wh f * who was workin, loods in the reo; king in old B 300 m (Seale not less than 1 em to 10m or 1/1000) xis in the region, evolvy * old Bombay Preg : da formula: ‘One half km or width between the banks whichever is more Q 125A (Seale 1 em to 50m or 1/5000 VA+10 reas should be used for computation. Where a ‘ction mn, the mean is arrived as follows. ‘here, Q = Run-offin m/s and © Catchment area in sq, km hs, ‘A’ mean area of flow in the stream, *A’,, ‘A’, etc. areas of hot plotted to the natural scale (the same scale horizontally and {(P) cannot be scaled directly from the section and has to be Jine into a convenient number of parts AB, BC and CD, etc. as day PQ, let PR or QR be its horizontal and vertical projections. PR can be measured on the horizontal scale of the given 28 29peace IRC:SP:82-2008 Appendix 4.3 DISCHARGE BY SLOPE-AREA METHOD. This method is applicable where reliable data regarding the highest level of discharge at or close to the site is available but not regarding velocity. It is generally easy to obtain highest level of \lischarge data by local enquiries from the oldest inhabitants in the area or by observing old flood narks on the trees and buildings near the project site. The determination of flood discharge can then be done by applying formulae for determining discharge in open channels. Site data to be wolleeted for this purpose are:~ 1, Cross-Section of the river at the site of the probable scoured bed line 2. Observation of the nature of river 3. Slope of the surface of the water in the stream noted by observations during floods or from flood marks For this purpose three crass-section of the river should be taken one at the proposed site of the crossing, one upstream and one downstream, distances being as given in Table 4.2 below Table 4.2: Spacing of Cross-Section on Streams Catchment Area of Distance apart for Cross-Seetion 3.0 km? or less 100 m (Scale not less than I.em to 10 mor (1/1000) 3.00 15.0 km® 300 m (Scale not less than I em to 10 m or 1/1000) Over 15.0 km? ‘One halfkm or width between the banks whichever is more (Scale 1 em to 50 m or 1/5000 ‘The average of the three cross-sectional areas should be used for computation. Where a ber of cross-section have been taken, the mean is arrived as follows A}t2A, PATA (nel) Where, ‘n’ number of cross sections, flowat different cross-sections. mean area of flow in the stream, ‘A’,, ‘A’, etc. areas of When the cross-section is not plotted to the natural scale (the same scale horizontally and ally), the wetted perimeter (P) cannot be scaled directly from the section and has to be calculated. Divide up the wetted line into a convenient number of parts AB, BC and CD, ete. as shown in Fig. 4.7. verti Consider one such part say PQ, let PR or QR be its horizontal and vertical projections. The PQ= (PR? + QR2). Now PR can be measured on the horizontal scale of the given 29IRC:SP:82-2008 cross-section and QR on'the vertical scale. PQ can then be calculated. Similarly the length of each partis calculated. Their sum gives the wetted perimeter. The velocity of discharge is calculated by Chezy Formula or Manning’s formula. Generally, Manning’s formula (in metric units) which is simpler, is used v= R® s¥ (4.1) Where, v Velocity of flow in m/sec considered uniform throughout the section R Hydraulic mean depth that is A/P (in m) n= Rugosity coefficient = Flood slope of the river usually taken as bed slope in absence of precise data (Fig 4.8), Fig. 4.8 Slope $ may be corrected for the kinetic energy difference at the two ends and is given by: Z,-Z,+ [ VEVe S- - au ud Ll (4.2) 30IRC:SP:82-2008 The second term is kinetic energy difference, which is negligible and can be neglected where the reach is sufficiently long or the slope is not too flat, The value of rugosity coefficient ‘y’ is given in the following Table 4.3, Table 4.3: Value of 1 (Rugosity Coefficient) SLNo. | Surface Perfect | Good | Fair Bad Natural Streams 1 Clean, straight bank, full stage, 02s | 00275 | 003 | 0033 no rifts or deep pools 2 Same as (1), but some weeds and stones 0.03 0033 | 003s | 004 % Winding, some pools and shoals, clean 0.035 0.04 oss | os 4 Same as (3), lower stages, more ineffective slope | 0.04 0.085 00s | 003s and sections 5 Same as (3), some weeds and stones 0.033 | 0.035, os | 004s 6 ‘ame as (4), stoney sections 0.045 0s o0ss_| 006 1 Sluggish river reaches, rather weedy or with 0.05 0.06 007 | 008 deep pools, 8 Very weedy reaches 0.075 o1 012s | 01s (4.3) 1 V= — ReS* = 48% Where, R* n is a function of the size, shape and roughness of the stream and is called its conveyance factor. Thus the discharge conveying capacity of a stream depends on its conveyance factor and slope. Ifthe shape of the cross-section is irregular as happens when a stream rises its banks and shallow overflows are created, itis necessary to sub-divide the channel into two or three subsections. hen ‘R’ and ‘n’ are found for each sub-section, and their velocities and discharges are computed eparately and then added together to get the total discharge. 31di IRC:SP:82-2008 Appendix 4.4 DISCHARGE BY RATIONAL METHOD. Arational method for estimation of flood discharge should take into account the intensity, tribution and duration of rainfall as well as characteristics of the catchment area, It should also take into account the discharge characteristics of the catchment area which depend on its shape, slope, permeability and initial wetness of the catchment, 1 Govardhanlal, in his method, applied the following Rational formula where by knowing the highest observed rain fall at a representative gauging station in an hour and knowing characteristics of the catchment area and rainfall, one find the discharge safely for areas upto 500 sq. km. ‘The formula is as follows Q=AI,2 a (4.4) Maximum flood discharge in m’ /s Catchment area in hectare Max. intensity of rainfall in centimeter per hour Function depending upon characteristics of the Catchment in producing peak run-offand given by 0.056 £P a= — ae (4,5) t +1 P= percentage coefficient of surface run-off for the catchment characteristics as given in (Table 4.4). Considerable judgment and experience are called for in assessment value of P. Any error in the later will diminish the reliability of the results of laborious calculations involved in this method. f = Factorto correct for the variation of intensity of rainfall over the area of the catchment. (Graph 4.1). time of concentration in hours Estimation of time of Concentration (t,) Itis the time taken by the run-off from the farthest point on the periphery of the Catchment (called the critical point) to reach site of Bridge. The concentration time depends on (i) the distance from the critical point upto the Bridge site and (2) the average velocity of flow which depends upon the slope, the roughness of drainage channel and depth of flood. Complicated formulae exists for determining the time of concentration (t,) from characteristics of the catchment.IRC:SP:82-2008 Table 4.4: Maximum Value of P in the Formula Steep, bare rock and also city pavements 090 Rock, steep but wooded 080 Plateaus, lightly covered 070 Clayey soils, stiff and bare 060 do- Tightly covered 030 Loam, lightly cultivated or covered 040 -do- Tightly cultivated 030 Sandy soil, light growth 020 “don covered, heavy bush 010 1 os os = rr ee CCATCHMENTAREAINHIECTARES — Feurve Graph 4.1 ‘The time of concentration (t,) can be obtained by using the State of California formula. This formula has also been recommended for application in India, in IRC:SP:13, para 4.7.5.2 and which is as follows: (vo 0.87 x | — + (4.6) (a Where, L distance from the farthest point in a catchment to the site in km H_ = fallinlevel from the farthest point to the bridge site in m and t, is the time of concentration 3IRC:SP:82-2008 The value of A,L, and H can be obtained from Survey of India Topographical maps. I, has to be obtained from Meteorology Department. I, of region have not to be found for each design problem, its characteristic of the whole region and applies to pretty vast areas having the same weather conditions. I, ofa region should be found once for all and should be known to local engineers. The Metrological Department, Govt. of India have data for heaviest rainfall in centimeter/ hour collected for various places in India and are to be obtained from them. Rational Method may be applied safely for areas upto 500 sq. km and upto 2000 sq. km in extreme cases. The use of Rational Method for small catchments have been advised in IRC:SP:13 vide clause 4.7.14 stating that since the average designer cannot rely so much on his judgement and intuition for selecting value of *C’ in Empirical formulae he should adopt ‘Rational Method” which has been outlined in detail in IRC:SP:13, Para 4.7. 34