STUDY OF LOOSENESS FACTOR FOR BARRAGES A DISSERTATION submitted in partial fulfilment of the requirements for the award of the degree of MAStER OF ENGINEERING in WATER RESOURCES DEVELOPMENT NRUSINGH CHARAN MOHANTY * yi~X h WATER RESOURCES DEVELOPMENT TRAINING CENTRE UNIVERSITY OF ROORKEE ROORKEE-247667 (INDIA) OCTOBER, 1984 CEATIFICAT E Certified that the dissertation entitled, I STUDY OF LOOSE.- US FACTORFOR BARRAGES' which is being submitted by Mr. Nru Singh Charan Mohanty in partial fulfilment for the award of the degree of master of Engineering ( Water Resources Development) of Univer- sity of Roorkee, as a record of student's own work carried out by him under our guidance and supervision, The matter embodied in this dissertation has not been submitted for the award of any other Degree or Diploma. This is further to certify that he has worked for a period from 16.6.1984 to 16.10.1984 for preparing this dissertation for Master of Engineering at this University. ( AYAN S.iMA) (C.P. SINHA) Reader Professor Water Resources Development Water Resources Development Training Centre, Training Centre, University of Roorkee University of Roorkee Roorkee, U.P. (India) Roorkee, U.P. (India) ROORKEE DATED .- Oct. 2,) , 1984 CKNOLDGNENTBS The author wishes to express his deep and sincere gratitude to Prof. C.P. Sinha, Professor in W.R.D.T.C., University of Roorkee-. and Sri Nayan Sarma, Reader in WRDTC, University of Roorkee for their valuable constant guidance and kind encouragement throughout the work. The author owes his gratefulness to officers of U.P. I.R,I., Roorkee for their useful discussion and valuable suggestion on the subject, 1 R©OBKEE Nrusingh Charan Mohanty Trainee Officer October ,1984 S Y N 0 P S I S Looseness factor of a barrage largely influences its hydraulic performance and, to a great extent, moulds the river behaviour near the site. In' absenceof any rational approach for its deter- mination, the waterway of a barrage is generally decided by the designer depending on his judgement and the past experience. The opinion is divided on the value of looseness factor to be adopted in design of a diversion structure. Also the discharge parameter for Lacey's wetted perimeter formula is not well defined and this creates room for vagueness for the value of looseness factor. The subjective approach to the problem of determining water way of a diversion structure has been mainly responsible for the undesirable shoal formations upstream of barrages which adversely affect the functioning of the works. In this dissertation an effort has been made to study the effect of looseness factor on sediment transporting capacity at barrage site, and also to evolve a ration- al procedure for determining waterway on the basis of existing sedi- ment transport theories* The consideration has been made that the greatest volume of sediment should be carried past the structure. A procedure has been suggested for arriving at the desired waterway by adopting the concept of bed. generative discharge of Schaffernack along with the simplified DuBoy' s formula. A study has also been made on the inter-relation among waterway, pond level and opera- tion from the consideration of minimising the silt deposition on the upstream. The 'studyhas indicated the desirability of adoption of a variable pond level, conforming to prevailing river water level as far as practicable with a view to preventing harmful siltation upstream of the barrage. C 0 N T E NT S Pages CE .TT Fl C \TL aCKNO`°1L2DG MSS IS SYXI PST S C: { 1PTcR — .1 INTRODUCTION Cj 1 APT =R — 2 LITERATURE RSVI E':! CHAPTER — 3 CASE STUDY CHAPTER — 4 DISCUSSION CH \PT ER — 5 CONCLUSION R-.F'-R'~1C35 FT GUl. S CHAPTER-1 I INTRODUCTION 1,1 GENERAL Barrages and weirs are man made barriers constructed across rivers for diversion of a part of the river flow to the offtaking canal. The diverted water may be utilised for irrigation and/or for power generation. Waterway of a barrage wields a close influence not only over the hydraulic design of the structure but on its performance as well. The waterway is generally decided consider- ing a suitable looseness factor, which is a ratio of the actually provided waterway to the Lacey's wetted regime perimeter. So far, there is hardly any consensus among river engineers and designers on the value of looseness factor that should be adopted in design. As a result the Looseness factor is often adopted by the designers banking upon his judgement and experience which differ widely from barrage to barrage. This is borne by the fact that looseness fac- tor, of some of the existing barrages and weirs varies between 0.45 to 2.2. Also, the status of design discharge for waterway is left to the discretion of the designer and is practically decided on conjecture. The subjective approach to the problem of determining the waterway has been responsible for Wanton shoal formation on the upstream of the diversion works thereby vitiating the hydraulics, functioning of'the structure. Obliquity of flow, shifting of deep channel, masking of some barrage bays, development of cross 2 flows are some of the problems which manifest in the wake of shoal formations in the upstream of a barrage. Other,attendent problems associated with shoal formations are increased in the depth of local scour, reduction in effective waterway of bas, increase in afflux, vitiating the hydraulic jump and increase in the value of downstream retrogression. In other words, a waterway which is not arrived at after well- thought-out and rational ana- lysis, but decided on some sort of thumb rule, may lead to a com.. plete failure of the intended hydraulic functioning of the barrage as outlined above. In the light of the above, it has been felt that a study on the looseness factor i.e. waterway of a barrage is of great importance to the designers of barrages. 1.2. PR:FSEXT PRACTICES. The present practice of design of barrage is to fix the water- way of barrages same as Lacey's wetted perimeter. Lacey's wetted perimeter is defined as 4,75/f, where 1 Q' is the design discharge considered for fixing the waterway and is normally 1 in 50 years, The practice of using Lacey's regime concept for fixing the water- way does not seem to be rational and in fact is very much arbitrary. This is because -- i) The constant of proportionality has a value in between 3.6 to 6.15. ii) Unlike a constant discharge in a canal, the discharge in a river is highly fluctuating. Hence the discharge to be considered for the purpose of fixation of waterway is of critical importance. iii) When a structure is built across a river, its regime gets disturbed. Hence, strictly speaking, it may not be reason- able to apply Lacey's regime theory for the post barrage condition. s per some of the recent studies a criteria may be fixed for fixation of waterway by studying the cost economics of the barrage and the waterway giving the lowest cost may be adopted. In each design, the designer designs a structure with two. aims in front of him, (i) safety against failure and against failure in functioning (ii) structure should be economical, Hence, even though a structure is economical, i may fail to function satisfactorily. The purpose of the construction of the structure is not fulfilled., 1.3. DESCRIPTION OF ANALYSIS i) The first aim of the analysis is to study to what extent a fixed fond level and the waterway affect the transporting capacity of the stream at different stages, 'Dakpathar' barrage has been adopted for this analysis, The analysis has been done by using shields modified curve for tractive stress. It has been found that available tractive stress varies inversely as the waterway, ii) The second aim of analysis is to fix a suitable waterway for .(a) minimum shoal formation in front of the weir or barrage (b) Least affect on the morphology of the barrage by construction of the structure. For the above studies I Bed generative discharge, as proposed by Schaffernack has been considered, This bed generative discharge is the discharge carrying the highest volume of silt. 4 For this study Bhimgoda weir site has been taken into consideration, The main difficulty faced here is the non-availability of silt data. Hence silt volume has been estimated by using Du-Boys' equation. Bed generative discharge has been found out by using the procedure proposed by Schafferneick. For a pond level corresponding to mean annual flood, silt carrying capacity of the river at post construction stage just upstream of the barrage has been estimated at bed generative dis- charge. From this the width needed for sediment carrying capacity at bed. generative discharge will give the required waterway of the river near barrage. Lanes, simple theory relating discharge, slope, sediment discharge, Median diameter of the bed materials has been applied for arriving at the waterway which will have least affect on the morphology of the river has been determined. The waterway obtained from the above two different ways of analyse are nearly equal. But it is very much different for the waterway required from the point of economy. Hence a compro- mise is to 'be made between the two. With the waterway as found, from above and the available discharge during monsoon, pond level that can be maintained for keeping all gates opened has been found out. Also the part of the waterway that is to be closed for maintaining the pond level has been determined. From the analysis of silt carrying capacity it was seen that a variable pond level is a better concept as far as silt transport across the diversion work is concerned.