Table Of Content

इंटरनेट मानक Disclosure to Promote the Right To Information Whereas the Parliament of India has set out to provide a practical regime of right to information for citizens to secure access to information under the control of public authorities, in order to promote transparency and accountability in the working of every public authority, and whereas the attached publication of the Bureau of Indian Standards is of particular interest to the public, particularly disadvantaged communities and those engaged in the pursuit of education and knowledge, the attached public safety standard is made available to promote the timely dissemination of this information in an accurate manner to the public. “जान1 का अ+धकार, जी1 का अ+धकार” “प0रा1 को छोड न’ 5 तरफ” Mazdoor Kisan Shakti Sangathan Jawaharlal Nehru “The Right to Information, The Right to Live” “Step Out From the Old to the New” IS 7365 (2010): Criteria for hydraulic design of bucket type energy dissipators [WRD 9: Dams and Spillways] “!ान $ एक न’ भारत का +नम-ण” Satyanarayan Gangaram Pitroda ““IInnvveenntt aa NNeeww IInnddiiaa UUssiinngg KKnnoowwlleeddggee”” “!ान एक ऐसा खजाना > जो कभी च0राया नहB जा सकता हहहहै””ै” Bhartṛhari—Nītiśatakam “Knowledge is such a treasure which cannot be stolen” IS 7365: 2010 Hkkjrh; ekud ckYVh Vkbi mtkZ {k;dkjdksa dh nzopkfyr fMtkbu ds ekin.M (nwljk iqujh{k.k ) Indian Standard CRITERIA FOR HYDRAULIC DESIGN OF BUCKET TYPE ENERGY DISSIPATORS ( Second Revision ) ICS 93.16 4 6 _ 9 0 © BIS 2010 0 2 S: B U R E A U O F I N D I A N S T A N D A R D S C HI MANAK BHAVAN, 9 BAHADUR SHAH ZAFAR MARG P A NEW DELHI 110002 GR E N HI S N March 2010 Price Group 11 U S Dams and Spillways Sectional Committee, WRD 9 FOREWORD This Indian Standard (Second Revision) was adopted by the Bureau of Indian Standards, after the draft finalized by the Dams and Spillways Sectional Committee had been approved by the Water Resources Division Council. This standard was first published in 1974. The first revision was taken up in 1985 to reflect the latest practices prevailing and to utilize the knowledge gained during the use of the standard. This revision has been taken up to utilize the experience further gained during the use of this standard. Energy dissipator at the toe of spillway of a hydraulic structure is necessary to minimize erosion of strata downstream of the structure. For this purpose there are different types of energy dissipators, such as hydraulic jump, impact, jet diffusion, interacting jets, hollow jet and multiple ski-jump. Generally bucket type energy dissipators are being used for energy dissipation in medium and high dams. Hydraulic behaviour of bucket type energy dissipator depends on dissipation of energy through: a) interaction of two rollers formed, one in the bucket, rolling anti-clockwise (if the flow is from the left to the right) and the other downstream of the bucket, rolling clockwise; or b) interaction of the jet of water, shooting out from the bucket lip, with the surrounding air and its impact on the channel bed downstream. Bucket type energy dissipators can be either: a) roller bucket type energy dissipator, or b) trajectory bucket type energy dissipator. 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, shall be rounded off in accordance with IS2:1960 ‘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. IS 7365 : 2010 Indian Standard CRITERIA FOR HYDRAULIC DESIGN OF BUCKET TYPE ENERGY DISSIPATORS ( Second Revision ) 1 SCOPE b) interaction between the jet and surrounding air, c) diffusion of the jet in the tailwater, This standard lays down criteria for hydraulic design of bucket type energy dissipators. d) impact of the channel bed, and e) pre-formed plunge pool. 2 TERMINOLOGY 3 SYMBOLS For the purpose of this standard the following definitions shall apply. The symbols used in the standard are defined as given below: 2.1 Solid Roller Bucket — An upturn solid bucket (see Fig. 1A) is used when the tailwater depth is much a = vertical distance from the lip level to the in excess of sequent depth and in which dissipiation of highest point of the centre of jet, in m; considerable portion of energy occurs as a result of d = critical depth, in m; c formation of two complementary elliptical rollers, one d = depth of scour below tailwater level, in m; in bucket proper, called a surface roller, which is anti- s clockwise (if the flow is from the left to the right) and Ds = horizontal distance from spillway crest axis to the other downstream of the bucket, called the ground the point of maximum probable scour, inm; roller, which is clockwise. d = depth of flow entering bucket, in m; 1 2.2 Slotted Roller Bucket — An upturn bucket with d2 = sequent depth, in m; teeth in it (see Fig. 1B) is used when the tailwater depth d = height of tailwater above bucket invert, inm; 3 is much in excess of sequent depth and in which the d = tailwater level minus bucket lip elevation, 4 dissipation of energy occurs by lateral spreading of jet in m; passing through bucket slots in addition to the F = Froude number of jet entering bucket; formation of two complementary rollers as in the solid j bucket. v = a 2.3 Trajectory Bucket/Flip Bucket — An upturn solid gd1 bucket (see Fig. 1C) is used when the tailwater depth F = discharge parameter; D is insufficient for the formation of the hydraulic jump, q the bed of the river channel downstream comprises = · 103 sound rock and is capable of withstanding, without gH3/2 1 excessive scour, the impact of the high velocity jet. g = acceleration due to gravity, in m/s2; The flow coming down the spillway is thrown away h = height of roller above bucket invert, in m; from toe of the dam to a considerable distance b downstream as a free discharging upturned jet which h = height of surge above bucket invert, in m; s falls into the channel directly, thereby avoiding H = depth of overflow over spillway, in m; excessive scour immediately downstream of the H = reservoir pool elevation minus bucket invert spillway. There is hardly any energy dissipation within 1 elevation, in m; the bucket itself. The device is used mainly to increase the distance from the structure to the place where high H2 = spillway crest elevation minus bucket invert velocity jet hits the channel bed, thus avoiding the elevation, in m; danger of excessive scour immediately downstream of H = reservoir pool elevation minus tailwater 3 the spillway. Due to the throw of the jet in the shape of elevation, in m; a trajectory, energy dissipation takes place by: H = reservoir pool elevation minus bucket lip 4 a) internal friction within the jet, elevation, in m; 1 IS 7365 : 2010 H = reservoir pool elevation minus jet surface S = horizontal distance from spillway crest axis 5 d elevation on bucket, in m; of the point of maximum probable surge, inm; H = velocity head of jet at bucket lip, in m; T = maximum tailwater depth, above bucket v Max P = pressure on the bucket, in t/m2; invert, for good performance of slotted roller bucket, in m; q = discharge intensity per metre of bucket width, in [m3/s]/m; TMin= minimum tailwater depth, above bucket invert, for good performance of slotted roller Q = total discharge, in m3/s; bucket, in m; R = radius of bucket, in m; 1A SOLID ROLLER BUCKET 1B SLOTTED ROLLER BUCKET 1C TRAJECTORY BUCKET FIG. 1 SKETCHES FOR BUCKET TYPE ENERGY DISSIPATORS 2 IS 7365 : 2010 T = tailwater sweep out depth, in m; 4.1.1.2 In the case of slotted roller bucket, a part of the s v = actual velocity of flow entering bucket, in m/s; flow passes through the slots, spreads laterally and is a lifted away from the channel bottom by a short apron v = theoretical velocity of flow entering bucket, t at the downstream end of the bucket. Thus the flow is in m/s; dispersed and distributed over a greater area resulting X = horizontal throw distance from bucket lip in a less violent ground roller. The height of boil is to the centre point of impact with tailwater, also reduced in case of slotted roller bucket. The slotted in m; bucket provides a self-cleaning action to reduce Y = difference between the lip level and abrasion in the bucket. tailwater, sign taken as positive for tailwater 4.1.1.3 In general the slotted roller bucket is an below the lip level and negative for tailwater improvement over the solid roller bucket for the range above the lip level, in m; of tailwater depths under which it can operate without g = specific weight of water, in t/m3; and sweep out or diving. However, it is necessary that = bucket lip angle (exit angle) with horizontal, specific model experiments should be conducted to f in degree. verify pressure on the teeth so as to avoid cavitation conditions. In case of hydraulic structures, slotted roller 4 HYDRAULIC DESIGN OF ROLLER BUCKET buckets should not be provided where heavy sediment/ TYPE ENERGY DISSIPATORS (SOLID AND gravel/boulder are expected. Heavy sediment load/ SLOTTED ROLLER BUCKETS) gravel/boulders rolling down the spillway face can cause heavy damage to the teeth thereby making them 4.1 General ineffective and on the contrary, increasing the chances 4.1.1 The following two types of roller buckets are of damage by impact, cavitation and erosion. adopted on the basis of tailwater conditions and 4.1.2 Drawal of Bed Materials importance of the structure: A major problem with the solid roller bucket would be a) Solid roller bucket, and the damage caused to the bucket due to churning action, b) Slotted roller bucket. because of the downstream bed material brought into Roller bucket type energy dissipator is preferred when; the bucket by the pronounced ground roller. Even in a slotted roller bucket, downstream material might get a) tailwater depth is high (greater than 1.1 times drawn due to unequal operation of gates. The channel sequent depth preferably 1.2 times sequent bed immediately downstream of the bucket shall be depth), and set at 1 to 1.5 m below the lip level to minimize the b) river bed rock is sound. possibility of this condition. Where the invert of the bucket is required to be set below the channel general 4.1.1.1 In the case of solid roller bucket, the ground bed level, the channel should be dressed down in one roller is more pronounced. It picks up material from level to about 1 to 1.5 m below the lip level for downstream bed and carries it towards the bucket where about15m length downstream and then a recovery it is partly deposited and partly carried away slope of about 3 (horizontal) to 1 (vertical) should be downstream by the residual jet from the lip. The given to meet the general bed level as shown in Fig. 2. deposition in roller bucket is more likely when the Model studies should be done to check this tendency. spillway spans are not operated equally, setting up If possible, even provision of properly anchored solid horizontal eddies downstream of the bucket. The picked concrete apron laid on fresh rock may be considered up material which is drawn into the bucket can cause to avoid river bed material drawing into the bucket, as abrasive damage to the bucket by churning action. FIG. 2 RECOVERY SLOPE DOWNSTREAM OF SOLID ROLLER BUCKET 3 IS 7365 : 2010 it may cause heavy erosion on the spillway face, bucket hydraulic phenomenon of sweep out or heavy and side training walls. submergence occurs depending upon which of the rollers is inhibited. 4.1.2.1 In the case where the bucket invert is substantially higher than the general channel bed level 4.2.2 Design Criteria and the channel bed is erodible, the cascading flow The principal features of hydraulic design of solid roller over the lip for small discharges may cause deep scour bucket consists of determining; very close to the bucket lip. A concrete apron of width 15 m or more if required may be provided downstream a) bucket invert elevation, and parallel to the end sill as shown in Fig. 3 to b) radius of the bucket, and minimize the possible scour in the bed near the bucket c) slope of the bucket lip or the bucket lip angle. lip. The apron should be keyed into good rock. 4.2.2.1 Bucket invert elevation 4.1.3 Precautions in Operation Normally the invert level of a roller bucket is so fixed It is necessary to operate all the spillways gates equally that the difference in the maximum tailwater level (under partial operation condition) to achieve corresponding to design flood and the invert level (d ) satisfactory performance of the bucket. Unsymmetrical 3 is between 1.1 to 1.4 times the sequent depth (d ). Thus operation of gates or operation of only a few gates at a 2 d = 1.1 to 1.4 times d . The design charts given in time may set up horizontal eddies in the channel 3 2 Fig.4 and Fig. 5 and sample calculations given in downstream which may bring debris into the bucket. Annex A may be used to determine the bucket invert All loose debris inside and just beyond the bucket elevation and probable roller and surge height for the should be removed after construction and before the expected range of spillway discharges. Satisfactory bucket is put to use. energy dissipation is obtained when the roller height 4.1.3.1 Divide walls would be necessary to segregate (h ) is between 75 and 90 percent of the tailwater depth b the spillway spans, if unequal spillway operations (d ). If the aforesaid two criteria are satisfied, then the 3 cannot be avoided with due consideration for building surge height (h) measured above the invert level is105 s of tailwater level for various discharges. Also when to 130 percent of the tailwater d , that is, h/d = 1.05 3 s 3 the invert levels of the buckets in adjacent bays are to 1.3. When the invert elevation arrived at to get the provided at different elevations, divide walls may be h /d ratio between 0.75 and 0.90, is considerably b 3 required depending upon the flow conditions. Model below the channel bed level, substantial excavation studies may be conducted for such cases. The divide would be involved for dressing downstream channel walls shall be designed for unequal operation condition bed down to 1.0 to 1.5 m below the endsill level in of the spillway spans and also for differential pressure minimum 15.0 m length or more in the downstream due to adjoining gates closed on one side and all gates depending upon the discharge intensity and the head. open on the other side. It would, therefore, be preferable if the invert level can be brought near to the channel bed level with the h /d b 3 4.2 Hydraulic Design of Solid Roller Bucket ratio still remaining within the prescribed limits, and d being about 1.2 to 1.4 times d . The charts are 4.2.1 General 3 2 applicable for the ranges of variables shown in Fig. 4 For effective energy dissipation in a solid roller bucket and Fig. 5. The channel bed elevation is believed to both the surface or dissipating roller and the ground or have negligible effect on roller and surge heights. stabilizing roller should be well formed, otherwise FIG. 3 CONCRETE APRON LAID ON FRESH ROCK 4 IS 7365 : 2010 RANGE OF H/R FOR SATISFACTORY 1 PERFORMANCE OF BUCKET q Equivalent Range of gH3/2 F H/R 1 1 0.090 5.4 2 to 5 0.060 6.7 2 to 6 0.040 8.3 2 to 6 0.026 10.3 3 to 6 0.013 14.7 3 to 8 Range of variables: Spillway slopes (S : 1) 1 : 1 to 1.67 : a Lip angle (f ) =45° [the curves may also be used for other lip angles (see 4.2.2.3)] H/H =0.68 to 0.93 2 1 FIG. 4 DESIGN OF SOLID ROLLER BUCKET — ROLLER DEPTH 5