Spillways

This presentation covers topic Spillway.

1. SPILLWAYS DIVYA VISHNOI Assistant Professor

2. SPILLWAYS Spillways are provided as integral part of a dam or as an auxiliary structure constructed separate from the main dam to release surplus flood water which are in excess of the storage space in the reservoir as provided in the operation plan and must be passed downstream. The primary function of spillways is to release surplus waters in conjunction with other discharging devices from the reservoir, in order to prevent rise of water level above a specified level in the reservoir to avoid unwanted submergence upstream or consequent overtopping and possible failure of the dam. Thus, spillways work as safety valves for a dam and the adjoining countryside. To dissipate this excessive energy and to establish safe flow conditions in the downstream of a dam spillway, energy dissipaters are used as remedial devices.

3. Necessity of spillways The height of the dam is always fixed according to the maximum reservoir capacity. The normal pool level indicates the maximum capacity of the reservoir. The water is never stored in the reservoir above this level. The dam may fail by over turning so, for the safety of the dam the spillways are essential. The top of the dam is generally utilized by making road. The surplus water is not be allowed to over top the dam, so to stop the over topping by the surplus water, the spillways become extremely essential. To protect the downstream base and floor of the dam from the effect of scouring and erosion, the spillways are provided so that the excess water flows smoothly.

4. Location of Spillway Spillways may be provided within the body of the dam. Spillways may sometimes be provided at one side or both sides of the dam. Sometimes by-pass spillway is provided which is completely separate from the dam.

5. Determination of discharge capacity and number of spillways By studying the flood hydrograph of past ten years, the maximum flood discharge may be computed which is to be disposed off completely through the spillways. The water level in the reservoir should never be allowed to rise above the maximum pool level and should remain in normal pool level. So, the volume of water collected between maximum pool level and minimum pool level computed, which indicates the discharge capacity of spillways. The maximum flood discharge may also be computed from other investigation like, rainfall records, flood routing, empirical flood discharge formulae, etc.

6. Determination of discharge capacity and number of spillways From the above factors the highest flood discharge is ascertained to fix the discharge capacity of spillways. The natural calamities are beyond the grip of human being. So, an allowance of about 25 % should be given to the computed highest flood discharge which is to be disposed off. The size and number of spillways are designed according to the design discharge.

7. Types of Spillways Free over fall or straight drop Spillways Ogee or overflow Spillway Siphon Spillway Side Channel Spillway Chute or Trough or Open channel Spillway Drop inlet or Shaft or morning glory Spillway

8. Free over fall or straight drop Spillways Water flows over a relatively thin spillway crest and falls freely to the downstream. Usually appropriate for thin dams having almost vertical downstream faces. This type of spillways may be economical for low heads as compared with overflow spillways because of saving in concrete. Not recommended for high heads because of structural instability problems. A basin is constructed on the downstream side to form a small artificial pool which is known as water cushion.

9. FREE OVERFALL SPILLWAY

10. Ogee or overflow Spillway The ogee spillway is a modified form of drop spillway. Overflow spillways also called ogee-shaped (S-shaped) spillways. This type of spillways allows the passage of the flood wave over its crest. Widely used on Gravity dams, Arch dams, and Buttress dams

11. Ogee or overflow Spillway The flow depth at the crest is slightly critical than hydrostatic pressure. Overflow spillways Controlled (gated, guided) Uncontrolled ( ungated , free) Almost all recently constructed dams are installed with crest gates to store more water in the reservoir.

12. Ogee or overflow Spillway The downstream profile of the spillway is made to coincide with the shape of the lower nappe of the free falling water jet from a sharp crested weir. The shape of the lower nappe is not same for all the head of water above the crest of the weir. It differs with the head of water. for the design of the ogee spillway the maximum head is considered.

13. Ogee or overflow Spillway If the spillway runs with the maximum head, then the overflowing water just follows the curved profile of the spillway and there is no gap between the water and the spillway surface and the discharge is maximum. When the actual head becomes more than the designed head, the lower nappe does not follow the ogee profile and gets separated from the spillway surface.

14. Ogee or overflow Spillway Thus a negative pressure develops at the point of separation. Due to the negative pressure, air bubbles are formed within the flowing water. These air bubbles air responsible for the frictional force (i.e. abrasion) which causes much damage to the spillway surface. if the head of water is less than the designed head, the waterjet adheres to the body of the spillway and creases positive pressure which reduces the discharge through the spillway.

15. Ogee or overflow Spillway

16. Ogee or overflow Spillway

17. Siphon Spillway It is designed by the principle of a siphon. When water rises over the FRL then water start spilling. There is a air vent for removing the entrapped pressure from the water. It is a closed conduit system formed in the shape of an inverted U-tube, positioned so that the inside of the bend of the upper passageway is at the normal storage level of the reservoir. The initial discharges from the siphon spillway are just like the discharge over a weir. After the air in the bend over the crest is exhausted, the water is discharged through siphonic action.

18. Siphon Spillway

19. Side Channel Spillway If a sufficient crest length is not available for an overflow or chute spillways in narrow valleys, floodwater is taken in a side channel. When the dam is not rigid and it is undesirable to pass flood water over the dam , this type of spillway is used. After passing crossing over the spillway crest ,water flows parallel to the crest. The crest is placed parallel to the discharge channel. Flow into the side channel might enter on only one side of the trough in the case of steep hillside locations, or on both sides and over the end of the trough if it is located on a knoll or gently sloping abutment. Discharge characteristics are similar to an ordinary overflow weir, except that at a high discharge the crest may be partly submerged.

20. SIDE CHANNEL SPILLWAY

21. SIDE CHANNEL SPILLWAY

22. Chute or Trough or Open channel Spillway In this type water is conveyed from the reservoir to the river or to nalla below the dam through an excavated open channel, through fairly steep slope. In case of having sufficient stiff foundation conditions at the spillway location, a chute spillway may be used in stead of overflow spillway due to economic consideration. A steep slope open channel is constructed in slabs with 25-50 cm thickness having lengths of approximately 10 m. When the horizontal distance between the upstream of the spillway and the tail water is considerable long, a long steep sloped chute usually follows the overflow spillway until the tail water.

23. Drop inlet or Shaft or morning glory Spillway If a sufficient space is not available for an overflow spillway, a shaft spillway may be considered. In the site of shaft spillway Seismic action should be small, Stiff geologic formation should be available, and Possibility of floating debris is relatively small. Flow conditions in the spillway: Level 1 - a weir flow Level 2 -midway between weir flow and pipe flow Level 3- pressurized pipe flow.

24. Drop inlet or Shaft or morning glory Spillway Flow conditions in the spillway: Level 1- a weir flow Cs: discharge coefficient for a shaft spillway. H : total head on the inlet R: radius of the shaft inlet Weir flow with air entrainment takes place until point A. Pressurized pipe flow starts after point B. Part of the curve between point A and B describes the combination of weir and pipe flows.

25. Drop inlet or Shaft or morning glory Spillway

26. Drop inlet or Shaft or morning glory Spillway When the shaft is completely submerged, further increase in head will not result in appreciable increase in discharge. This type of spillway is not suitable for large capacity and deep reservoirs because of stability problems. Special designs are required to handle cavitations damage at the transition between shaft and tunnel. Repair and maintenance of shaft spillways are difficult.

27. SHAFT SPILLWAY

28. Factor affecting selection of spillway Safety Considerations Consistent with Economy Hydrological and Site Conditions inflow flood availability of tail channel, its capacity and flow hydraulics power house, tail race and other structures downstream Topography Type of Dam Purpose of Dam and Operating, Conditions Conditions Downstream of a Dam Nature and Amount of Solid Materials Brought by the River

29. Special consideration Ogee spillway a) It is most commonly used with gravity dams. However, it is also used with earth and rockfill dams with a separate gravity structure; b) The ogee crest can be used as control in almost all types of spillways; and c) It has got the advantage over other spillways for its high discharging efficiency.

30. Special consideration Chute Spillway a) It can be provided on any type of foundation, b) It is commonly used with the earth and rockfill dams, and c) It becomes economical if earth received from spillway excavation is used in dam construction. Following factors limit its adaption: a) It should normally be avoided on embankment; b) Availability of space is essential for keeping the spillway basins away from the dam paving; and c) If it is necessary to provide too many bend, in the chute because of the topography, its hydraulic performance can be adversely affected.

31. Special consideration Site Chanel Spillway a) This type of spillway is preferred where a long overflow crest is desired in order to limit the intensity of discharge, b) It is useful where the abutments are steep, and c) It is useful where the control is desired by the narrow side channel. Note: - The factor limiting its adoption is that this type of spillway is hydraulically less efficient.  

32. Special consideration Shaft Spillway ( Morning Glory Spillway) a) This can be adopted very advantageously in dam sites in narrow canyons, and b) Minimum discharging capacity is attained at relatively low heads. This characteristic makes the spillway ideal where the maximum spillway outflow is to be limited. This characteristic become undesirable where a flood more than the design capacity is to be passed. So, it can be used as a service spillway in conjunction with an emergency spillway. Note: - The factor limiting its adoption is the difficulty of air-entrainment in a shaft, which may escape in bursts causing an undesirable surging motion.

33. Special consideration Siphon Spillway a) Siphon spillways can be used to discharge full capacity discharges, at relatively low heads, and b) Great advantage of this type of spillway is its positive and automatic operation without mechanical devices and moving parts. The following factors limit the adoption of a siphon spillway: a) It is difficult to handle flows materially greater than designed capacity, even if the reservoir head exceeds the design level; b) Siphon spillways cannot pass debris, ice, etc; c) There is possibility of clogging of the siphon passage way and breaking of siphon vents with logs and debris; d) In cold climates, there can be freezing inside the inlet and air vents of the siphon; e) When sudden surges occur and outflow stops; f) The structure is subject to heavy vibrations during its operation needing strong foundations; and g) Siphons cannot be normally used for vacuum heads higher than 8 m and there is danger of cavitation damage.

34. Special consideration Over fall or Free Fall Spillway a)This is suitable for arch dams or dams with downstream vertical faces; and b) This is suitable for small drops and for passing any occasional flood. Note: -The factor limiting its adoption is that, ordinarily, the maximum hydraulic drop from head pool to tail pool water should not exceed 20 m,

35. ENERGY DISSIPIATORS Hydraulic structures constructed at the D/S end of a dam to dissipate the huge K.E. of water Due to this provision large scale scour near the toe of dam can be prevented.

36. In general, K.E. of this supercritical flow can dissipated by By converting the supercritical flow by hydraullic jump to sub critical flow, i.e. provision of Stilling Basin . By directing the flow of water in the air and then making it fall away from toe of the Dam, i.e. provision of Bucket type energy Dissipators .

37. Stilling B asin T ype Energy D issipators They may fundamentally be divided into two types: Horizontal apron type 2. Sloping apron type

38. Bucket type energy dissipators can be either: a ) Roller bucket type energy dissipator ; or b ) Trajectory bucket type energy dissipator . The following two types of roller buckets are adopted on the basis of tailwater conditions and importance of the structure: a ) Solid roller bucket, and b) Slotted roller bucket

39. SELECTION Of An ENERGY DISSIPATOR An energy dissipator is selected based upon the following 2 parameters 1. Jump Height : which is fixed for a discharge intensity and height of a spillway. 2. Tail Water Depth : which is calculated by actual discharge observations.

40. Now a Graph is plotted between: 1. Jump Heights (depths) v/s Discharge. 2. Tail Water Depth v/s Discharge. The above Graphs are superimposed and following cases are analysed :

41. CASE I

42. This is the ideal case In this, the horizontal apron provided on the riverbed downstream from the toe of the spillway would suffice. The length of the apron should be equal to the length of the jump corresponding to the maximum discharge over the spillway.

43. Case II

44. It is apparent that the tail water depth as provided by the natural river is not sufficiently for the jump to form. This may be over come by providing: a stilling basin apron that is depressed below the average riverbed level, or a sill or baffle of sufficient height at the end of the spillway, or A ski jump, i.e. Trajectory roller bucket.

45. CASE III

46. Since this situation results in submergence of the jump , Therefore, it is necessary to raise the floor in order to form a clear jump. In practice, it is done by providing AN INCLINED APRON OF THE STILLING BASIN or A ROLLER BUCKET

47. C ASE IV

48. This situation may be taken care of by providing an inclined floor in the upper portion of the stilling basin and providing either a depressed floor in the lower portion of the basin or provide a baffle at the end of the basin.

49. CASE V

50. In this case a sloping apron may be provided which lies partly above and partly below the riverbed . So that the jump will form on the higher slope at low discharges and on the lower slope at high discharges

51. RTU Questions What are the essential requirements of a spillway? Describe the process of determining spillway capacity? Describe various types of spillways with sketches? Describe and sketch ogee shaped spillways with equations of its downstream and upstream. Describe the factor affecting the required spillway capacity. What is the role of flood routing in spillways design? Enumerate the various types of spillways. Describe the Chute spillway with a neat sketch? Also describe the design of its various component.

52. References Irrigation Engineering & Water Power Engineering By Prof. P.N.MODI and Dr. S.M. SETH --- Standard Book House Delhi Irrigation Engineering & Hydraulic Structures By Prof. Santosh Kumar Garg Khanna Publishers Irrigation, Water Power Engineering & Hydraulic Structures By Prof K.R. Arora Standard Publishers Distributions Internet Websites http://www.aboutcivil.org/ http://nptel.ac.in/courses/105105110/

53. Thanks