Spillways

Topics:
1. Types of Spillways
2. Design Principles of Ogee Spillway
3. Spillway Gates
4. Energy Dissipaters
5. Significance of Jump Height Curve & Tail Water Curve
6. Stilling Basins

1. SPILLWAYS (water resources engineering – ii) Unit – Iv Rambabu Palaka , Assistant Professor BVRIT

2. Learning Objectives Types of Spillways Design Principles of Ogee Spillway Spillway Gates Energy Dissipaters Significance of Jump Height Curve & Tail Water Curve Stilling Basins

3. Introduction A Spillway is the overflow portion of dam, over which surplus discharge flows from the reservoir to the downstream. For safety, spillways should have sufficient capacity to discharge floods, likely to occur during the lifetime of the dam.

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. Types of Spillways Free Overfall or Straight Drop Spillway Ogee or Overflow Spillway Side Channel Spillway Chute or Open Channel or Trough Spillway Conduit or Tunnel Spillway Drop Inlet or Shaft or Morning Glory Spillway Siphon Spillway

6. 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. Suitable Artificial Protection should be provided to avoid SCOUR Free Overfall or Straight Drop Spillway

7. 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 in Gravity dams, Arch dams, and Buttress dams Overflow spillways: Controlled (gated, guided) Uncontrolled (un-gated, free) Almost all recently constructed dams are installed with crest gates to store more water in the reservoir Overflow or Ogee Spillway

8. Side Channel Spillway Discharge characteristics of a side channel spillway are similar to those of an ordinary overflow spillway and are dependent on the selected profile of the weir crest. Although the side channel is not hydraulically efficient, nor inexpensive, it has advantages which make it adoptable to spillways where a long overflow crest is required in order to limit the afflux (surcharge held to cause flow) and the abutments are steep.

9. Chute Spillway Chute spillways are used in flow ways where water is to be lowered from one level to another and where it is desirable to avoid a stilling basin. These are mostly used with earth dams Advantages: It can be provided on any type of foundations. Simplicity of design. However this type of spillway should not be provided where too many bends are to be given as per topography.

10. Conduit or Tunnel Spillway Where a closed channel is used to convey the discharge around a dam through the adjoining hill sides, the spillway is often called a tunnel or conduit spillway. The closed channel may take the form of a vertical or inclined shaft, a horizontal tunnel through earth or rock, or a conduit constructed in open cut and backfilled with earth materials. Tunnel spillways are advantageous for dam sites in narrow gorges with steep abutments or at sites where there is danger to open channels from rock slides from the hills adjoining the reservoir.

11. Shaft Spillway If 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. This type of spillway is not suitable for large capacity and deep reservoirs because of stability problems. Special designs are required to handle cavitation damage at the transition between shaft and tunnel. Repair and maintenance of shaft spillways are difficult.

12. Siphon Spillway A siphon spillway may be constructed in the body of a concrete dam when space is not available for an overflow spillway. Disadvantage of siphon spillway: The flow would result excessive vibrations in the dam body which may cause expansion problems in the joints. There is a possibility of cavitation for negative pressures, which is affected by the head between upstream and downstream water levels. Repair and maintenance of siphon spillways are difficult. It has a limited capacity.

13. Ogee Spillway (Design Principles)

14. Hydraulic Jump When flood discharge passes over the spillway crest, it has high potential energy which gets converted into kinetic energy as it glides along it. This high energy has to be dissipated otherwise it would cause erosion at the downstream toe.

15. JHC vs TWC Jump Height Curve (JHC): It is a curve representing post jump depth (d 2 ) with discharge (q) Tail Water Curve (TWC): It is a curve representing tail water depth (D) with discharge (q)

16. Five conditions that govern the relationship between JHC and TWC

17. JHC lies lower than TWC at all Discharges (PROTECTION MEASURES)

18. JHC lies above TWC at all Discharges (PROTECTION MEASURES)

19. JHC lies lower than TWC at Small Discharges & JHC lies above TWC at large discharge & Vice-Versa (PROTECTION MEASURES)

20. Stilling Basins

21. USBR Stilling Basin Type – II (Fr > 4.5) for Medium and High Dams

22. USBR Stilling Basin Type – IV (2.5 < Fr < 4.5) for Weirs and Barrages

23. Spillway Gates Flash Boards, Stop Logs and Needles Radial Gates Drum Gates Vertical Lift Gates Bear Trap Gates Rolling Gates

24. Reference Chapter 11 Irrigation and Water Power Engineering  By Dr. B. C. Punmia , Dr. Pande Brij Basi Lal , Ashok Kr. Jain, Arun Kr. Jain