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In this article we will discuss about the main types of spillways used to handle surplus water.
1. Overflow or Ogee Type Spillway:
This spillway consists of a control weir whose shape is ogee or S-shaped. The shape of the ogee spillway conforms to the profile of lower nappe of water, falling from a sharp crested weir. The profile of the spillway is so shaped that the overflowing water remains in touch with the spillway surface throughout the fall, when the rate of overflow corresponds to the maximum designed capacity.
When head on the spillway crest is less than the maximum designed head, the falling nappe would remain adhering to the profile of the spillway. This causes positive hydrostatic pressure and thus reduces the discharging capacity of the spillway. On the contrary, when head on the crest exceeds the maximum design head during unexpected large floods, the lower nappe of falling water may not remain in touch with the spillway surface.
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This phenomenon causes negative pressure and cavitation. Cavitation has a damaging effect which is caused at points where lower surface of the falling nappe does not remain in touch with the profile. At such points vacuum is developed which results in cavitation effect. In addition to cavitation, vibrations are also set up in the dam due to alternate making and breaking of the contact between falling nappe of water and profile of the spillway.
The D/S curve of the ogee follows following equation:
x1.85 = 2H0.85 x y (14.1)
Where x and y are the co-ordinates of the crest profile measured from the apex of the crest. H is the design head.
Other elements of the crest as shown in Fig. 14.2 are as follows:
It should be taken care of, that the upper curve at the crest should be neither too sharp nor too broad.
Discharge over an ogee spillway is found out by following equation:
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Q = CLeHe1.5
This spillway is the most common type of spillway used in gravity dams.
2. Free Overfall or Straight Drop Spillway:
It is a low height weir, whose D/S face is either vertical or nearly vertical. In this spillway the flow drops freely from the crest. The under-side of the falling nappe is sufficiently ventilated to prevent a pulsating jet. The crest is also sometimes extended in the form of an overhanging slab on D/S side so as to keep the nappe of falling water away, from coming in contact with the D/S surface of the spillway. The falling jet of water hits the D/S floor almost vertically and thus lot of energy is generated due to impact.
This energy can be dissipated by adopting any of the following measures:
(i) Provide concrete apron for some length on the D/S side.
(ii) Develop a small pool of water, below the falling jet of water, by constructing a small secondary dam at some distance on the D/S side.
(iii) Impact blocks of small heights are constructed on the D/S side. The energy is dissipated by means of turbulences caused by the impingement of the flow against the impact blocks.
(iv) By developing hydraulic jump.
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This type of spillway is recommended for large heads.
The difference between ogee type and free overfill spillway can now be easily understood. In case of free overfall spillway, the water jet falls clearly away from the D/S face and space between D/S face and underside of the jet is ventilated.
In ogee spillway nappe of falling water remains in touch with the D/S surface of the spillway. In case of ogee spillway a smooth gradual reverse curvature is given on the D/S face of the spillway so as to cause dissipation of excess energy.
3. Side Channel Spillway:
This spillway is suitable for earth or rock fill dams, in narrow canyons and other situations, where installation of direct overflow spillway is not possible. This spillway is also the best choice where a long overflow crest is required so as to limit the surcharge head and also under the situation where abutments are steep and precipitous.
In this spillway the control weir is kept along the side and approximately parallel to the upper portion of the spillway. The flow after passing over the crest turns at approximately 90°. Flow may enter the side channel from one side only or from both the sides and one end also.
Discharge characteristics are similar to ordinary overflow weir, except that at high discharge its crest may be partly submerged. Generally a trough is formed on D/S side of the crest and water enters it before taking a turn to flow in the side channel.
4. Chute Spillway:
It is also called open channel or trough spillway. It consists of a steep sloped open channel which is called trough or chute. It is placed in the dam abutment, or in a saddle. The discharge flown over the chute, is carried to the D/S of the dam through a natural channel which may be meeting the river on D/S side.
The channel may be formed artificially or by excavating the trench, if natural channel does not exist. This spillway is provided for earth or rock fill dams and is always isolated from the main dam.
This spillway has following advantages:
(i) It is simple in design and construction.
(ii) It is adoptable to almost any foundation condition.
(iii) It is economical.
5. Tunnel Spillway:
In this spillway a closed channel is used to convey surplus water from U/S to D/S of the dam. The closed channel may be vertical, inclined, or horizontal. When the closed channel is carried under the dam, it is known as conduit spillway. Conduit spillway is suited best for darn in wide valley.
Tunnel spillway is considered best when dam site is narrow canyon. This spillway is designed to flow partly full. Full flow is not allowed as it will develop siphonic action and negative pressures may be created in the conduit.
6. Siphon Spillway:
This spillway consists of one or a number of units of conduits formed by inverted U-tubes. The initial discharge through the conduits is similar to that of a weir, but when air is drawn away from above the end over the crest, the discharge starts flowing by siphonic action.
The inside of the bend of inverted U-tube is at normal reservoir level. When water level in the reservoir rises, the water first flows as for the weir spillway and when air inlet is blocked by rising water, siphonic action starts and spillway starts discharging with full capacity. Spillway continues to discharge with full capacity unless the reservoir level drops below the air inlet pipe, when air enters the conduit and breaks the siphonic action.
Siphon spillway can be classified into two categories:
(i) Saddle Siphon Spillway:
Saddle siphon spillway can be constructed in many designs but basic concept for all of them is the same. One type of saddle siphons is shown in Fig. 14.8. The diagram consists of a U-shaped siphon pipe whose D/S end remains submerged in tail water. An air inlet is provided near the top of the bend.
Crest of the inside of U-tube is maintained at the normal reservoir level and air inlet slightly above this level. At normal reservoir level no waterflows through the siphon. During floods as the water level in the reservoir rises above the normal reservoir level, water will start flowing through the conduit of the spillway.
When rising water level blocks the air inlet, the flow through the conduit immediately starts under siphonic head. A number of such siphonic units may be installed depending upon the diameter of the conduit of the siphon and the amount of flood water to be handled. When water level in the reservoir drops below the air inlet, siphonic action is broken. Closing of the air inlet by rising water is known as priming and opening of the air inlet by dropping level of water is known as depriming of the siphon spillway.
Discharging capacity of a saddle siphon spillway is obtained by following formula –
Where A = Area of cross-section at crown.
H = Operating head. It is taken as vertical height from reservoir level to the centre of the outlet if outlet is discharging in atmosphere and upto tail water level if outlet is submerged.
C = Coefficient of discharge whose average value is taken as 0.65.
Siphon spillways can be further classified as high head, medium head and low head siphons.
(ii) Volute Siphon Spillway:
It is special type of siphon spillway. It comprises a vertical shaft which is bent at discharging end. Its top end consists of funnel shaped lips in which a number of volutes are developed. Due to volutes in the funnel shaped lip, a spiral motion to the water passing along them is set up. The funnel shaped lip remains covered by a concrete dome supported on number of pillars.
A deprimer is also installed over the dome which connects the outer air with the funnel shaped lip. The outer end of the deprimer is kept just at the reservoir level. When water level in the reservoir rises, it closes the deprimer and entry of air is prevented.
Now water flowing over the volutes sucks off the entrapped air due to spiral flow of water, and siphonic action starts and siphon start discharging at its full capacity. When reservoir level falls below the air inlet, air enters and breaks the siphonic action.
Discharging capacity of this siphon is found out by following formula –
= 
A = area of the cross-section of the pipe.
C = coefficient of discharge.
H = Maximum operating head.
HL = Head loss through the siphon which is generally very small and may be ignored.
Advantages of Siphon Spillway:
i. Since its discharge does not depend upon the head above the crest, it starts discharging with full capacity once siphonic action starts.
ii. Since these spillways starts discharging with full capacity right from the beginning, the reservoir level will not rise much. This will cause less submergence of U/S areas during floods.
iii. River training works of low heights will have to be constructed.
Disadvantages:
i. They do not allow additional waters to be stored in the reservoir to augment the supplies at a later stage.
ii. They cause lot of vibrations when in action. This may cause cracks in the dam and may lead to their ultimate failure.
iii. They have to be maintained regularly.
7. Shaft Spillway:
This spillway consists of a vertical shaft and a horizontal conduit. The top of the shaft is specially designed, through which water enters and then drops through a vertical shaft and finally gets carried to the D/S side by a horizontal conduit. This spillway is also sometimes known as drop inlet or morning glory spillway.
The shaft spillway may be standard crest type or flat crest type. Small shaft spillway may be made from steel or concrete pipes, but large spillways are made from heavily reinforced pipes. Diversion tunnels used for diverting the river water during the construction of the dam may be plugged and then used as shaft spillway by attaching vertical shafts to the horizontal tunnels. Top flared inlet of the vertical shaft is known as morning glory and is considered as an essential feature of the shaft spillway.