Spillways: Types, Force and Gates | Geography

In this article we will discuss about:- 1. Introduction to Spillways 2. Types of Spillways 3. Dynamic Force 4. Energy Dissipation 5. Spillway Gates.

Contents:

1. Introduction to Spillways
2. Types of Spillways
3. Dynamic Force on Spillways
4. Energy Dissipation below Spillways
5. Spillway Gates

1. Introduction to Spillways:

Spillway is a structure, used to pass surplus flood water from reservoir to the downstream side of the dam. It is sometimes also known as surplussing work, as its main job is to handle the surplus water. It is a sort of safety valve for a dam. During floods, the reservoir level goes on increasing, as more and more water enters the reservoir.

If spillways had not been there, the level in the reservoir will go on rising even beyond the maximum reservoir level. This may lead to over topping of the dams. Additional stresses may be induced in the dam and this may cause failure of the dam. More rises in water level beyond the maximum flood level will cause submergence of vast additional areas and may cause suffering to the people living on the U/S side.

Spillways act as safety valve for the safety of the dam and also for the adjoining areas. When water level in the reservoir reaches some specified level, the surplus water automatically starts escaping through the spillways. As more and more flood water enters the reservoir, there is more and more rise in water level and consequently more and more water starts escaping over the spillways.

At maximum flood level (H.F.L.) of reservoir, the outflow rate over spillways reaches the inflow in the reservoir and thus further rise in the water level of the reservoir is stopped. The spillways should be of sufficient capacity so as to be capable to effectively handle the largest floods, without causing any damage to the dam or any appurtenant structure.

Spillways are also designed according to the safe capacity of the D/S channel, otherwise D/S areas may get flooded. The design flood discharge required to be handled by the spillway can be found by flood routing.

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2. Types of Spillways:

The spillways may be broadly classified as:

1. Emergency Spillways:

Main spillway is the spillway which is called upon to work under normal conditions of flood. Emergency spillway is provided to deal with abnormal conditions of flood. Under normal flood conditions, the emergency spillways are not required to work.

They may be called upon to work under following abnormal conditions:

(i) Rise is water level in the reservoir is continuing above the maximum permitted level even when main spillway and other outlet works are working at their full capacity. This happens when flood greater than design flood has occurred.

(ii) When, due to certain reasons, enforced shut down of the outlet works have to be applied and flood water is entering the reservoir with full vigour.

(iii) If spillway gates have got struck down and cannot be opened immediately.

In all the above three conditions the level of reservoir may rise above the maximum contemplated level and thus may cause failure of the dam and extra submergence on the U/ S side. In all such conditions, the emergency spillways are opened. Emergency spillways are generally formed by lowering the crest of a dike section below that of the main embankment.

Natural saddle, if available, is an ideal site for emergency spillway. Emergency spillways are constructed as low height earth embankments, but before earth embankments are made the surface of the saddle or lowered dike section is made pucca, so that when emergency spillway is opened it is not unnecessary eroded and widened.  

The crest or top of the emergency spillway is kept at or little above the designed maximum reservoir water surface. When rising water level in the reservoir reaches the maximum high flood level and is still rising, the emergency spill­way, which is an ordinary earth fill low dam, gets either automatically washed away or is deliberately cut.

This causes very large amount of flood water to escape, and further rise in reservoir level stops. When flood waters are dis­charged D/S, and the reservoir level has dropped below the maximum flood level, the flow through the emergency spillway stops. After this main spillways are capable to handle the situation.

When flow through emergency spillway has stopped, it is again filled with earth embankment and emergency spillway again restored. Since emergency spillway section either breaches itself or is deliberately cut, it is sometimes also known as breaching section.

2. Main Spillways:

We have already stated that these spillways work under the designed flood conditions. Emergency spillways only help the main spillways to save the situation created by abnormal conditions. There are several types of spillways which may be used as main spillways.

They are as follows:

i. Overflow or ogee spillway.

ii. Free overfall or straight drop spillway.

iii. Side channel spillway.

iv. Chute spillway, which is also called trough or open channel spillway.

v. Tunnel spillway.

vi. Siphon spillway.

vii. Shaft, or drop inlet or morning glory spillway.

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3. Dynamic Force on Spillways:

There is a continuous change in the velocity of water when water flows over the curved surface of ogee spillway. The continuous change in velocity phenomenon induces change in momentum from section to section and thus causes a force on the spillway structure. This force is known as dynamic force.

Consider two sections A and B of water on the curved surface.

The resultant of the forces on the element of water is as follows:

The forces F_h and F_v are the forces acting on free body of water. These forces include gravity force, hydrostatic pressures and reactions of any object in contact with water.

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4. Energy Dissipation below Spillways:

When stream of water moving with hyper-critical velocity, meets another stream, moving with sub-critical velocity there is abrupt rise in the water level of the stream. This abrupt rise of water is known as hydraulic jump. Hydraulic jump is a very useful measure of dissipating the energy of flowing water.

Flood water at the level of the crest of the spillway has potential energy proportionate to the rise of the crest above the D/S floor of the spillway. When flood water passing over the crest of spillway reaches the bottom of spillway on D /S side, the potential energy of water is converted into kinetic energy, as velocity is greatly increased by the time water reaches from crest to D/ S floor of the spillway.

This high kinetic energy may cause very deep erosions on D/S if measures to dissipate it are not taken. There are several methods of dissipating the energy of the shooting flow of water.

The following are the most important measures:

1. Formation of hydraulic jump.

2. Developing water cushion on D/S side.

3. Stilling basins with or without blocks of different sizes and shapes arranged in different manners.

4. Bucket type energy dissipators.

Where excess energy of gliding water is to be dissipated before the flow joins the D/S main river channel, the hydraulic jump is considered one of the most effective measures. If spillway discharge is directly being spilled into the river on D/S side, water cushion and other measures are considered best.

Hydraulic Jump Computations:

The hydraulic jump that is formed at the stilling basin has some distinctive characteristics. The jump assumes a definite form depending upon the energy of flow to be dissipated in relation to the depth of flow.

The form of jump depends upon the following factors:

(i) Discharge passing over the crest per metre length of the spillway (g),

(ii) Critical depth of flow (d_e), and

(iii) Froude number parameter which is, various elements of the jump are shown in Fig. 14.12.

The depth of water jump can be computed as follows:

(a) For a given discharge q per metre length of the spillway, find out the total head of water (H_e) at the crest of the spillway including head due to velocity of approach.

U/S total energy line (T.E.L.) = crest level + H_e. If we consider that no loss of energy has taken place while transition from crest to toe, the specific energy (E₁) at toe of the spillway will be equal to the T.E.L. at U/S.

E₁ = U/S T.E.L.

After having found out E₁ and q the depth (d₁) of water at pre-jump position can be found by trial and error method.

Determine Froude number F₁

The depth of water at post jump (d₂) can be found out from following equations –

The values d₂ can be plotted for various values of q and a curve known as jump height curve (J.H.C.) is plotted as shown in Fig. 14.12 (b).

The height of the tail water for each discharge may or may not correspond to the height of perfect jump.

A curve relating depth of tail water (D) with discharge (q) may be drawn. Such curves are known as tail water curves (T.W.C.).

By comparing J.H.C. and T.W.C. following five conditions of jumps are possible:

1. Both the curves coincide.

2. T.W.C. curve lies above the J.H.C. for all the discharges.

3. T.W.C. lies below the J.H.C. for all the discharges.

4. T.W.C. lies above for small discharges and then below at higher discharge.

5. Reverse of case 4.

Case I – Both the Curves (J.H.C. and T.W.C.) Coincide for All the Discharges:

The jump will be formed at the toe of the spillway. The post jump depth will be already available in the channel. The jump formation will be perfect for all the discharges. This condition requires horizontal apron of length 5(d₂ – d^) beyond toe. See Figs 14.12 and 14.16 (d).

Case II – T.W.C. Curve Lies above the J.H.C. for All the Discharges:

In this case the value of jump height (d₂) is less than the tail water depth. In this case jump will be completely submerged. In this case very little energy will be dissipated. But this condition can be made effective by reducing the depth of tail water at the point of formation of the jump. See Figs. 14.13, 14.14, 14.15 and 14.16 (b).

The protection works in this case may be in the form of:

(i) A sloping apron instead of horizontal apron. See Fig. 14.13.

(ii) Using a low level bucket which is sharply turned up. It acts as a deflector. See Fig. 14.14. (iii) Providing end sills with baffles. In this case energy is dissipated by impact and friction. See Fig. 14.15.

Case III – T.W.C. Lies below the J.H.C. for All the Discharges:

In this case depth of tail water level is smaller than the depth of the jump for all the discharges.

The protection of the bottom can be accomplished by any of the following measures – See Fig. 14.16 (e):

(i) Provide a depressed cistern having its bed below the level of the bed level of the river. Toe of the spillway and bed of cistern are joined by slopping glacis. See Fig. 14.17 (a).

(ii) Providing a cistern like (i) but instead of joining toe and bed of cistern by sloping glacis, baffle walls are provided in the cistern. See Fig. 14.17 (b).

(iii) Baffles and low weir may be provided. See Fig. 14.17 (c).

(iv) Up-turned or Ski-jump bucket is formed in the spillway structure at toe. See Fig. 14.17 (d)

Case IV – T.W.C. Lies above for Small Discharges and below for Large Discharges:

In this case, jump depth d₂ is smaller than D for small discharges and larger for large discharges. Thus jump will remain submerged at low discharges whereas at high discharges tail water depth will be insufficient.

In this case a slopping apron partly below and partly above the bed level of river is constructed. Horizontal length of the apron is sufficiently provided. If need be, end sill is also provided. In this case jump will be formed at higher up point on the sloping apron for small discharges. At higher discharges the jump will be carried further on D/S side. See Figs 14.16 (c) and 14.18.

Case V – T.W.C. Lies below for Small Discharges and above for Large Discharges:

This case is just the reverse of case IV. Protective measure is also the same as in case IV. The formation of jump will also be reverse in order i.e. at high up points for high discharges and at lower points for low discharges. See Fig. 14.16 (f)

IS 4997 —1968 has given the criteria for design of hydraulic jump type stilling basins with horizontal as well as for sloping aprons.

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5. Spillway Gates:

The spillways may be uncontrolled or controlled types. Uncontrolled spillways do not require any gate. Controlled spillways are provided with gates. Controlled spillways are considered superior to uncontrolled ones, as better control on the reservoir level can be exercised with gates.

During low flows, additional storage can be done in the reservoir with the help of gates. However, during high floods gates are fully lifted or opened so that the full discharging capacity of the spillway may be used to dispose of flood water. In case of earth dams, spillways are not provided with gates. Slackness on the part of lifting the gates may cause over-topping of the earth dam and may lead to its failure.

Following are the various types of gates, which are commonly used:

1. Flash Board:

It is a temporary arrangement which is adopted to store extra water during low flows in the river. At large floods they are either removed or allowed to drop on D/S side if hinged at the crest. Flash Boards are made by joining wooden planks which are fixed against the pins. The pins are installed over the crest at suitable intervals. Flash Boards when in position may be vertical or inclined.

2. Stop Log or Needle Gates:

In this case, piers are located at suitable intervals on the crest and stop logs which are nothing but timber planks fixed horizontally in the grooves of the piers, one above the other. While opening, the stop logs are removed one by one. In the case of needle gates, wooden planks are not used horizontally but vertically.

A groove is developed at the crest of the spillway. Bridging girder is also put parallel to the crest of the spillway. The wooden needles are fixed with their bottom in a groove in the crest and top against the bridging girder.

3. Tainter Gates or Radial Gates:

This gate is in the form of a sector of a circle. A curved plate is used to support water. The curved plate is given adequate support by a frame work of steel sections. The gate remains hinged at the piers, on both the ends. This gate can thus rotate about horizontal axis. Water pressure is ultimately transferred to the piers, through bearings fixed in piers. The tales are generally very heavy and can be lifted or lowered with the help of power driven winches only.  

4. U.S.B.R. Drum Gates:

This gate remains hinged at the top of the crest of the spillway. This gate when open i.e. when not in position, remains swung into a ditch formed at the crest of the spillway. In open position drum of the gate itself forms the crest of the spillway while frame work supporting the skin plate remains in the ditch. See Fig. 14.22 dotted position. The gate remains hinged at point A. The gates are put in closed position with the help of power driven winches only.

5. Bear Trap Gate:

This gate consists of two parts – one part slipping over the other for some length. It is mostly used for low Navigation dams.

6. Vertical Lift Gates:

This is the gate which is in most common use in all the irrigation structures. It consists of a Rectangular skin plate of mild steel. This plate remains supported on a frame work of I-girders or channel sections. Grooves are constructed on the crest as well as along the inside of piers. The gates slide, or move, vertically up end down in these grooves.

Water pressure acting on the gate is transferred to the crest and piers. In order to facilitate lifting and lowering of the gates, rollers are fitted in the grooves. The gates are lifted with the help of winches. Counter weights are also used to ease the lifting of the gates. Counter weights remain suspended with one end of the ropes.

The other end of ropes being connected to the gates. If gates are very big in size, they may be constructed in more than one part and each part moves separately. These gates may be constructed in sizes varying form 6 m to 15 m height and 15 m width.  

7. Rolling Gate:

It consists of a steel cylinder as large in diameter as the height of the opening and spanning between the piers. Each pier has an inclined rack. The gate is wound round the periphery of the cylinder. The gate is also attached to the inclined rack with the help of toothed gears. The gate can be rolled while opening and unrolled which is closing the gate.  

8. Trash Rack:

It is not a gate but a sort of permanent structure which is installed at the mouth of the opening. Its main purpose is to prevent entrance of debris into the openings. A framed trash rack structure of steel sections or of R.C.C. members is constructed and iron bars are put in the openings of the frame work to prevent entrance of debris. This structure is usually constructed at the entrances of sluice ways or penstocks etc.  

9. Fish Belly Flap Gate:

This gate is also known as Bascule type of gate.  This is used at the top of the weir crest to store extra storm water. The gate is fitted on the crest with the help of hinged joint. It is operated with the help of a lever rod.