Definition of Spillway
A Spillway is a channel or passageway through which flood/surplus water escapes or release safely from a reservoir or dam . Theyare provided for all dams as a safety measure against overtopping and the consequent damages and failure. It acts as a safety value for the dam. The spillway must be hydro-dynamically & structurally safe.
Classification of Spill way:
There are 9 different types of Spill-ways based on the pertinent feature. These are described in detail below:
Free Overfall Spill way:
A free overfall spillway (or a straight drop spillway) is a type of spill way in which flow drops straight or freely from crest into the stream bed, sometimes scours occur & thus there is a possibility of formation of a pool. This type of Spillway is suitable for Arch dam.
An Ogee-shaped (or Overflow) Spill way is the most commonly used spillway. It is widely used with gravity dams, arch dams & buttress dams. Several Earth & Rock fill dams are also provided with this type of spillway as a superstructure. An Ogee-shaped Spillway has a control weir of ogee-shaped, which is like the elongated English letter “S”. The upper part of the spillway surface matches closely to the profile of the lower nappe of a ventilated sheet of water falling freely from a sharp-crested weir. Downstream & Upstream profile of Ogee spillway :
Downstream Profile : The D/S profile of the spillway can be represented by the following general eqn.,
- Upstream Profile : The Upstream profile should confirm to the following eqn. ,
Chute Spillway :
A Chute Spillway (or Open Channel Spill way) consist of a open channel , through which the water discharge. For earth dams and rock fill dams a separate spill way is generally constructed in a flank for saddle away from the dam if suitable site exists.
Side Channel Spill-way:
In a side channel spill-way, the control weir is placed approximately parallel to the upstream portion of the spill-way discharge. Thus the flow after passing over the crest is carried in a discharge channel running parallel to the crest. The spillway discharge flows over the weir crest and falls into a narrow trough.
A Shaft Spill way consist of a horizontal crest & vertical shaft, with its top surface at the crest level of the spillway and its lower end connected to a vertical shaft. The other end of the vertical shaft is connected to a horizontal conduit or tunnel, which extends through or around the dam and carries the water to the river downstream. A shaft spillway is used at the sites where the conditions are not favorable for an overflow or a chute spill-way.
A Siphon spillway operates on the principle of siphonic action. It is a closed conduit of the shape of an inverted U-tube with unequal legs. It is commonly used in practice. The siphon duct is formed by an air tight RCC cover, called hood over an oggy-shaped body wall made of concrete. The top of the body wall forms the crest of the spill-way and kept at the F.R.L. of the reservoir.
A conduit Spill-way consist of a closed conduit to carry the flood discharge to the downstream channel . It is constructed in the abutment or under the dam . The closed conduit may take the form of a vertical or inclined shaft, a horizontal tunnel, or a conduit constructed in an open cut and then covered. Such a spill-way is suitable for dam sites in narrow canyons with steep abutments.
A culvert spill-way is a special type of conduit or tunnel way in which the inlet is horizontal. Moreover, the profile grade of a culvert type is uniform. It should not be used for drops exceeding 8, because negative pressure may develop along its boundary when it is placed on steep slopes and it runs full.
Essential Requirements of Spillway:
- In addition to providing sufficient capacity, the spill-ways must be hydraulically and structurally sound and located such that the discharge will not cause objectionable erosion downstream near the toe of the dam causing the failure of the dam and other appurtenant structures.
- Uncontrolled erosion of bed and bank materials due to faulty design of spill-way and energy dissipation devices has caused not only serious safety problems, heavy maintenance cost are also to be incurred annually after the monsoon when the spill way is in operation.
- The Spill way’s bounding surface must be erosion resistant to withstand the high velocity flow created due to the drop in the water surface from the reservoir level upstream to the tail water level downstream to the dam.
- They are to be designed as transitions structures for smooth passage of flow from upstream to downstream of a storage reservoir without causing any damage to the structure or endangering the river system.
Determining Spill-way Capacity:
The required spill-way capacity is usually determined by “Flood Routing”. The process of computing the reservoir stage, storage, volume and outflows rates corresponding to a particular hydrograph of inflow is commonly referred to as flood routing. The spill-way capacity should be equal to the max. outflow rate determined by flood routing. The following data req. for the flood routing :
- Inflow Flood Hydrograph, indicating the rate of inflow with respect to time. It is the same as the design flood hydrograph of the spillway.
- Reservoir Capacity Curve, indicating the reservoir storage at different reservoir elevations.
- Outflow Discharge Curve, indicating the rate of outflow through spillways at diff. reservoir elevations. By flood routing, the max. Overflow rate and the maximum rise in water surface can be determined.
Factors affecting the required spill way capacity:
- Inflow Flood Hydrograph: The Inflow Flood Hydrograph should be selected according to the degree of protection that ought to be provided to the dam. It will depend upon the type and height of the dam, its location with respect to inhabited and developed area, and consequence of its failure.
- Available Storage Capacity: If the available storage capacity of the reservoir is quite large as compared to the inflow; a spill way of smaller capacity will normally be required.
- Capacity of Outlets: If the dam outlets can be used to discharge a portion of the flood, its capacity can be correspondingly reduced.
- Gates in Spill-way: If the it is gated; its discharge capacity can be modified. For a gate controlled spill way the water can be stored up to the top of the gates, whereas in the case of an ungated spill-way, the water can be stored only up to the crest level.
- Possible Damage: If there is possibility of extensive damage on the downstream, large spill-way capacity should be provided.