Spillways 1.0 Definition A spillway is an engineering structure used to provide the controlled release of surplus water flow that cannot be contained in a storage basin, (usually originating from a river or reservoir) from a dam or levee[footnoteRef:1], into a downstream area. They are employed to avoid overtopping of the retained or dammed water, which can destroy the dam and to also regulate the level of water held behind the dam. There are a few types of spillways utilised today, and are categorised based on the intake mechanism and structure used to release the water. The following will cover the various disciplines of spillway usage. [1: Levee an embankment built or deposited as a result of sedimentation, used to prevent the overflow of a river]

2.0 Types of Spillways Comment by Jaikeshan Takchandra: This entire section is under revision and will be updated at a later timeI also think once we see the maps and so it will give us a better idea as to what is relevant here1. Uncontrolled Spillways 2. Controlled Spillways 3. Auxiliary Spillways 4. Concrete dams I. Open channel spillways Open channel spillways are dam spillways that utilize the principles of open channel flow to convey impounded water in order to prevent dam failure. They can function as principal spillways, emergency spillways, or both. They can be located on the dam itself or on a natural grade in the vicinity of the dam.

II. Side channel spillways Side channel spillways are located just upstream and to the side of the dam. The water after glowing over a crest enters a side channel which is nearly parallel to the crest. This is then carried by a chute to the downstream side. Sometimes a tunnel may be used instead of a chute.

III. Ogee spillways This spillway is generally provided in rigid dams and forms part of the main dam itself, if sufficient length is available. The crest of the spillway is shaped to conform to the lower nappe of a water sheet flowing over an aerated sharp crest weir.

IV. Chute (trough) spillways In this type of spillway, the water, after flowing over a short crest or other kind of control structure, is carried by an open channel (called a chute or trough (to the downstream side of the river. The control structure is generally normal to the conveyance channel. The channel is constructed in excavation with stable side slopes and invariably lined. The flow through the channel is super-critical. The spillway can be provided close to the dam or at a suitable saddle away from the dam where site conditions permit.

V. Stepped spillways

VI. Bell-mouth spillways

VII. Siphon spillways This spillway works on the principle of a siphon. A hood provided over a conventional spillway forms a conduit. With the rise in reservoir level, water starts flowing over the crest as in an ogee spillway. The flowing water, however, entrains air and once all the air in the crest area is removed, siphon action starts. Under this condition, the discharge takes place at a much larger head. The spillway thus has a larger discharging capacity. The inlet end of the hood is generally kept below the reservoir level to prevent floating debris from entering the conduit. This may cause the reservoir to be drawn down below the normal level before he siphon action breaks and therefore arrangement for de-priming the siphon at the normal reservoir level is provided.

3.0 Construction Spillways are expected to be composed of the following structures (see Fig. 1-3.0): I. Approach channel and safety boom II. Control structure such as: a crest structure gates bulkheads stop-logs grade soil III. conveyance features e.g. floor, walls conduits or tunnels IV. terminal structure/energy dissipater: hydraulic jump stilling basin flip bucket plunge pool V. downstream channel

FIG 1- 3.0: Parts of a typical spillway construction (needs citation; all images will be redrawn)

4.0 Design Procedures and Considerations I. Design data collection II. Feasibility design III. Final design IV. Cost estimating V. Safety of dam project

5.0 Spillway Design and Analysis The type, location, and size of spillway will be dependent on the evaluation of a number of factors, including:I. Site conditions (geology and topography)II. Dam type III. Hydrologic considerations IV. Hydraulic considerations V. Seismic considerationsVI. Constructability considerations VII. Project objectives VIII. Risk analysis IX. Operation maintenance considerations X. Economics 5.0.1 Parameters considered in Designing spillways 1. The inflow design flood hydrograph2. The type of spillway to be provided and its capacity 3. The hydraulic and structural design of various components 4. The energy dissipation downstream of the spillway 5.0.2 Basic expected design features 1. Smooth flow contraction towards crest of dam 2. Critical flow conditions at crest 3. Converging conveyance walls 4. Energy dissipation in the channel centerline For a given inflow flood hydrograph, the maximum rise in the reservoir level depends on the discharge characteristics of the spillway crest and its size and can be obtained by flood rioting. Trial with different sizes can then help in getting the optimum combination. 5.1 Inflow flood design The criteria for inflow design is given below in IS: 11223-1985 dams maybe classified by using the hydraulic head and the gross storage behind the dam as given.ClassificationGross StorageHydraulic Head

SmallBetween 0.5 and 10 million m3Between 7.5 m and 12 m

IntermediateBetween 10 and 60 million m3Between 12 m and 30 m

LargeGreater than 60 million m3Greater than 30 m

The inflow design flood for safety of the dam would be as follows:Size as determined aboveInflow design flood for safety of dam

SmallGenerally 50 years return period flood should be adopted for design of surplus arrangement. Where dam breach may cause loss of human lives or great damage to property etc.

IntermediateStandard project flood (SPF)

LargeProbable maximum flood (PMF)

5.2 Design of side walls The profile of flow on spillway surface determines the height of side walls required to retain flow on the spillway. These are designed as retaining walls with water side face to be vertical or near vertical for perfect energy dissipation.The bottom width of side wall is decided as per the safe bearing capacity of soil at foundation level.The stability should be checked at foundation level, top of bed concrete level and at water side floor level etc. The foundation level of downstream side wall should be kept at downstream floor foundation level. Uplift pressures should always be considered at all elevations while checking stability. Foundation of upstream side walls should be kept at foundation level of upstream impervious floor.5.3 Energy dissipater Hydraulic jump type stilling basinHydraulic jump may be defined as a phenomenon, which is a distinct rise or jump of water, accompanied by a great deal of turbulence. This phenomenon may occur when a shallow stream of water moving with a high velocity strikes a stream of water moving with a low velocity. When a fast moving wall of water has to be slowed down to prevent scour damage below a work, the hydraulic jump can be used with great advantage to destroy the kinetic energy. Horizontal apron type Slopping apron type

5.4 Factors affecting design I. Safety considerations consistent with economy Many failures have resulted from improperly designed spillways, or those of inadequate capacity. II. Hydrological and site conditions The spillway design and its capacity depend on: Inflow discharge, frequency and shape of the hydrograph of the river Height of the dam Capacity curve Geological and other site conditions Important topographical features Steepness of terrain Amount of excavation and possibility of its use as embankment material Stability of slopes and safe bearing capacity of soils Permeability of soils III. Type of dam The type of dam influences the design of the spillway. Earth and rock fill dams have to be provided with ample spillway capacity IV. Purpose of dam and operating conditions The ungated spillway should be provided, except in special circumstances when gated spillways may be provided 6.0 Hydraulic Design and Analysis I. Develop and verify discharge curves for the river II. Prepare initial flood routings of frequency floods to verify appropriateness of the spillway type and sizeIII. Refine spillway control structure layout and associated discharge curves based on results from previous stepsIV. Prepare flood routings to estimate maximum reservoir water surfaces and discharge ranges for various operational conditionsV. Prepare initial water surface profiles and layout the spillway conveyance features and terminal structure size and type VI. Refine spillway conveyance features and terminal structure based on results from previous steps VII. Prepare final water surface profiles to finalise size and type of spillway conveyance features and terminal structure.7.0 Selecting a spillway 8.0 Summary and Conclusion Overtopping protection system must be adequate Minimum energy loss spillway Design construction must be sound Comment by Jaikeshan Takchandra: Will be updated upon completion of final report