Spillways 1.0 Defnition A spillway is an engineering structure used to provide the controlled release of surplus water ow that cannot be contained in a storage basin, (usually originating from a river or reservoir) reservoir) from a dam or levee 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 intae mechanism and structure structure used to release the water. water. The following will cover the various disciplines of spillway usage. 2.0 Types o Spillways 1. !ncont !ncontro rolle lled d Spillway Spillways s ". #ontr #ontroll olled ed Spill Spillway ways s $. Au%ili Au%iliary ary Spillw Spillways ays &. #onc #oncrrete ete dam dams s '. pen c ch hannel s sp pillways ays pen channel spillways are dam spillways that utilie the principles of open channel ow to convey impounded water in order to prevent dam failure. They can function as principal spillways, emergency emergency spillways, or both. They can be located on the dam itself or on a natural grade in the vicinity of the dam. ''.
Side ch channel sp spillways ays Side channel spillways are located *ust 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 carried by a chute to the downstream downstream side. Sometimes a tunnel may be used instead of a chute.
'''.
gee sp spillways This spillway is generally generally provided in rigid rigid dams and forms part of the main dam itself, if su+cient length is available. The crest of the spillway spillway is shaped to conform to the lower nappe of a water sheet owing over an aerated sharp crest weir.
'. '.
#hut #hute e (tr (trough) ugh) s spi pill llw ways ays
1 Levee - an embanment built or deposited as a result of sedimentation, used to prevent the overow of a river
'n this type of spillway, the water, after owing over a short crest or other ind 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 e%cavation with stable side slopes and invariably lined. The ow through the channel is supercritical. The spillway can be provided close to the dam or at a suitable saddle away from the dam where site conditions permit. . '. ''.
Stepped spillways /ellmouth spillways Siphon spillways This spillway wors on the principle of a siphon. A hood provided over a conventional spillway forms a conduit. 0ith the rise in reservoir level, water starts owing over the crest as in an ogee spillway. The owing water, however, entrains air and once all the air in the crest area is removed, siphon action starts. !nder this condition, the discharge taes place at a much larger head. The spillway thus has a larger discharging capacity. The inlet end of the hood is generally ept below the reservoir level to prevent oating debris from entering the conduit. This may cause the reservoir to be drawn down below the normal level before he siphon action breas and therefore arrangement for depriming the siphon at the normal reservoir level is provided.
3.0 Construction Spillways are e%pected to be composed of the following structures (see ig. 1$.2)3 '. ''.
Approach channel and safety boom #ontrol structure such as3 a crest structure gates bulheads stoplogs grade soil conveyance features e.g. oor, walls conduits or tunnels • • • • •
'''.
'.
terminal structure4energy dissipater3 hydraulic *ump stilling basin ip bucet plunge pool downstream channel • • •
.
FIG 1
$.23 Parts o a typical spillway construction (needs citation; all imaes will !e
redrawn"
#.0 Desin Procedures and Considerations '. 5esign data collection ''. easibility design '''. inal design '. #ost estimating . Safety of dam pro*ect
$.0 Spillway Desin and %nalysis The type, location, and sie of spillway will be dependent on the evaluation of a number of factors, including3 '. ''.
Site conditions (geology and topography) 5am type
'''. '. . '. ''. '''. '9. 9.
6ydrologic considerations 6ydraulic considerations Seismic considerations #onstructability considerations 7ro*ect ob*ectives 8is analysis peration maintenance considerations :conomics
&.1.1 Parameters considered in Desinin spillways 1. The inow design ood hydrograph ". The type of spillway to be provided and its capacity $. The hydraulic and structural design of various components &. The energy dissipation downstream of the spillway $.0.2 1. ". $. &.
'asic epected desin eatures Smooth ow contraction towards crest of dam #ritical ow conditions at crest #onverging conveyance walls :nergy dissipation in the channel centerline
or a given inow ood hydrograph, the ma%imum rise in the reservoir level depends on the discharge characteristics of the spillway crest and its sie and can be obtained by ood rioting. Trial with di;erent sies can then help in getting the optimum combination. $.1 )n*ow *ood desin The criteria for inow design is given below in 'S3 11""$1<=> - dams maybe classi?ed by using the hydraulic head and the gross storage behind the dam as given. @ross Storage /etween 2.> and 12 million m$ /etween 12 and 2 million m$ @reater than 2 million m$ The inow design ood for safety of the dam would be as follows3 Sie as determined above
'now design ood for safety of dam
Small
@enerally $0 years return period ood should be adopted for design of surplus arrangement. 0here dam breach may cause loss of human lives or great damage to property etc.
'ntermediate
Standard pro*ect ood (S7)
Barge
7robable ma%imum ood (7C)
$.2 Desin o side walls The pro?le of ow on spillway surface determines the height of side walls reDuired to retain ow 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 checed at foundation level, top of bed concrete level and at water side oor level etc. The foundation level of downstream side wall should be ept at downstream oor foundation level. !plift pressures should always be considered at all elevations while checing stability. oundation of upstream side walls should be ept at foundation level of upstream impervious oor. $.3 +nery dissipater ,ydraulic -ump type stillin !asin 6ydraulic *ump may be de?ned as a phenomenon, which is a distinct rise or *ump of water, accompanied by a great deal of turbulence. This phenomenon may occur when a shallow stream of water moving with a high velocity stries a stream of water moving with a low velocity. 0hen a fast moving wall of water has to be slowed down to prevent scour damage below a wor, the hydraulic *ump can be used with great advantage to destroy the inetic energy. •
6oriontal apron type
•
Slopping apron type
$.# actors a/ectin desin '. Safety considerations consistent with economy Cany failures have resulted from improperly designed spillways, or those of inadeDuate capacity. ''. 6ydrological and site conditions The spillway design and its capacity depend on3
'now discharge, freDuency and shape of the hydrograph of the river 6eight of the dam #apacity curve @eological and other site conditions 'mportant topographical features Steepness of terrain Amount of e%cavation and possibility of its use as embanment material Stability of slopes and safe bearing capacity of soils 7ermeability of soils Type of dam The type of dam inuences the design of the spillway. :arth and roc ?ll dams have to be provided with ample spillway capacity 7urpose of dam and operating conditions The ungated spillway should be provided, e%cept in special circumstances when gated spillways may be provided •
• • • • • •
• •
'''.
'.
.0 ,ydraulic Desin and %nalysis '. 5evelop and verify discharge curves for the river ''. 7repare initial ood routings of freDuency oods to verify appropriateness of the spillway type and sie '''. 8e?ne spillway control structure layout and associated discharge curves based on results from previous steps '. 7repare ood routings to estimate ma%imum reservoir water surfaces and discharge ranges for various operational conditions . 7repare initial water surface pro?les and layout the spillway conveyance features and terminal structure sie and type '. 8e?ne spillway conveyance features and terminal structure based on results from previous steps ''. 7repare ?nal water surface pro?les to ?nalise sie and type of spillway conveyance features and terminal structure. .0 Selectin a spillway .0 Summary and Conclusion vertopping protection system must be adeDuate Cinimum energy loss spillway 5esign construction must be sound • • •
