Design Of Spillway

Y.C.Agarwal*

1. Function:
   Spillway are provided for storage and detention dams to release surplus or flood water which cannot be contained in the allotted storage space and at diversion dams to by-pass flows exceeding those which are turned into the diversion dam.
   The primary function of spillway is to release surplus waters from the reservoir in order to prevent overtopping and possible failure of the dam.
   The water discharged over the spillway of a dam attains a very high velocity due to its static head, which is generally much higher than the safe non-eroding velocity in the downstream. This high velocity flow may cause serious scour and erosion of river bed downstream. To dissipate this excessive energy and to establish safe flow conditions in the downstream of a dam spillway, energy dissipaters are used as remedial devices.
2. Inflow design flood: The criteria for inflow design flood is given in IS:11223-1985 "Guidelines for fixing spillway capacity". The dams may be classified according to size by using the hydraulic head (from normal or annual average flood level on the downstream to the maximum water level) and the gross storage behind the dam as given below. The overall size classification for the dam would be greater of the following two parameters

Classification	Gross Storage	Hydraulic Head
Small	Between 0.5 and 10 million m3	Between 7.5 m and 12 m
Intermediate	Between 10 and 60 million m3	Between 12 m and 30 m
Large	Greater than 60 million m3	Greater than 30 m

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

Size as determined above	Inflow design flood for safety of dam
Small	Generally 50 years return period flood should be adopted for design of surplussing arrangement. Where dam breach may cause loss of human lives or great damage to property etc, the inflow design flood may be adopted as per IS 11223 : 1985
Intermediate	Standard project flood (SPF)
Large	Probable maximum flood (PMF)

3. Factors affecting design:
   1. Safety considerations consistent with economy
      Many failures of dams have resulted from improperly designed spillway or spillways of inadequate capacity. Properly designed structure of adequate capacity may be found to be only moderately higher in cost than a structure of inadequate capacity.
   2. Hydrological and site conditions
      The spillway design and its capacity depend on
      • Inflow discharge, its frequency, and shape of hydrograph
      • Height of dam
      • Capacity curve
      • Geological and other site conditions
        Important topographical features, which affect spillways design, are
      • Steepness of terrain
      • Amount of excavation and possibility of its use as embankment material.
      • The possibility of scour
      • Stability of slopes, safe bearing capacity of soils.
      • Permeability of soils.

      For example, incase of narrow valley dams, side or chute channel spillway is very seldom possible, because of steepness of banks and their insufficient stability.

   3. Type of Dam
      The type of dam influences the design flood and spillway. Earth and rockfill dams have to be provided with ample spillway capacity.
   4. Purpose of dam and operating conditions
      The ungated spillway should be provided, except in special circumstances when gated spillway may be provided.
4. Combined service and auxiliary spillway:
   When site conditions are favourable, the possibility of gaining over all economy by utilising an auxiliary spillway in conjunction with a smaller service type structure should be considered. In such cases, service spillways should be designed to pass floods likely to occur frequently and the auxiliary spillway control set to operate only after such small floods are exceeded. For this purpose a saddle or depression along the rim of the reservoir is a favourable condition.
5. Spillway components:
   Spillway can be built as part of main dam or separately. Concrete or masonry overflow spillway can be built in the river section where rock foundations are suitable even though adjacent section of the dam may be embankment type. Separate spillways are required for all types of embankment dams.
   Major components of spillways are as follows

• Entrance channel: It admits reservoir water to the spillway and controls the discharge.
• A conduit, which carries the spillway discharge from the entrance structure to a low level outlet downstream of the dam.
• An outlet structure to dissipate the energy of the high velocity flow from the conduit and conveys the water to the channel downstream

6. Type of spillways:
   Spillways are classified according to their most prominent feature either as it pertains to the discharge carrier or some other component. These may be gated or ungated. Common types, which are generally constructed in MIW, are as below:
   • Free overfall or straight drop spillways broad crested
   • Overflow or ogee spillways.
   • Chute spillways
   • Saddle spillway
   • Bye wash spillway
   1. Free overfall or straight drop spillway
      In this type, water drops freely from the crest. Occasionally the crest is extended in the form of overhanging lip to direct small discharges away from the face of overfall section. Ordinarily the use of this structure for hydraulic drops (head pool to tail water) in excess of 20 meters should not be considered.
   2. Ogee or overflow spillway
      This type comprises a control weir, which is ogee or 'S' shaped. The ogee shape conforms closely to the profile of aerated lower nappe and falling from a sharp crested weir. The upper curve at the crest may be made either broader or sharper than the nappe. A broader curve will support the sheet and hydrostatic pressures will occur along the contact surface. Support sheet thus creates a backwater effect and reduces the coefficient of discharge. The sharper crest on the other hand creates negative pressure, increases the effective head and thereby discharge. This type of spillway should be constructed in the Nalla itself as far as possible.
   3. Chute spillway
      In this type water is conveyed from the reservoir to the river or to nalla below the dam through an excavated open channel, through fairly steep slope and placed either along the abutment or through a saddle in the rim of reservoir. Chute spillway ordinarily consist of an entrance channel either straight or curved in alignment, a control structure, a terminal structure, and an outlet channel. The main design consideration would be to fix the longitudinal bed profile of the channel and its sectional dimensions. The energy of the flow has to be suitably dissipated at the outlet, before the flow enters the downstream channel. These are mostly used with earth dams and have the following merit.
      • It can be provided on any type of foundations.
      • It becomes economical if earth received from spillway excavation is used in dam construction.
      • Simplicity of design.
      • However this type of spillway should not be provided where too many bends are to be given as per topography. This type should be avoided on embankments.
   4. Saddle spillways
      In some basins formed by a dam, there may be one or more natural depressions or saddles in the rim of the basin, which can be used as spillway. It is usually necessary for the saddle to be on firm rock.
   5. Bye wash and waste weir :
      Bye wash spillway may be provided near the either flank of dam embankment, where the rocky strata is available (i.e. approx. 1 m below the N.S.L.) and the NSL are near FTL. The bed bar may be provided adjacent with wing wall. It is very economical, less complicated construction and less maintenance. No energy dissipation is generally necessary if rock is available. For small discharges per unit length of bye wash, the availability of hard soil, instead of rocky strata, may be sufficient. Where near the flanks, the NSL is lower than FTL, then upto 1.5 m drop a waste weir may be provided with downstream energy dissipating cistern.
7. Type of energy dissipators:
   These can be classified as below:
   1. Hydraulic jump type stilling basin
      Hydraulic 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

      Factors affecting the design of energy dissipators are:
      (a) Nature of foundations (b) Magnitude of floods and their recurrence (c) Velocity of flow (d) Orientation of flow (e) Elevations of tail water at various discharges (f) Type of dam and its spillway.

8. 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 design loads and load combination should be as per IS: Code 12720- 1993
   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.
9. Upstream and downstream pile line :
   In pervious foundations these should be provided. Downstream pile should be provided of R.C.C. Depth of downstream pileline should be provided as per Khosla's theory and below maximum scour level.
10. Thickness of concrete floor :
    In pervious foundations these should be provided as per Khosla's theory.

REFERENCES:

1.	"Design of small dams" by United States Bureau of Reclamation
2.	Theory & design of irrigation structures, Volume II by R S Vashney, S.C. Gupta & R.L. Gupta
3.	IS 11223 : 1985	Guidelines for fixing spillway capacity
4.	IS 12720 : 1993	Criteria for structural design of spillway training walls and divide walls
5.	IS 11155 : 1994	Construction of spillways and similar overflow structures - Code of practice
6.	IS 4997 : 1968	Criteria for design of hydraulic jump type stilling basins with horizontal and sloping apron
7.	IS 12804 : 1989	Criteria for estimation of aeration demand for spillways and outlet structures
8.	IS 7365 : 1985	Criteria for hydraulic design of bucket type energy dissipators
9.	IS 11527 : 1985	Criteria for structural design of energy dissipators for spillways
10.	IS 13551 : 1992	Criteria for structural design of spillway pier and crest
11.	SP 55 : 1993	Design aid for anchorages for spillway piers, training walls and divide walls
12.	IS 5186 : 1994	Design of chute and side channel spillways - Criteria
13.	IS 11772: 1986	Guidelines for design of drainage arrangements of energy dissipators and training walls of spillways
14.	IS 10137 : 1982	Guidelines for selection of spillways and energy dissipaters
15.	IS 12731 : 1989	Hydraulic design of impact type energy dissipators- Recommendations

* Director, Minor Irrigation Schemes, Jaipur - 302015