Assoc. Prof. Dr. Tarkan Erdik Spillways Assoc. Prof. Dr. Tarkan Erdik Department of Civil Engineering, I.T.U

Spillway: Structural component of a dam that evacuates flood wave from reservoir to a river at the downstream. Spillway is safety valve of a dam DESIGN RETURN PERIOD From 100 yrs for diversion weir to 15,000 yrs or more (Probable Maximum Flood-PMF) for earth-fill dams. If failure is tolerated, engineering judgement cost-benefit analysis Use certain return period TYPES OF SPILLWAYS The more common types are: 1)Overflow (Ogee crested) 2)Chute 3)Side Channel 4)Shaft 5)Siphon

Spillways are desinged by taking into the following concepts Spillway design data -Storage volume vs elevation -Developed from topograpic maps -Requires operation rules for modeling Spillway discharge rating curve

Reservoir capacity curve

Spillway discharge rating

Most of the spillways are overflow types. Overflow spillways Spillways of Keban Dam.

Most of the spillways are overflow types. Overflow spillways Have large capacities Have higher hydraulic conformities Can be used successfully for all types of dams Allow the passage of flood wave over its crest Are often used on concrete gravity, arch and buttress dams Are constructed as a separate reinforced concrete structure at one side of the fill-typed dams Are classified as uncontrolled (ungated) and controlled (gated)

A.Design Discharge of Spillway Design discharge of an overflow spillway can be determined by integrating velocity distribution over the cross-sectional flow area on the spillway from the crest to the free surface. The equation can be obtained as below where Qo is the design discharge of a spillway Co is discharge coefficient L is the effective crest length Ho is total head over the spillway crest

The effective crest length can be computed as following equation where L’ is the net crest length which is equal to the total crest length. N is the number of bridge piers. Kp is presence of piers coefficient Ka is abutments coefficient Ho is total head over the spillway crest

Usually, a bridge is constructed over the spillway crest to provide the transportation on the crest between two sides of a dam. Piers are constructed on the crest of an overflow spillway to mount gates, to divide the spillway in various chutes such that gentle flow conditions prevail in narrow chutes.

The noses of piers and abutments should be rounded sufficiently to minimize hydraulic disturbances due to seperation

Abutments are responsible for generating gentle flow. If contraction takes place due to the curvature of streamlines local pressure reduce Abutments are responsible for generating gentle flow.

If contraction takes place due to the curvature of streamlines local pressure reduce

Discharge coefficients for vertical faced crest P/Ho

Discharge coefficients with sloping upstream face Cinc/Co P/Ho

Discharge coefficients for varying heads Cme/Co He/Ho Spillways are rarely operated with their design head since the design head corresponds to very large return periods having relatively small risk.

Discharge coefficients due to apron effect Cma/Co (hd+d)/He

Discharge coefficients due to eubmergence effects Cms/Co hd/He

Discharge coefficients for flow under gates d/H1

B.Design Discharge of a Spillway If the gates are partially opened, the discharge can be computed as follows where g is the gravitational acceleration C is the discharge coefficient for a partially open gate L is the effective crest length H1 and H2 are the heads

CREST GATES Provide additional storage above the crest Common types: radial and rolling CREST PROFILES The ideal shape of overflow spillway crest under design conditions for a vertical upstream face is recommended by USBR (1987)

Radial gate

Drum gate

The upstream quadrant of the crest may confirm to the equation of an ellipse as given below: Where the values of A1 and B1 may be determined from the graphs.The downstream profile of the ogee crest may confirm to the following equation:

The values of “K” and “n” in the parabolic relation given in the graphs above. The pressure distribution on the bottom of the spillway face depends on the smoothness of the crest profile. Important Note: •The upstream face of the crest is formed by smooth curves in order to minimize the separation •For a smooth spillway face, the velocity head loss over the spillway can be ignored.

To prevent cavitation, sets of ramps are placed on the face of overflow spillways. Air is introduced by suction into the nappe created by the ramp through vertical shafts to increase the negative pressure to atmospheric pressure Ramp application at the face of Atatürk Dam spillway

If H<Ho streamlines are small pressure over spillway is greater than atmosperic but still less than hydrostatic.

Variation of % energy loss against Froude number

Sequent depth of the hydraulic jump, y2, can be determined from the momentum equation between sections (1) and (2). Ignoring the friction between these sections, the momentum equation for a rectangular basin can be written as

and after simplification Where Fr1 is the flow Froude number at section (1). The energy loss through the hydraulic jump in a rectangular basin is computed from:

Case 4: y2 is greater than y3 at low flows and smaller at high flows

Type 1

Blocks Sill Type 2 The basin length is smaller than basin I by 33%

Sill Pier Blocks Type 3 The basin length is smaller than basin I by 60% but more difficult to construct

Type 4

Overflow Spillway The ogee shape which approximates the profile of the lower nappe of a sheet of water flowing over a sharp-crested weir provides the ideal form for obtaining optimum discharges

The Chute Spillway The Chute Spillway is an obvious choice wherever the foundation strata pose difficult problems. Chutes are also employed in canals for conveying water from a higher to lower elevation with a consequent energy dissipation. The U.S.B.R. specifies broadly, that when canal drops are about 60 m the Chute discharge carriers should be employed

The Chute Spillway A chute spillway is a common and basic design which transfers excess water from behind the dam down a smooth decline into the river below. 

The Side Spillway If sufficient crest length is not available for overflow or chute spillways in narrow valleys, flood water is taken in a side channel. A side channel spillway is one in which the control weir is placed approximately parallel to the upper portion of the discharge channel. The flow over the crest falls into a narrow trough opposite to the weir, turns an approximate right angle, and then continues into the main discharge channel

SHAFT SPILLWAYS A shaft spillway may be constructed in locations where sufficient space is not available for an overflow spillway. In a shaft spillway, water drops through a vertical shaft made of reinforced concrete or steel to a horizontal conduit or to the diversion tunnel which conveys water to the downstream. In this case, the discharge through the inlet may be given as Where, Cs is the discharge coefficient for a shaft spillway which is different from the aforementioned spillway coefficients. Ho is the total head on the inlet (h+ha) and R is the radius of the shaft inlet as follows

Ladybower Dam, West Spillway Shaft

Siphon Spillways A siphon spillway, as demonstrated in next Figure, may be constructed in the body of a concrete dam at a site where there is no enough space for an overflow spillway. Since it is a closed conduit, which has a limited size, its capacity is not as high as that of an overflow spillway. Whenever there is enough head at the crown of the siphon, it operates like an overflow spillway and flow

Tunnel Spillway Where a closed channel is used to convey the discharge around a dam through the adjoining hill sides, the spillway is often called a tunnel or conduit spillway.

•Provides added crest length for a given width Labyrinth  Spillway •Provides added crest length for a given width ◦ Less head needed to pass a given discharge •Additional crest length obtained by series of trapezoidal or triangular walls •Suitable for use anywhere ◦ Particularly when width fixed and dealing with large discharge