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Spillways Abdüsselam ALTUNKAYNAK, PhD Associate Professor, Department of Civil Engineering, I.T.U. Department of Civil Engineering, I.T.U. altunkaynak.net

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Spillway: Structural component of the dam that evacuates flood wave from reservoir to river at the downstream. Is safety valve of the dam DESIGN RETURN PERIOD From 100 yrs for diversion weir to 15,000 yrs or more (Probable Maximum Flood-PMF) for earth-fill dams TYPES OF SPILLWAYS More common types are: (1) Overflow (Ogee crested) (2) Chute (3) Side Channel (4) Shaft (5) Siphon altunkaynak.net

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1) OVERFLOW SPILLWAYS Allows the passage of flood wave over its crest Used on often concrete gravity, arch and buttress dams Constructed as a separate reinforced concrete structure at one side of the fill-typed dams Classified as uncontrolled (ungated) and controlled (gated) Most of the spillways are overflow types They have large capacities They have higher hydraulic conformities They can use successfully for all types of dams

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altunkaynak.net Ideal Spillway Shape The underside of the nappe of a sharp-crested weir when Q=Q max H Sharp crested weir AIR H P HoHo yoyo haha E G L The Overflow spillway

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altunkaynak.net 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. Q o = C o L H o 3/2 The equation can be obtained as below where Q o is the design discharge of spillway C o is discharge coefficient L is the effective crest length Ho is total head over the spillway crest

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altunkaynak.net USE Fig. to modify C o for inclined upstream face. USE Fig. to obtain C o for heads other than design head. USE Fig 4.8 to reflect apron effect on C o. USE Fig. to reflect tail-water effect on C o. Determined from Figures for the vertical overflow spillways as a function of P (spillway height) / H o (total head) The overall C o multiplying each effects of each case

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altunkaynak.net B-Design Discharge of Spillway If the gates are partially opened, the discharge can be computed as following Q = 2/3 (2g) 0.5 C L (H 1 3/2 - H 2 3/2 ) C: Discharge Coefficient determined from Figure 4.10 Where g is the gravitational C is the discharge coefficient for a partially open gate L is the effective crest length H 1 and H 2 are the heads as defined in Figure 4.4

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altunkaynak.net CREST GATES Provide additional storage above the crest See Fig for Primitive types of gates. See Fig for Underflow gates. 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) x y R=0.5 H o R=0.2 H o H o H o Origin of Coordinates

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altunkaynak.net The values of K and n in the parabolic relation given in Fig can be determined from Figure The pressure distribution on the bottom of the spillway face depends on The smoothness of the crest profile. 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. Important Note :

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altunkaynak.net If H (head) > H o p < p atm. overflowing water may lose contact with the spillway face, which results in the formation of a vacuum at the point of separation and CAVITATION may occur. In order to prevent cavitation, sets sets of ramps are placed on the face of overflow spillways so that the jet leaves the contact with the surface. Subatmospheric Pressure Zone Spillway Crest Flow Direction HoHo H>H o EGL Development of negative pressures at the spillway crest for H>H o

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altunkaynak.net Energy Dissipation at the Toe of Overflow Spillway Excessive turbulent energy at the toe of an overflow spillway can be dissipated by a hydraulic jump, which is a phenomenon caused by the change in the stream regime from supercritical to subcritical with considerable energy dissipation. should be done to prevent scouring at the river bed. P E1E1 y2y2 y1y1 E2E2 HoHo hLhL ΔE EGL (O)(1) (2)

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altunkaynak.net Sequent depth of the hydraulic jump, y 2 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 with

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altunkaynak.net and after simplification and F r1 is the flow Froude number at section (1). Where The energy loss through the hydraulic jump in a rectangular basin is computed from

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altunkaynak.net Case 1: If the tailwater depth, y 3, is coincident with the sequent depth, y 2, the hydraulic jump forms just at toe of the spillway as shown in Figure below y 2 =y 3 y1y1 (1) (2) Flow conditions for y 2 =y 3

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altunkaynak.net Case 2: If the tailwater depth is less than the required sequent depth, the jump moves toward the downstream, as can be seen from Figure below. This case should be eliminated, because water flows at a very high velocity having a destructive effect on the apron. y 2 >y 3 y1y1 (1)(2) Flow conditions for y 2 >y 3

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altunkaynak.net Case 3 : If the tailwater depth is greater than the required sequent depth as shown in Figure below y 2 < y 3 Flow conditions for y 2

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altunkaynak.net REMINDERS: 2) If y 2 (tailwater depth) is subcritical a HYDRAULIC JUMP between y 1 and y 2 (toe and tailwater, see case1). 3) y 2 (conjugate depth) determined from Eq. for rectangular basin. 1)y 1 (depth at the toe) a supercritical depth and determined from Energy Eq. between upstream of spillway and the toe

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altunkaynak.net CHUTE SPILLWAYS SIDE CHANNEL SPILLWAYS SHAFT SPILLWAYS SIPHON SPILLWAYS

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