Understanding Spillways: Types, Design, and Functions

 Lecture 10 & 11

Spillway

·        A spillway is a waterway designed to release excess floodwaters from a reservoir once it reaches full capacity.

·        It serves as a safety mechanism for dams, preventing potential structural damage.

·        A spillway can be integrated into the dam itself, positioned at one end, or constructed as a separate structure away from the dam.

Necessity of Spillways

(a)

·         The height of a dam is determined based on the maximum storage capacity of the reservoir.

·         The normal pool level represents the reservoir's maximum capacity, and water is never stored above this level.

·         To prevent dam failure, such as by overturning, spillways are crucial as they ensure the safe discharge of excess water.

(b)

·         The top of the dam is often used for constructing roads.

·         To prevent surplus water from over-topping the dam, spillways are absolutely necessary.

(c)

·         Spillways are provided to safeguard the downstream base and floor of the dam from scouring and erosion.

·         They ensure that excess water is discharged smoothly, minimizing potential damage to the dam structure.

Essential requirements of a spillway

The key requirements of a spillway are as follows:

1.     Adequate Capacity:

o    The spillway must be designed to handle the maximum expected flood discharge, ensuring it can release surplus water without over-topping the dam.

2.     Hydraulic and Structural Soundness:

o    The spillway must be both hydraulically efficient and structurally robust to withstand the forces exerted by flowing water.

3.     Safe Location and Disposal:

o    It should be positioned in a way that ensures safe water discharge, preventing erosion or undermining of the dam's downstream toe.

4.     Erosion-Resistant Surfaces:

o    The spillway's surfaces must be constructed to resist erosion caused by high-velocity water flow, especially due to the drop from the reservoir level to the tailwater level.

5.     Energy Dissipation Mechanisms:

o    Energy dissipation devices (e.g., stilling basins) are typically required downstream of the spillway to reduce the water's kinetic energy and prevent scouring or damage to the surrounding area.

Location of Spillway

• Spillways are typically located in the following configurations:
• Within the dam structure: Integrated directly into the body of the dam.
• At one or both sides of the dam: Positioned adjacent to the dam, either on one side or both sides.
• As a by-pass spillway: Constructed as an entirely separate structure, independent of the dam.

Components of spillway

1.     Control Structure:

o    This is a key part of the spillway that manages and regulates the release of water from the reservoir.

o    It ensures no water is released below a set reservoir level and allows outflow only when the water level exceeds this threshold.

o    Typically, the control structure includes a weir, which can be sharp-crested, ogee-shaped, or broad-crested.

o    In some cases, gates are installed on the weir's crest to further control the water flow.

2.     Discharge Channel:

o    The water released through the control structure is carried to the river downstream via a discharge channel or waterway.

o    This channel ensures the safe transfer of water from the reservoir to the riverbed below the dam.

o    The discharge channel can take various forms, such as:

§  The downstream face of the spillway.

§  An open channel dug into the ground.

§  A closed conduit running through or under the dam.

§  A tunnel built through an abutment.

3.     Terminal Structures (Energy Dissipators):

o    As water moves from the reservoir level to the downstream river level, its potential energy converts into kinetic energy, resulting in high flow velocities.

o    These high velocities can cause severe erosion or scouring of the riverbed and banks, potentially damaging the spillway, dam, and nearby structures.

o    To prevent this, energy dissipators are installed at the spillway's downstream end to reduce the flow's energy before it re-enters the river.

4.     Entrance and Outlet Channels:

o    Entrance channels may be needed to draw water from the reservoir and direct it to the control structure.

o    Similarly, outlet channels may be required to carry the spillway flow from the terminal structure back to the river downstream.

o    These channels are unnecessary in cases where the spillway directly draws water from the reservoir and releases it straight into the river (e.g., overflow spillways).

o    However, for spillways located through abutments, saddles, or ridges, entrance and outlet channels are often essential.

Determination of discharge capacity and number of spillways

The maximum discharge capacity and the number of spillways is determined by studying the following factors:

1)    By studying the flood hydrograph of past ten years, the maximum flood discharge may be computed which is to be disposed off completely through the spillways.

2)    The water level in the reservoir should never be allowed to rise above the maximum pool level and should remain in normal pool level. So, the volume of water collected between maximum pool level and minimum pool level computed, which indicates the discharge capacity of spillways.

3)    The maximum flood discharge may also be computed from other investigation like, rainfall records, flood routing, empirical flood discharge formulae, etc.

4)    From the above factors the highest flood discharge is ascertained to fix the discharge capacity of spillways.

5)    The natural calamities are beyond the grip of human being. So, an allowance of about 25 % should be given to the computed highest flood discharge which is to be disposed off.

6)    The size and number of spillways are designed according to the design discharge.

 

TYPES OF SPILLWAYS

Classification based on the time when the spillways come into operation

ü Service Spillway (Primary Spillway)

·         Operation Time: Regularly used during normal and moderate flood conditions.

·         Purpose: Designed to handle frequent water releases and maintain the reservoir at safe levels.

·         Features: Typically has a well-defined control structure (e.g., gates or weirs) to regulate flow.

ü Auxiliary Spillway

Purpose:

o    Acts as a secondary spillway to assist the primary spillway during unusually high flood events.

o    Provides additional discharge capacity when the reservoir water level exceeds the design capacity of the primary spillway.

Operation:

o    Activated only during rare or extreme flood conditions.

o    May operate automatically when water levels rise above a predetermined threshold.

ü Emergency Spillway

Purpose:

§  Serves as a last-resort safety feature to prevent over-topping and catastrophic failure of the dam during extreme flood events.

§  Designed to handle flows that exceed the capacity of both the primary and auxiliary spillways.

Operation:

§  Activated only in dire situations, such as when the reservoir level rises to a critical level that threatens dam safety.

§  Often designed to operate automatically without human intervention.

Some of the situations which may lead to emergency are

(i)               An enforced shutdown of the outlet works

(ii)             A malfunctioning of spillway gates

(iii)          The necessity for bypassing the regular spillway because of damage or failure of some part of that structure.

 

Classification based on the flow through the spillway being controlled or uncontrolled

ü Controlled or Gated spillway:

A spillway having means to control the outflow from the reservoir is known as controlled or gated spillway.

ü Uncontrolled or Ungated spillway:

A spillway, the crest of which permits water to escape automatically, as the water level in the reservoir rises above the crest is known as uncontrolled or ungated spillway.

Classification based on the prominent features pertaining to the various components of the spillway

 

1.     Free overfall or straight drop spillway

2.     Overflow or Ogee spillway

3.     Chute or Open channel or trough spillway

4.     Side channel spillway

5.     Shaft or Morning glory spillway

6.     Conduit or Tunnel spillway

7.     Siphon spillway.

8.     Labyrinth Spillways

9.     Baffled Chute Spillways

 

Free Overfall or straight drop spillway

A free overfall spillway (or straight drop spillway) features a low-height, narrow-crested weir with a vertical or near-vertical downstream face. Water flows over the crest and falls freely as a jet, clear of the spillway's downstream face. To direct small discharges away, an overhanging lip is sometimes added. Proper ventilation beneath the falling jet prevents pulsation or fluctuations.

Without downstream protection, the falling jet can scour the streambed, forming a deep plunge pool. To prevent this, an artificial pool can be created using a low auxiliary dam, an excavated basin with a concrete apron, or by ensuring sufficient tailwater depth to form a hydraulic jump. Floor blocks and an end sill may also be added to stabilize the jump and reduce scouring.

This spillway type is commonly used for low earth dams, thin arch dams, or structures with near-vertical downstream faces. However, it is unsuitable for high drops on weak foundations due to the risk of large impact forces, vibrations, and potential apron failure. Free overfall spillways are typically limited to hydraulic drops of up to 6 meters to avoid these issues.

Overflow or Ogee spillway

An overflow or ogee spillway is an enhanced version of the free overfall spillway and is commonly utilized in conjunction with gravity, arch, and buttress dams. It is also incorporated into several earth dams. The key distinction between the free overfall spillway and the overflow spillway lies in the behavior of the water as it passes over the crest. In a free overfall spillway, the water flows over the crest and falls freely, creating a clear separation from the downstream face of the spillway. In contrast, an overflow spillway directs the water smoothly over the crest and ensures it flows gently along the downstream face of the structure.

Crest Shape of Overflow Spillway

An overflow spillway features a control structure shaped like an ogee or S-curve weir. The crest of this spillway is designed to closely match the profile of the lower surface of a water nappe flowing over a ventilated sharp-crested weir at the design head. This nappe-shaped profile is ideal because, at the design head, the water flows smoothly over the crest, maintaining contact with the spillway surface, ensuring optimal discharge and atmospheric pressure along the contact surface.

When the discharge head is less than the design head, the water remains in contact with the spillway, but positive hydrostatic pressure occurs, creating a backwater effect that reduces discharge. Conversely, at heads greater than the design head, the water may separate from the spillway surface, forming a zone of negative pressure, which increases the effective head and discharge.

The nappe-shaped profile depends on factors like head, upstream face inclination, and spillway height above the stream-bed, which affects approach velocity. Extensive experiments by the U.S. Bureau of Reclamation (U.S.B.R.) determined these profiles for vertical or inclined upstream faces. Based on this data, the U.S. Army Corps of Engineers established standard crest shapes for overflow spillways at the Waterways Experiment Station in Vicksburg (U.S.W.E.S.).

Such shapes designated as the WES standard spillway shapes, can be represented by the following equation

Xⁿ = KH_d^n-1 Y ………………...(i)

Where,

§  X and Y are coordinates of the profile of the crest of the spillway with the origin at the highest point of the crest;

§  H_d is design head excluding head due to velocity of approach;

§  K and n are constants whose values depend on the slope of the upstream face of the spillway.

The values of K and n are given as follows:

Fig.  WES-standard spillway shapes

As shown in Fig. above X is taken as positive in the downstream direction and Y is taken as positive in the downward direction.

Since Eq. (i) is applicable only for the positive values of X and Y, it gives the crest shape downstream from the origin of the coordinates.

For intermediate slopes, approximate values of K and n may be obtained from the plots of the above values against the corresponding slopes

Fig: Plots of n and K versus upstream slope

The radius R (in m) of the bucket may be obtained approximately by the following empirical formula:

Where,

§  V is the velocity in m/s of the flow at the toe of the spillway; and

§  H is head in m excluding head due to the velocity of approach, on the spillway crest.

Neglecting the energy loss involved in the flow over the spillway the velocity of flow V at the toe of the spillway may be computed by

Where,

§  Z is fall, or vertical distance from the upstream reservoir level to the floor at the toe;

§  Hα is head due to velocity of approach on the upstream side;

§  g is acceleration due to gravity; and

§  y is depth of flow at the toe.

The crest shape upstream from the origin of the coordinates is usually in the form of a compounded circular curve as shown in Fig. WES-standard spillway shapes, or in the form of an elliptical curve. However, on the basis of the recent studies, U.S. Army Crops of Engineers has proposed an equation for the crest shape upstream from the origin of the coordinates which is as follows:

In above Eq.  only negative values of X are to be used. The minimum value of X being (–0.270Hd) corresponding to which the value of Y is equal to 0.126Hd. Further this Eq. is applicable only to the spillways which have their upstream face vertical.

The upstream face of the spillway may sometimes be designed to set back as shown by dotted lines in Fig. (WES-standard spillway shapes) which will result in an overhang called ‘corbel’ on the upstream side of the spillway.

Discharge of Overflow Spillway

Q = CLH_e^3/2

Where,

§  Q is discharge;

§  C is coefficient of discharge;

§  L is effective length of crest of spillway; and

§  H_e is total head of flow on crest of spillway including head due to velocity of approach.

The discharge coefficient of an overflow spillway is typically quite high, with a maximum value reaching approximately 2.2, provided that no negative or suction pressure is generated. However, the actual value of this coefficient is influenced by several factors:

                   I.            Height of spillway above stream-bed or bed of approach channel

The height of the spillway above the stream-bed or approach channel bed influences the velocity of approach, which in turn affects the coefficient of discharge. As the spillway height increases, the velocity of approach decreases, leading to an increase in the coefficient of discharge. Model tests indicate that the velocity of approach has a negligible effect when the spillway height is equal to or greater than 1.33 H_d, where H_d is the design head excluding the velocity head.

Figure below illustrates the relationship between the coefficient of discharge (C) and the ratio of spillway height (P) to the design head including velocity head (H_D), where H_D = H_d + H_a. The plot shows a significant increase in C as the spillway height reaches up to 2H_D, beyond which further increases in height result in minimal changes to C.

                II.            Ratio of the actual total head of flow over spillway crest to the design head

Figure below illustrates the relationship between C/C′​ and H_e/H_D​​ for a spillway with a height P above the stream-bed greater than 1.33H_D​. Here, C is the discharge coefficient for the actual total head H_e​, and C′ is the discharge coefficient for the design head H_D​, including the velocity head. The plot shows that as H_e/H_D​​ increases, C/C′​ also increases, meaning the discharge coefficient rises with higher heads. For H_e<H_D, C<C′, and for H_e>H_D​, C>C′.

Designing the spillway with a lower design head can yield higher discharge coefficients for most flow heads. However, the design head should not be less than 80% of the maximum head to prevent cavitation. Model tests indicate that for spillways with P>1.33H_D, the velocity head is negligible, and when H_e=H_D​, the discharge coefficient is 2.2. For other heads, the coefficient can be determined from Figure below. Similar curves are available for spillways with P<1.33H_D​.

             III.            Slope of upstream face of spillway

For small values of the ratio P/H_D​, a spillway with a sloping upstream face has a higher discharge coefficient than one with a vertical upstream face. However, as P/H_D​ increases, the discharge coefficient for spillways with a sloping upstream face tends to decrease.

             IV.            Extent of downstream submergence of spillway crest

The coefficient of discharge decreases with submergence. As shown in Figure below, even with a submergence degree of up to 60%, the reduction in the coefficient of discharge is only around 5%.

 

Effective Length of Crest of Overflow Spillway

The effective length of crest of an overflow spillway is given by the following equation:

Where,

§  L = effective length of crest

§  L´ = net length of crest which is equal to the sum of the clear spans of the gate bays between piers

§  H_e = total head of flow on crest including head due to velocity of approach

§  N = number of piers

§  K_p = pier contraction coefficient, and

§  K_a = abutment contraction coefficient.

The pier contraction coefficient, K_p depends on several factors such as

(i)               Shape and location of pier nose:

(ii)             Thickness of pier;

(iii)          Approach velocity; and

(iv)           Ratio of actual head on crest to design head.

 

For the flow at the design head the average values of K_p may be assumed as follows

The abutment contraction coefficient K_a also depends on several factors such as

(i)               Shape of abutment;

(ii)             Angle between upstream approach wall and axis of flow;

(iii)          Approach velocity; and

(iv)           Ratio of actual head on crest to design head.

 

For the flow at the design head the average values of K_a may be assumed as follows.

§  r = radius of abutment rounding and

§  H_d = design head.

In above the discharge Equation provides the discharge for an ungated overflow spillway or for a gated overflow spillway when the gates are fully open.

However, for a gated spillway operating with partial gate openings, the flow behavior resembles that of a low-head orifice. In such cases, the discharge can be calculated using the following equation.

Where,

§  Q is discharge;

§  C_d is coefficient of discharge;

§  g is acceleration due to gravity;

§  L is effective length of crest of spillway; and

§  H₁ and H₂ are total heads (including head due to velocity of approach) above bottom and top of the opening, respectively.

The coefficient of discharge (C_d) varies depending on the specific arrangement of gates and the spillway crest. It is also affected by the approach and downstream conditions, as these influence the contraction of the flow jet. Additionally, the effective length (L) of the spillway crest is determined by the total clear spans of the gate bays located between the piers.

 

Example

Design a suitable section for the overflow portion of a concrete gravity dam having the downstream face sloping at a slope of 0.7H:1V. the design discharge for the spillway is 8000 cumecs. The height of the spillway crest is kept at RL 204 m. the average river bed level at the site is 100 m. The spillway length consists of 6 spans having a clear width of 10 m each. Thickness of each pier may be taken to be 2.5 m.

Solution:

Q = CLH_e^3/2

L ≈ L = 6 x 10 = 60 m

8000 = 2.2 x 60 x H_e^3/2

H_e = 15.5 m ≈ 16 m

Now effective length of spillway

L_e = L´ – 2 (NK_p + K_a) H_e

Assume that 90⁰ cut water nose piers and rounded abutments shall be provided.

K_p =0.01

K_a = 0.10

N =5

L_e = 55.2 m

Hence again,

Q = CLH_e^3/2

8000 = 2.2 x 55.2 x H_e^3/2

H_e = 16. 3 ≈ 16 m

Downstream profile

The WES equation

Xⁿ = KH_d ^n-1 Y

n = 1.85

K = 2

 

H_d = H_e = 16 m

 

Y = X^1.85 /21.11

dY/dX = 1/0.7

1.85X^1.85-1 /21.11 = 1/0.7

X = 22.4 m

Y = X^1.85 /21.11 = 22^1.85 /21.11 = 14.6 m

The coordinates from X=0 to X= 22.4 m is tabulated below

The upstream profile

A reverse curve at toe with a radius equal to h/4 = 104/4 = 26 m can be drawn at angle 60⁰.

[In the above example, the effect of factors in C_d has not been calculated. However, it must be calculated and checked to determine whether they have an impact or not] 

Chute or Open Channel or Trough Spillway

A chute spillway, also known as an open channel or trough spillway, is a type of spillway where water flows through an open, lined channel or chute from the reservoir to the downstream river. It is one of the most common and versatile spillway designs, particularly suitable for earth-fill and rockfill dams.

• The spillway consists of an open channel (often lined with concrete) that carries water from the reservoir crest to the downstream side.
• The channel is typically steep to maintain high flow velocities and prevent sediment deposition.
• It may include energy dissipators (e.g., stilling basins) at the downstream end to reduce the flow’s kinetic energy and prevent erosion.

 

Side Channel Spillway

A side channel spillway is a type of spillway where water flows over a weir or crest and is carried away through a channel that runs parallel to the spillway crest, rather than directly downstream. This design is particularly useful in narrow valleys or where space constraints make traditional spillways impractical.

• Water flows over a weir or crest and is diverted into a side channel that runs parallel to the spillway.
• The side channel collects the water and conveys it to a chute or tunnel for safe discharge downstream.
• Often includes energy dissipators (e.g., stilling basins) to reduce flow energy before releasing water into the river.

 

Shaft or Morning Glory Spillway

A shaft spillway, also known as a morning glory spillway, is a unique and efficient type of spillway used in dam engineering to safely discharge excess water from a reservoir. It is characterized by its distinctive funnel-shaped structure, which is both functional and aesthetically pleasing. The design of a shaft spillway consists of several key components that work together to manage water flow effectively.

The shaft spillway is composed of three main parts:

a.     Flaring Funnel (Crest)

• The topmost part of the spillway is a flaring funnel, which is circular in shape and serves as the crest of the spillway. This funnel is typically designed to flare outward, resembling the shape of a morning glory flower, hence the name "morning glory spillway."
• The crest is positioned at the reservoir's maximum water level, allowing water to flow over it when the reservoir reaches its capacity.
• The circular design of the crest ensures uniform distribution of water as it enters the spillway, minimizing turbulence and optimizing flow efficiency.

b.     Vertical or Inclined Shaft

• Below the flaring funnel, the spillway transitions into a vertical or inclined shaft. This shaft is a long, cylindrical structure that extends downward from the crest.
• The shaft is designed to carry water from the crest to the lower levels of the dam. Its diameter is carefully calculated to handle the expected volume of water flow without causing excessive pressure or cavitation.

c.      Horizontal or Near-Horizontal Conduit or Tunnel

• At the base of the shaft, the spillway connects to a horizontal or near-horizontal conduit or tunnel. This conduit is typically constructed to pass through the dam or around it, depending on the dam's design and the surrounding topography.
• The conduit directs the water from the shaft to the river or downstream area below the dam, ensuring that the water is discharged safely and efficiently.

Key Considerations for Selecting a Shaft Spillway Site:

1.     Seismic Action Should Be Small

o    Shaft spillways involve deep underground tunnels that could be vulnerable to seismic activity.

o    In earthquake-prone areas, ground movement can cause cracks, misalignment, or even failure of the structure.

o    Proper geotechnical investigations and seismic design reinforcements are essential if used in seismic zones.

2.     Stiff Geologic Formation Should Be Available

o    A strong, stable rock foundation is required to support the vertical shaft and underground conduit.

o    Weak, loose, or highly fractured rock formations may lead to settlement, leakage, or collapse.

o    If soft soil or unstable geology is present, additional reinforcement or a different spillway type may be needed.

3.     Possibility of Floating Debris Should Be Relatively Small

o    Shaft spillways can be prone to clogging by logs, ice, and other debris.

o    If the reservoir frequently collects floating debris, protective measures such as trash racks, floating booms, or periodic debris removal must be implemented.

o    Blockage can reduce efficiency and cause unexpected water level rises, increasing dam safety risks.

 

Conduit or tunnel spillway

A conduit or tunnel spillway utilizes a closed channel to divert discharge either around or beneath a dam. This enclosed channel may consist of a vertical or inclined shaft combined with a horizontal tunnel or conduit. The spillway's control structure can take the form of an overflow crest, a vertical or inclined orifice entrance, or a side channel crest. The conduit or tunnel is designed to operate partially full to prevent siphonic action, which may occur due to negative pressure within the system. To maintain free flow conditions, the flow area is typically limited to about 75% of the total conduit area. Additionally, air vents are strategically placed at critical locations to ensure sufficient air supply, preventing unsteady flow within the spillway.

Siphon spillway

There are two types of siphon spillways as indicated below.

(i)               Saddle siphon spillway

(ii)             Volute siphon spillway

 

Saddle Siphon Spillway

A saddle siphon spillway, also known as a hood siphon spillway, is designed with an inverted U-tube shape featuring unequal legs. It consists of a reinforced concrete cover called a hood or cowl placed over an ogee-shaped body wall, forming a siphon duct. The crest of the spillway is located at the full reservoir level. The inlet and outlet of the hood are shaped to allow smooth bell-mouth entry and exit. The inlet is submerged below the reservoir level to prevent debris entry, vortices, and drawdowns that could disrupt siphonic action. The outlet is submerged in a cup-like basin or a cistern with a low weir to create a water seal, preventing air from entering the siphon duct.

A deprimer hood is placed above the main hood, connected via an air vent. Its inlet is slightly above the full reservoir level. When water rises above the crest, it submerges the deprimer hood, sealing air entry. As water flows over the crest, air in the siphon duct is gradually sucked out, reducing pressure and creating suction. This suction draws more water, initiating siphonic action, a process called priming. Priming depth, the water level above the crest required for priming, varies based on the priming device and water level rise rate. Gradual water level rise facilitates quicker priming.

Once primed, the siphon continues operating as long as the reservoir level is above the crest. The operating head is the difference between upstream and downstream water levels. When the reservoir level drops, exposing the deprimer hood, air enters the siphon duct, breaking the siphonic action in a process called depriming.

Another variant, the tilted outlet type saddle siphon spillway, is integrated into the dam body. It features a vertical lower limb for natural priming and an upward-tilted outlet to create a water seal, aiding early priming. An air inlet pipe is provided for depriming.

 

Discharge through Saddle Siphon Spillway

Q = CA (2gH)^0.5

Where,  

§  Q = discharge

§  C = coefficient of discharge the value of which may be taken as 0.65

§  A = area of cross-section of throat = L × b, where

§  L = the length of the spillway and

§  b = the height of the throat

§  g = acceleration due to gravity, and

§  H = operating head for the siphon spillway

 

Volute Siphon Spillway

The volute siphon spillway, also called the Ganesh Iyer siphon, is a specialized type of siphon spillway designed by Ganesh Iyer in India. It consists of a vertical pipe (shaft or barrel) with a funnel-shaped opening at the top and a right-angled bend at the bottom, which connects to a horizontal outlet conduit for water discharge.

Key Features:

• The top lip of the funnel is set at the full reservoir level to control overflow.
• Volutes (curved vanes) on the inner funnel surface create a spiral motion in the water, forming a vortex in the vertical pipe.
• A dome with a cylindrical drum is placed above the funnel, allowing an annular space for air movement.
• Small air pipes on the dome act as deprimers, stopping siphonic action when water levels drop.

Functioning:

• As water rises above the full reservoir level, it spills over the funnel lip and follows the volutes, creating a vortex.
• This vortex generates strong suction and vacuum, initiating the siphon action.
• When the reservoir level lowers to full capacity, air enters through the pipes, breaking the vacuum and stopping the siphon.

 

The discharge through a volute siphon spillway is given by

Where,

§  Q = discharge

§  A = area of cross-section of pipe

§  g = acceleration due to gravity

§  H = maximum operating head

§  H_L = head loss through the siphon, and

§  C = coefficient of discharge.

Qualities of a Good Siphon Spillway

A well-designed siphon spillway should possess the following characteristics:

1.     Automatic Operation: It should function automatically without requiring manual intervention.

2.     Quick Priming: The spillway should prime rapidly to initiate siphonic action efficiently.

3.     Low Priming Depth: It should begin siphonic action with only a minimal rise in the reservoir water level.

4.     High Discharge Coefficient: The spillway should have a high efficiency in discharging water.

5.     Smooth Priming and Depriming: The transition between priming and depriming should occur without shock, and the flow should remain steady and continuous once primed.

6.     Vibration-Free Operation: There should be no vibrations in the spillway or the dam when the siphon is running full.

7.     Cost-Effective: It should have low initial construction costs as well as minimal operation and maintenance expenses.

8.     Self-Cleaning: The design should prevent the accumulation of silt or debris in the siphon duct.

9.     Easy Accessibility: All parts of the spillway should be easily accessible for repairs and maintenance.

10. Minimal Protective Works: The cost of protective works in the rear should be kept to a minimum.

11. Simple Design and Construction: The spillway should be straightforward in both design and construction.

12. Consistent Priming and Depriming Depths: The priming and depriming depths should be repeatable, ensuring reliable and predictable operation.

Advantages of Siphon Spillway

Here are the benefits of a siphon spillway compared to other types of spillways:

1.     It operates automatically without requiring mechanical components or moving parts.

2.     It allows maximum discharge within a small range of water level fluctuations in the reservoir.

3.     Due to its higher operating head, it provides a greater discharge per unit length.

4.     It is virtually maintenance-free and ensures a leak-proof system.

5.     The land acquisition cost for areas submerged between the maximum water level and the full reservoir level is minimal.

6.     The required dam height above the spillway crest is comparatively lower.

7.     During flood conditions, it helps remove sediment from the reservoir bed.

8.     Its construction cost is competitive with other spillway types.

Limitations of Siphon Spillway

The limitations of a siphon spillway include the following:

1.     It is ineffective in passing ice and debris.

2.     There is a risk of blockage in the siphon duct and siphon breaker vents due to debris or leaves.

3.     The intermittent priming and breaking of the siphon can cause sudden variations in discharge, leading to significant fluctuations in downstream water levels.

4.     If only one siphon is installed, the reservoir outflow might exceed the inflow. However, a more balanced flow can be achieved by using multiple smaller siphons with siphon breaker vents positioned at varying elevations, ensuring they activate sequentially as the reservoir water level rises.

5.     A robust foundation is necessary to withstand the strong vibrations typically associated with siphon spillways.

6.     Even a minor crack in the cover can disrupt the siphon’s operation by allowing air to enter.

7.     In volute siphon spillways, negative pressure may develop at the bend, potentially causing operational issues.

Labyrinth Spillways

A labyrinth spillway is a type of spillway used in dams and other hydraulic structures to manage and control the release of water. It is characterized by its unique design, which features a series of zigzag or folded walls that increase the effective length of the spillway crest. This design allows for a higher discharge capacity compared to a traditional straight spillway of the same width.

Key Features:

1.     Increased Discharge Capacity: The labyrinth design increases the effective length of the spillway crest, allowing more water to pass through for a given head (water level above the crest). This is particularly useful in situations where space is limited.

2.     Space Efficiency: Labyrinth spillways can be constructed in a relatively compact area, making them suitable for sites where a longer straight spillway would not be feasible.

3.     Hydraulic Performance: The design helps to dissipate energy as water flows over the spillway, reducing the risk of erosion and damage to the downstream area.

4.     Adaptability: Labyrinth spillways can be adapted to various site conditions and can be designed to handle a wide range of flow rates.

 

 

Baffled Chute Spillways

A baffled chute spillway is a type of spillway designed to control and dissipate the energy of flowing water as it moves downstream. It incorporates baffles (obstructions or deflectors) along the chute (the inclined channel) to reduce the velocity of the water and minimize erosion or damage to the spillway and downstream areas. This design is particularly useful in steep terrains where high-velocity flows can cause significant erosion and structural damage.

Key Features of a Baffled Chute Spillway

1.     Baffle Blocks or Deflectors:

o    Baffles are installed along the chute to disrupt the flow of water, creating turbulence and reducing the kinetic energy of the water.

o    These blocks are typically arranged in a staggered pattern to maximize energy dissipation.

2.     Energy Dissipation:

o    The primary purpose of the baffles is to dissipate the energy of the flowing water, preventing high-velocity flows from causing erosion or damage downstream.

3.     Steep Slopes:

o    Baffled chute spillways are often used in steep terrains where traditional spillways might struggle to control the high-velocity flow.

4.     Compact Design:

o    The use of baffles allows for a more compact design compared to other energy dissipation structures like stilling basins.

Advantages of Baffled Chute Spillways

• Effective Energy Dissipation: The baffles significantly reduce the velocity of the water, protecting the downstream area from erosion.
• Adaptability to Steep Slopes: Ideal for sites with steep gradients where conventional spillways may not be effective.
• Cost-Effective: Can be more economical than constructing large stilling basins or other energy dissipation structures.
• Low Maintenance: The design is robust and requires minimal maintenance compared to other spillway types.

Design Considerations

1.     Flow Velocity and Discharge:

o    The design must account for the maximum expected flow velocity and discharge to ensure the baffles can effectively dissipate energy.

2.     Baffle Geometry:

o    The size, shape, and spacing of the baffles are critical to their effectiveness. They must be designed to create the right amount of turbulence without causing excessive splash or overflow.

3.     Slope of the Chute:

o    The slope of the chute must be carefully calculated to balance the flow velocity and the energy dissipation provided by the baffles.

4.     Material Durability:

o    The spillway and baffles must be constructed from durable materials (e.g., reinforced concrete) to withstand the forces of high-velocity water and potential debris.

5.     Downstream Protection:

o    Even with energy dissipation, additional measures (e.g., riprap or aprons) may be needed to protect the downstream area.

 

Image Source: https://www.mdpi.com/