Journal of Water Resource and Protection, 2012, 4, 1051-1060 Published Online December 2012 (
Modeling the Mangla Dam Spillway for Cavitation and
Aerators Optimization*
Mohammad Rafi1, Akhtar Ali2#, Ghulam Qadir3, Rafaquat Ali1
1Water Resources Division, National Engineering Services of Pakistan (NESPAK), Lahore, Pakistan
2Asian Development Bank, Manila, Philippines
3WAPDA Model Study Cell, Irrigation Research Institute, Irrigation Secretariat, Lahore, Pakistan
Received September 15, 2012; revised October 16, 2012; accepted October 23, 2012
This study evaluated the effects of increased reservoir conservation level by 40 ft (12.2 m), on spillway velocities; it’s
discharging capacity and associated cavitation risk. The study optimized the aerators size and shape to avoid cavitations.
The mathematical model was used to estimate the flow velocities and cavitation risk, when scale model study assessed
the spillway discharging capacity and optimized the performance of the aerators for modified conditions. The mathe-
matical model simulations showed increased flow velocities and damage index for modified conditions. The damage
potential was 2 - 3 times higher with modifications and falls within the major to catastrophic region. The scale model
study showed that discharging capacity of the spillway can effectively be restricted to original design by raising spill-
way crest by 5.0 ft (1.52 m). The scale model study also showed that the two aerators near sluice and at the chute with
an air duct pipe of 3.0 ft diameter can improve the free surface flow profile reducing the risks of cavitation. Simulations
for several configurations demonstrated clearer affect of aerators ramps on flow trajectory and gate opening. It also de-
picted that the height of the ramp of sluice aerator has a positive effect on the flow performance to about 7.5 inches (19
cm), when further increase in the ramp height reduced the flow performance.
Keywords: Spillway; Model Studies; Discharging Capacity; Cavitation Risk; Aerators Optimization
1. Introduction
High flow velocities can induce cavitations and cause
serious damages to the spillways of high dams. Forma-
tion of flow bubbles indicates spillway surface deforma-
tion [1]. Increasing flow velocities and as a result de-
creasing pressures may pass through a critical value ini-
tiating cavitation, which is known as incipient cavitation.
Conversely, decreasing velocity resulting in increasing
pressure may arrive to a point to disappear cavitation and
is called desinent cavitation. Surface roughness of spill-
way floor and water impurities aggravate cavitations,
accelerate damages and can result is spillway failure.
Interactions between the flowing water and the at-
mosphere may lead to significant air-water mixing and
complex multiphase flow situation [2,3]. The cavitation
can be prevented either reducing the flow velocity or
increasing the flow pressure or with combination of both.
The studies on the effect of variable spillway width and
invert curvature on the flow pressure for Amaluza dam
spillway in USA indicated that dispersion of a small
amount of air through water prism can significantly re-
duce for the risks of cavitation damage [4]. It was found
that about 7.5% of air by volume was needed to stop
cavitation damages in a 28-day concrete surface with a
compressive strength of 17 mega-Pascals [5]. The re-
quired air quantity to protect a spillway surface from
cavitation increases with decrease in surface strength [6].
Application of aerators to prevent cavitation damage was
successfully tested for Grand Coulee Dam in USA [7].
Bottom aerators are provided when natural aeration of
the high velocity spillway chute does not satisfy the
minimum air concentration requirements to develop posi-
tive pressures. The aerators for the first time were tested
at Yellowtail dam following high discharges in 1967 [8].
The minimum air concentration is function of Froude
Number [9].
90 min0.015 2
*Disclaimer: The ideas and finding presented in this paper are of the
authors and do not necessarily reflect the views and policies of the
organizations, they belong.
#Corresponding author.
where 90 min is minimum air concentration, F0 inflow
Froude number and FCmin is Froude number at the incep-
tion point.
opyright © 2012 SciRes. JWARP
Mangla dam reservoir in Pakistan has a gross storage
capacity of 5.88 millions AF (7.25 km3), it supplies irri-
gation water to over 4 million ha and can generate up to
1000 mega watt of electricity. The dam’s reservoir area
of 100 mi2 (160 km2) creates a live storage capacity of
5.34 millions AF (6.58 km3). Since its inception in 1967,
the reservoir sedimentation has reduced its storage ca-
pacity by 20% or 1.15 MAF (1.42 km3). The reduced
water storage implicated water shortages for irrigation
and hydropower. The studies indicated that increasing
the dam height by 30 ft (9.5 m) and the reservoir conser-
vation level (RCL) by 40 ft from 1202 to 1242 ft is pos-
sible and it can refurbish the lost capacity [10]. Never-
theless, raising the dam height and RCL can increase the
spillway discharge, discharge intensities and flow veloci-
ties, which may cause spillway cavitations and structural
damages. This study 1) checked the effect of raised dam
height and the reservoir conservation level on the spill-
way discharge, discharge intensities and velocities through
hydraulic design computations; 2) tested the effect of
different gate openings and reduced orifice areas on
spillway discharge, discharge intensities and velocities
on a scale model; 3) assessed the cavitation risk due to
increased velocities by using a mathematical model and 4)
optimized the size and shape of the bottom aerator for
reduced cavitation risk by using the scale model.
2. Material and Methods
2.1. Mangla Dam Spillway
The Mangla dam embankment is 380 ft (115.83 m) high
above river bed and 10300 ft long (3140 m) long. It was
proposed to raise the embankment height by 30 ft (9.14
m) and pool conservation level by 40 ft (12.20 m). The
dam spillway is orifice type headworks, two-stage still-
ing basin and sloping side walls. The headworks of the
main spillway are 444 ft (135.33 m) long. It consists of
three monoliths separated by 24 ft (7.3 m) wide piers.
Each monolith comprised three orifices of 36 ft (10.97 m)
width and 40 ft (12.2 m) height, which are equipped with
radial gates. Each orifice within the monoliths is sepa-
rated by 12 ft (3.66 m) wide pier. Parabolic chute follows
the headworks crest. An intermediate weir divides the
chute into two and creates a stilling basin and water pool
at an elevation of 1000 ft (304.8 m). The spillway plan
and the longitudinal sections are in Figure 1.
2.2. Spillway’s Hydraulic Design
In the original design, the probable maximum flood (PMF)
discharge was fixed as 1.01 million ft3·sec–1 (28,600
m3·sec–1) and the discharge intensities over the upper and
lower chutes were fixed as 2275 ft3·sec–1·ft–1 (211.4
m3·sec–1·m–1) and 1443 ft3·sec–1·ft–1 (134 m3·sec–1·m–1),
respectively. The hydraulic design computations showed
that the raised dam height and RCL may increase the
maximum discharge through the existing spillway to 1.31
million ft3·sec–1 (37,095 m3·sec–1)—27% higher than the
original design discharge and corresponding increase in
discharge intensities and flow velocities. The design con-
sidered reducing the orifice area to restrict the spillway
discharge and discharge intensities within the original
design limits. The hydraulic computations showed that
raising the floor level of the spillway crest by 5 ft (1.524
m) from 1086 to 1091 ft can reduce the orifice area to
control the spillway discharge to original design limits.
Therefore, the hydraulic design suggested raising the
invert level by 5 ft to the end of gate piers with 2 ft high
ramp at an angle of 10 degree from end (Figure 2). This
modification1 may have only reduced the spillway dis-
charge to the original design limit, but not the flow ve-
locities, as the velocities are function of total head across.
Hydraulic design computations for the proposed modifi-
cations indicated that the flow velocities are likely to
exceed from original designed velocity of 100 ft·sec–1
(30.48 m·sec–1). This increase in velocity could induce
cavitation risk in the sluice bays and on the parabolic
2.3. Mathematical Model and Cavitation Risks
The cavitation risk due to increased flow velocities along
the spillway chute were assessed by using a mathematic-
cal model-USBR-EM42 [1]. The cavitation risk to a hy-
draulic structure is function of flow velocities, hydrody-
namic pressures and surface irregularities. Mathemati-
cally, the pressure coefficient (Cp)—basis for the cavita-
tion index, can be derived from the Bernoulli equation
for conditions that reference elevation is equal to the
elevation in question.
DD tt
where P and P0 pressure intensity and reference pressure
and Vo is reference velocity considering elevation differ-
ence is negligible. The pressure coefficient or pressure
parameter is also referred as Euler number. Damage in-
dex was assumed as a quasi-quantitative measure of the
severity of the cavitation damage as a function of dis-
charge and time. It can be used to differentiate between
minor and major damages. The model computes the cav-
ity damage index from
tte (4)
where, tc is cumulative time of operation, Di is damage
1The overall modifications include raising the dam height and reservoi
conservation level, raising the spillway invert level by 5 ft and intro-
duction of bottom aerator.
Copyright © 2012 SciRes. JWARP
Copyright © 2012 SciRes. JWARP
Figure 1. Plan and longitudinal section of Mangla spillway.
Figure 2. Sectional model of modified spillway and sluice and chute ae r ators.
Copyright © 2012 SciRes. JWARP
Copyright © 2012 SciRes. JWARP
index at end of previous discharge, Dp is damage potential
for next discharge. The integrating constant to allows the
equation to incorporate the cumulative effect of flow at
various discharges. At the start of operation, Di = 0 at tc=
0 and t0 = –1. Damage potential Dp in Equation (2) was
computed from
˗ Discharge for variable gate openings (5, 10, 15, 20, 25
and 30 ft); with reservoir level maintained at 1242 ft.
˗ Discharge for variable gate openings (5, 10, 15, 20, 25
and 30 ft); with reservoir level maintained at 1260 ft.
Francis formula was used to compute discharge from
the observed water levels on the weir.
 (5)
where, σs is cavitation index for initiation of damages, σf
is cavitation index of the flow and V and Vr are flow and
reference velocities, respectively. The model sets cavita-
tion index “σ” equal to pressure coefficient with minus
sign in Equation (3) (σ = –Cp).
The spillway chute was divided into 22 sections lon-
gitudinally created by 23 station points to determine the
locations of highest flow velocity and cavitation. Dam-
age potential and damage risks were assessed for incipi-
ent, major and catastrophic damages from the criteria
given in Table 1.
2.4. The Scale Model
The scale model was built and operated at Hydraulic Re-
search Station, Nandi Pur, Pakistan. The scale model was
used to verify the spillway discharge and velocities and
to test the various options of the bottom aerators to re-
duce cavitation risk. A sectional model of the main
spillway was constructed on an undistorted scale of 1:36
(Figure 2). It comprised of 2 bays—one full central bay
and two half bays on either side of the central bay. The
model included a portion of the reservoir, proportionate
central part of the approach channel, headworks, radial
gates, hoisting arrangement, parabolic chute and upper
stilling basin and weir. The model was fabricated in
transparent Perspex in order to minimize the frictions and
facilitate visual observation.
A suppressed sharp-crested rectangular weir of 9 ft
(2.74 m) length and 5 ft (1.52 m) height was constructed
at immediately downstream of the sectional model to
determine the stage-discharge relationship of the modi-
fied spillway (with 5 ft rise in invert level) for the fol-
lowing conditions.
˗ Discharge for different reservoir levels with fully
opened gates (35 ft opening means full open gate).
Table 1. Damage potential and damage index criteria (after
Falvey, 1990).
Damages Damage potential Damage index
Incipient 500 5000
Major 1000 10,000
Catastrophic 2000 20,000
32 32
0.171 aa
where, Q is discharge in m3·sec–1, L is length of the weir
in m; H is head on weir crest in m and ha is approach
velocity head in m.
A scale of 1:36 was used for geometric similitude be-
tween the model and the prototype. In free surface flows,
most laboratory studies are based on a Froude similitude
since gravity effects are important [11,12]. The same
concept was used in this study, but due to scale effect, it
may not be able to achieve true dynamic similarity. In
fact in geometrical similarity models, it may not be pos-
sible to satisfy simultaneously Froude and Reynolds
similarities unless at full scale [13]. Froudian equations
represented the mathematical relationship between di-
mensional and hydraulic quantities of the model and
F (7)
 (8)
where Fr is Froude number, V is velocity, g is gravita-
tional force and h is head of water column. Subscripts M
and P represents model and prototype respectively. The
relationships for the transference of model data to proto-
type equivalents are given Table 2.
2.5. Aerators Optimization
The aerators entrained air into the flow through side
ducts due to the pressure difference. This arrangement
generally functions effectively except for submergence
Table 2. Mathematical relationships for dimensional and
hydraulic quantities.
Dimension Ratio
Time 12
Velocity 12
Discharge 52
Roughness (Manning’s n) 16
conditions, which may reduce the aerators effectiveness.
The aerators in this study consist of a ramp, circular air
duct under the horizontal floor and an air supply gallery.
The ramp was likely to create sub-pressure region by
lifting up the high velocity water jet above the chute
floor. The scale model study optimized the aerators pa-
rameters, evaluated the air entrainment and checked the
performance of the spillway with modified design. Two
aerators at sluice gate and end of horizontal floor were
tested for several combinations including pipes of 2.25 ft
(0.70 m) and 3 ft (0.91 m) diameters and for different
ramp heights (Table 3). The pipes connected the air duct
with the 5 ft (1.52 m) high vertical step at the raised floor
end. The flow velocity on the model was measured by
using Kempton probes. Graduated staff gauges were used
to measure the water levels at different sections of the
model. Validyne transducers were used to measure the
pressures that recorded magnified signals on strip chart
3. Results
3.1. Spillway Discharge Capacity
The spillway stage-discharge relationship at full gate open-
ing and with modifications was superimposed on the
original spillway rating determined in 1967 at the time of
the dam construction (Figure 3). It shows that the stage-
discharge relationship in both the cases with and without
modifications reasonably match and falls on the pro-
jected original rating. Results showed that discharging
capacity of the modified spillway was within the design
limits of its original capacity at maximum pool level of
1260 ft and it was about 0.16% higher at modified reser-
voir conservation level of 1242 ft (Tabl e 4 ). Insignificant
increase in discharging capacity of the modified spillway
at the reservoir conservation level was less likely to
negatively impact the hydraulic performance of the en-
ergy dissipaters. The results based on scale-model study
indicated that the proposed raising of the crest level by 5
ft can effectively curtail the spillway discharging capac-
ity to its original design level.
3.2. Cavitation Risk
Mathematical model study showed that velocities vary
from 100 ft·sec–1 to 125 ft·sec–1 (30.48 to 38.1 m·sec–1)
for different discharges at the lower part of the chute near
the toe under existing conditions. It showed a velocity up
to 138 ft·sec–1 for a discharge of one million ft3·sec–1
with modified conditions. This velocity is most likely to
create cavitation. The logarithmic equations with an r2
value of 0.95 reasonably fit to the discharge velocity re-
lationship for both existing and modified conditions
(Figure 4). However, the trend lines show scatter be-
tween –5% and 18% under existing conditions and be-
tween –5% and 23% for modified conditions. The cavita-
tion risks were estimated by damage potential and dam-
age index under existing and with modifications. It
shows that damage potential for existing spillway were
below 600, which sharply increases up to discharge of
Table 3. Aerators parameters tested on the scale model for gate opening of 5, 10, 15, 20, 25, 30 and 35 ft (gates fully open).
Run no Chute aerator Sluice aerator
Base length = 11.3 ft (3.44 m) Base length = 3.0 ft (0.91 m)
Air duct pipe diameter = 2.25 ft (0.68 m)
Run 1: base case No ramp No ramp
Run 2 Ramp of 2 ft height and 10˚ back slope No ramp
Air duct pipe diameter = 3.0 ft (0.91 m)
Run 3 Ramp of 2 ft height and 10˚ back slope Ramp of 3.6 inches and 5.7˚ back slope
Run 4 Ramp of 2 ft height and 10˚ back slope Ramp of 5.5 inches and 8.68˚ back slope
Run 5 Ramp of 2 ft height and 6˚ back slope Ramp of 5.5 inches and 8.68˚ back slope
Run 6-1 Ramp of 2 ft height and 10˚ back slope Ramp of 6.5 inches and 10.2˚ back slope
Run 6-2 Ramp of 2 ft height and 10˚ back slope Ramp of 7.5 inches and 11.77˚ back slope
Run 6-3 Ramp of 2 ft height and 10˚ back slope Ramp of 8.5 inches and 13.28˚ back slope
Run 6-4 Ramp of 2 ft height and 10˚ back slope Ramp of 10 inches and 15.52˚ back slope
Run 6-5 Ramp of 2 ft height and 10˚ back slope Ramp of 12 inches and 18.43˚ back slope
Run 7 No inverse slope Ramp of 7.5 inches and 11.77˚ back slope
Copyright © 2012 SciRes. JWARP
M. RAFI ET AL. 1057
Table 4. Comparison of spillway discharges with and with-
out modifications
Damage potential (D
Discharge (ft3/ s ec)
1242 936,027 937,513 +0.16
1260 1,010,000 1,009,630 –0.036
Existing DpModified Dp
Poly. (Existing Dp)Poly. (Modified Dp)
The original discharge is based on model study in 1967; and modified on the
basis of model study in 2004.
Discharge (ft
Reservoir levels (ft)
Poly. (Original)
Figure 3. Spillway rating curve with and without modifica-
Velocity (ft/sec)
Discharge (ft3/ sec)
Existing V22Modified V22
Log. (Existing V22)Log. (Modified V22)
Figure 4. Comparison of chute velocities for existing and
modified spillway at 22 ft from crest.
350,000 ft3·sec–1 and flattens down with almost horizon-
tal slope with the further increase in discharge beyond
350,000 ft3·sec–1 (Figure 5). However, the damage po-
tential for modified conditions sharply increases to about
1400 to a discharge of 350,000 ft3·sec–1 and then flattens
down afterwards. It also shows that the damage potential
for modified conditions is more than double the damage
potential under existing condition. Comparing with dif-
ferent cavitation damage levels in Table 1, the damage
potential under modified conditions was between major
Figure 5. Damage potential in relation to spillway dischar-
ges without and with modification scenarios.
and catastrophic levels. Relationship between damage
index and spillway discharge also shows relatively higher
risks of cavitation in case of modifications (Figure 6).
Both analysis for damage potential and damage index
infer that proposed modifications are most likely to cause
cavitation problem. It necessitated for testing the provi-
sion of appropriate aerators to avoid cavitations.
3.3. Optimizing the Aerators
The scale model tested two aerators at sluice gate and
end of parabolic chute for modified pool conservation
level of 1242 ft (378.6 m). The optimization included
two diameters of air supply duct, without ramp and with
different configurations of the ramps.
Run 1: Air duct dia 2.25 ft, both aerator without
ramp (Base trial)
A pipe of 2.25 ft diameter served as air duct. The
model was operated at reservoir conservation level for
gate openings varying from 5 to 35 ft. The results with 5
ft gate opening showed low pressure air pocket under the
flow jet emerging from the gate and the sluice aerator
drew adequate air with hissing noise. Low pressure air
cavity was also noted at the chute aerator. At 10 ft gate
opening, the length of air pocket was reduced and some
back flows were accumulated. Increased flow depth and
back flows caused reduction in air supply to sluice and
chute aerators. At 15 ft gate opening, further increase in
back flows and flow depth reduced the air supply tre-
mendously and the sluice aerator was almost ineffective.
Nevertheless, flow jet drew some air on the spillway
chute showed sign of poor functioning of the chute aera-
tor. At 20 ft gate opening, piling up of back flow at the
tail end of the air duct reduced the air supply to insig-
nificant level and water almost filled the air pocket at the
location of chute aerator. The results showed inefficacy
of the aerators arrangement in supplying the air. Specifi-
cally, the chute aerator was ineffective to supply air to
the duct.
Copyright © 2012 SciRes. JWARP
D amage in dex (D i)
Discharge (ft3/ sec)
Existing DiModified Di
Log. (Existing Di)Log. (Modified Di)
Figure 6. Damage index in relation to spillway discharges
without and with modification scenarios.
Run 2: Air duct dia 2.25 ft and chute aerator with
ramp of 2.0 ft height &10˚ back slope
The chute aerator was modified to improve the air
supply by providing a ramp of 2 ft height and a back
slope at an angle of 10 degree at the end of raised floor.
The model was run at the pool conservation level and the
gate opening varied from 5 to 35 ft. The results showed
better air flow and longer flow trajectory at the end of
raised floor for 5 ft gate opening. At 10 ft gate opening,
the length of the air cavity on the chute was reduced, but
no back flow was noted. At 15 ft gate opening the size of
the air cavity was further reduced, but the air pocket at
the end of the raised floor was still effective in drawing
the air. At 20 ft gate opening, there was more reduction
in air cavity and accumulation of back flows also started.
At 25 ft gate opening, accumulation of back flow was
increased and the length of flow trajectory tremendously
reduced, but the trajectory was still instrumental to
pushing some air on the top of the parabolic chute. At 30
ft gate opening, the air cavity at the end of raised floor
was almost filled with the back flows. Nevertheless, the
modified ramped aerator was still able to draw some air.
The results from run 2 showed improved air supply to the
parabolic chute as compared with run 1.
Run 3: Air duct dia 3.0 ft and chute aerator with
ramp of 2.0 ft height and 10˚ back slope and sluice
ramp of 3.6 inches and 5.7˚ back slope
In this run, the diameter of the air supply duct was in-
creased from 2.25 ft to 3.0 ft. A ramp of 3.6 inches (back
slope 1:10 or 5.7 degree) in the sluice aerator and 2.0 ft
in the chute aerator with 10 degree back slope were also
provided. The results indicated that the increased pipe
diameter enhanced the air supply to the air duct of the
sluice aerator at all gate openings. The length of flow
trajectory at the end of raised floor was increased from
30 ft with 2.25 ft diameter pipe (run 2) to 33 ft with 3.0 ft
diameter pipe at 5 ft gate opening. Further, air suction
limit of the sluice aerator was increased from 10 ft gate
opening in case of 2.25 diameter pipe to 12 ft gate open-
ing for 3.0 ft diameter air duct. This proved superiority of
3.0 ft diameter air duct over 2.25 ft diameter air duct.
However, it was almost ineffective at gate opening more
than 12 ft.
Run 4: Air duct dia 3.0 ft and chute aerator with
ramp of 2.0 ft height and 10˚ back slope and sluice
ramp of 5.5 inches and back slope of 8.68˚ (1 :1 0)
In this run the ramp height of the sluice aerator was in-
creased from 3.6 inches to 5.5 inches increasing the back
slope of the ramp from 5.7 to 8.69 degree, when all other
features remained same as in case of run 3. The results
showed the length of air cavity as 33 ft, when effective
gate opening increased from 12 to 16 ft. Raising the ramp
height of the sluice gate did not show negative effect on
the discharge rating of the spillway.
Run 5: Air duct dia 3.0 ft and chute aerator with
ramp of 2.0 ft height and 6˚ back slope and sluice
ramp of 5.5 inches and back slope of 8.68˚)
In run 5, the ramp slope of the chute aerator was
changed from 10 to 6 degree with horizontal, when all
other features remained same as in case of run 3. The
results of this run for same set of flow conditions showed
remarked reduction in the length of air cavity as com-
pared with ramp slope of 10 degree (Table 5). The re-
sults showed disadvantages of changing the ramp slope;
therefore, further tests were not conducted with 6 degree
ramp slope and the 10 degree ramp slope was found
more suitable (Run 4). This indicated that the perform-
ance of chute aerator was at maximum for parameters set
in run 4. Therefore, further simulations focused to im-
prove the performance of the sluice aerator, keeping
other parameters constant.
Run 6: O ptimizing the ramp of sluice aerato r, when
keeping the air duct dia 3.0 ft and chute aerator with
ramp of 2. 0 ft h eight and 1 0˚ back slope
The simulation from run 1 to run 5 revealed that in-
crease in ramp height of the sluice aerator from 3.6
inches to 5.5 inches improved the effective gate opening
from 12 to 16 ft. It showed potential to further manure it.
Therefore, a number of simulation runs were made by
increasing the ramp height to 6.5, 7.5, 8.5, 10 and 12
inches without changing the base length of 3.0 ft. The
results of these simulations are given in Table 6. Rela-
tionship between the ramp height, effective gate opening
and discharge is in Figure 7.
Table 5. Comparison of air cavity for different ramp slopes
at the chute.
Gate opening (ft) Length of air cavity (ft)
10 degree slope 6 degree slope
5 131 93
10 143 100
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Table 6. Results of optimization of the ramp height of the sluice aerator.
(Degree) Observations on model
6.5 10.23
˗ Cavity length immediate below the spillway gates was increased from 40 ft for 5.5 inch ramp to 54 ft with 6.5 inch
˗ Effective gate opening at which the aerator sucked the air was increased from 16.0 to 19.75 ft.
˗ The aerator submerged and became ineffective at 20 ft gate opening.
˗ The change in ramp height did not affect the maximum discharge outflow at the highest pool level of 1260.
7.5 11.77
˗ Cavity length downstream of spillway gates was increased 62 ft.
˗ Effective gate opening for air sucking increased to 22.75 ft, which can release a discharge of 475,000 ft3·sec–1 at
conservation pool level of 1242 ft.
˗ The change in ramp height did not affect the maximum discharge outflow at the highest pool level of 1260.
8.5 13.28 ˗ No significant improvement noted when effective gate opening changed from 22.75 to 23.25 ft.
10.0 15.52
˗ Cavity length downstream of spillway gates was increased 67 ft.
˗ Effective gate opening for air sucking increased to 24.25 ft, which can release a discharge of 510,000 ft3·sec–1 at
conservation pool level of 1242 ft.
˗ Increased back flow along the bed caused pulsation of the flow jet in the sluice bays. It adversely affected the
discharging capacity of the spillway.
12.0 18.43
˗ The flow jet landed on 2 ft high ramp at the end of the chute aerator resulting in high splashes. Pulsating of the
flow jet was increased. Outflow discharge capacity of the spillway reduced by 7755 ft3·sec–1. Therefore, it was not
acceptable scenario.
ramp was worse in function and reduced the spillway
discharging capacity by 7756 ft3·sec–1 at maximum res-
ervoir level. The above discussion reveals that a sluice
ramp of 7.5 inches with an angle of 11.77˚ performed
better of all the scenarios and appears to be the appropri-
ate ramp height for sluice aerator.
3.6 5.56.5 7.58.510
Discharge (ft
/ sec)
Ramp height (Inches)
Effective gate opening
Poly. (Discharge)
Poly. (Effective gate ope
Effective gate open ing (ft)
Run 7: Sluice aerator with inverse slope eliminated
Elimination of inverse slope (1:20) and provision of
horizontal floor at downstream of sluice aerator at an
elevation of 1091 ft, with all other optimized parameters
resulted in increased cavity length, but decreased cavity
depth, small improvement in effective gate opening and
non-uniform distribution of sucked air along full width of
the bay. It indicates that inverse slope performed better in
cavity depth and is instrumental in uniform air distribu-
tion along the bay width.
Figure 7. Optimization of ramp height of the sluice aerator
in relation to spillway discharge and effective gate opening
(Air duct 3.0 ft diameter). 4. Conclusions
Five feet rise in the bed of the headworks bay from
1086 to 1091 ft effectively restricted the maximum
spillway discharge to its original design capacity limit
of 1.01 million ft3·sec–1 at the maximum reservoir
level of 1260 ft. This rise did not significantly change
the flow conditions of the spillway.
The observations on the scale model have shown that
an increase in sluice ramp height increased the limit of
effective gate opening. Flow conditions in the sluice bays
within the piers remained smooth and stable to ramp
height of 7.5 inches. Pulsation of flow jet started at ramp
height of 8.5 inches and above. For 5 ft gate opening and
10 inches ramp height, the flow jet landed on the slope of
2.0 ft high ramp at chute aerator and increased splashing.
The ramp height of 10 inches increased the heaving and
pulsating in the headworks bays and reduced the dis-
charging capacity of the spillway. However, 12 inches
Mathematical model study depicted high cavitation
risks and damage potential for modified spillway and
showed a need for introduction of aerators to avoid
Scale model study indicated that a chute aerator of 2.0
ft height and an angle of 10˚ was highly effective in
inducing air over the parabolic chute up to gate open-
ings of 25 ft. However, the air suction was reduced at
the gate opening of more than 25 ft.
Among the several ramp heights and slope of the
sluice aerator, an aerator with ramp height of 7.5 inches
performed best of all. It did not negatively affect the
spillway discharging capacity at the conservation level of
1242 ft and maximum reservoir level of 1260 ft. The
sluice aerator with less ramp height (3.6 and 5.5 inches)
failed to lift the flow jet and submerged at 10 ft gate
opening and above. On the other hand high ramp height
for example 10 and 12 inches increased the heaving and
pulsation in the bay and reduced its discharging capacity
significantly. It revealed that 7.5 inches was the optimum
ramp height for the sluice aerator.
5. Acknowledgements
The authors acknowledge the technical support by Mangla
Joint Venture-Mangla Dam Raising Project, Pakistan,
assistance by the technical staff of the Hydraulic Re-
search Station Nandi Pur and financial support by the
Water and Power Development Authority, Pakistan.
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