M. RAFI ET AL.
1052
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
chute.
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.
0
22
p
PP
CV
0
In
ip
DD tt
(2)
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
(3)
0
ip
DD
c
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.
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