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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