Chapter 6
Structure and Layout
6.1 General
The Krishna (Almatti) – Pennar link canal is proposed to off-take from the
right flank of the Almatti dam in Karnataka State. As the Almatti dam is
proposed to be used without any modification to the structure, a brief
description of the design features of the head works of the Almatti project,
as given in the project report, is presented here. However, detailed
surveys of the proposed link canal were carried out by NWDA and based on
the surveys, the designs of the canal and other CD/CM works have been
carried out and are presented in this Chapter.
6.2 Structure, Layout and Design Features of the Head
Works
6.2.1 Location of the Head Works
The Almatti dam is an ongoing project on Krishna River just downstream of
its confluence with Ghataprabha River near Almatti village in Bagewadi
taluk of Bagalkot district in Karnataka State. The construction of Almatti
dam has been taken up by the Government of Karnataka in two stages.
The link canal is proposed to off-take from Almatti reservoir. The FSL of the
canal at off-take is proposed to be 510.00 m. A new reservoir is proposed
at RD 386.400 km on river Pennar to serve as a balancing reservoir. The
existing Bukkapatnam tank at RD 536.200 km across Chitravati River is also
proposed to be utilised as a balancing reservoir.
6.2.2 Components of the Head Works
a) Almatti Dam
The Almatti dam comprises of a concrete spillway in the centre and
masonry non-overflow section as well as earthen dams on either flank. The
FRL of reservoir is 519.6 m. The total length of the dam is 1564.83 m and
its height is 52.24 m. The gross storage capacity of the reservoir is 3439.7
Mm3 and the live storage 3104.70 Mm3.
Length of the spillway of Almatti dam is 486.50 m. The non-overflow dam is
of length 676.33 m and the earthen dam is of 402.00 m including dykes.
There will be regulated release of water from Almatti dam to be picked up
downstream at Narayanpur dam for diversion into the left bank canal.
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b) Link Canal Off-take
The link canal is proposed to off-take from the right bank of Almatti Dam.
The required discharging capacity of the link canal has been worked out to
be 230 cumec. A powerhouse is proposed at the head of the link canal.
The installed capacity of the powerhouse has been estimated to be 13.5
MW and it is proposed to install 3 generating units of 4.5 MW each. The link
canal off-takes from 200m upstream of right flank of the dam. The length
of approach channel is about 700m followed by 1125 m long tunnel. A head
regulator is proposed at the off-take point.
The link canal is proposed to be operated for 6 months i.e., from June to
November. The withdrawal pattern of link canal varies from peak demand
of 230 cumec during August to lean demand of 30 cumec during June. The
reservoir simulation study conducted reveals that the reservoir level
fluctuates from 519.60 m to 514.49 m during June to November.
c) Kalvapalli Balancing Reservoir
The Kalvapalli balancing reservoir is proposed across Pennar River in
Anantapur district. The FRL of the reservoir will be 475 m with gross
storage capacity of 83 Mm3 and live storage capacity of 73 Mm3. NWDA has
carried out a detailed topographical survey of the reservoir. The proposed
link canal falls into Kalvapalli reservoir at RD 386.400 km with its FSL at
475.606 m and again off-takes from right bank of the reservoir with FSL
463.000 m. The inflow into the reservoir from its own catchment is fully
earmarked for uses downstream of the reservoir and will not be diverted
into the link canal. The dam is a composite dam with 2825 m earthen
portion and 400 m masonry. The non-spillway section is for 300 m and
spillway for 100 m in masonry portion. A head regulator for releasing water
to the link canal has been provided with two openings of 2 m x 3 m each.
The sill level of the regulator is kept at the DSL of 466.00 m. Since the bed
level of the canal at the off-take from Kalvapalli dam is 459.4 m, there will
be a fall of 6.6 m. Hence, a stilling basin is provided at the outlet for
dissipation of energy before water is let into the canal.
d) Bukkapatnam Tank
The Bukkapatnam tank is an existing tank across Chitravati River to store
15.30 Mm3 of water near Kothacheruvu village, Bukkapatnam Mandal in
Anantapur district. The tank is formed by an earthen bund 5.60 km long.
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A free over fall masonry weir is located on the left flank and 3 weirs at right
flank to discharge a maximum flood of 566 cumec. There are four sluices
located in earthen dam to feed eight canals. The FTL of the tank is
448.07m.
The proposed Krishna (Almatti) – Pennar Link canal outfalls into
Bukkapatnam tank on its left bank at RD 536.200 km with FSL 449.766 m
and again off-takes from the right bank of the tank with FSL 440.00 m.
Thereafter the link canal traverses in north direction through the Kottakota
reserved forest.
6.3 Design Features of Link Canal
6.3.1 Description of the Canal System
The Krishna (Almatti) – Pennar link canal off-takes from right bank of the
Almatti dam on river Krishna with FSL 510.00 m. The general topography
of the area through which the link canal passes through is mostly plain.
Five tunnels with a total length 35.66 km are proposed along the alignment
to cross the ridges. The canal for most of its length runs in south east
direction.
6.3.2 Utilisation of Water Potential from the Streams Crossed by
the Canal
Various streams and rivulets crossed by the Krishna (Almatti) – Pennar link
canal are not perennial. The yields are undependable and the streams are
prone to flash floods. One balancing reservoir is proposed across the river
Pennar near Kalvapalli. The inflow from its catchment is earmarked for
downstream uses. Therefore, augmentation of the flows in the link canal
from the waters available in the streams crossed by the canal is planned.
6.3.3 Description of Soil Profile along the Canal Alignment
The details of sub-surface strata have been collected by the CSMRS by
excavating open trial pits along the canal alignment. A total of 44 samples
have been collected. The trial pits were made where the bed level of the
canal is up to 3 m from the NSL. The reaches where the depth is more
than 3 m geophysical investigation was carried out.
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6.3.4 Evaluation of the Design Parameters based on Samples
Collected Enroute
The canal alignment generally runs in partial cutting and embankment. The
soils available from cutting and adjoining borrow areas are found to be
suitable for embankment purposes. When the canal runs in embankment
higher than 8 m, the soils available in the adjoining fields are proposed to
be used for formation of the embankment.
6.3.5 Lining
100 mm thick concrete (CC 1:3:6) lining is proposed for both bed and sides
throughout the length of the canal.
6.3.6 Design Calculations for Adequacy of Canal Section
(a) Formulae used
The canal sections for various reaches are designed by using
Manning’s formula for velocity
V = (1/n) R 2/3 S ½ where
V = Velocity
n = Rugosity co-efficient
S = Bed slope
R = Hydraulic mean depth (A/P)
A = Area of cross section = bd + d2 (θ + cot θ)
P = Wetted perimeter = b + 2d (θ + cot θ)
b = bed width
d = depth of water
θ = Angle of the side slope
The critical velocity for the canal is given by Vo = 0.55 d0.64
A rugosity co-efficient of 0.018, side slope of 1.5 horizontal to 1 vertical and
bed slope of 1 in 20000 up to Kalvapalli dam, 1 in 12000 from Kalvapalli to
Bukkapatnam and 1 in 15000 from Bukkapatnam to tail end have been
adopted. Different bed slopes for three different reaches have been
adopted considering the natural soil level to avoid high embankments and
deep cutting.
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(b) Design of Canal Section in Various Reaches
The peak discharge required at the head of the canal is worked out based
on crop water requirement. The design discharge is considered to be
greater of the following:
i) 1.1 times the peak discharge and
ii) 1.25 times the average discharge
The design discharge at the head thus calculated is 230.00 cumec, against
the actual peak discharge.
As the canal advances from the Almatti dam, the discharge in the canal
gets reduced at different reaches due to drawl of water to meet the
irrigation requirement of enroute command. This necessitates reduction in
canal section in order to affect economy. Hence, the canal is broadly
divided into suitable hydraulic reaches and sections are designed to carry
the required discharge in that particular reach.
The entire canal is divided into six reaches based on the following
consideration.
i) Downstream of major off-take points
ii) Considerable reduction in the discharge
The velocity at different reaches is in the range of 1.02 to 0.740 m/sec.
The full supply depth provided is 5.25 m in the head reach and 3.00 m in
the tail reach. A free board of 0.75 m above FSL is provided throughout
the length of the canal.
6.4 Canal Structures
6.4.1 Cross Drainage Works
As the link is aligned as a contour canal, it crosses several streams, rivers,
roads and hills. The type of cross drainage work depends upon the HFL
and bed level of the stream with reference to full supply level and bed level
of the canal at the crossing. In its entire length of 587.175 km of the link
canal, 99 cross drainage works are provided, which comprise of 21
aqueducts, 30 super passages and 39 canal syphons and 9 syphons
aqueducts. In addition, 31 under tunnels are provided.
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6.4.2 Cross Masonry Works
The link canal crosses a number of roads and railway lines enroute. The
location of road / rail bridges has been ascertained from topographical
surveys. A total of 97 road bridges, 5 nos. of cross regulators and 2 nos.
escape regulators have been provided.
6.4.3 Tunnels
The link canal along its entire length of 587.175 km, pass through 5 tunnels
for a total length of 35.66 km. Shortest tunnel is of 1.125 km and longest
is of 20 km length. Altogether there are 220 structures. Details are given in
Table 6.1
Table 6.1
Number of structures
Type of structure Numbers
Head works 3
CD works 99
CM works 113
Tunnels 5
Total 220
6.4.4 Power House
It is proposed to generate power from the discharge of link canal at the exit
of tunnel no.1 at the off-take point from Almatti reservoir. An approach
channel of 700 m long and 32.00 m wide off-takes from the foreshore of
Almatti dam, followed by a tunnel of 1.125 km and 9.0 m dia. A fore bay of
12 m long and 63.6 m wide is provided at the exit of tunnel. A reinforced
concrete intake structure consisting of 6 nos. of vents of 2.0 m x 5.0 m is
provided at the end of the intake fore bay to feed turbines at 2 vents for
each turbine. Three nos. of turbines of 4.50 MW each has been proposed.
The details are given under Chapter on ‘Power’.
6.4.5 Head Loss at Different Structures
Head losses due to various CD/CM works have been worked out using the
formulae given in I.S. Codes and text books.
The head loss due to various structures works out to 20.366 m, including
the bed slope within the structures.
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6.5 Alternative Proposal to be considered for Detailed
Investigation at the time of Preparation of DPR
6.5.1 Canal instead of Tunnel between RD 77.9 to 97.9 km
The alignment of the canal from RD 77.90 km to RD 97.90 km passes
through the tunnel with less core overburden. Hence, instead of tunnel the
canal can be detoured. The length of detoured canal works out to
approximately 100 km. The comparative costs of the alternatives are to be
worked out.
6.5.2 Crossing Point of Road connecting Arod to Gaddi Village
at RD 154 km.
The proposed canal at RD 150 km runs along the foothills and after
traversing for a distance of approximately 6 km, it takes a ‘U’ turn and at
RD 154.057 km it takes a 60o turn and moves away from the hills.
Between RD 150 and 166 km, the link canal passes through high
embankment in the order of 30 m for about 3 km around the periphery of
the hillock. The alignment along the foothills was proposed to cover the
command area around the periphery of the hillock. Alternatively a tunnel
can be provided from RD 154 km to avoid the provision of high
embankment and this will also help in reducing length of the canal from 18
km to approximately 5 km. Since the Tungabhadra Left Bank Canal passes
in close proximity with the proposed link canal, the command area around
the foothills is very marginal. Small distributaries from the off-take and exit
of tunnel may be able to cater to the irrigation needs of the command area
around the foothills. The provision of tunnel appears to be a better
proposition and detailed investigation may be carried out at the time of
preparation of DPR.
6.5.3 Provision of Tunnel between R.D. 211 to 214 km
As per pre-feasibility report the canal alignment at this reach passes
through the foot hill very near to the Tungabhadra High Level Canal. The
link canal at this reach will run in deep cutting. Virtually there is no space
for canal excavation between the hill and the High Level Canal. The canal
has, therefore, been diverted through the hillock. Since, the overburden is
more than 50 m, a tunnel can be provided at this reach.
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6.5.4 Possible Reduction in Cost of Project
In the pre-feasibility report, the off-take level at Almatti was taken as 510
m considering the FRL of 512.256 m. Subsequently, the FRL has been
raised to 519.60 m and there is a proposal to further increase the FRL to
524.256 m. The off-take level of the link canal can, therefore, be raised to
516.00 m which will enable raising of bed level of the canal in the initial
reach, where it is running in deep cutting, besides helping to increase the
gradient of four tunnels to 1:3,500 from 1:10,000 enabling considerable
reduction in the diameter of tunnel which will also cause smooth transition
from open channel to tunnel and from tunnel to open channel at the intake
and exit of tunnels, respectively. Since the CCA available in all the en route
sub-basins are much more than as envisaged in the PFR, the canal can be
terminated at Bukkapatnam tank and the CCA in the en route sub-basins
increased proportionately so that the total CCA of the link canal remains
unchanged. The savings from the excavation and lining of the canal and
tunnels by increasing the off-take level from 510.0 to 516.0 m is estimated
to be Rs. 53,000 lakhs and from curtailment of canal beyond Bukkapatnam
is Rs. 66,600 lakhs approximately. The total cost of the project works out to
Rs. 540,380 lakhs against Rs. 659,980 lakhs and the corresponding B.C.
Ratio will be 1.35.
