CHAPTER – IV
DESIGN ASPECTS
4.0
Engineering assessment
As already discussed in the foregoing chapters, the Burhi GandakNoon-Baya-Ganga link project envisages diversion of the 492 cumec of flood
water from the proposed barrage across river Burhi Gandak, to the river
Baya/Ganga. The design aspects of various components of the project have
been carried out departmentally by NWDA as per the provisions laid down in
relevant IS codes. Design philosophy for the components, their design,
alongwith underlying consideration, assumptions and parameters and the
basis for selecting the parameters form the subject of the chapter. The salient
details of the design in respect of the various components are briefly
discussed in this chapter.
4.1
General
The link project mainly comprises of construction of a barrage and
head regulator across river Burhi Gandak near Muriaro village in Samastipur
district and a 28.95 km long canal. The link canal has been designed keeping
in view the worst scenario i.e. when both Burhi Gandak and Ganga are in
spate. It is observed that during flood times the river Ganga inundates the
whole area near the outfall of the link canal and the river Baya and Ganga
become one river.
The HFL of the river Burhi Gandak at the Samastipur Railway bridge
which is very near to the proposed barrage site is recorded as 49.38 m. The
HFL of the river Baya/Ganga near the outfall point of the link canal has been
computed with respect to HFL recorded at Burha sluice on river Baya which
is in the upstream of the outfall point and HFL recorded at Hathidah G&D
site on river Ganga which is in the downstream of the outfall point and is
found to be 43.15 m. and the same was also locally confirmed during the
course of field surveys and investigations.
As the difference between HFL of the river Burhi Gandak at the offtake
point and the HFL of the river Baya/ Ganga at the outfall point of the link
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canal is only 5.85 m, considering the head losses of 2.845 m due to canal
slope and various hydraulic structures across the link canal and the required
full supply depth of 5 m of the link canal, the flow in the canal has been
considered to be non-uniform. The canal, therefore, has been designed
considering the non-uniform nature of the flow and to counter the backwater
effect of the flood in Baya/Ganga on the flow in the canal. A number of cross
drainage structures and various road bridges have been proposed across the
link canal which are described in the following paragraphs.
4.2
Structures
(a) A barrage of 611 m length has been proposed across river Burhi
Gandak at the latitude of 25049′ 22″ N and longitude 85053′56″ E, out of
which 189 m is the overflow section and the remaining 422 m is the non
overflow section. Detailed design of the barrage has been carried out and the
″hydraulic details are furnished in Table- 4.1:
Table – 4.1
Hydraulic details of Burhi Gandak barrage
Barrage
Total length
Deepest Pond Free board No. and size of
bed
level
vertical gates
level of
river
Burhi
189 m overflow
35.0 m 49.0m
2.00 m
Undersluice
Gandak section.
(As per IS :
gate - 3 nos.
barrage 422 m non overflow
10635(8m x 14m)
section.
1983)
Other bays
Total : 611m
gate- 16 nos.
(8m x 12.8 m)
(b) One head regulator has been proposed at the right side of barrage at an
angle of 900 with the barrage axis. The head regulator is 46.10 m long , 27 m
wide with crest level of 43.00m and 3 nos. of vertical gates of size 9 m x 6m
for the offtake of main canal.
(c) 28.95 km long gravity lined canal offtaking from Burhi Gandak barrage
has been proposed.
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(d) 4 drainage syphons, 3 canal syphons, 2 head regulators for branch
canal, 1 cross regulator, 1 canal escape, 1 bridge on national highway, 6
bridges on state highway 7 village road bridges and 5 pipe culverts have been
proposed.
The barrage, cross drainage works and their vicinity are discussed
briefly in the subsequent paragraphs.
4.3
Geology, seismicity and foundations
4.3.1 Geology
The proposed site of the barrage across river Burhi Gandak is narrow
and bowl shaped. At both the banks of the river, flood protection bunds have
already been constructed by the Govt. of Bihar and now the river flows within
the bunds. The project area belongs to a part of Burhi Gandak-Gangetic
alluvial plain lying between Himalayan foothills in the north and
Chnotanagpur plateau in south. This area exhibits a flat country and shows
presence of Neogene-Quaternary terraces with master slope towards Ganga
river. The area belongs to Ganga Kosi formation representing older flood
plain deposits which consists of unoxidised to feebly oxidised dark grey
clayey silt, sandy silt and silty clay. From this study, it appears that the entire
canal alignment alongwith CD/CM structures will pass through Gangetic
alluvial reach.
4.3.2 Seismicity
The project area lies in seismic zone IV as per the seismic zoning map
of India as incorporated in IS criteria for earthquake resistant design
structures [IS : 1893-(Part-I)2002]. For this seismic zone, the value of
horizontal seismic coefficient α0 and seismic zone factor for average
acceleration spectra, F0 are 0.05 and 0.25 respectively. In view of target date
of completion of DPR of this project, the above parameters recommended for
seismic zone IV have been considered in the design of the barrage. The site
specific seismic study report of the project is being referred to National
Committee on Seismic Design Parameters (NCSDP) for necessary approval.
In case of any deviation in the considered seismic design parameters, the
designs of the project adopting the eismic parameters recommended by
NCSDP will be modified at the time of detailed design of the project at the
time of construction.
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4.3.3 Foundations
The investigations of the foundation of the barrage and CD/CM
structures have been carried out by CSMRS, New Delhi and GSI, Kolkata.
Exploratory drilling at 22 locations, 5 at barrage axis and remaining 17 along
the link canal at various CD/CM structures have been carried out. The SPT
tests and water permeability tests have been conducted at the project sites and
undisturbed and disturbed samples collected. The undisturbed and distrubved
soil samples have been tested in the laboratory of CSMRS, New Delhi and
various tests such as mechanical analysis, atterberg limits, insitu density and
moisture content, specific gravity, triaxial shear, one dimensional
consolidation etc. have been carried out.
4.4
Design flood and sediment studies
4.4.1 Design flood
Bihar is India’s most flood prone state, with 76 percent of the
population, in the north Bihar living under the recurring threat of flood
devastation. About 68,800 sq km out of total geographical area of 94,160 sq
km comprising 73.06 percent is flood affected. The plains of Bihar, adjoining
Nepal, are drained by a number of rivers that have their catchments in the
steep and geologically nascent Himalayas. Kosi, Gandak, Burhi Gandak,
Bagmati, Kamla Balan, Mahananda have a very high sediment load and drop
it down in the plains of Bihar. About 65% of catchment area of these rivers
falls in Nepal/Tibet and only 35% of catchments area lies in Bihar. A review
by Kale (1997) indicated that the plains of north Bihar have recorded the
highest number of floods during the last 30 years. In the years 1978, 1987,
1998, 2004 and 2007 Bihar witnessed high magnitudes of flood. The total
area affected by floods has also increased during these years. Flood of 2007
demonstrates the severity of flood problem when a vast area of 23,490 sq km
was badly affected by the floods of Bagmati, Kamla & Adhwara groups of
rivers causing loss of about 800 human lives, even when Ganga, the master
drain was flowing low.
The design flood studies for river Burhi Gandak have been carried out
by NIH. Design flood was arrived through flood frequency analysis using
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Gumbel and Log Pearson type- III distribution. The flood discharge
corresponding to 50 years return period at Sikandarpur G&D site has been
estimated as 4920 cumec, of which 10% i,e 492 cumec has been considered
for diversion through the link canal.
4.4.2 Sediment studies
The Burhi Gandak river is a left bank tributary of river Ganga and
outfalls into Ganga at Khagaria. As per the report of the Committee (2002) on
“Silting of River in India” constituted by MOWR, GOI. The annual average
sediment load at Sikandarpur and Rosera G&D sites are 9.13 million ton and
13.12 million ton of which 98% and 99% sediment load observed during the
monsoon period respectively. The average sediment during the decade of
sixty is minimum and had gradually increased over the years and is maximum
during the decade of eighties. The average monsoon sediment is 97% to 99%
of the annual sediment.
The analysis of sediment data of Sikandarpur G&D site indicates that
out of annual average suspended load of 9.13 million tone, 4.9% is coarse,
17.9% is medium and 77.2% is fine. Similarly, Rosera site for the period
1978-89 indicates that out of annual average suspended sediment load of
13.12 million ton, 2.8% is coarse, 9.4% is medium and 87.8 is fine. These
analysis show that the sediment load at Rosera is more than the sediment load
at Sikandarpur and the percent fraction of medium sediment at Rosera is 9.4
% whereas for Sikandarpur it is 17.9%. However, at both the sites the
suspended sediment mainly consists of fine sediment, the contents of which
vary from 65 % to 91 % in case of Sikandarpur and 77% to 94% in case of
Rosera site.
4.5
Barrage
The Burhi Gandak-Noon-Baya-Ganga link project envisages the
construction of a barrage across the river Burhi Gandak.
4.5.1 Selection of barrage site
The barrage site has been selected by examining location and site visits
with the team of Engineering Geology Division, GSI, Kolkata. The barrage is
proposed across river Burhi Gandak at the latitude of 25049′22″ N and
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longitude 85053′56″E. The river is narrow and bowl shaped at the proposed
barrage site and therefore will be cost effective at the time of construction.
Detailed design of the barrage has been carried out and furnished at
Annexure 4.1
4.5.2 Topography of barrage site
The Burhi Gandak barrage is proposed across the river Burhi Gandak
19 km downstream of Samastipur town where the topographical feature of
river is bowl shaped, near village Muriaro, at latitude 25049′27″ N and
longitude 85053′48″ E. The pond level of barrage is kept at 49.00 m i.e.
slightly lower than the highest flood level recorded at Samastipur Railway
bridge, CWC Gauge site which is located about 19 km in the upstream.
Rosera G&D site maintained by CWC is situated about 24 km downstream of
the proposed barrage site. The flood discharge for 50 years return period at
Sikandarpur G&D site has been estimated as 4920 cumec. It is proposed to
divert 10% of the estimated flood discharge i.e. 492 cumec to minimize the
flood damages in lower reaches of basin area.
4.5.3 Hydraulic aspects of barrage
It is proposed to construct a 611 m long barrage across river Burhi
Gandak. Out which 189 m is the overflow section and the remaining 422 m is
the earthen non overflow section. The length of undersluice bays and other
bays are 160 m and 187.56 m respectively. The general layout plan of the
barrage and head regulator is appended as Plate 4.1 in Drawing Volume.
The barrage is designed considering the estimated flood discharge of
4920 cumec corresponding to 50 years return period. 3 number of under
sluice bays of size 8 m x 14 m and 16 other bays of size 8 m x 12.8 m each
have been provided. Undersluice bays have been designed to pass about 20%
of flood discharge and the balance through other bays. The discharge passing
through undersluice bays and other bays have been calculated by the broad
crested weir and sharp crested weir formula respectively.
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The broad crested weir and sharp crested weir formulae are
Q
=
1.705 (L-0.1 n H ). H3/2 and
Q
=
1.84 (L-0.1 n H)H3/2 respectively.
Q
=
L
=
Clear waterway
n
=
The number of end contractions
H
=
Head over crest
Where
Discharge
The contour plan, L-section & cross section of river Burhi Gandak at
proposed barrage axis have been appended as Plate 4.2 and 4.3 respectively
in the Drawing Volume. To keep adequate regulation of link canal, the pond
level of barrage has been proposed as 49.00 m so that the canal may be
regulated adequately at FSL 47.50 m. The crest level of undersluice is kept at
RL 35.00 m and of other bays at RL 36.20 m. The height of gates in under
sluice and other bays are 14.0 m and 12.8 m respectively. The design of
barrage is given in Annexure 4.1. Typical plan of barrage and head regulator
is appended as Plate 4.4 in Drawing Volume.
The total floor length through undersluices is provided as 160 m where
upstream floor length is 95.49 m and downstream length is 64.51 m. The
other bays’ total floor length is provided as 187.56 m where upstream and
downstream floor length is 123.05 m and 64.51 m respectively. Crest width of
other bays is kept as 4 m. The thickness of downstream floor is varying from
3.23 m to 5.16 m for undersluice and 3.23 m to 5.16 m for other bays
downstream floor. Section of undersluice and other bays are appended as
Plate 4.5 and 4.6 in Drawing Volume.
4.5.4 Non overflow section of the barrage
The HFL at the proposed barrage site is considered as 49.38 m. The
distance between the river banks at highest flood level is measured as 611
m at the barrage axis. The diversion structure is constructed with a restricted
waterway of 189 m for economy as well as better flow conditions. The un95
bridged width of the river is blocked by means of non overflow or marginal
bund. A 14 m wide road is proposed on the top of embankment. The drawing
is appended as Plate 4.5 in Drawing Volume.
4.6
Head regulator and cross regulator
Head regulator has been proposed on the right flank of the barrage.
The designed discharge of head regulator has been considered as 590 cumec
including 20% extra for silt ejector. The clear waterway for passing the
design discharge is calculated by shallow waterway drowned weir formula
which is as follows:
Q
=
2/3 x C1 x L 2g { h+ ha) 3/2 -ha 3/2}
+ C2 x L d 2g (h+ ha)
Where C1
h
d
=
0.577, C2 = 0.80 are constant.
=
difference in FSL of canal and pond level
= difference in FSL and crest level of canal
of head regulator
L
= clear water way
Thus, providing 3 bays of 9.0 m each with 2 nos. of piers of width 1.5
m have been provided to pass the required discharge. Total waterway will
become 30 m. The crest level of undersluice is 35.00 m and head regulator is
kept at 43 m i.e. 8.0 m above the bed level of river. The canal is proposed to
off-take at RL 47.50 m. The length of downstream floor of head regulator is
computed as 27.09 m and thickness ranging between 1.5 m to10 m. The
height of gate is kept 6.00 m. Two head regulators and one cross regulator
have also been proposed at R.D. 3.0 km of the link canal from where branch
canals will take off for enroute irrigation. The design of head regulators are
furnished at Annexure- 4.2, 4.2.1, 4.2.2. A cross regulator is proposed for the
diversion of main canal water to branch canal for the irrigation of command
area. The design of cross regulator is furnished at Annexure 4.2.3. The plan
and section of head regulator is appended as Plate 4.8 & 4.9. The joint plan
and section of head regulators and cross regulator are appended as Plate 4.10.
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4.7
Silt ejector
A silt ejector has been proposed at the canal at RD 1.00 km for the
extraction of silt from the canal as a remedial measure for silting of the canal.
The design discharge for the silt ejector has been considered as 98 cumec
which is 20% of the design discharge of 492 cumec of the link canal. In the
silt ejector, 11 main channels of size 6.55m x 1m each have been provided
and under these main channels 22 sub channels of size 3.6 m x 1m each have
been provided. The velocities in the main channels at entry point and in the
sub channels at exit points are 0.73 m/s and 1.24 m/sec respectively. The
overall waterways for main channels and sub channels are 72m and 79m
respectively. The design of the silt ejector is given at Annexure-4.3 & 4.3.1.
The plan and section of the silt ejector is appended as Plate 4.11 in the
Drawing Volume.
4.8
Canal
4.8.1 Canal alignment
The layout plan of link canal is furnished at Plate 4.11. The alignment
of the proposed link canal has been finalised on the basis of field survey. The
contour plan and longitudinal section of the link canal have been prepared
based on field surveys and appended as Plate 2.51 to 2.58 in Drawing
Volume. The canal off takes through head regulator of the barrage from right
bank of river Burhi Gandak at FSL 47.50 m. It is aligned as contour canal
with bed slope 1:20,000 throughout its length. The canal passes through
agricultural land. The entire section of link canal from RD 0.0 km to 28.95
km is in partly cutting and partly filling. The canal crosses a number of roads,
streams, railway lines requiring construction of cross-drainage cross masonry
works. Statement showing location, type of CD/CM works, FSL, head losses
provided for various CD works etc. are furnished at Annexure 4.4. The link
outfalls into river Baya at R.D. 28.95 km. The FSL of link canal at the outfall
point is 44.66 m.
Canal escape has been provided along the canal at RD 7.30 km for
safety of canal. The design of the escape is furnished at Annexure 4.10 and
plan and section are appended as Plate 4.13 in the Drawing Volume.
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4.8.2 Canal capacity
The Burhi Gandak-Noon-Baya-Ganga link canal will carry 492 cumec
discharge from Burhi Gandak barrage.
4.8.3 Canal section
The canal section will be trapezoidal with rounded corners and lined on
sides as well as bed. The section of canal at head is 82.10m(bed width) X
5.00 m (full supply depth) and at the outfall point is 94.30 m X 5.00 m
excluding free board of 1.20 m. The statement showing the hydraulic details
of canal is shown in Annexure 4.5. Typical sections in partial cutting and
partial filling are appended as Plate 4.14 to 4.16 in Drawing Volume.
4.8.4 Details of lining provided
It is proposed to provide cast-in-situ cement concrete lining for the
entire length of canal to minimize seepage to avoid water logging in the areas
adjacent to the canal lining with cement concrete of M 15 grade with LDPE
films in canal bed as well as in side slopes. The thickness of lining is kept as
12 cm as per IS code 3873-1978.
4.8.5 Brief description of canal design
(a)
Design calculation for adequacy of canal section
The canal section in various reaches is designed using Manning’s
formula.
V = (1/N) R 2//3 S 1/2
Where
V = Velocity in m/sec
N = Rugosity coefficient, i.e.0.018 for lined canal
R = Hydraulic radius = Area/wetted perimeter
S = Bed slope of canal
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(b)
Side slope
Side slopes are assumed as 1.5:1 (H:V) throughout the main canal.
(c)
Velocity allowed
The velocity of canal is 1.07 m/sec.
(d)
Critical velocity ratio
The critical velocity ratio is found to be ranging from 0.69 to 0.70 in
main canal.
(e)
Full supply depth and free board
The full supply depth of canal is kept as 5.00 m throughout the length.
The free board of 1.20 m has been provided as per IS : 10430-1982.
(f)
Transitions in canal section and head losses
The loss of head due to contraction is computed as
= 0.2(V2 2 – V1 2 )/ 2g
where V2
V1
= Velocity of flow of canal at the exit of contraction
= Velocity of flow of canal before contraction
The loss of head due to expansion is considered as
0.3 ( V 2 2 – V 1 2 ) /2g
The loss due to difference in velocity heads is considered as
( V 22 – V12 )/2g
(g)
Head loss provided at canal structures
The head losses at various canal structures across the link canal were
also computed while designing the structures. The head loss provided for the
various cross drainage structures are as follows
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1.
Syphon aqueduct
0.015 m
2.
Road bridges
0.01 m
3.
Cross regulator
0.05 m
4.
Canal syphons
0.38 m.
4.8.6 Canal structures
There are 1 number of silt ejector, 2 number of head regulators and 1
number of cross regulator along the alignment of the canal.
4.9
Canal operation
The canal will be in operation in monsoon period for a peak discharge
of 492 cumec.
4.10 Cross drainage/masonry works
The type of cross drainage structures to be provided depends upon the
position of bed level of streams, drains, road, railway etc. The typical designs
of some of the following cross drainage works have been carried out.
4.10.1 Syphon aqueduct (drainage syphon)
Four syphon aqueducts have been proposed along the link canal. The
designs of the syphon aqueducts have been carried out and furnished in
Annexure 4.6.1 to 4.6.4. The plan and section of syphon aqueducts are
appended as Plate no 4.17 to 4.20 in Drawing Volume.
4.10.2 Canal syphons
Three canal syphons have been proposed in link canal. The designs of
canal syphons have been carried out and are furnished in Annexures 4.7.1 to
4.7.3. The plan and the section of canal syphons are appended as Plate 4.21
to 4.23 in Drawing Volume.
100
4.10.3 Bridges
Bridges on 1 national highway, 6 state highways and 7 village roads
have been proposed along the canal. Head loss of 0.01 m is considered at
each bridge. The designs of the road bridges have been carried out for canal
discharge of 541 cumec and bed width ranging from 82.10 m to 94.30 m.
The waterway is estimated using the formula.
Water way=b+2[s (d+d1 + Afflux ) + bearing of abutment]
Where b = Bed width of canal
s = Side slope of canal
d = Full supply depth of canal
d1 = Free board
Designs of the road bridges are given in Annexures 4.8.1 to 4.8.13 .
The various drawings are appended as Plate 4.24 to 4.37 in Drawing Volume.
4.10.4 Pipe culverts
Five hume pipe culverts have been proposed along the main link canal
for the easy movement of the surface water from one side of the canal to other
side. No head loss for hume pipe culverts is provided. The designs of the pipe
culverts have been carried out and are furnished in Annexure 4.9. The
sections of hume pipe culverts are appended as Plate 4.38 to 4.42 in
Drawing Volume.
4.11 Reachwise description of the link canal
(a)
Reach from RD 0.0 km to 7.85 km
The link canal offtakes from river Burhi Gandak and runs in the south west direction for a length of about one kilometer. In this each, the canal
crosses various village roads at RDs 1.69, 2.87, 3.50, 6.40, 6.90 and 7.45 km.
The canal also crosses a railway line (Samastipur-Khagaria) at RD 2.23 km.
101
The canal also crosses a minor stream at RD 4.8 km where pipe culvert has
been provided. A syphon aqueduct and escape across a local stream viz.
Jamwari nadi is proposed at RD 7.35 km.
(b)
From RD 7.85 km to 21.45 km
In this reach , the link canal crosses 6 number of village roads at RDs
7.95, 8.30, 11.55, 12.75, 15.10 and 20.95 km. The canal also encounters 4
number of road bridges across the state highways at RDs 9.95, 15.50, 17.90
and 19.90 km, 3 number of pipe culverts are also proposed at R.Ds 10.65,
17.10 and 17.80 km. The canal also encounters two numbers of syphon
aqueducts at RDs 13.00 (on a local stream) and 19.70 km (on Kabrain nalla)
and a canal syphon at RD 20.90 km on railway crossing , near village Ajnaul.
(c)
From RD 21.45 km to 28.95 km
In this reach, the link canal crosses river Noon by way of a syphon
aqueduct at RD. 21.50 km. The link canal encounters two village roads at
RDs 22.10 and 22.60 km where road bridges have been provided. The canal
crosses a stream by a pipe culvert at RD 24.3 km .A canal syphon is also
proposed at RD 27.3 km across Hajipur –Barauni railway line .A road
bridge across NH-28 (Hajipur- Barauni) is also proposed at RD 28.4 km.
Finally, the link canal outfalls into river Baya at RD 28.95 km.
102