1.3 Topographic Survey (on river) Topographic survey has been carried out to understand the river bed profile and to develop numerical model for each river. Survey lines are decided from the hydraulic point of view, which are also close to past survey line so that a comparative study can be easily carried out. The schematic diagram of survey lines is shown in Figure 1.3.2 and Figure 1.3.3 for Meghna Bridge and Gumti Bridge, respectively. As there are no survey results/ survey records for Kanchpur Bridge, therefore, the survey lines have been selected at 200 m intervals in u/s and d/s of existing bridges, which are shown in Figure 1.3.4. Survey type - River Bathymetric Survey (under the river flow, water) - Using Echo- Sounding device (Figure 1.3.1) - River Topographic Survey (on the land) The above survey results will be formulated soon. Echo-sounding device -Echotrac DF3200 MKII (GPS) Odom Hydrographical Systems Inc.) Figure 1.3.1 bathymetric survey device. $3
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1.3 Topographic Survey (on river)
Topographic survey has been carried out to understand the river bed profile and to develop numerical model for each river. Survey lines are decided from the hydraulic point of view, which are also close to past survey line so that a comparative study can be easily carried out. The schematic diagram of survey lines is shown in Figure 1.3.2 and Figure 1.3.3 for Meghna Bridge and Gumti Bridge, respectively.
As there are no survey results/ survey records for Kanchpur Bridge, therefore, the survey lines have been selected at 200 m intervals in u/s and d/s of existing bridges, which are shown in Figure 1.3.4.
Survey type
- River Bathymetric Survey (under the river flow, water)
- Using Echo- Sounding device (Figure 1.3.1)
- River Topographic Survey (on the land)
The above survey results will be formulated soon.
Echo-sounding device -Echotrac DF3200 MKII
(GPS) Odom Hydrographical Systems Inc.)
Figure 1.3.1 bathymetric survey device.
Source Edited JIC
A Report 2by JICA Team
Figure 1.3.2 Topographic Survey Line at M
eghna Bridge
2 B
asic Design StudyR
eport On The Project For Protection W
orks For Meghna B
ridge In Prople's Republic O
f B
angladesh, Feb 1998
Source :Edited RHD Report3 by JICA Team
Figure 1.3.3 Topographic Survey Line at Gumti Bridge
3 Rehabilitation of Existing Meghna Bridge & Gumti Bridge Protective Works of Pier Foundation Design Report, Roads & Highways Department, Feb 2011
Figure 1.3.4 T
opographic Survey Line at Kanchpur B
ridge
1.3.1 Kanchpur Bridge
Figure 1.3.5 Topographic Survey Line at Kanchpur Bridge
Table 1.3.1 Longitudinal River Bed Profile of Lahkya River Distance Between
Figure 1.3.14 Cross Section Profile of Gumti River
1.4 River Current Velocity Survey
1.4.1 Overall
In order to protect the existing bridge and new bridge from scouring around piers, it is necessary to predict current velocity in design flood event. River current velocity survey is held to collect the basic hydrological data of the river, and data measured will be used to check the numerical analysis.
River current velocity survey is carried out from July 30th 2012 by ADCP device which can measure river current velocity, flow direction and total discharge measurement along the cross section of the river.
1.4.2 Method
Discharge measurement has been conducted at Kanchpur, Megnha and Meghna-gumti Rivers using Acoustic Doppler Current Profiler (ADCP) and DGPS System on 30th July 2012.
The R D Instruments state-of-the earth Workhorse Rio-grande ADCP (600 Khz) with bottom tracking option has been used for the survey. The instrument is capable of velocity profiling upto 45m depth. The measurement has been done following WinRiver User Guide supplied by the manufacturer of the instrument. The instrument is used for measuring velocity at a fixed time interval (known as ensemble) without anchoring at desired transects location (moving boat condition). It measures flow velocity at each ensemble (around 2.5 second intervals) at 50cm interval throughout the water column.
Thus a series of velocity data is recorded along the whole transect line. However, it estimates the discharge of unmeasured areas (the top of instrument face, near bottom part and at edges). The river flow is calculated online by the WinRiverII Software adding discharge of each ensemble as it moves along the transect line. As the river carries considerable amount of sediment and have a higher velocity, the bias bottom tracking condition was observed. To overcome this, GPS (GGA) has been used as reference for velocity and discharge computation. Necessary compass calibration has been made earlier as per guidelines set in the User Manual supplied by RD Instruments. The observed data has been found very consistent. The discharge data provides good indication of the distribution of flow. It also provides velocity distribution across the full channel.
Table 1.4.1 Instruments and Software on Current Velocity Survey
Supplier ADCP Device Workhouse Rio Grande 600 khz RTK GPS Device Trimble 5700 Software Winriver II
Figure 1.4.1 ADCP Device (Workhorse Rio Grande 600khz)
Figure 1.4.2 ADCP Software Main Window
Cross Section Curent Velocity
Cross Section Curent Direction
1.4.3 Survey Result
(1) Overall
Total discharge , average flow area and average velocity of three bridges by ADCP survey are shown in Table 1.4.2.
In this result, discharge at Meghna bridge is about 10 times more than discharge at Gumti bridge,. Both river flow originally one channel at upstream of the bridges, and after Bhairab Bazar, station, both river channel are separated. Hence it seems that the discharge at Bhairab Bazar almost flows along Main Meghna river through Meghna Bridge.
Table 1.4.2 Measurement Result at Meghna Bridge
Unit Meghna Meghna-Gumit Kanchpur
Water Level M.S.L 3.72 3.69 3.96
Total Discharge m3/s 11637.0 1063.5 1248.4
Average Flow Area m2 13245.9 7416.0 2267.5
Average Velocity m/s 0.892 0.143 0.554
Megh
na M
eghna-G
umti
Kanch
pur
Table 1.4.3 C
ross Section Current V
elocity Survey Result
(2) Meghna Bridge (Meghna River)
Table 1.4.4 Measurement Result at Meghna Bridge
Unit Result
Water Level M.S.L 3.72
Total Discharge m3/s 11637.0
Average Velocity m/s 0.892
Figure 1.4.3 Transect Lines at Meghna Bridge Site
Meghna River SEC-09 (Meghna Br Center Line)
-25
-20
-15
-10
-5
0
5
10
15
20
-800 -600 -400 -200 0 200 400 600 800 m
PW
L
Figure 1.4.4 Water Level on the Measured Period (July 30th 2012)
Chittagong Dhaka
Chittagong
Dhaka
Chittagong side of Meghna river on the shore line Water level on measured day from Chttagong
side
Chittagong side of the river shoreline Dhaka side of the river shoreline
Figure 1.4.5 Water Level on the Measured Day at Meghna Bridge (July 30th 2012)
Figure 1.4.6 Cross Section Velocity Contour at Meghna Bridge
FLOW FLOW
FLOW
FLOW
Chittagong Dhaka
Main Channel Fast Flow-> about1.2m/sec
Chittagong side of bank Slow Flow-> about 0.5m/sec
Dhaka side of bank Slow Flow-> about 0.5m/sec
(3) Gumti Bridge
Table 1.4.5 Measurement Result at Gumti Bridge
Unit Result
Water Level M.S.L 3.69
Total Discharge m3/s 1063.5
Average Velocity m/s 0.143
Figure 1.4.7 Transect Lines at Gumti Bridge Site
Meghna-Gumti River SEC-04 (Meghna-Gumti Br Center Line)
Figure 1.4.8 Water Level on the Measured Period (July 30th 2012)
Chittagong
Dhaka
ChittagongDhaka
Chittagong side of river on the shore line Water level on measured day from Chttagong
side
Chittagong side of the river shoreline Dhaka side of the river shoreline
Figure 1.4.9 Water Level on the Measured Day at Gumti Bridge (July 30th 2012)
Figure 1.4.10 Cross Section Velocity Contour at Gumti Bridge
FLOW
FLOW
FLOW
FLOW
Chittagong Dhaka
Main Channel Very Slow Flow-> about0.3m/sec
Chittagong side of bank Fast Flow-> about 0.9m/sec
Dhaka side of bank Slow Flow-> about 0.5m/sec
(4) Kanchpur Bridge
Table 1.4.6 Measurement Result at Kanchpur Bridge
Unit Result
Water Level M.S.L 3.96
Total Discharge m3/s 1248.4
Average Velocity m/s 0.554
Figure 1.4.11 Transect Lines at Kanchpur Bridge Site
Kanchpur SEC-11 (Kanchpur Br Center Line)
-15
-10
-5
0
5
10
15
20
25
-300 -200 -100 0 100 200 300 400 500 m
PW
L
Figure 1.4.12 Water Level on the Measured Period (July 30th 2012)
Chittagong
Dhaka
Chittagong Dhaka
Chittagong side of Lahkya river on the shoreline Water level on measured day from Chittagong
side
Chittagong side of the river shoreline Dhaka side of the river shoreline
Figure 1.4.13 Water Level on the Measured Day at Kanchpur Bridge (July 30th 2012)
Figure 1.4.14 Cross Section Velocity Contour at Kanchpur Bridge
FLOW
FLOW
FLOW
FLOW
Chittagong Dhaka
Main Channel Fast Flow-> about1m/sec
Chittagong side of bank Slow Flow-> about 0.5m/sec
Dhaka side of bank Very Slow Flow-> about 0.1m/sec
2. HYDROLOGICAL AND HYDRAULIC ANALYSIS
2.1 Design Criteria
Bridge: Design water level is 100-year return period water level + 2.0m freeboard (minimum) up to the bottom of bridge girders, in reference to Japanese "Government ordinance for structural standards for river administration facilities".
Motorway Alignment: The elevation at the bottom of sub base level is to be above the 100-year return period of high water level.
Protection of Road Embankment: It is provided for inundated areas except reaches where flow velocity is low. The elevation of protection level is 1.0m above 100-year return period flood level.
2.2 Hydrological Analysis
Hydrological analysis was carried out to estimate the design discharges of streams at motorway crossing points which are required to estimate the hydraulic design outputs at bridges and also water levels. The corresponding catchment area of stream was measured using topographic maps or/and referring the data from nearby gauging stations.
In this section, the discharge around three bridges are re-examined by discharge data which is collected in chapter 4.
2.2.1 Kanchpur Bridge
There is no existing river plan at Kanchpur Bridge and Lahkya river, hence the discharge value for Kanchpur Bridge is estimeted in this study by using recently collected reference data.
There are two water level and discharge measuring station, Demra at Lahkya river and Demra at Balu river which is maintained by BWBD. The discharge at Kanchpur Bridge is sum of discharge of Lahkya and Balu river.
100-year return period discharge at Demra(Lahkya) and Demra(Balu) is estimated according to Japanese Government's technical standard shown in Table 2.2.2 and Table 2.2.3, and the 100-year return period discharge at Kanchpur Bridge is 3480 [m3/s] , shown in Table 2.2.1 and Figure 2.2.1.
Table 2.2.1 100-year return period discharge at Kanchpur Bridge
Source: Estimated by the study team according to Japanese Gorvernment technical standard
Kanchpur Bridge Q=2596 + 884 = 3480 [m3/s]
Demra(Lahyka) Q=2596 [m3/s]
Demra(Lahyka) Q=884 [m3/s]
Balu River Lahkya River
Figure 2.2.1 Discharge measuring station map around Kanchpur Bridge
Source: Estimated by the study team according to Japanese Gorvernment technical standard
Figure 2.2.2 Frequency Curve for Design Discharge (W=1/100) at Demra(Lahkya) St.
Return Period [Year]
Probability [%]
Q=2596[m3/s] in LogP3 Distribution
Discharge [m3/s]
Source: Estimated by the study team according to Japanese Gorvernment technical standard
Figure 2.2.3 Frequency Curve for Design Discharge (W=1/100) at Demra(Balu) St.
Return Period [Year]
Probability [%]
Q=884[m3/s] in Gunbel Distribution
Discharge [m3/s]
2.2.2 Meghna & Gumti Bridge
The design discharge should be determined by the maximum number considering the former plan discharge and latast discharge data.
In upper Meghna river, there is only one discharge measuring station at Bhairab Bazar which is maintained by BWBD. Hence, the discharge is estimated according to the ”Feasibility study on Meghna, Gumti Bridges construction project - final report (1985), JICA”. In this report, discharge at Bhairab Bazar is estimated at first, then discharge at Meghna and Gumti Bridge is estimated by considering the flow distribution to main channel and branch of Meguna river and rest of catchment area after Bhairab Bazar Station.
100-year return period discharge at Bhairab Bazar Station is estimated by three method in Table 2.2.4, the maximum discharge is 23700[m3/s] in 1985's JICA Report.
100-year return period discharge at Bridge is estimated below, relationship between discharge and catchment area, discharge distribution is shown in Figure 2.2.5.
100 year return period discharge of Meghna Bridge Q= 15,200m3/sec.
100 year return period discharge of Gumti Bridge Q= 12,400m3/sec.
Table 2.2.4 100-year return period discharge at Bhairab Bazar
Discharge at Bhirab Bazar [m3/s]
Remarks
1985 JICA report 223,700 Adaption in this research 1992 FAP9B 20,300 Estimated using by collected data
22,848 See Table 2.2.5 and Figure 2.2.4
Table 2.2.5 Occurrence Probability for Discharge at Bhairab Bazar (Estimated)
Source: Estimated by the study team according to Japanese Gorvernment technical standard
Table 2.2.6 Catchment Area of the Meghna River
Outside of Bangladesh[km2]
Inside of Bangladesh[km2]
Total [km2]
Catchment Area at Bhairab Bazar St.
41,390 21,570 62,960
Rest of Catchment Area Between Bridge Site And Bhairab Bazar St.
2,760 4,170 6,930
Catchment Area at Bridge Sites 44,150 25,740 6,9890
Source: 1985's JICA report
Source: Estimated by the study team according to Japanese Gorvernment technical standard
Figure 2.2.4 Frequency Curve for Design Discharge (W=1/100) at Bhairab Bazar St.
Return Period [Year]
Probability [%]
Q=22848[m3/s] in Gunbel Distribution
Discharge [m3/s]
Source: Estimated by the study team according to1985's JICA report
Figure 2.2.5 Relationship Between 100-year period Discharge and Catchment Area
2.3 High-Water Level Calculation
2.3.1 Method
Hydraulic design was carried out to obtain the design outputs at the three bridges using Nays2D on i-Ric software platform developed by Professor Yasuyuki SHIMIZU of Hokkaido University(Japan). Nays2D is free software, which is capable of calculating unsteady horizontal two-dimensional river flows and riverbed variation / lateral erosion. The software can be downloaded from website: http:// http://i-ric.org/en/
Water surface profiles are computed from horizontal two-dimentional lattice by solving the 2d-unsteady equation of motion. Energy losses are evaluated by friction (Manning’s equation) and contraction/expansion coefficients. Nays2D requires inputs for boundary conditions of upstream discharge and either downstream water level.
The following procedure was adopted in the modelling.
Discharge 23800[m3/s] Catchment Area 62960[km2]
Gumti Bridge Discharge Q=27600 45%=12,400 [m3/s]
Meghna Bridge Discharge Q=27600 55%=15,200 [m3/s]
55% of discharge flows to Meghna Bridge
45% of discharge flows to Gumti Bridge
Catchment Area 69890[km2] Discharge at Bridge site 23800[m3/s] 69890/62960=27600[m3/s]
River profile is set up in the model using the river cross-sections taken at upstream and downstream. Cross section data is used (will be updated by using new survey results)
Channel roughness “Manning’s n” is adopted according to the bed material diameter d=0.167[mm]. Manning's n value for d=0.167[mm] is n=0.020 according to Japanese Gorvernment's Technical Standard, that nunber is applied for main river and flood plain.
The upstream boundary condition is applied for 100-year return period discharge for each bridges.
The downstream boundary condition is applied for 100-year return period watar level for each bridges which is estimated according to Japanese Gorvernment's Technical Standard, shown in Table 2.3.2 to Table 2.3.4 and Figure 2.3.2 to Figure 2.3.4.
The boundary conditon for numerical model simulation is are summarized in Table 2.3.1. Water Level measured at BWDB's station is transformed from PWD.m to R.L.m by relationship shown in Figure 2.3.1.
Table 2.3.1 Boundary Condition for Hydraulic Analysis at Each Bridge (100-Year Return Period)
Source: Estimated by the study team according to Japanese Gorvernment technical standard
Source: Estimated by the study team according to Japanese Gorvernment technical standard
Figure 2.3.2 Frequency Curve for Design Water Level (W=1/100) at Meghna Ferryghat St.
Return Period [Year]
Probability [%]
H=6.984[PWDm] in SqrtEt Distribution
Water Level [×1000 PWDm]
Source: Estimated by the study team according to Japanese Gorvernment technical standard
Figure 2.3.3 Frequency Curve for Design Water Level (W=1/100) at Daudkandi St.
Return Period [Year]
Probability [%]
H=7.355[PWDm] in Gunbel Distribution
Water Level [×1000 PWDm]
Source: Estimated by the study team according to Japanese Gorvernment technical standard
Figure 2.3.4 Frequency Curve for Design Water Level (W=1/100) at Demra(Lahkya) St.
Return Period [Year]
Probability [%]
H=7.469[PWDm] in SqrtEt Distribution
Water Level [×1000 PWDm]
2.3.2 Numerical Simulation Results in 100-year return period condition
To estimate scour around new bridge pier, hydraulic values in 100-year return period flood is calculated by Nays2D software for each bridges.
(1) Kanchpur Bridge
The hydraulic value of numerical analysis results at bridge center line in 100-year return period discharge is shown in Table 2.3.5, and cross section bed profile, water level, current velocity and water depth at same line is shown in Figure 2.3.5.
Contour map of bed elevation, current velocity, water depth, and water surface level in 100-year return period flood in this model is shown in Source: Estimated by the study team
Figure 2.3.6 and Source: Estimated by the study team
Figure 2.3.7.
The hydraulic value shown in Table 2.3.5 will be used to estimate the local scouring around each pier at Kanchpur Bridge.
Table 2.3.5 Numerical Analysis Result in 100-year return period at Kanchpur Bridge
Pier No Water Depth
[MSL.m] Bed Elevation
[MSL.m] Water Elevation
[MSL.m] Current Velocity
[m/s]
A1 1.65 4.60 6.25 0.49
P1 2.76 3.49 6.25 0.62
P2 5.76 0.48 6.24 0.89
P3 12.04 -5.80 6.24 1.22
P4 14.90 -8.66 6.23 1.33
P5 16.44 -10.20 6.24 1.23
P6 12.67 -6.42 6.25 0.77
P7 2.61 3.65 6.26 0.28
A2 2.61 3.65 6.26 0.28 Source: Estimated by the study team
Elevation WaterSurfaceElevation
(m)
Elev
atio
n (R
.L.m
)
Velocity
(m)
Velo
city
(m/s
)
Depth
(m)
Dept
h (m
)
Source: Estimated by the study team
Figure 2.3.5 Numerical Analysis Result along Bridge Axis at Kanchpur Bridge (100-year)
Dhaka SideChittagon Side
Bed
Ele
vatio
n [M
SL.
m]
Cur
rent
Vel
ocity
and
Dire
ctio
n V
ecto
r [m
/s]
Source: Estimated by the study team
Figure 2.3.6 Bed Elevation and Current Velocity Contour around Kanchpur Bridge (100-year)
Kanchpur Bridge
Kanchpur Bridge
Wat
er D
epth
[m]
Wat
er S
urfa
ce L
evel
[MS
L.m
]
Source: Estimated by the study team
Figure 2.3.7 Water Depth and Water Surface Level around Kanchpur Bridge (100-year)
Kanchpur Bridge
Kanchpur Bridge
(2) Meghna Bridge
The hydraulic value of numerical analysis results at bridge center line in 100-year return period discharge is shown in Table 2.3.6, and cross section bed profile, water level, current velocity and water depth at same line is shown in Figure 2.3.8.
Contour map of bed elevation, current velocity, water depth, and water surface level in 100-year return period flood in this model is shown in Figure 2.3.9 and Figure 2.3.10.
The hydraulic value shown in Table 2.3.6 will be used to estimate the local scouring around each pier at Meghna Bridge.
Table 2.3.6 Numerical Analysis Result in 100-year return period at Meghna Bridge
Pier No Water Depth
[MSL.m] Bed Elevation
[MSL.m] Water Elevation
[MSL.m] Current Velocity
[m/s]
A1 1.80 4.69 6.50 0.02
P1 2.87 3.62 6.50 0.13
P2 4.86 1.63 6.49 0.62
P3 7.78 -1.33 6.46 0.61
P4 13.91 -7.45 6.45 0.42
P5 14.27 -7.81 6.47 1.13
P6 15.23 -8.76 6.48 1.30
P7 18.54 -12.05 6.48 1.21
P8 23.89 -17.40 6.49 1.14
P9 27.23 -20.76 6.48 1.23
P10 25.97 -19.51 6.46 1.44
P11 16.41 -9.95 6.46 1.47
P12 1.36 5.11 6.48 1.07
A2 0.00 9.13 9.13 0.00 Source: Estimated by the study team
Elevation WaterSurfaceElevation
(m)
Elev
atio
n (R
.L.m
)
Velocity
(m)
Velo
city
(m/s
)
Depth
(m)
Dept
h (m
)
Source: Estimated by the study team
Figure 2.3.8 Numerical Analysis Result along Bridge Axis at Meghna Bridge (100-year)
Dhaka SideChittagon Side
Bed
Ele
vatio
n [M
SL.
m]
Cur
rent
Vel
ocity
and
Dire
ctio
n V
ecto
r [m
/s]
Source: Estimated by the study team
Figure 2.3.9 Bed Elevation and Current Velocity Contour around Meghna Bridge (100-year)
Meghna Bridge
Meghna Bridge
Wat
er D
epth
[m]
Wat
er S
urfa
ce L
evel
[MS
L.m
]
Source: Estimated by the study team
Figure 2.3.10 Water Depth and Water Surface Level around Meghna Bridge (100-year)