Finding a sustainable navigation route in Jamuna river using two-dimensional hydrodynamic and sediment models Md. Mostafa Ali1, Muhammed Ali Bhuiyan2

1 Assistant Professor, Department of Water Resources Engineering, BUET, Dhaka 1000, Bangladesh 2 Professor and Head, Department of Water Resources Engineering, BUET, Dhaka 1000, Bangladesh ABSTRACT: Erosion and deposition processes of braided rivers are two intricate morphological phenomena. Erosion of banks and formation of bars on channels of Jamuna are posing serious problems in Bangladesh. Frequent shifting of navigational routes and harbors (ferry ghat) costs not only money and time but also lots of concern and hassle. As the eastern and western parts of Bangladesh are being connected by ferry services through the confluence of Jamuna and Ganges rivers (Fig. 1), this paper deals with modeling of the lower Jamuna along with confluence using two-dimensional finite element method. Being a confluence of two mighty rivers of the world, the site is morphologically very unstable. Under SMS (Surface-water Modeling System) environment, RMA2 (hydrodynamic) and SED2D (sediment transport) models have been used to simulate the morphology of the river. Finite element meshes have been developed using LANDSAT image of the 17th November 2000. Scattered survey data of April 2001 by BIWTA (Bangladesh Inland Water Transport Authority) provided the initial bathymetry of the finite element meshes. The hydrodynamic model developed in this study has been satisfactorily calibrated for 2001and validated for 2002 and 2003, respectively against the observed water surface elevations at Aricha. Sediment model has been calibrated using simulated bathymetry with the measured bathymetry of the 23rd August 2001. The model has also been validated with the measured bathymetries of Novembers 2002 and 2003. Sediment rates of Baruria generated from SED2D, satisfactorily matched with FAP-24 data (FAP, 1996). Subsequently, three options have been investigated to evaluate a sustainable navigable route and thus to locate a ferry ghat. Existence of a perennial deep-pocket near Naradaha (Fig. 3a) motivates this study to take those options and among them Option2, which is a dredging alternative in the upstream channel connecting upper segment of the deep-pocket, has been found with least sedimentation compared to other options. Therefore the option in question presents a potential alternative for developing a sustainable ferry route. 1 INTRODUCTION Jamuna is one of the greatest rivers in the world ranking fifth in terms of discharge (mean flow 20,400 m3/s) and eleventh in terms of drainage area (666,000 km2), is happened to be a braided alluvial river (Thorne et al., 1993). The dominant discharge for the Jamuna at Bahadurabad was found to vary between 36,000 and 38,500 m3/s. The usual average peak discharges is in the order of 55,000 m3/s, with a minimum of 43,100 m3/s in 1994, and maximum of 102,000 m3/s in 1998. The peak flows that occur during late July or early August cause over-bank spillage. These prevailing circumstances cause erosion either at banks or at bars and a deposition in the channel. Bifurcation of channels and meandering growths are the natural fluvial phenomena occurring in the channel. However, development of large bars and deposition in main channels of Jamuna are posing serious navigational problems. Jamuna being the largest (6–15 km wide) river of Bangladesh has divided the country into eastern and western parts structurally. It is so, in addition to a bridge which has recently been constructed, a perennial navigational route have to exist there as an alternative communication system by means of ferry services.

As shown in Fig. 1, Protabpur and Kazirhat are two temporary locations for existing ferry ghats. In the past, BIWTA maintained a ferry route from Aricha to further up at Nagarbari. This route has long been abandoned due to large-scale siltation and shifting of the western anabranch. A recent study (BIWTA, 2003) suggested construction of a permanent ferry ghat at Kazirhat. As alternatives, three more locations (Khanpura, Khayerchar, and Natibpur) have been identified for possible ghat construction; however, preferences of these locations are less compared to Kazirhat.

Different aspects of the morphological behavior of the Jamuna river were studied by a large number of researchers over the past decades: Coleman (1969), Bristow (1987), Klaassen et al. (1988), Hossain (1992), Thorne et al. (1993), Hoque (1999), BIWTA (2003) etc. This research deals with the morphological analysis of Jamuna using finite element method and hence the calibrated model is applied to find a sustainable navigable

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channel for BIWTA. In this study RMA2 and SED2D models under SMS environment, which are finite element programs for hydrodynamic and sediment transport respectively, are used to simulate the flow field and morphology of the river Jamuna. 2 SURFACE-WATER MODELING SYSTEM SMS, an acronym for Surface-water Modeling System, is used for pre- and post-processing for different surface water related models. SMS provides various tools and modules for editing, visualizing the data and results. The following models are in use for this study.

GFGEN is the Geometry File GENeration program. GFGEN’s purpose is to create geometry and finite element mesh files for input to the RMA2 and SED2D models. It supports both 1-D and 2-D elements of either straight or curved edges.

RMA2 is a two-dimensional depth averaged finite element hydrodynamic model. It computes water surface elevations and horizontal velocity components for sub-critical, free-surface flows in 2-D flow fields. RMA2 computes a finite element solution of the Reynolds form of the Navier-Stokes equations for turbulent flows. Friction is calculated with the Manning’s or Chezy’s equation, and eddy viscosity coefficients are used to define turbulence characteristics. Both steady and unsteady flow problems can be analyzed by RMA2.

SED2D is a two-dimensional depth averaged finite element sediment transport model. It computes bed level cand water depths. SED2D depends on solutions of hydrodyna

3 MODEL DEVELOPMENTS Three models have been developed for the present study: a sta sediment transport models. The steady model, which has tmodel. Unsteady model is used to obtain hydrodynamic generated under the steady model is used for all the subseqparameters such as boundary conditions, material roughness,wetting/drying parameters, time-step, sediment characteristimesh module. SMS saves all the files required to run RMA2

Mesh generation: The study area has been defined usinbranches of Jamuna are separated by a large permanent islanotherwise, remains dry. To reduce the computational time ansimulation grids. The image has been imported in the backgtools for defining the study area boundaries and features, froseveral trials, the appropriate mesh has been generated witstudy area.

Bathymetry: Once the mesh has been finalized the bathmodule. BIWTA lean period bed elevation charts, surveyeinitial bathymetry of the model. The distribution of measureand 2b. Areas where data are missing, February 1999 survehave been used to fill-up the gaps.

Initial and boundary conditions: There are three dischararea. Two discharge boundaries are in Jamuna and one inPadma river. As Jamuna has got two open boundaries, the to

Figure 1. Location of ferry ghats (contour usingdepths 2.3 m below and above)

hanges, sediment concentrations, bed shear stresses mic models, like RMA2.

eady hydrodynamic, an unsteady hydrodynamic and o be generated first, is used to hot-start the unsteady solution files for sediment transport model. Mesh uent modeling. Once the mesh is generated, various turbulence exchange coefficients, initial conditions, cs, sediment concentrations etc. are assigned using and SED2D models. g a LANDSAT image of 17th November 2000. Two d, which is only submerged during very high flood, d grid, the large middle island is excluded from the

round using map module. The map module provides m which a finite element mesh can be created. After h a total of 4185 elements and 12940 nodes in the

ymetry is interpolated to the mesh nodes by scatter d during April 2001, have been used to set-up the d and interpolated bathymetries is shown in Figs. 2a yed data of BIWTA and of BWDB cross-sections,

ges and one water level boundaries used in the study Ganges; the remaining water level boundary is in tal flow of the river has been divided into two equal

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parts according to BIWTA (2003). Discharges and water levels of 2001 to 2003 are used as boundary conditions in those boundaries. Two forms of initial conditions have been used in RMA2. In cold-start simulation it has been assumed that initial water levels are same everywhere as the downstream boundary water level and velocities at all nodes are equal to zero. In hot-start, previous solution file obtained from a cold-start run has been used as initial condition.

Roughness parameters: Depth varying Manning’s n has been assigned as roughness. After calibration the lower and upper values of n have been assigned as 0.029 and 0.045.

Turbulence: Turbulence is used to maintain the stability of the model in RMA2 and in this study after several trials, an automatic transfer of turbulence has been assigned using Peclet number equal to 20.

Wetting/drying parameters: To simulate wetting/drying phenomena, a drying depth of 0.01 m and a rewetting depth of 0.11 m have been found suitable in the study.

Time step: Time step is very important in unsteady modeling. After several trials, a time step for both hydrodynamic and sediment models have been taken as one-hour. This time step has provided stable solutions.

Calibration and validation: The unsteady hydrodynamic model has been calibrated by comparing the simulated and observed water surface elevations of 2001 at Aricha. Further the model has been successfully validated for 2002 and 2003. Sediment model has been calibrated by comparing simulated and observed bathymetries for the August 2001. Validation of the same sediment model has also been done for 2002 and 2003 bathymetries. It is imperative to mention here that all the morphological changes found to occur only for monsoon flows, so the above calibration and validation runs have been given using monsoon data (June-October) only. Sediment model is also satisfactorily compared for simulated sediment rates with the observed rates (FAP 24, 1996) at Baruria.

Surveyed Lines April 2001

Data from Feb 1999

BWDB X-Sections

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Figure 2a. BIWTA measured bathymetry on to the satellite image of 17th November 2000

SUSTAINABLE NAVIGABLE CHANNELS

he main objective of the study is to find a suitable ferryorphological simulation (2001-2003), it has been observe

onfluence. It is also observed that the old ferry ghats ieposition in its navigational channels. According to BIWT.3 m. These minimum depths usually occur during the le

Figure 2b. Interpolated bathymetry from surveyed data (colors using 2.3 m depth below and above)

route for navigational purpose. After three years of d that a huge deposition has been occurred around the n the upstream locations have been blocked due to A the minimum depth requirement for a ferry route is an seasons in the months of February or March. As

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simulation runs are given for monsoon periods, the depths are obtained for those monsoon periods only. Therefore, to obtain depths at lean periods, several steady solutions have been generated for initial (April 2001) and simulated bathymetries (for October 2001, October 2002, and October 2003), using lowest water level and corresponding discharge of the three years in question. Lowest water level of 2.6 m PWD (Public Works Datum) at the downstream boundary of Padma river has been applied for the above steady runs. The corresponding discharges at Jamuna (combined) and Ganges are 4100 m3/sec and 2110 m3/sec, respectively.

The above steady solutions show that a perennial deep-pocket (shown in Fig. 3a) exists at downstream location close to confluence near Naradaha. This deep-pocket is a part of a one-time navigation channel, which was formed after 1998 flood and then since following year getting deposition from its upstream locations. Nominal deposition has been observed in the deep-pocket for the 2001-2003 morphological simulation. The upstream deposited area is becoming integral part of the country land and thus not supplying much sediment to the deep-pocket. It is also found that the influence of the sedimentation near the confluence is reducing significantly in the deep-pocket. It seems the deep-pocket is in the leeway besides the heavy siltation region.

The deep-pocket location is at a 3.3 km less distance than existing ferry ghat location at Kazirhat (shown in Fig. 1). If the location is chosen, distance, travel time and fuel cost can be kept at minimum and greater turnout of ferry operations may be achieved. During the study by BIWTA (2003), the existence of such a deep-pocket was overlooked. So this deep-pocket motivates this study to take into account three possible options, as shown in Table 1, to find a sustainable navigable route. The length and amount of capital dredging required to implement those options, as calculated from 2003 simulated bathymetry, are also given in Table 1. All these options have been run, using boundary conditions of 2003 monsoon, to see the impact of interventions.

Table 1. Options with the amount of capital dredging, interventions and their locations

Options Location Intervention Amount of dredging, Mm3

Option1 near Naradaha (635 Lat.) dredging (2438 m) 3.57

Option2 near Natibpur (637.5 Lat.) dredging (1474 m) 0.90

Option3 dredging in both Natibpur and Naradaha, and spur at Natibpur

dredging and spur (3912 m) 4.47

650 645 640 Natibpur Option1 635 Naradaha 630 625 465 470 475 480 Figure 3a. Navigable depths on to 2003 bathymetry (using depths 2.3 m below and above)

650 645 640 635 630 625 465 470 475 480 Figure 3b. Navigable depths after 1-year simulation (using depths 2.0 m below and above)

Deep pocket

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650 645 640 Option2 Natibpur 635 Naradaha 630 625 465 470 475 480 Figure 4a. Navigable depths on to 2003 bathymetry (using depths 2.3 m below and above)

650 645 640 635 630 625 465 470 475 480 Figure 4b. Navigable depths after 1-year simulation (using depths 2.0 m below and above)

650 645 640 Spur Option3 Natibpur 635 Naradaha 630 625 465 470 475 480 Figure 5a. Navigable depths on to 2003 bathymetry (using depths 2.3 m below and above)

650 645 640 635 630 625 465 470 475 480 Figure 5b. Navigable depths after 1-year simulation (using depths 2.0 m below and above)

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The first option (Fig. 3a) is a dredging of the channel near Naradaha connecting the lower segment of the deep-pocket. The second option (Fig. 4a) is also a dredging of the existing channel near Natibpur to connect the upper segment of the deep-pocket. The third option (Fig. 5a) has got both upper and lower segments of the deep-pocket connected with proposed dredged channels, along with a spur placed in the left bank near Natibpur. The results after one-year simulation for each option have been shown in Figs. 3b, 4b and 5b. In Option1 (Fig. 3b), the proposed channel completely closed up due to heavy siltation of the order of 2.5 m. In Option2 (Fig. 4b) dredged channel shows little deposition of the order of 0.35 m. Scrutinizing these two options in the model runs, in a bid for further improvement of Option2, third option has been taken. But lower dredged channel in Option3 (Fig. 5b) has been blocked-up again due to huge deposition near the confluence. Upper dredged channel shows more deposition compare to Option2 of the order of 0.45 m due to presence of scoured soils from the spur in front of the channel. Therefore Option2 has come-out as an alternative route, which required a total of 0.18 Mm3 (0.35 m×350 m width×1474 m length) dredging in a year to maintain the channel as an entrance to the deep-pocket. So, future ferry ghat has been proposed besides the deep-pocket at Naradaha/Khayerchar. The new route to deep-pocket is 3.3 km less than the route followed in Kazirhat by BIWTA (2003). This site poses a new alternative compared to Kazirhat. 6 CONCLUSIONS Hydrodynamic and sediment transport models have been used to find a morphologically sustainable navigable channel. Once the models are adequately calibrated and validated, can be used for future simulations. Three options have been examined using a deep-pocket found near west bank of Naradaha. Option1 is the dredging of a channel connecting the lower segment of the deep pocket near Naradaha. This channel fills with severe deposition within a season. Option2 is the dredging of an upper channel connecting the upper segment of the deep-pocket. This dredged channel shows little deposition and can be maintained with yearly dredging of the order of 0.18 Mm3. To see further intervention, Option3 has been designed as a blend of Option1 and Option2 along with a spur in the left bank of the permanent island (char). However, the option has been found ineffective to improve the situation. Lower channel again silted up within a season, and the upper channel showed higher deposition compare to Option2 due to the presence of a spur in the mouth of the channel. Therefore, Option2 has been considered for using as a navigation route towards Khayerchar/Naradaha instead of Kazirhat proposed by BIWTA (2003). The new route towards deep-pocket is found to be 3.3 km less than the route followed in Kazirhat. This site represents a new alternative compared to Kazirhat. Satellite images near Naradaha/Khayerchar show that the location of the confluence is ever changing, in future flow patterns would force the confluence to attain new positions. Therefore any new development particularly construction of roads and ferry terminals necessitates the need for further study by including these new infrastructure and bathymetry under the model construction. This forthcoming study may suggest further means for sustainability of the Naradaha/Khayerchar area for ghat construction. 7 REFERENCES BIWTA. (2003). “Hydraulic and morphological study for the selection of a site for ferry ghat alternative to

Nagarbari/Notakhola.” Final Report prepared by department of Water Resources Engineering, BRTC, BUET, June 2003.

Bristow, C.S. (1987). “Brahmaputra River: Channel migration and deposition.” Proc. Conference on Fluvial Sedimentolgy.

Coleman, J.M. (1969). “Brahmaputra River: Channel processes and sedimentation.” Sedimentary Geol., Vol. 3, Nos. 2-3, pp. 129-239.

Klaassen, G. J., and Vermeer, K., and Uddin, N. (1988). “Sedimentological processes in the Jamuna (Lower Brahmaputra) River, Bangladesh.” Proc. Intern. Conf. On Fluvial Hydraulics, Budapest, Hungary, pp. 381-394.

Thorne, C. R., Russel, A.P.G., and Alam, M.K. (1993). “Planform pattern and channel evaluation of the Brahmaputra River, Bangladesh.” in ‘Braided Rivers’J.L. Best and C.S. Bristow (eds), Geological Society of London Special Publication No. 75, ISBN 0-9003317-93-1, pp. 257-276.

Hoque, N. (1999). “Application of remote-sensing and GIS for monitoring fluvial processes.” PhD. Thesis, Physics department, BUET.

Hossain, M.M. (1992). “Total sediment load in the lower Ganges and Jamuna.” Jr. Institution of Engineers, Bangladesh, vol. 20(1-2), pp. 1-8.

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