Keywords

1 Introduction

Socioeconomic upliftment due to rapid urbanization in the metro areas is a prime concern in today’s era. The infrastructure development in the cities helps in the growing economy of the individuals as well as enhancing the lifestyle of the citizens. In the north-eastern part of the country, the Brahmaputra River provides immense opportunity in the development of the tourism as well as in some other sectors. The riverfront protection or development in this dynamic river is a critical task because of its unpredicted nature. Near the city of Guwahati, during monsoon, generally the river carries more than 40,000 cumecs of water with sediments, whereas during lean period, the flow recedes to near about 3000 cumecs. Actual variation in flow however ranges from nearly 2000 cumec to 70,000 cumec in this reach. A detail study is needed to be carried out before implementing any near bank infrastructure developmental work at the waterfront of Brahmaputra River. Physical modelling of a braided river is not quite cost effective and is tedious in nature. Under this circumstance, numerical modelling is a better alternative to the former with some amount of uncertainty. Out of different hydrodynamic modelling tools, MIKE21C model is used for the simulation in this study. MIKE21C is a 2D software developed by the Danish Hydraulics Institute, Denmark, used for simulating free-surface flows, water quality, sediment transport and waves in rivers, lakes, estuaries, bays, coastal seas and other water bodies. It solves the 2D depth averaged unsteady free surface flow equations in curvilinear coordinates by finite difference implicit method. Hassan and Dibike [2] used MIKE21C to simulate the confluence of the rivers Ganges and Yamuna, situated in Bangladesh, and showed its potential in river dredging and navigation studies. Hye and Jahan [3] used MIKE21C to simulate Meghna River in Bangladesh and investigate the response to flow and sediment regime changes due to construction of a road bridge upstream of an existing railway bridge. Beck and Basson [1] used MIKE21C to determine the effectiveness of flushing of sediments during breaching, by investigating the breaching process at different water levels for the Klein river estuary in South Africa.

In the present study, a 2D hydrodynamic modelling is done at a reach length of 1.00 km at Brahmaputra River near Guwahati. Simulation is carried out for two scenarios to study the possibility of changes in the flow characteristics near the bank due to possible development/protection activities taken up at the riverfront. A critical discussion is made based on the observed results. Figure 1 shows the satellite images of the modelled reach in different years.

Fig. 1
figure 1

Historical satellite imageries depicting the site for riverfront development

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2 Study Area and Model Description

Brahmaputra River is one of the largest rivers in the world carrying huge amounts of sediments. The river crosses the Indo-China Border near Pasighat, entering Assam and flows westward for about 700 km downstream towards Dhubri leaving Assam and entering Bangladesh. For the present study, a river reach of around 1.00 km on the main stem Brahmaputra has been considered (Fig. 2). The area lies between 91° 44′ 28.51″ E, 26° 12′ 05.91″ N and 91° 42′ 04.45″ E, 26° 10′ 15.64″ N in the Guwahati region. The bed slope in this portion is around 0.1 m/km. The temperature varies from 10 °C in the winters to 38 °C in the summers.

Fig. 2
figure 2

Study area of the Brahmaputra River, Assam

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The advancement of the study in understanding the changes within the river and its effects has been possible through model study. Hydrodynamic models solve complex equations using numerical methods with the advantage of calculating various flow parameters. MIKE 21C has been used to carry out the simulations in different scenarios in the present study.

MIKE 21C simulates the 2D flow in curvilinear grid and studies the sediment transport and morphological processes in rivers. It is a 2D depth averaged flow model based on the Saint Venant equation (continuity and conservation of momentum). The equations solved are:

$$\frac{\delta p}{\delta t}+\frac{\delta }{\delta s}\left(\frac{{p}^{2}}{h}\right)+\frac{\delta }{\delta n}\left(\frac{pq}{h}\right)+2\frac{pq}{h{R}_{n}}+\left(\frac{{p}^{2}-{q}^{2}}{h{R}_{s}}\right)+gh\frac{\delta H}{\delta s}+\frac{g}{{C}^{2}}\frac{p\sqrt{{p}^{2}+{q}^{2}}}{{h}^{2}}=\mathrm{RHS}$$
$$\frac{\delta q}{{\delta t}} + \frac{\delta }{\delta n}\left( {\frac{q^2 }{h}} \right) + \frac{\delta }{\delta s}\left( \frac{pq}{h} \right) + 2\frac{pq}{{hR_s }} + \left( {\frac{q^2 - p^2 }{{hR_n }}} \right) + gh\frac{\delta H}{{\delta n}} + \frac{g}{C^2 }\frac{{q\sqrt {p^2 + q^2 } }}{h^2 } = {\text{RHS}}$$
$$\frac{\delta H}{{\delta t}} + \frac{\delta p}{{\delta s}} + \frac{\delta q}{{\delta n}} - \frac{q}{R_s } + \frac{p}{R_n } = 0$$

where s,n—coordinates of curvilinear coordinate system, p,q—mass fluxes in s and n direction, H—water level, h—water depth, g—gravitational acceleration, C—Chezy Roughness coefficient, Rs, Rd—radii of curvature of s and n lines, RHS—Reynold stresses.

3 Methodology and Applications

An important stage in simulation of the hydrodynamic model is the model set up and grid generation. A suitable curvilinear grid is developed over the study area whose coordinates are derived from the satellite imageries (Fig. 3). A dense grid provides accurate simulations of the water flow. For the model set up, a bathymetric file is required as an input over which the simulation run is dependent. The bathymetric survey was conducted along the study reach, and the simulation was performed with the necessary boundary conditions. The model calibrated and validated for this portion of the river reach under a different project was run at two different discharges given as an input in the upstream boundary condition. The observed water level at the downstream boundary for the corresponding discharges was used as the downstream boundary condition.

Fig. 3
figure 3

Curvilinear grid and Bathymetry of the study area

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The highest discharge recorded in the Brahmaputra River in Pandu (near Guwahati) is nearly 70,000 cumec and lowest recorded discharge is 1800 cumec (source: waterresources.assam.gov.in). The simulation was, however, performed for two different discharges: (a) considering high-flow scenario commensurate with the bank full discharge, which is about 50,000 cumec for the study area and (b) for a discharge of 30,000 cumec, at which the river water touches the bank with significant depth. Two scenarios were taken into consideration under which the entire study was performed. Scenario 1 represents the present condition of the river, wherein the area marked (Fig. 2) is studied for changing flow alteration near the bank. Scenario 2 represents a condition where the bank (encircled portion) is elevated to the road level and the flow conditions are checked. This study primarily focusses on evaluating changes in the present flow condition if the bank is raised in this relatively stable portion (marked in the Fig. 2) to develop the same for recreation and other social activities. This would attract investments, provide employment opportunities, tourism and add economic growth of the city.

4 Results and Discussion

It is observed from the historical images that the morphology of the modelled portion has not changed much for several years. Therefore, scope of utilizing this portion for various activities can be explored. The simulated water depth under different discharges are shown in Figs. 4 and 5 under the existing scenario and road elevated scenario. Figures 6 and 7 represent the simulated velocity vectors under the road elevated scenario and present scenario. An inundation in the modelled area is observed under high discharges for present condition, and therefore, raising of this portion is necessary to utilize that portion for some activities. Figures 8 and 9 show the scour depth at the u/s and d/s of the obstruction present in the reach for the present and proposed scenario. Under the proposed new scenario, no inundation is noticed in the modeled area; however, some variation in the velocity has been observed (Fig. 10).

Fig. 4
figure 4

Simulated water depth under varying discharges under the existing scenario

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Fig. 5
figure 5

Simulated water depth under varying discharges under the road-elevated scenario

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Fig. 6
figure 6

Simulated velocity vectors under varying discharges under the present scenario

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Fig. 7
figure 7

Simulated velocity vectors under varying discharges under the road-elevated scenario

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Fig. 8
figure 8

Scour depth at u/s and d/s of the statue at present scenario a Q = 30,000 cumec, b Q = 50,000 cumec

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Fig. 9
figure 9

Scour depth at u/s and d/s of the statue at proposed scenario a Q = 30,000 cumec, b Q = 50,000 cumec

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Fig. 10
figure 10

Velocity plots of the present and proposed scenario at a 30,000 cumec and b 50,000 cumec

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5 Conclusion

The geoinformatic study has shown that the stable area, identified from the historical planform study, can be protected from flooding and can be utilized for recreational and other economic activities to attract tourists. As the river is quite wide as compared to this narrow strip considered for raising, no significant changes in near bank velocity is noticed. The bank is not inundated at discharges of 30,000 and 50,000 cumecs under the proposed scenario. Of course, with higher discharges, flow will overtop. However, raising of the narrow stable strip to the present road level will prevent flooding of this area for a discharge up to 50,000 cumec. Maximum scour depth under the present scenario is observed within the range of 10–20 m and in the proposed scenario, scour depth is observed within the range of 7–15 m at the upstream. As the scour depth is high, necessary protection measures will have to be adopted for ensuring stability of the raised portion.