1 INTEGRATING 1D-2D HYDRODYNAMIC MODEL FOR SABARMATI UPPER RIVER BASIN WITH SPECIAL REFERENCE TO AHMEDABAD CITY AREA. Sejal Chandel ˡ, Dr. Suvarna Shah² ˡ PG Student, Department of Civil Engineering, Faculty of Technology and Engineering , The Maharaja Sayajirao University of Baroda, Vadodara, Gujarat, India ²Associate Professor, Department of Civil Engineering, Faculty of Technology and Engineering ,The Maharaja Sayajirao University of Baroda, Vadodara, Gujarat, India [email protected] [email protected] Abstract In recent study, Gujarat has become one of the India’s most urbanized state, causing severe flash flooding. The Sabarmati river is one of the major west-flowing rivers in India and biggest river of north Gujarat. Urbanization should meet the population’s need by enlargement of paved areas, which has unusually changed the catchment’s hydrological and hydraulic characteristic. Therefore, the frequency of flash flooding in Sabarmati river has been increased. The Sabarmati river basin experienced eight times devastating flooding condition between 1972 to 2020.Among which July 2017 flooding event breakdown a 112 years old record of 1905. The Dharoi dam and Wasna barrage on Sabarmati river and surrounding district Kheda, Mehsana, Gandhinagar, Ahmedabad received a huge rainfall caused anomalous inflow to tributary which forced the dam authorities to release huge discharge in short duration. The Sabarmati riverfront of Ahmedabad had been going under water for five days due incessant rainfall in the city that leads to swelling of the Sabarmati River in 2017. In order to determine extent of Inundation, Hydrodynamic Model HEC-RAS(5.0.6) with Arc GIS was used. Various scenarios were run to study the impact of simulation on flood inundation(with & without riverfront project).The simulated flood depths have been compared with actual depths obtained at gauging station, which were collected from Government authorities. Ultimately, the analysis was used to create maps for flood of different return periods with RAS Mapper and ArcMap that visually show the reach of the floodplains, illustrating the affected areas. Results demonstrate the usefulness of modelling system to predict the extent of flood inundation and thus support analyses of management strategies to deal with risk associated with infrastructure in an urban setting. Keywords: Hydrodynamic Model, Sabarmati Riverfront, HEC-RAS, Flood Mapping 2 1 Introduction: Flooding is a common natural disaster with a devastating and widespread effect responsible for economic losses and mortality [1,2]. It is the most frequent natural disaster among all, to be faced by India[3]. Over the years, the adverse effects of flooding have increased due to changing climate conditions and human interventions[4]. The major factors which lead communities to increased exposure of such flooding risks include urban expansion, changing demographic features within floodplains, changes in flood regime, and human intervention (social and economic developments) in the ecological system [5]. Planning decisions such as the construction of dams, weirs, canals and houses in floodplains can also increase the risk of floods[6]. Flood modelling has provided an indispensable tool to inform the development of the robust flood risk management strategies to avoid or mitigate the adverse impacts of floods on individuals, communities, and critical infrastructures such as transportation routes, hospitals, and others[7]. Flood water surface elevation information is important to know the depth of the flooding[8]. Hydraulic models play an important role in determining accurate water surface elevations as well as flood inundation areas using sets of hydrodynamic equations. With the advancement in the computer technology, computation of river hydraulics and modelling became easier now by use of various one-dimensional (1D), two-dimensional (2D), 1D-2D coupled, and three-dimensional (3D) models. The 1D models are more popular because of their simplicity for set up and calibration [13], but due to certain limitations, these models are not suitable for floodplain modelling [14]. The 1D models that use only channel geometry are ideal for forecasting accurate water levels until the water is confined within the bank of channel [15]. Whereas the 2D Hydrodynamic models are widely used to simulate urban flood inundation [16-18]. However, 2D calculation can be time consuming, and large variations in cell sizes may occur because of the complex urban underlying surface or local refinement, which leads to lower efficiency because the time step is limited by numerical stability and is largely determined by the smallest cells in the mesh. The 1D-2D Integrated hydrodynamic models have advantage of both the 1D and 2D hydrodynamic model because of its similarity between physical and model behaviour. Although in the above-mentioned studies, the Integrated 1D–2D models can dynamically represent coastal, urban, river and floodplains interactions and are therefore well suited to assess the impact of flooding from different sources. Studies on 1D and 2D modelling of river flow are limited in India due to unavailability of good quality of measured data. Availability of limited number of river cross-sections and scarcity of hourly time series of water level and flow data are the main causes, limiting the river hydraulic studies in India[20].The Present Study was undertaken keeping in view the flood problem also the impact of Sabarmati riverfront of the semi-arid region of Sabarmati basin located in northern part of the Indian state of Gujarat. To overcome the problem of less available surveyed river cross-section, in the present study, river cross-sections were extracted from Shuttle Radar Topography Mission (SRTM) Digital Elevation Model (DEM) of 30 m spatial Resolution Using ARC-GIS software developed by ESRI( 3 Environmental Systems Research Institute).The HEC-RAS software (which is developed by Hydrologic Engineering Centre of U.S.Army corps of Engineers) is used to develop the 1D hydrodynamic model for the Sabarmati River. Finally, the calibrated and validated 1D hydrodynamic model was coupled with the 2D floodplain model to assess the effect of the Sabarmati riverfront on flood inundation extent at downstream reach. Flood frequency analysis is carried out to calculate the flow magnitudes of 100-year recurrence interval using a statistical technique known as Gumbel distribution, subsequently which has been used for flood simulation . Water surface elevations greater than the terrain elevation are included in the inundation depth grid. 2 Study Area and Data Used: 2.1 Sabarmati Basin The Sabarmati River is one of the four main rivers which traverse the alluvial plains of Gujarat. It rises in the Aravalli hills at a north latitude of 24° 40' and an east longitude of 73° 20' in the Rajasthan state at an elevation of 762 meters near the popular shrine of Amba Bhavani. After traversing a course of about 48 km in Rajasthan, the river enters the Gujarat State. Wakal River joins it from the left, near village Ghonpankhari. It receives the Sei River from the right near Mhauri and then the Harnav River from the left at about 103rd km from the source. Thereafter, it enters the Dharoi reservoir. Downstream of the Dharoi reservoir, Sabarmati is joined the Hathmati River. The river passes through Ahmedabad at about 165 km downstream of Dharoi dam. Further 65 km downstream, another tributary, the Watrak River joins it from the left. Flowing for a further distance of 68 km, the river outfalls into the Gulf of Cambay in the Arabian Sea. The Sabarmati River has a length of 371 km and the drainage area is of 21674 sq. km. The Sabarmati basin extends over parts of Udaipur, Sirohi, Pali and Dungarpur districts of Rajasthan, Sabarkantha, Kheda, Ahmedabad, Mahesana, Gandhinagar and Banaskantha districts of Gujarat. In Gujarat, the basin occupies an area of 17,550 Sq. km accounting to 81% of the total basin area. In Rajasthan, it covers an area of 4,124 Sq. km which accounts for 19% of the total basin area. In the Figure 1, the index map of the Sabarmati basin has been shown along with latitudes and longitudes i.e., where the basin is located in India. It also shows the elevation map of Sabarmati basin and locations of the tributary, dam and barrage site. The Average annual rainfall of the Sabarmati basin is 689.90 mm. The average annual mean temperature for this period is 26.33°C. The Study area for the present study starts from the downstream of Dharoi Dam to Vautha. In this Study, there are two major hydraulic structures in the Sabarmati river basin, the Dharoi dam and the Wasna Barrage. The Dharoi Dam is located on Sabarmati river at Kheralu taluka of district Mehsana, 103 km from the source of the river with a catchment area of 5475 km2 . The Wasna Barrage is located on Sabarmati river at Ahmedabad, 165 4 km from the Dharoi Dam with catchment area of 10619 km2. From the elevation map of the basin, it can be clearly seen that the area after the confluence of the river is almost flat with a low elevation, which makes the region more vulnerable to flooding. The downstream boundary of the study area is the Vautha gauging station at distance of 67 km from the Wasna barrage in the Sabarmati River. The 1D hydrodynamic model was calibrated for 1992,1994,1997,2005,2011,2016,2020 flood year at Three gauging station(Derol bridge, Subash bridge, Rasikpura ) which is located on the Sabarmati River. Passing through the centre of Ahmedabad city, the Sabarmati River is a major source of water for the city. The river has been subjected to severe pressure and abuse owing to then fast pace of urban and industrial growth of the city. Sabarmati Riverfront is a waterfront being developed along the banks of Sabarmati river in Ahmedabad about 9 km from Subhash Bridge up to the Wasna Barrage through the city with an average width varying from 325 to 500 m, with two meandering loops at Gaikwad Haveli and Wadaj.