Integrating 1D-2D Hydrodynamic Model for Sabarmati Upper River Basin with Special Reference to Ahmedabad City Area

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. The height of the bank’s ranges from 4 to 9 m. The edge is not clearly defined by embankments or retaining walls at most places, and the river edge gently slopes down to the riverbed at several places, which have vegetations.

Fig 1 Index map of Sabarmati Basin with elevation map and locations of Dharoi dam and

Wasna barrage

2.2 Data used :

The data used for the present analysis was collected from the Dam authorities, Dharoi Dam, Wasna Irrigation Department Ahmedabad, the Central Water Commission (CWC-Gandhinagar) and the State Water Data Center (SWDC). The present study area starts from the Downstream of Dharoi Dam, the structural details and releases from this dam was obtained from the respective dam authority. The SRTM DEM of 30 m spatial Resolution was downloaded from the United States Geological Survey (USGS) for extraction of cross-sections, river alignment and floodplain bathymetry Using ARC-GIS. The land use and land cover for flood plains have been developed using ARC-MAP(ARC GIS). The details about Sabarmati riverfront(Including bridge details) such as location, cross-section detail and elevation of embankment of riverfront have been collected from the SRFDCL House (Sabarmati River Front Development Corporation Limited- Ahmedabad).

3 Methodology:

The methodology involves the development and calibration-validation of 1D hydrodynamic model using HEC-RAS, 2D floodplain model using HEC-RAS, and coupling of these 1D and 2D models using HEC-RAS . Figure 2 shows the flowchart of methodology used in the present study.

Case 2 Case 1 1D Hydrodynamic Model 2D Floodplain Model

Roughness Parameter Bathymetry Cross- Boundary (Manning’s n-value from River Mesh-Creation land use-land cover sections conditions network classification

With Channel modification

Model Calibration using Manning’s n-values Coupling of 1D-2D Hydrodynamic model Validation of Hydraulic model

(w Flood Inundation Mapping For 100 years Recurrence Interval

3.1 Development of 1D Hydrodynamic Model:

The 1D hydrodynamic model was developed for 232 km of river reach using HEC-RAS Software. The SRTM DEM downloaded from the USGS was pre-processed using Spatial Analyst Tools featured in ArcGIS Desktop v10.3.1. This pre-processed DEM was then imported into Ras-Mapper tool(HEC-RAS) to trace river reach(Center line, Bank line and Flow Line) and generate a cross-section. Cross sections for the model were extracted from DEM using the Ras-mapper “Auto-generate cross sections” tool. Figure 2 shows the location and chainage of the cross-sections generated for the 1D hydrodynamic model. The Cross-sections were of 500 m interval and width of 1200m. There are two boundary conditions, Rating Curve of Dharoi dam as upstream boundary conditions, while the normal depth at Vautha gauging station was given as downstream boundary condition. The initial value of Manning’s n is taken as 0.03 for bank and 0.025 for channel for the simulation of the 1D hydrodynamic model. The model was calibrated for flood year of 1992,1994,1997,2005,2011,2016,2020 at Three gauging station(Derol bridge, Subash bridge, Rasikpura ) with different roughness coefficient. The range of Manning’s n was taken from literature (Chow 1959) for calibration of the model. The performance evaluation of the model was carried out by calculating the performance indices i.e., RMSE (Root Mean Square Error). The calibrated 1D hydrodynamic model is then validated for individual flood event of 1993,2006,2015,2017. This calibrated and validated 1D hydrodynamic model was used for coupling with the 2D floodplain model.

SABARMATI RIVER

Fig 2 1D Hydrodynamic Model (With geometric data including Centerline, Flow path, Bank line and Cross-sections) 7

3.2 Development of 2D Hydrodynamic Model :

The 2D floodplain model has been developed for Sabarmati river reach using HEC- RAS. The floodplain bathymetry was generated from SRTM DEM using 2D Mesh generator tool in HEC-RAS .The cell size of mech was taken as 100 m. Due to unavailability of data, the infiltration and evapotranspiration were not simulated in the model as well as negative slope is neglected. The scour depth considered as 1 m. The LULC map of the study area was prepared using FAO soil data images that were downloaded for the study area from (http://www.fao.org/soils-portal/en/) to set the bed resistance value for the floodplain. This image was then converted to a raster format and the study area was extracted using ‘Extract by Mask’ tool of ArcGIS spatial analyst toolbox.

In the present analysis, two cases are considered in the 2D Hydrodynamic model. i.e., case: 1) 2D floodplain model without considering Sabarmati Riverfront, and case: 2) 2D floodplain model with Sabarmati Riverfront. The Sabarmati riverfront was developed as channel modification tool in the HEC-RAS model. The elevation of the top of the embankment and side slope were obtained from the authorities of the Sabarmati riverfront (SRFDCL house). Figure 3 shows the of the cross-section of channel modified in HEC-RAS model. Figure 4 represent the actual Photograph of Sabarmati River-front.

Fig 3. Cross-section of Channel modified (Sabarmati Riverfront Creation) 8

3.3 Coupling of the 1D and 2D hydrodynamic models: The efficient hydrodynamic coupling between the 1D hydrodynamic model of the river and the 2D hydrodynamic model of the floodplain can be carried out using the HEC- RAS v.5.0.6 model. The 1D hydrodynamic model was coupled with the 2D floodplain model. The HEC-RAS Ras-Mapper was used to couple these models with synchronized simulation period. The Ras-Mapper connects the two models by providing the outflow from either bank of 1D model (output from HEC-RAS 1D hydrodynamic model) as the inflow boundary condition of the 2D floodplain model. The MODEL was calibrated for 1992,1994,1997,2005,2011,2016,2020 flood year and validated for 1993,2006,2015,2017 flood year. The model is Calibrate against the observed & simulated water levels at Key location (Derol bridge, Subhash bridge, Rasikpura). In that 3 different manning’s n-value taken was summarised below; [A] 0.030 for bank and 0.025 for channel(throughout river except channel modified), 0.015 for channel modified section. [B] 0.026 for bank and 0.021 for channel up to Dharoi dam to Subash bridge, Form Subash bridge to Wasna barrage 0.011(channel modified). For downstream of Wasna barrage 0.040 for bank and 0.035 for channel [C] 0.040 for bank and 0.035 for channel(throughout river except channel modified), 0.013 for channel modified section. The performance of the model has been assessed using RMSE (Root Mean Squared Error) as the performance index indices. The RMSE describes the spread of the residuals. For the best performance of the model, RMSE is zero. The RMSE is defined as:

where, S0 = observed stage, SS = simulated stage and N = number of observations

4 Result Analysis 4.1 Results of 1D Hydrodynamic Model: The 1D hydrodynamic model was developed and calibrated in HEC-RAS for the Eight flood year such as 1992,1994,1997,2005,2007,2011,2016,2020. Calibration of hydrodynamic model includes the choice of an appropriate value of manning’s ‘n’ such that simulated stage obtained from the HEC RAS model should be close to the observed stages. The 1D hydrodynamic model was simulated for different values of Manning’s roughness coefficient 9 n and comparison of water levels was made at three different gauging station such as Derol bridge, Subash bridge and Rasikpura for calibration and validation. Table 1 represent the results of RMSE value of Different Flood year with different roughness coefficient Such as Case [A], Case[B], Case[C]. It can be observed from Table that the absolute error between simulated results and observed values are decreases with decrease in roughness value. The RMSE corresponding to the Manning’s n of 0.026 for bank and 0.021 for channel as well as for modified channel section’s n is 0.011 and downstream of Wasna barrage taken as 0.040 for bank and 0.035 for channel( case [B] was selected) is the minimum amongst all. The 0.021 value of Manning’s n represents that the river bed is clean and straight with full stage without any drift or deep pools. The 0.035 for downstream of Wasna barrage suggests river bed is of cultivated areas. The 0.011 for modified channel section suggest concrete embankment constructed at both sides of river bed. These results have been validated for four flood year such as 1993,2006,2015,2017 of shown in Table 2 and the value of n as 0.021(Case [b]) is selected as roughness value for channel bed in the present study for further investigation. Figure4,5 and Figure 6 shows the graphical representation of observed and simulated stage values with Manning’s n as 0.021(case[B]) at Derol bridge, Subash bridge and Rasikpura gauging station respectively for different flood year that taken as calibration and validation. The flow becomes 2D in such cases, which can be simulated accurately by 1D-2D coupled hydrodynamic model.

n-value YEAR 1992 1994 1997 2005 2007 2011 2016 2020

Case-[A] 1.015 1.092 2.25 1.059 1.066 1.2 2.08 1.1

RMSE

Case-[B] 0.658 0.74 1.67 1.014 1.03 1.11 1.70 0.88

Case-[C] 1.8 1.52 2.8 1.108 1.12 1.49 2.3 1.172

Table-1 RMSE values for different flood year of different manning n’s value (calibration result) 10

Year 1993 2006 2015 2017 RMSE(n-value 1.23 0.80 0.454 0.53 CASE[B])

Table-1 RMSE values for different flood year of manning n’s value selected case[B] (validation result)

DEROL BRIDGE 100

85 WATER LEVEL(m) WATER 80 1992 1993 1994 1997 2005 2006 2007 2011 2015 2016 2017 2020 YEAR

DEROL BRIDGE COMPUTED W.L DEROL BRIDGE OBSERVED W.L

Fig. 4 Observed and simulated water level at Derol bridge gauging station for case[B]

SUBASH BRIDGE 60 50 40 30 20

WATER LEVEL(m) WATER 10 0 1992 1993 1994 1997 2005 2006 2007 2011 2015 2016 2017 2020 YEAR

SUBASH BRIDGE COMPUTED W.L SUBASH BRIDGE OBSERVED W.L

Fig. 5 Observed and simulated water level at Subash bridge gauging station for case[B] 11

RASIKPURA 25

WATER LEVEL(m) WATER 5

0 2005 2006 2007 2011 2015 2016 2017 2020 YEAR

RASIK PURA COMPUTED W.L RASIK PURA OBSERVED W.L

Fig. 6 Observed and simulated water level at Rasikpura gauging station for case[B]

4.2 Results of 1D-2D coupled hydrodynamic model:

The 1D-2D coupled hydrodynamic models have been developed for twelve flood years (1992,1993,1994,1997,2005,2006,2007,2011,2015,2016,2017,2020)out of which three flood years(2006,2016&2017) shown here with and without incorporating the Sabarmati riverfront(channel modification) using HEC-RAS. Whereas 2006 flood came before development of riverfront and 2016 & 2017 flood came after the development of Sabarmati riverfront. The flood wave propagation of simulated model near the Sabarmati riverfront are shown here. The simulated flood inundations at these places have been compared with actual flood inundation depths obtained by government authorities to identify the effect of the Sabarmati riverfront on flood inundation extent. The velocity extent also been compared for both the cases. The geographic locations of these places have been identified with the help of ARC-GIS imaginary and digitised using Ras-Mapper(HEC-RAS v.5.0.6) and ArcGIS desktop 10.3.1.

Fig 5 & 6 represent the flood extent and velocity extent at Ahmedabad city during 2006 Flood (without Sabarmati riverfront and with Sabarmati riverfront). Fig 7 & 8 represent the flood extent and velocity extent for Ahmedabad city during 2016 Flood(without Sabarmati riverfront and with Sabarmati riverfront). Fig 9 & 10 represent the flood extent and velocity extent for Ahmedabad city During 2017 Flood (without Sabarmati riverfront and with Sabarmati riverfront). Figure 11 to 16 is graphical presentation of water level extent and velocity extent at the downstream of Wasna barrage for flood year 2006,2016 and 2017 respectively.

Fig 5 2006 flood [ flood depth extent without Sabarmati riverfront (case 1) and with riverfront (case 2)]

Fig 6 2006 flood [ Velocity extent without Sabarmati riverfront (case 1) and with riverfront (case 2)]

Fig 7 2016 flood [flood depth extent without Sabarmati riverfront (case 1) and with riverfront (case 2)]

Fig 8 2016 flood [ Velocity extent without Sabarmati riverfront (case 1) and with riverfront (case 2)]

Fig 9 2017 flood [flood depth extent without Sabarmati riverfront (case 1) and with riverfront (case 2)]

Fig 10 2017 flood [ Velocity extent without Sabarmati riverfront (case 1) and with riverfront (case 2)]

14 Water Level Chart for downstream of Wasna Barrage

50 40 30 20 10 Water-level(m) 0

73500 71500 69500 67500 65500 63500 61500 59500 57500 55500 53500 51500 49500 47500 45500 43500 41500 39500 37500 35500 33500 31500 29500 27500 25500 23500 21500 19500 17500 15500 13500 Cross-section

without RF with RF

Fig 11 2006 flood (Water level comparison for both cases at downstream of Wasna barrage)

Velocity Chart for Downstream of Wasna Barrage 6 5 4 3 2

Velocity(m/s) 1 0 73500 71500 69500 67500 65500 63500 61500 59500 57500 55500 53500 51500 49500 47500 45500 43500 41500 39500 37500 35500 33500 31500 29500 27500 25500 23500 21500 19500 17500 15500 13500 Cross-section

without RF with RF

Fig 12 2006 flood (Velocity comparison for both cases at downstream of Wasna barrage)

Water-level chart for downstream of Wasna barrage 50 40 30 20 10 Water-level(m) 0 73500 71500 69500 67500 65500 63500 61500 59500 57500 55500 53500 51500 49500 47500 45500 43500 41500 39500 37500 35500 33500 31500 29500 27500 25500 23500 21500 19500 17500 15500 13500 cross-section

With RF Without RF

Fig 13 2016 flood (Water level comparison for both cases at downstream of Wasna barrage)

Velocity chart for downstream for Wasna barrage 5 4 3 2 1 Velocity (m/s) Velocity 0

73500 71500 69500 67500 65500 63500 61500 59500 57500 55500 53500 51500 49500 47500 45500 43500 41500 39500 37500 35500 33500 31500 29500 27500 25500 23500 21500 19500 17500 15500 13500

Cross-section With RF Without RF

Fig 14 2016 flood (Velocity comparison for both cases at downstream of Wasna barrage)

Water level chart for downstream of Wasna barrage 50 40 30 20 10 0 Water level(m) Water 73500 71500 69500 67500 65500 63500 61500 59500 57500 55500 53500 51500 49500 47500 45500 43500 41500 39500 37500 35500 33500 31500 29500 27500 25500 23500 21500 19500 17500 15500 13500 Cross-section

With RF Without RF

Fig 15 2017 flood (water level comparison for both cases at downstream of Wasna barrage)

Velocity chart for downstream of Wasna barrage 5 4 3 2 1 Velocity (m/s) Velocity 0 73500 71500 69500 67500 65500 63500 61500 59500 57500 55500 53500 51500 49500 47500 45500 43500 41500 39500 37500 35500 33500 31500 29500 27500 25500 23500 21500 19500 17500 15500 13500 Cross-section

With RF Without RF

Fig 16 2017 flood (Velocity comparison for both cases at downstream of Wasna barrage) 16

It has been observed from the simulated results that flood inundated area in the Ahmedabad City are decreasing due to development of Sabarmati riverfront( channel modification). Also, the velocity is increasing range 0.50 to 0.87 m/s thus it suggests the water will pass rapidly within City Area which reduce the impact of flood. It has been noticed from the simulated results that the villages which are located on downstream side of the Sabarmati riverfront are not affected because of the Sabarmati riverfront(channel modification). The simulated flood depths of these villages are having minor difference( 1 m to 1.5 m) which can be neglected which justifies that these villages are not affected because of the Sabarmati riverfront.

The flood frequency analysis(Gumbel distribution) has been caried out for Dharoi dam and Wasna barrage using 35 years of data(received from dam authorities).Thus, the integrated 1D- 2D hydrodynamic models have been developed for 100 years recurrence interval with and without incorporating the Sabarmati riverfront(channel modification) using HEC-RAS. Figure 17&18 represents the inundated area(flood depth map) of Ahmedabad city for both the cases(without riverfront and with riverfront). Figure 19&20 represents the flood velocity map of Ahmedabad city for both the cases. Figure 21 represent Water Surface Profile plot of C/S 83000 for both the cases (without riverfront and with riverfront). Figure 22 and 23 comparison charts (for both the cases -without Riverfront and with riverfront) of water surface elevation and velocity for downstream of Wasna barrage respectively.

Fig 17 Flood depth map for 100-year recurrence interval without riverfront (case 1) 17

Fig 18 Flood depth map for 100-year recurrence interval with riverfront (case 2)

Fig 19 Flood Velocity map for 100-year recurrence interval without riverfront (case 1)

Fig 20 Flood Velocity map for 100-year recurrence interval with riverfront (case 2)

Fig 21 Water Surface Profile plot of C/S 83000 for both the cases (without riverfront and with riverfront

Water level Chart for downstream of Wasna barrage 50 40 30 20 10 Water level(m) Water 0 73500 71500 69500 67500 65500 63500 61500 59500 57500 55500 53500 51500 49500 47500 45500 43500 41500 39500 37500 35500 33500 31500 29500 27500 25500 23500 21500 19500 17500 15500 13500 Cross-section

with rf without rf

Fig 22 Comparison chart of water level (for both the cases) for downstream of Wasna barrage

Velocity Chart for downstream of Wasna barrage 6 4 2 0 Velocity (m/s) Velocity

73500 71500 69500 67500 65500 63500 61500 59500 57500 55500 53500 51500 49500 47500 45500 43500 41500 39500 37500 35500 33500 31500 29500 27500 25500 23500 21500 19500 17500 15500 13500 Cross-section

with rf without rf

Fig 23 Comparison chart of Velocity (for both the cases) for downstream of Wasna barrage

5 Conclusions:

Following findings can be summarized from the present study:

The Sabarmati River basin of India has experienced the severe flash flooding due to urbanization which unusually changed the catchment’s hydrological and hydraulic characteristic. The Dharoi dam and Wasna barrage on Sabarmati River and seven tributary of Sabarmati River received unprecedented inflow which forced the dam authorities to release huge discharge in short duration. The development of Sabarmati Riverfront which increase the beautification of city as well as reduce the flood condition throughout the city. The flood modeling system presented in this study is an integrated system to stakeholders to investigate potential mitigation options and strategies in response to expected flooding scenarios. The 1D hydrodynamic model has been developed for Sabarmati River reach from downstream of Dharoi dam to Vautha gauging station. The developed model has been calibrated for Eight flood year(1992,1994,1997,2005,2007,2011,2016,2020)and identified best performance for Manning’s n value of 0.021for channel and 0.026(from Dharoi dam to Subash bridge),0.011 for modified channel section(Subash bridge to Wasna barrage) and 0.035 for channel and 0.040 for bank for Wasna barrage to Vautha gauging station. The performance of the 1D hydrodynamic model has been assessed by indices like RMSE. The developed model has been validated for four years (1993,2006,2015,2017) shows good results for with RMSE of 1.23,0.80,0.454 and 0.53 respectively. The calibrated and validated 1D hydrodynamic model is then coupled with 2D floodplain model for simulation of flood inundation depths at various locations using HEC-RAS V5.0.6. Two different 2D floodplain models have been coupled with 1D hydrodynamic model, i.e., 2D floodplain model without incorporating the Sabarmati Riverfront, and 2D floodplain model with incorporation of the Sabarmati Riverfront. The simulated flood inundation depths from these two models have been compared with the actual flood inundation depths obtained by Government authorities. The comparison of results shows that the villages which are located downstream side of the Sabarmati riverfront are not affected due to Sabarmati riverfront-Waterfront. The Flood frequency analysis has been carried out for Dharoi dam and Wasna barrage using GUMBEL DISTRIBUSTION method. The integrated 1D-2D hydrodynamic model has been developed for 100 years recurrence interval for both the cases(without riverfront and with riverfront). The simulated flood inundation depths from these two models have been compared with each other and it shows due to development of riverfront the inundated area get reduced and velocity has been increasing in that region. Due to that flood impact get reduces at Ahmedabad city. Also, there is no Hazard impact imposed on downstream of Wasna barrage due to development of waterfront. From the model , the area of Ahmedabad City like Vasna, Gayaspur, Paldi, Vadaj, Chalod are low laying area so its highly flood risk zone of city. The results of 1D hydrodynamic model can be useful to understand the behaviour of river flow, 20

discharge carrying capacity of river and required height of the levees at different cross- sections for flood prevention. The results from 1D-2D coupled hydrodynamic models are useful to develop flood inundation maps for identifying depths of flood in 232 km stretch area . This will help in taking measures for evacuation of people of the study area. The comparison of simulated flood depths with and without incorporation of the Sabarmati Riverfront-Waterfront can be useful to SRFDCL authorities, State Government authorities and people from nearby City and villages to take necessary flood preventive measures.

Conflicts of Interest: The authors declare no conflict of interest.

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