National Conference on Water Resources & Flood ...

National Conference on Water Resources & Flood Management with special reference to Flood Modelling October 14-15, 2016 SVNIT Surat

HYDRODYNAMIC SIMULATION OF PILI RIVER FOR RIVERBED DEVELOPMENT USING HEC-RAS Siddhant Dash1, Chetan Topre1, Ritesh Vijay2 and Rajesh Gupta1 1

Civil Engineering Department, Visvesvaraya National Institute of Technology, Nagpur 440010, India Email: [email protected] 2 Cleaner Technology and Modeling Division, National Environmental Engineering Research Institute, Nagpur 440020, India Email: [email protected]

ABSTRACT Nagpur, the winter capital of Maharashtra, is rapidly developing towards mega cities. With the rapid urbanization and the smart city development plan, the impact on the natural resources and environment is of prime importance. Amongst these, the rivers in the city will have significant effect. The possibility of flooding in the vicinity of the bank of river increases due to settlement and encroachment on either side of the river. So, proper river training works must be established, and boundaries of the river bank shall be fixed in order to prevent flood during peak flows in the river. The study area considered is Pili River, which originates from Gorewada Tank and runs towards the eastern side of the city. It carries sewage and storm water from north zone of Nagpur through minor and major drains/nallahs. The hydrodynamic simulation of the Pili River was carried out using ArcGIS and HEC RAS considering floods for 5yr, 10yr, 25yr, 50yr & 100yr return period under existing and modified river geometry. The results in terms of water profile, flow and velocity at different cross sections of the river were transferred to Google Earth for delineation of flood lines for peak flows of 100yr return period. The flood lines provide potential warning to help river training works by considering dredging, slope stabilization and construction of levees for future planning and construction activities along the river. Key Words – River, Hydrodynamic Simulation, Encroachment, Flood, Flood Lines

1. INTRODUCTION Urbanization associated with economic growth, particularly in developing countries, has become an inevitable fact of progress in the past half century (United Nations, 2010). With more than 50% of the world’s population now residing in urban areas, the hazards of environmental degradation and changes are increasing rapidly. One of the major natural calamities which the world is facing in this 21st century is Floods. In recent past human activities have emerged as one of the major cause of floods. The increasing human activities along the floodplains of river along with heavy rainfalls as a result of climate change effect have given rise to major risks of flooding during the storms in monsoon. It has impacted the humans and development on a large scale with loss of lives and economy of the country. India has witnessed serious flooding events in last decade. Floods of Mumbai (2005), Uttarakhand (2013, 2016), Kashmir (2014), Chennai (2015) and Madhya Pradesh (2016) have proved the adverse effects it has on living beings and economy of the states. With the rise in such events and noticing the common cause of it, the government and local bodies have started giving importance to the flood management. With rapid advancement in computational technology and research in numerical techniques, various one dimensional (1D) hydrodynamic models based on hydraulic routing have been developed, calibrated, validated and successfully applied for flood forecasting and inundation mapping (Vijay et. al, 2007). Hydrodynamic models WRF-28-1

National Conference on Water Resources & Flood Management with special reference to Flood Modelling October 14-15, 2016 SVNIT Surat

that reproduce the hydraulic behavior of river channels have proved to be effective tools in floodplain management. In the present study, the hydrodynamic simulation of the Pili River in the city of Nagpur, Maharashtra, India has been carried out for storm of 5yr, 10yr, 25yr, 50yr and 100yr return periods respectively with existing and modified river geometry.

2. STUDY AREA Nagpur is located at the exact center of the Indian peninsula and at a mean altitude of 310 meters above sea level, with an area of 217.56 sq.km and a population of 25.49 Lakhs (Year 2011). The average rainfall is 1242 mm. Water supply to the Nagpur city is about 700 MLD and sewage generation is around 550 MLD. There are several natural and artificial lakes in the city. The city is divided into three zones namely; north zone, central zone and south zone based on the drainage pattern of the three rivers namely Pili, Nag and Pora respectively flowing through the city. The Pili River is approximately 18 km, originates from Gorewada Lake at North West Part of the city and flows from west to east in the northern part of Nagpur. It confluences with Nag river and finally merges into Kanhan river. Figure 1 represents catchment of Pili river and the study area of 5 km from Gorewada lake. Three drains entering into the rivers at 2.1km, 2.4km and 3.9 km chainage respectively which contributes to the river stream during the storm. Four sub-watersheds; one due to Gorewada Catchment and three of the drains entering into the river are considered for the present study area as shown in the Figure 1.

STUDY AREA

Figure 1: Catchment of Pili River and Study Area

3. METHODOLOGY 3.1 Flood Frequency Analysis Flood frequency analysis was used for the determination of maximum rainfall intensities. The peak WRF-28-2

National Conference on Water Resources & Flood Management with special reference to Flood Modelling October 14-15, 2016 SVNIT Surat

flow was calculated using rational method (Subramanya 2008). The available data acquired from Pune Meteorological department includes 24-hour Rainfall data basis from 1969-2000 for the Nagpur city. From the available data, the maximum daily rainfall recorded for each of the year is tabulated and the hourly maximum rainfall is evaluated. From the evaluated maximum hourly rainfall data, the mean and standard deviation are evaluated. The hourly Precipitation depth analysis is carried out for various return periods using the Probability Distribution Function. Maximum precipitate depths are converted into hourly intensities and tabulated. A graph is plotted using the values of hourly intensities and duration for different return periods in MS Excel. As per Intensity- Duration- Frequency (IDF) generalized formula

𝑖=

𝑎

(1)

𝑑𝑛

𝑎 = 𝑐 ∗ 𝑇𝑚 (2) Where, i – Intensity in mm/hr. T- Return period in Years c, m, n are regional coefficients d- Duration in Hours The values of the regional coefficients ‘c’, ‘m’ and ‘n’ are obtained as 34.371, 0.1992 and 0.67 respectively and are substituted in equation to obtain the generalized equation for Nagpur City (NMC 2009). Based on the above coefficient, the intensities for various return periods and durations are evaluated. Intensities are plotted for various return periods and durations as shown in Figure 2. In current simulation, watershed characteristics were delineated using CartoDEM and with the help of ArcGIS, the area contributing to proposed drain was calculated in hectares based on topography. The composite coefficient of runoff ‘C value’ was calculated for as 0.7 considering different land uses contributing to a single catchment.. For calculation of storm flow, rational method was used as per the following equation: 𝑄 = 10 𝐶 𝑖 𝐴

(4)

Where, C =Coefficient of runoff A =Area of catchment (Gorewada lake) i =Intensity of rainfall for tc Based on the equation, storm flows were calculated for once in 5 yr, 10 yr, 25 yr, 50 yr and 100 yr return period.

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National Conference on Water Resources & Flood Management with special reference to Flood Modelling October 14-15, 2016 SVNIT Surat

Figure 2: Intensity- Frequency-Duration Curves by Gumbel’s Extreme Value (type 1) distribution

3.2

Model Conceptualization

The model was conceptualized in HEC-RAS (5.0.1) by using the data obtained from the above processes as an input. The river length and its cross sections were used as an input in geometric data. The base flow of the river and peak flood flows of corresponding sub-watersheds pertaining to different return periods were given as an input for steady flow data. The flow profiles for 5 yr., 10 yr., 25 yr., 50 yr. and 100 yr. storm was created and the model was simulated for steady state conditions considering sub-critical flow regime. For the modification of the riverbed, the river bed was dredged for 0.5 m depth from invert level in order to reduce the flooding effect. The hydrodynamic steady state simulation was run again and the difference between water levels at cross sections due to 5 yr., 10 yr., 25 yr., 50 yr. and 100 yr. storm for original condition and modified condition was observed. Also levees were provided at appropriate locations in order to check the flooding of river water at several points in the stretch and the water surface profiles at all the cross sections for the 5 yr., 10 yr., 25 yr., 50 yr. and 100 yr. storm were observed. The results of the simulation were then transferred on GIS Platform; here ArcGIS. The width of submergence at each cross section was noted and was plotted in ArcGIS. The joining of these points gave flood lines for5 yr., 10 yr., 25 yr., 50 yr. and 100 yr. storm on ArcGIS. Similarly the same procedure was repeated for the modified sections after dredging of the river and provision of levees.

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National Conference on Water Resources & Flood Management with special reference to Flood Modelling October 14-15, 2016 SVNIT Surat

4. RESULTS AND DISCUSSION Model simulations for water surface elevations along the stretch of the river were used to determine the extent of the floods. The outputs such as the water surface profile, flow and velocity at the cross sections were used to determine the critical points of flooding along the stretch of the river. The simulation was done for the existing cross section, cross section with the provision of levee and modified cross section with the provision of levees. Cross section is divided into three portions as main channel, left bank and right bank. 4.1

Cross Section Geometry

The existing and modified cross sections with the simulated water level obtained due to all the storms are shown in Figure 3. The cross section geometry of the river was not adequate to accommodate the flow due to all the storms. For example at cross section at Chainage 4300, the water surface profiles were observed to overflow for all discharges. Even, water surface profile was generated for depressed portion of cross section in right and left banks for once in 5 yr, 10 yr and 25 yr return period which was not supposed to get submerged (Figure 3a).To overcome this situational problem, levee points were defined for the existing cross section which resulted in the checking of submergence on those depressed portion (Figure 3b).Further, cross section geometry was modified by keeping the main channel points as it is and dredging of the channel by 0.5m from the invert with the provision of levee points (Figure 3c). 4.2

Longitudinal Profiles

The longitudinal profile plots were obtained after the simulation for base flow (dry weather flow) and once in 5 yr, 10 yr, 25 yr, 50 yr and 100 yr return period storm for existing and modified sections of the river (Figure 4). Figure 4a describes the water surface profiles for existing riverbed and Figure 4b provides the same for modified riverbed. The difference in the water surface profile can be observed in the insets considering river dredging. A drop of 0.3 to 3.6 m in the water surface profile is observed due to modification in the riverbed considering all the cross sections.

(a): Existing Cross section

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National Conference on Water Resources & Flood Management with special reference to Flood Modelling October 14-15, 2016 SVNIT Surat

(b): Existing Cross section with provision of Levees

(c): Modified Cross section with provision of Levees Figure 3: Cross section details at Chainage 4300

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National Conference on Water Resources & Flood Management with special reference to Flood Modelling October 14-15, 2016 SVNIT Surat

(a): Existing river bed (before dredging)

(b): Modified river bed (after dredging) Figure 4: Longitudinal Profiles of the River

4.3 Flood Lines The simulation results of water surface profile for once in 100 years were used to delineate the flood lines along the river using flood points. The floods points are those, which matches the ground elevation with the flood water surface elevation of that particular storm. The flood points were marked on the Google Earth based on the results obtained from the simulation before and after dredging of the sections. Joining of these flood points along the length of the study area delineated flood lines due to 100 year return period storms for existing and modified river geometry (Figure 5). A significant difference in the flood lines can be seen at some points along the stretch of the river due to modification in the river bed geometry. Similarly the flood lines for once in 5, 10, 25 and 50 years were also delineated for immediate and long term measures.

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National Conference on Water Resources & Flood Management with special reference to Flood Modelling October 14-15, 2016 SVNIT Surat

Before dredging After After dredging dredging

Figure 5: Delineation of flood lines for once in 100 year storm (Source: Google Earth)

5. CONCLUSION The hydrodynamic simulation of Pili River has been attempted to compute the water level and discharge profile of normal and flood conditions along the existing river geometry. The water surface profile was computed along the cross section of main channel and adjacent river banks. Profile was also computed for depressed portions beyond the elevated main channel. This is observed as inappropriate representation of water surface profile along the cross section. To overcome this problem, levee points were assigned to the existing cross sections as a result of which it was observed that computation of water surface profile was checked beyond the levee points. Further, the simulation was performed for modified river bed geometry with a dredging of 0.5m below invert level and provision of levee points. There was a significant decrease in the water surface elevation due to modified cross section and as a result, flooding was checked up to a great extent. The water surface elevations were transferred on Google earth and flood lines were delineated for extent of submergence under different storms. This will help in determining the land use under submergence and river training works by considering dredging, slope stabilization and construction of levees for future planning and construction activities along the river. ACKNOWLEDGEMENTS Authors are acknowledged to Director VNIT, Nagpur and Director CSIR-NEERI, Nagpur for their constant support and infrastructural facility to carry out this research study.

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National Conference on Water Resources & Flood Management with special reference to Flood Modelling October 14-15, 2016 SVNIT Surat risk assessment in Lighvan Chai River, 4(4), 38–45. Kute, S., Kakad, S., Bhoye, V., &Walunj, A. (2014). Flood Modeling of River Godavari Using Hec-Ras. International Journal of Research in Engineering and Technology, 3(9), 81–87. Midaoui, et al. (2015). Integration of GIS and HEC-RAS In floods modeling of the Ouerghariver, northern Morocco. European Scientific Journal, 11(2), 196–204. NMC (2009).Preparation of master plan / perspective plan for drainage system and rejuvenation of lakes and rivers of Nagpur city for the year 2041. Draft detailed project report of storm water drainage system central zone volume – 1 –Report,Nagpur Municipal Corporation. Subramanya K. (2008). A Text book on Hydrology Third Edition, Tata McGraw Hill Publications, New Delhi, India. U.S. Army Corps of Engineers (USACE). (2016). ‘‘HEC-RAS River Analysis System.’’ User’s manual, Version 5.0.1, February 2016, Hydrologic Engineering Center. U.S. Corps of Engineers. 2016. “HEC-RAS River Analysis System, Hydraulic Reference Manual.” Vijay, R., Sargaonkar, A., & Gupta, A. (2009). A hydrodynamic approach to address Yamuna riverbed development in Delhi. Canadian Journal of Civil Engineering, 36(7), 1155–1163 Vijay, R., Sargoankar, A., & Gupta, A. (2007). Hydrodynamic simulation of river Yamuna for riverbed assessment: A case study of Delhi region. Environmental Monitoring and Assessment, 130(1-3), 381–387

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