See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/316282942 Development of Flood Forecasting Model for Narayani Basin, Nepal Conference Paper · April 2017 CITATION READS 1 1,835 3 authors: Binod Parajuli Dilip Kumar Gautam Department of Hydrology and Meteorology Practical Action Consulting 15 PUBLICATIONS 108 CITATIONS 30 PUBLICATIONS 221 CITATIONS SEE PROFILE SEE PROFILE Rocky Talchabhadel Texas A&M University 115 PUBLICATIONS 594 CITATIONS SEE PROFILE Some of the authors of this publication are also working on these related projects: S4W-Nepal View project Multi-hazard risk assessment along Bhotekoshi River Corridor (Sino-Nepal border region) View project All content following this page was uploaded by Binod Parajuli on 18 June 2019. The user has requested enhancement of the downloaded file. A Technical Report On Development of Flood Forecasting Model in Narayani River Basin, Nepal Submitted By: Binod Parajuli Hydrologist Flood Forecasting Section Department of Hydrology and Meteorology (DHM), Nepal May 2016 Executive Summary This study was undertaken with an aim to prepare a flood forecasting model along with flood hazard and inundation maps in the flood affected region of Narayani and East Rapti River. The ultimate purpose is to enhance the flood forecasting and early warning system in the department of Hydrology and Meteorology identifying flood susceptible areas and support as an early warning decision support system for the communities and other stakeholders. The potential flood hazard and inundation maps enables Department of Hydrology and Meteorology to identify or update the warning and danger levels in the respective flood forecasting stations, hence pre-informing of the impending flood event ‘if and when’ the threshold warning and danger levels exceed. This enables the disaster responders to raise alarm to the communities for evacuation and to take precautionary measures, thus saving lives and properties with longer lead times. As a decision support tool, the flood forecasting model and flood hazard maps pre-informs the communities and stakeholders at different levels to plan and implement their programs avoiding the potential flood hazard areas. As an instance, vulnerability assessment of the households living within the potential flood hazard zones can be done to develop and implement mitigation and resilience programs. In other case, identify those vulnerable households and landowners for resettlement and/or implement various programs for resilience capacity buildings at the community and household levels. Department of Hydrology and Meteorology currently have no models for flood forecasting in the country. The United States Army Corps of Engineers (USACE) Hydrologic Engineering Center’s Hydrological Modeling System (HEC-HMS) was examined for streamflow forecasting using Narayani basin. Observed precipitation data was obtained from DHM. Calibration and verification of the modeling system was done through an operational perspective to test the model’s applicability at DHM. Model development was done using observed precipitation and was conducted in several stages. The Nash Sutcliffe for the main Devghat Flood Forecasting Station is 0. 892 for calibration (2008-2011) and 0.893 for the validation (2012-2013) for model without snowmelt consideration. The result is also similar for the snowmelt consideration. After the model error correction using ARIMA modules the Nash-Sutcliffe is 0.97 for both calibration and validation periods. The calibrated model was tested with bias corrected WRF one day precipitation forecast estimations for the same periods. The bias correction scheme was computed and applied at the basin scale. When used as input to the HEC-HMS model the Nash-Sutcliffe is 0.33 for the calibration period and 0.66 for the validation period. But the result is satisfactory when the model error correction is done again for the, which results Nash-sutcliffe to be 0.82 for calibration period and 0.87 for validation period. With above described applications, the key outputs of the study are highlighted hereunder: a. The study has developed a flood forecasting model with satisfactory result for Narayani River basin. This indicate that the model can be steadily used for one day flood forecasting. The study has prepared flood hazard and inundation maps of the settlements along the Narayani/East rapti, rivers covering all communities along the River upto Indo-Nepal Boarder. The flood hazard maps were prepared based on the flood forecast modelling and Geographic Information System (GIS) based analysis of flood inundation extent and depth. b. The warning level of Narayani River was found at 2 years return period with estimated discharge of 8,898 cumecs with the surface water level at 7.5 m from the mean sea level. The danger level is observed in inundation scenario of 5 years return period with the discharge of 10769 cumecs with the surface water level rising to 8.3 m from the datum of the gauging station. Similarly the warning level of East Rapti was found at 5 year return year flood with estimated discharge of 1035 cumecs with the surface water level of 3.0m. The danger level is observed in inundation scenario of 10 return year flood with estimated discharge of 1333 cumecs at 3.4m water level in the gauging station. With this the existing warning and danger level of both the rivers seems to be changed. c. For the inundation scenario at danger level in Narayani River and East Rapti River the flooding scenarios are: Most of the settlements of both the banks of Narayani in Chitwan and Nawalparasi and settlements along right bank of East Rapti River in Chitwan districts are at potential risks of flooding. TABLE OF CONTENTS 1. Introduction 1 1.1 Background 1 1.1.1 Causes of flood in Nepal: 3 1.1.2 Flood problems in Nepal Terai: 5 1.2 Objective 7 1.3 Expected Output 7 2. Study Area 8 2.1 River System 8 2.2 Climate and Hydrology 9 2.3 Topography 16 2.4 Flood and Inundation 17 2.5 Flood Forecasting and Warning System 18 3. Data Preparation 20 3.1 Downloading and Processing of Digital Elevation Model and land Use Data 20 3.2 Hydro-meteorological Data Preparation 21 3.2.1 Hydro-meteorological Data for Hydrological Model 21 3.2.2 Hydro-meteorological Data for Hydrodynamic Model 22 4. Development of Hydrological Model 23 4.1 Methodology Flow Chart 23 4.2 Terrain Preprocessing Using HEC-GeoHMS 23 4.3 HEC-GeoHMS Project Setup 24 4.4 Basin Processing 24 4.5 Extracting Basin Characteristics 24 4.6 Hydrologic Parameters 27 4.7 HMS 28 4.8 HEC-HMS Modelling 29 4.8.1 Editing a Basin Model 30 4.8.2 Creating a Meteorologic Model 31 4.8.3 Assigning the Discharge, Water level and rating Table 32 4.8.4 Control Specifications and Model Run 32 4.9 HEC-HMS Calibration and Validation 32 5. Development of Hydraulic Model 37 5.1 Methodology Flow Chart 37 5.2 Flood Frequency Analysis of Discharge Data in Different Return Period 37 5.3 Preparation of DEM 40 5.4 Land Cover/Use 41 5.5 Manning’s Roughness n 42 5.6 Hydrodynamic Modelling using HEC-RAS 43 5.6.1 Theoretical Background 43 5.6.2 One dimensional flow calculations in HEC-RAS 43 5.6.3 Pre-processing to develop the RAS GIS import file 44 5.6.4 Post-processing to generate GIS data from HEC-RAS results 44 5.6.5 HEC-RAS Simplifications of St. Venant Equation 44 5.6.6 Steps Involved in Hydrodynamic Model setup 45 5.6.7 Flood Danger Level and Warning Level Assessment 47 Flood Danger Level and Warning Level of Narayani River: 47 6. Conclusion and Recommendation 54 6.1 Synopsis of the Results and conclusion 54 6.2 Recommendation 55 ACKNOWLEDGEMENTS I wish to express my sincere gratitude to Dr. Rishi Ram Sharma, Director General, Department of Hydrology and Meteorology (DHM) and Mr. A.R Subbiah, Director of RIMES (Regional Integrated Multi-Hazard Early Warning System) for providing us an opportunity to attend the secondment program on “Hydrological and Hydrodynamic Modelling for Operational Flood Forecasting” in RIMES, Thailand. This project bears on imprint of many peoples. I sincerely wish to express my deepest sense of gratitude especially to Dr. Dilip Kumar Gautam, Hydrology- Team Leader (RIMES), Hydrologist Dr. Anshul Agrawal (RIMES) Mr. Itesh Das (RIMES) and Mr Niraj Shakya as well as other staff members of RIMES who rendered their help during the period of my secondment. The financial support from the project “Development of flood forecasting generation and application system for disaster mitigation in Nepal” is gratefully acknowledged. I would like to thank DHM and RIMES for providing necessary data to conduct this research. Lastly I would like to thank Mr Dinkar Kayastha, Hydrologist and secondment colleague from DHM for his constant co-operation during the study period. 1. Introduction 1.1 Background Nepal is a small country with an area of 147,181 sq. km. Its mean width is 193 km only. In such a short stretch, the elevation varies from 60m to 8848m. Nepal is a rich country in water resources. Nepal is also characterized by sources of many small to large-sized rivers, which flow from north to south. There are over 6,000 rivers and their total length exceeds more than 45,000 km. Of these rivers, about 1,000 are more than 10 km long and approximately 100 are more than 160 km long. The entire territory of Nepal is a part of the Ganges basin. The rivers of Nepal (Figure 1.1) can be classified into three categories: 1. Rivers originating from Himalayas: e.g. Koshi, Narayani, Karnali, and Mahakali. These are perennial rivers. They originate from the Himalayas and, after descending from the hills, flow through the Terai plains.