Rapid Assessment of Flood Damage and Community-based Risk Mapping in Flood Diversion Area-Case Study in Wuhan, Hubei Province January, 2018 1 / 41 Contents 1. Project Overview ........................................................................................................................................................... 3 2. Introduction of the study case ................................................................................................................................. 3 2.1 Introduction of the 2016 Wuhan Floods ................................................................................................. 3 2.2 Introduction of Dujiatai Flood Diversion Area.................................................................................... 5 3. Review of previous studies ........................................................................................................................................ 6 4. Data and Methods ......................................................................................................................................................... 7 4.1 Technology roadmap of the project .......................................................................................................... 7 4.2 Data collection .................................................................................................................................................... 8 4.3 Methods for damage assessment .............................................................................................................. 11 4.4 Methods for risk assessment ...................................................................................................................... 12 4.5 Field investigation of the flood diversion area ................................................................................... 13 5. Results for flood damage assessment ................................................................................................................. 16 5.1 Rapid assessment of the flood disaster with remote sensing ........................................................ 16 5.2 Rapid assessment of the flood disaster with social media ............................................................. 20 5.3 GIS platform for flood rapid assessment ............................................................................................. 26 6. Results for flood risk assessment .......................................................................................................................... 30 6.1 Analytical risk mapping ............................................................................................................................... 30 6.2 Community-based risk mapping .............................................................................................................. 33 7. Conclusion and Discussion ...................................................................................................................................... 35 References ........................................................................................................................................................................... 36 Appendix .............................................................................................................................................................................. 38 2 / 41 1. Project Overview Wuhan, the largest city in Central China, is located at the confluence of the Hanshui and Yangtze rivers and thus has always been a hotspot of flood risk in China. During the 2016 summer, heavy rainstorms and great floods hit Wuhan and its surrounding areas, communities in both urban and rural regions were severely affected and thousands of people in flood diversion area were urgently evacuated, leaving 14 mortalities and 2.26 billion RMB Yuan of economic losses. In the southwest corner of Wuhan, there lies the Dujiatai Flood Diversion Area between Yangtze River and Han River, which is the low- lying rural areas near rivers or lakes that are deliberately flooded in emergencies in order to protect the city. Even though planned as restricted development region, the residents in flood diversion area also increase significantly in recent years. They face greater risk of flood and should be given particular attention during flooding. This project focuses on two major issues that are important and challenging for the flood- prone city of Wuhan and especially the flood diversion area. Firstly, it aims to develop models and tools for rapid assessment of flood damage using modern remote sensing and GIS technology, which is often urgently needed for evacuation and relief after the occurrence of the disaster. Secondly, this project highlights the community participation in disaster reduction and aims to promote the integration of expert’s knowledge and local experience. To achieve this goal, a new flood risk mapping method will be designed in the project that combines local community’s risk perception and traditional risk analysis mainly from expert’s opinion. This project is conducted by researchers from Wuhan University (WHU), China Meteorological Administration (CMA), Beijing Normal University (BNU), University of Southampton and Newcastle University. International cooperation is an important part of this project. It would promote the knowledge exchange and experience sharing in the field of flood risk reduction, and motivate the application of more advanced techniques of flood assessment and risk m apping for the communities in flood diversion area. 2. Introduction of the study case 2.1 Introduction of the 2016 Wuhan Floods The 2016 Wuhan floods lasted about a month from June 30 to the end of July with flooding emergencies evolving with time and space. The accumulated rainfall reached over 560 mm within a week from June 30 to July 6, and 757,000 people were affected with 14 killed during this period. Economic loss reached 2.26 billion Yuan RMB. Some important sites and events are shown in Figure 1 and the process and timeline are illustrated below. 3 / 41 Figure 1. Important sites and events during the 2016 Wuhan Floods (1) July 1 On July 1st, in the northeast part of Wuhan, the water level of Jushui River reached 33.35 meters, exceeding the guaranteed water level with 0.24 meters, and caused severe burst in the phoenix section of the river, greatly threatening Xinzhou District downstream. (2) July 5 After several days of persistent heavy rains, the main districts of Wuhan suffered severe waterlogging. The traffic was disrupted and some communities were severely flooded, especially for those near the South Lake. In addition, thousands of residents in Xiaosi Town (rural area in the southwest) were emergently evacuated in the night of July 5th, as their home would be severely inundated according to the early warning. (3) July 6 Emergency of seepage was found in the main levee of the Yangtze River and if it was not 4 / 41 handled in time, the water in the Yangtze River would directly rush into the main district of Wuhan city and caused disastrous consequences. Premier Li Keqiang inspected the emergency site and demanded more efforts to fight against the floods. (4) July 14 The dyke and separation of Liangzi Lake and Niushan Lake was successfully demolished with blasting on July 14th. This was the emergency measure in response to high water level and great threat of Liangzi Lake. Five hours after the blasting, the water level in Liangzi Lake dropped by 0.17 meter. (5) July 8 and July 21 Great danger of breaching was found in the Tongjiahu Lake on July 8th and 21st. It is near the Tianhe International Airport, and if the water in the lake spilled out, the airport would be severely damaged and it would seriously affect the air transportation. At the end there was luckily no dike breach. 2.2 Introduction of Dujiatai Flood Diversion Area Dujiatai Flood Diversion Area, located in the southwest of Wuhan, is designated to protect Wuhan from the threat of peak discharge from Han River coinciding with high water lever in the Yangtze River (Figure 2). Once enabled, it can store extra floodwater of 1.6 billion cubic meters, which will greatly relieve the flood pressure of the Han River. It is low-lying area between Yangtze River and Han River, and was natural floodplain wetland in history. Now it has a total area of 614 square kilometers and involves 21 villages with total population of 160 thousand from Xiantao city, Caidian and Hannan District of Wuhan. Since its completion in 1956, Dujiatai Flood Diversion project has been used 19 times, and has regulated flood water of 19.125 billion cubic meters. It has played a huge role in guaranteeing flood control and security in the lower reaches of Han River and Wuhan city. From June 30 to July 5 2016, torrential rain hit the Caidian District with accumulated rainfall of 382.2 mm, and it rained 264.7 mm in a single day of July 5th, which is the highest amount of rainfall ever recorded on a single day at this location. Several dangerous situations were found in many places of this region and the water lever in Chen Lake increased over 40 cm and it was still rising, making several breaches in the dike. In the night of July 5th, the local government decided to take emergency measures: 12,000 residents from 12 villages of the Xiaosi Town within the Dujiatai Flood Diversion Area were emergently evacuated as water level was predicted to exceed its historical records and there was high potential of overtopping