Spatial modeling and analysis on flood risk assessment using fuzzy analytic hierarchy process: a case study of Bang Rakam Model in Thailand

Abstract

Flood events are a common global natural disaster with a high frequency and broader geographical distribution. In 2011, Thailand encountered the worst flood, the Thai government developed the “Bang Rakam Model 54” to be used as a guideline model to solve flood problems at Bang Rakam district in Phitsanulok. Subsequently, on September 20, 2016, the Ministry of Agriculture and Cooperatives (MOAC) and Royal Irrigation Department (RID) collaborated to resolve the menace of flood in the provinces of Sukhothai and Phitsanulok. The main idea of “Bang Rakam Model 60” was to adjust the cropping plan over the low-lying areas by allocating water for irrigation earlier than the usual cropping period. These low-lying areas then will be used as Monkey Cheek areas to retard the floods. The purposes of this study are to prioritize factors influencing flood hazard and flood vulnerability using fuzzy AHP, create a flood hazard map and flood vulnerability map, generate a flood risk map, and analyze the perception of farmers on flood risk. The study area covered the area of the Bang Rakam Model 60 project (2 provinces, 5 districts, 20 sub-districts, 93 villages). The fuzzy analytic hierarchy process (fuzzy AHP) based on Chang’s extent analysis was combined with geographical information systems (GIS). Eight factors were considered for the flood hazard map, i.e. 1) distance from drainage network 2) drainage density 3) elevation 4) flow accumulation 5) land use 6) slope 7) soil water infiltration and 8) average annual rainfall. Five factors were considered for the flood vulnerability map including 1) age group 2) dependency ratio 3) gender ratio 4) population density, and 5) road density. Each factor was weighted to obtain the final maps. The obtained flood hazard map and the flood vulnerability map were used to generate and assess the flood risk map. The opinions of 102 sampled farmers who cultivated in the Phitsanulok and Sukhothai provinces were collected and analyzed with the flood risk map to assess farmers' perception of flood risk.

The result showed that annual rainfall with a weight factor of 0.1879 was the most important factor influencing flood hazard. These were followed by flow accumulation (0.1667), drainage density (0.1611), elevation (0.1423), slope (0.1206), soil water infiltration (0.0988), distance from drainage network (0.0632), and land use (0.0594). Prioritization of factors influencing flood vulnerability revealed that population density and road density were the most important factors and recorded the same fuzzy weights of 0.3107. These were also followed by age group (0.1322), dependency ratio (0.1252), and gender ratio (0.1212). The total study area of 695.55 km2 was assessed into five flood risk levels of very high, high, moderate, low, and very low. Moderate level covered an area of 225.67 km2 (32.44%), high level covered 139.60 km2 (20.07%), very high level covered 119.12 km2 (17.13%), very low level covered 111.05 km2 (15.97%), and low level covered 100.11 km2 (14.39%). The results also showed that most of the very high-risk areas were along the Yom River and the border between Kong Krailat and Phrom Phiram districts. The analysis of the farmers’ perception of flood risk revealed that farmers in high-risk level areas have a high-level perception of flood risk.

Flood risk assessment at Bang Rakam Model 60 provided important information to guide future projects aimed at flood prevention, mitigation, preparation, response, and recovery. Additionally, the risk analysis results of the study will serve as baseline information for people to understand the level of damages and losses that can happen during a flood.