APPLICATION GIS AND REMOTE SENSINGTO ESTABLISH FLASH FLOOD HAZARD MAP IN TUYEN QUANG PROVINCE

Tuyen Quang is one of the provinces at high risk of flash floods in the Northern Midlands and Mountains of Vietnam. In the rainy season, like other localities in the region, Tuyen Quang has a long, concentrated rainfall combined with steep hills and mountains, large divisions, many rivers, and streams; In addition, the thinning of the vegetation cover due to excessive exploitation of the forest by the local people causes flash floods to appear more and more. Applying GIS and remote sensing to establish a map of flash flood risk is a quantitative approach and high reliability. This article has established a flash flood hazard map at a scale of 1/100,000 in Tuyen Quang province. In the map database, districts with a high risk of flash flood were identified, including Na Hang, Chiem Hoa, Ham Yen, and Lam Binh, the average flash flood hazard level included districts: Yen Son, Son Duong; Tuyen Quang city has a low risk of flash floods.


Introduction
Flash flood is a form of geological and hydrographic catastrophe with great devastation, causing serious consequences for the people. Tuyen Quang province is located in the northern mountainous region, the topography is quite steep and strongly divided. In addition, heavy rainfall, highly concentrated in the rainy season months, makes many areas at high risk of flash floods and landslides, especially in areas with devastated vegetation. Over the years, people here have always faced many risks of flash floods, along with property damage, changing the face of the landscape in the area. Nowadays, the application of GIS technology in flash flood research is becoming more popular and also brings many clear advantages.
Recently, there have been a number of research works related to the construction of flash flood warning models such as: Application of GIS and Remote Sensing in mapping the potential of flash flood in Son La Province, Vietnam by a team of authors from Faculty of Geography, Hanoi National University of Education; or Building an early warning system for flash flood in the mountainous area, a case study in Thuan Chau district, Son La province by a group of authors from Thuy loi University, University of Science, Vietnam National University, Hanoi Institute of Mechanism, Vietnam Academy of Science. These works have used methods such as the FFPI model, which is a quantitative model to determine the risk of flash flood generation based on the inherent characteristics of the study area such as slope, soil texture, type land use, vegetation cover; or MCA multi-criteria integration model. This is a model that integrates hydrological and geomorphological models basin with the help of GIS technology.
In this paper, we use spatial data that can be modeled such as DEM, mean catchment slope map, landslide hazard map and mean annual precipitation map, combined with weights for each factor. The factors affecting flash flood by analytical hierarchical method (AHP) to map flash flood risk used in this paper show reliable results. The combination of using DEM model, hierarchical analysis method combined with satellite image analysis to create flash flood hazard map is the novelty of this study.

Data
Data sources used for the study include: -Data collected from field survey.

Research Methods
The research methods of the topic include: remote sensing image analysis method, field survey method, AHP analysis method, GIS spatial data analysis method.
-Remote sensing image analysis is used to interpret flash flood locations in the study area.
-Field survey includes monitoring, conducting detailed measurements, determining the scale and characteristics of flash floods and the impact factors that generate flash floods. Since then, to conduct an assessment of the current state and changes of flash floods in the Tuyen Quang province.
-The method of hierarchical analysis (AHP) is to determine the role of each factor in the factors that generate flash floods on the basis of weighting and scoring.
Geographic Information System (GIS) is used to build spatial analysis, management, integration and overlapping layers of map information.
The AHP model combined with GIS will help to select elements, synthesize information suitable for research subjects.
To determine the level of risk of flash flooding, the thesis integrates the criteria according to the formula (1) (according to Patrono, et al., 1995): (1)

In which: FSI (Flash flood Susceptibility Index): is the index of risk of flash flood occurrence
Wj: is the weight of factor j Xij: is the number of grades i in the factor causing slip j The integration of AHP into GIS via linked formula (1) and calculated by Raster Caculator tool of ArcGis 10.5 software

The main factors causing flash floods in Tuyen Quang province
The main factors that generate flash floods include: the risk of landslides, slope and precipitation. In which rainfall is the main factor; slope plays an important role in influencing flow rate; a landslide risk factor provides reinforcement for flash flood generation. [3] Each of the factors mentioned above has a different role and influence on the generation of flash floods. Therefore, it is necessary to analyze, evaluate the weight and score each factor accordingly.
Using the AHP assessment method, we assigned scores to each factor based on the importance of flash flood formation (in Table 1). Each factor will be weighted by AHP method, comparing the correlation between factors by making a matrix, calculating the corresponding weight score of each factor (in Table 2 and Table 3).

The Average annual rainfall
Rainfall factor plays a key role in generating flash floods. Rainfall includes the intensity of rain and the time of rain being observed, measured and assessed the impact on the generation of flash floods. [2] Flash floods usually form in a short, sudden, high speed period, and often occur in areas with heavy, intense and prolonged rain. According to monitoring results over the past 20 years, the annual average rainfall of Tuyen Quang province is 1,600 -2,100 mm / year. Rainfall is unevenly distributed throughout the year, concentrating mainly in the rainy season months (July, August and September), accounting for 72% of the total annual rainfall.
On the basis of the observed data, a map of the annual average rainfall of Tuyen Quang province was made. Analysis and statistics results from the map are shown in Table 4   Table 4. Hierarchy effect and area of annual average rainfall factor The whole province 5,868.0 100

The Average slope of the sub-basins
The average slope of the river sub-basins plays an important role in the generation of flash floods. The average slope map of the sub-basins was developed using ArcGIS 10.5 software in combination with Mapinfo 15.0 software.
The thesis has divided the average slope of the sub-basins into 5 levels affecting the possibility of flash flooding in Tuyen Quang province. The analysis results from the slope map are shown in the data sheet (Table 5).  [2,4] After the landslide map was created and based on the score value, the author divided into 5 levels corresponding to its impact on the hazard of flash floods from low to high (table 6)   In terms of administrative units, we conduct statistics on flash flood risks in each district and city (Table 8).  Table 8 shows