##plugins.themes.huaf_theme.article.main##
Abstract
In the Lai Giang river basin, floods are the most common natural disasters (3 - 4 events per year) and cause the most severe consequences. One of the causes of high rainfall and flooding in the Lai Giang river basin has a great influence on the topography and morphology of the basin. The paper aims to use the Synop method to clarify the air masses and weather patterns that cause rain and floods, in which cold air is the factor that causes the most rain and floods (accounting > 33%); use geomorphological methods and geographic information system (GIS) to determine the topography of the river basin has a lower direction from west to east, in the west of the river basin are mountain ranges in the South Truong Son which is distributed in the North - South direction, with an altitude of 500-700m and Binh De mountains have a horizontal direction to the East Sea, they have made winter moist air masses from the sea easily go deep into the basin and meet the terrain barrier from these mountains, which has caused heavy rain. In addition, Lai Giang river basin has a rather large slope (13,9o) in the upper, the shape of the basin expands in the upper and middle areas but narrows in the downstream. This has made the process of flood concentration fast, the flood transmission time is short (6 - 8 hours), the flood area in the downstream is greatly affected with the flooding time of about 3 - 5 days.