Establishment of A Hierarchical Drainage Density Map, a Case Study of the Ngan Sau - Ngan Pho River Basin

Drainage density (DD) is a key morphometric parameter that reflects watershed hydrology and geomorphology, directly influencing runoff, flood hazards, soil erosion, and water resource management. This study establishes a hierarchical drainage density map for the Ngan Sau and Ngan Pho river basins in Ha Tinh Province, Vietnam, using GIS-based morphometric and hydrological analysis. Topographic maps and Digital Elevation Models (DEM) were processed to delineate sub-watersheds, extract stream networks, and calculate drainage density at multiple spatial levels. DD values were classified into four hierarchical categories to visualize spatial variability and identify zones with differing hydrological responses. The results show that the Ngan Sau and Ngan Pho basins exhibit the high drainage density values in the Lam River system, making them the most flood-prone areas. Validation with historical flood records and hydrological station data confirmed strong consistency between calculated DD-based flood risk and observed flood events. The hierarchical DD map provides a practical and reliable tool for watershed management, flood hazard zoning, erosion control, and sustainable land-use planning, offering transferable insights for other river basins in Vietnam and beyond.