APPLICABILITY OF GROUNDWATER FLOOD INTENSITY IN THE UPPER RIVERSIDES AFTER A LARGE WEIR CONSTRUCTION IN THE NAKDONG RIVER, SOUTH KOREA

Increasing the river cross section by a river excavation and water-filling by a large barrage construction in the Nakdong river, South Korea, brings the rise of the average river water level and the concomitant rise, averagely 2~3m, of groundwater levels in the riverside alluvium. Shallow groundwater level can increase a moisture content of shallow soil and affect the scale of surface flooding due to a decrease of infiltration rate. There have been a lot of public grievance on the inundation possibility by groundwater level rise in the riverside’s rice field with a flat topography formed by alluvium deposits. Total 109 groundwater monitoring wells were constructed along the river and groundwater level data measured at 1 hour interval are collected for last 3 years. To define groundwater flood intensity (GFI), a simple equation (GFI=tanα/DTW_min) is designed with the upward slope (tan α) of the water levels’ time series and the minimum depth to groundwater (DTW_min) at each rising event of the time series. The GFIs for each monitoring site along the river are calculated using groundwater level time series considering the base groundwater level, which means the depth from land surface, 2.5 m, considering the capillary effect. The relationship between GFI and impact factors, which include soil’s discharge grade, effective depth of soil, soil’s hydrologic class, land use type, air entry pressure class, soil’s composition, alluvium thickness, hydraulic conductivity, distance and height difference between a well and a river, groundwater level, and CN value, are analyzed and a simple meaningful equation is finally produced, log(GFI) = 2.94-0.85√(GWL) - 3.88√(SL-RL) + 2.55√(GWL-RL). Therefore, it is possible to evaluate the vulnerability of groundwater flooding for the whole riversides along the Nakdong river, if collecting the terrain elevation and estimated groundwater level’s data.