Flood Possibility Due to Shallow Groundwater Levels at the Riverside Plain in South Korea

The Four Major Rivers Restoration Project (4MRRP) was planned as a key project of the Korean government in 2008 with a vision of “Reviving Rivers of a New Korea”. This project was designed for most of the Korean main rivers, Han, Nakdong, Geum and Yeongsan rivers, including13 major tributaries of the rivers. The main contents of this project include a weir construction, 16 large weirs. Increasing the river water levels by the weir construction can cause the increase of groundwater levels at the riverside plains. The depth to groundwater ranges generally from 3 m to 10 m due to groundwater pumping for the last few decades before a weir construction, but it becomes shallower, 2 m to 5 m. Considering that the rainfall accompanies groundwater recharge, the reduced pore capacity by the shallow groundwater levels will not introduce enough water during a heavy rainfall and make direct runoff increase. This condition may produce the groundwater overflow at the land surface during a heavy rainfall and bring about a flooding. Numerical model using MODFLOW is used to estimate the rapid change in short-term (several days) groundwater levels during a heavy rainfall in the riverside plains. This estimated groundwater levels will be used to forecast the groundwater flooding possibility.