Study on the hydraulic characteristics of natural levees by field surveys: Case study of the Kokai River, Japan

As compensation for the limited experimental data from the borehole logs, geological knowledge is expected to contribute to better risk assessment of the river levee system. Although natural levees have long been recognized as related to the underseepage risk of river levees, limited quantitative knowledge of hydraulic characteristics is available. Field surveys are conducted at a site along the Kokai River in Japan to reveal the hydraulic characteristics of natural levee sediments. Sounding tests allow us to capture laminar structures in the natural levee sediments. Through in-situ seepage tests, it is found that the silty materials in the natural levee sediments have hydraulic conductivity in the order of around E-06 m/s. Based on the results from the field surveys, numerical spatial hydraulic conductivity models of natural levees are built by applying geostatistical methods, including indicator Kriging and indicator simulations. The methodology in this study shows a possibility of quantifying geological knowledge, which finally contributes to the quantitative risk assessment against underseepage and internal erosion.