As a result of Hurricane Katrina, many sections of the flood protection systems in New Orleans were eroded due to plunging water, and sections of flood walls were determinately damaged. Therefore, mitigating this type of erosion and failure is necessary for counteracting similar catastrophic events. This study evaluated the method to mitigate erosion due to plunging water by strengthening the soil with ground modification. The Vetiver plant and Polyhedral Oligomeric Silsesquioxanes (POSS) were the two main ground modifiers used in this test. Test results showed that both POSS and the Vetiver were effective in reducing erosion. POSS showed good erosion resistance with good applicability to field soils; Vetiver showed higher resistance to erosion by plunging water, but required time to achieve a well established root/stem system.

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At a watershed underlain by weathered granitic bedrock (5.99 ha) and covered by Japanese cypress forest, we estimated apparent bedrock infiltration (QB) by extracting evapotranspiration measured by eddy covariance (EEC) from the residual of the water budget (EWB = precipitation - runoff) over an 8-year period (2001-2008). The degree of interannual variation in EEC was relatively small throughout this period. Conversely, EWB showed larger annual variations and positive dependence on annual precipitation. The difference between EEC and EWB was greater in the years with more precipitation. Therefore, estimated values of QB also showed positive dependence on annual precipitation. Observed average annual precipitation, runoff, EEC, and estimated QB were 1549, 722, 742, and 86 mm year-1, respectively. Consequently, QB is believed to have amounted to 0-10% (average 5%) of total precipitation. The findings suggest that episodes of rainfall with medium and high intensity induce lower and higher QB values, respectively, with similar amounts of precipitation.

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Sakurajima is an active volcano on which even a small amount of rainfall can cause a debris flow due to the remobilization of accumulated volcanic ejecta on its slopes. The Japanese national government has constructed sabo facilities in the area because of frequent debris-flow disasters in the past and has conducted various types of debris-flow monitoring programs. Various types of sensors, such as wire, optical, vibration, and acoustic instruments, have been installed to detect debris flows, while camcorders and mud sampling systems have been installed to track the debris-flow process. Observations using these monitoring instruments have enabled clarification of debris-flow characteristics. Even so, more data on debris-flow discharge are required to improve the accuracy of debris-flow monitoring and to determine when and how debris flows occur.

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