To deal with transitional flow from debris flow through sheet flow to bedload flow, a riverbed shear stress formula was developed by applying the existing theory of debris flow to the sediment layer and Manning's theory to the water layer. We also propose a new method for determining the transitional condition of the sediment transport region to a continuous transition of equilibrium concentration. Numerical simulations were performed using an existing model and our new model. Results for conditions where the riverbed gradient changed gradually showed that the existing model indicated irregular deposition deformation. Moreover, simulation results of flow depth and sediment concentration oscillated around transition boundaries. When discontinuities of riverbed shear stress and equilibrium sediment concentration coincided,numerical oscillations became larger. Our model yielded smooth and stable simulation results without these problems.

Although the crush and abrasion phenomena associated with the downstream flow of riverbed gravel have long been studied, they still present a challenge. The crush and abrasion phenomena of gravel are not considered in river channel and erosion control plans in Japan. However, evaluating the crush and abrasion phenomena of gravel that flows down from mountain rivers is important from the viewpoint of the integrated management of sediment in a watershed. Thus, we conducted rotation crush and abrasion tests on nine cobble gravel rock types, ～200 mm in diameter, which are normally distributed in mountain rivers, and determined the characteristics of the weight reduction rate (βr) of cobble gravel by rock type and the grain size distribution of the sediment particles produced. Then, we conducted channel crush and abrasion tests to convert the crush and abrasion phenomena associated with the rotational movement of cobble gravel into those associated with the downstream flow of gravel in mountain rivers. The result of comparing the rates of weight reduction (βr and βs) of cobble gravel in both tests showed that the rate of weight reduction (βr) associated with rotation can be converted to the rate of weight reduction (βs)closertothedownstreamflow of cobble gravel in mountain rivers by multiplying the conversion rate α^＊ with the rotation-associated crush and abrasion coefficient αr using Sternberg's law.

On the afternoon of Friday September 25, 2015, a landslide occurred in Lilisland, part of the Wedamulla Estate of Kotmale, Nuwara Eliya District, Sri Lanka ; seven people were killed. Immediately after the disaster, the National Building Research Organization (NBRO) conducted a survey and produced the “Preliminary Land Survey Report on the Landslide Occurrence in Lilisland of Wedamulla Estate of Kotmale AGA Division, Nuwara Eliya District” (hereafter, the NBRO report). A Japan International Cooperation Agency (JICA) survey team performed a field survey after the landslide disaster in Koslanda, Badulla District, at the end of October 2014, and introduced a sediment disaster survey method that is now widely used in Japan and Sri Lanka. Also at this time, we (i. e., a JICA survey team) conducted a 2-day field survey on September 30 and October 1, 2015, for the purpose of elucidating the type of disaster and its underlying mechanism. In addition, we conducted an after-action review (AAR) on September 30 and October 1, 2015, and on December 19, 2016, and confirmed the situation at the time of the disaster. We compiled the results into a report for use in countermeasures against sediment disasters in Sri Lanka.