Afforestation works on seadikes along the coastal line in China aid soil erosion control and help to maintain the stability of seadikes. Tree species currently used in the afforestation are black locust, cryptomeria, water fir, bamboo, and a few chinese fir. In this connection, various laboratory tests and field researches were conducted to study the effect of roots of the different tree species on soil stability, and to know which type of tree species is the most appropriate for soil erosion control on seadikes.
Results of research works show that the tree roots, particularly those with diameter smaller than 1mm, known as “rootlets” increase soil anti-erosivity efficiently. Soil anti-scouribility raises with the increase in rootlet length and weight. The correlation coefficients between soil anti-scouring index and the rootlet length and weight is found to be 0.93 and 0.83, respectively. Soil with more tree roots usually has higher porosity, thereby having higher infiltration capacity. The correlation coefficient between soil non-capillary porosity and steady infiltration rate is 0.86.
Among the tree species tested, black locust forest has the smallest function on soil erosion control, and bamboo has the largest function. The main afforestation tree species which functions most in soil erosion control are in the order, bamboo>;cryptomeria>water fir>black locust.

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Oxidation and reduction of iron occurs relatively easily in natural water. Ferrous ion is the dominant form under reduced conditions. Ferric iron is the dominant ionic form under oxidized conditions. Under neutral pH value, ferrous iron is transformed to ferric iron by oxidation, joining with the hydroxide ion in surface water to form reddish-brown ferric hydroxide or ocher. Ocher is insoluble and either settles to the river bed or remains in suspension, leading stream water to turbidity. Such stream water degradation is predominant in the upper basin of the Kabamuta River and is attributable to ferric iron contained in spring water inflows. Thus, the purpose of this paper is to demonstrate the chemical processes of water degradation and its improvement methods from a view point of erosion control engineering.
Ferrous iron concentration, soluble iron concentration, and turbidity were measured from 1995 to 1997 at various sampling points chosen according to geomorphological features of the stream. Altogether, eleven experiments were conducted. Most of the ferrous iron was transformed to ferric iron by oxidation in the first 50m from the spring water inlet point and ferric iron was hydrated instantaneously. The maximum level of water turbidity was observed at a point about 600 m downstream from the inlet point, where is the just before injection of Takachiho River. The maximum level of turbidity decreased as the amount of oxidized ferrous iron became greater.
Since the ferrous iron oxidation rate proved to be the function of residence time and dissolved oxygen concentration of the stream water, it is reasonable to assume that the model based on convection-diffusion equation incorporated these variables in order to predict the value of maximum turbidity. Theoretical values calculated by the model agree well with the actual values, and it is suggested that maximum turbidity is decreased less than one-fourth of the ordinary value when atmospheric conditions accelerate the oxidation of ferrous iron due to long residence time in the first 100m of the stream.

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The paper discusses experimental and DEM analysis results of impact load due to debris flow on a sabo dam. The impact load experiment was performed with a wall type dam and four types of permeable dams. The simulation analysis was performed for the wall type dam by the modified DEM analysis which takes into consideration the rolling resistant moment, buoyancy and drag of water.
The results obtained are as follows: 1) Two models of impact load on the wall type dam were investigated. The analytical results for the impact load agree well with the experimental results. 2) A model considering the probability of gravel contact with the permeable dam is proposed to calculate the impact load. The results calculated by this model agree well with the measured load. 3)The relation between the behavior of gravel and the impact load is well reproduced by the modified DEM. The stress propagation mechanism from flow layer to sediment layer at overflow time was verified by DEM analysis and shows good agreement with theory.

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The bedrock of Kitamata Vally in the Name River is formed by granite, which is easily weathered for its soil condition. Therefore, many typical stony debris flows has been generated especially in Kitamata Valley. Since the debris flow observation has been started in 1982, seven debris flows has confirmed, and four debris flows was filmed in the VTR.
This paper presents the condition of generation and the discharge of debris flow, using analyzing VTR, aerial photo interpretation, also fields surveying. As a result, we recognized two-generation case of debris flows. The first type was that the debris flows generated by the slope collapses and the second was the riverbed deposits were turned to debris flow. These debris flows were divided into the three phases of the primary flow, the flow of large stone concentrated and the following flow consisted of water and fine materials, from the VTR observation. And also, there were many variations as the remarkable peak runoff, few times peak runoff, and many abrupt waves, in these flow shapes. Further, the maximum each value of the debris flows velocity, discharge and wave height generally appeared at the coming down of the debris flow peak with concentrating large stone, but these values partially appeared at the primary flow and the following flow. The behavior of the large stones of which the diameter of 1.0-1.5m or less flew down with revolving movement and the diameter more than 1.5 m flew down as if they were floating on the water.

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When a debris flow containing woody debris reached the Nakatani-gawa open-type steel-pipe gridded sabo dam in the Nakatani River, a lot of sediment and woody debris were trapped by the dam. The sediment deposited at the dam was later excavated to observe its vertical structure. It was concluded from the observation that : 1) the sediment transport was caused by a typical debris flow containing woody debris, and very large, mile-sized, and small rocks, and that : 2) not only the sediment in the frontal part of debris flow, but also the successive trains of debris flow was trapped by the woody debris that occluded the opening sections of the dam.

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