Raindrops are erosive agents that initiate the movement of soil. Several experimental studies are made on the splash behavior of soils with changes in the rainfall intensity. A “splash ratio”, for evaluating the splash behavior of soils is proposed. Using principal component analysis and multiple regression analysis, the relationship between the “splash ratio” and the physical and mechanical properties of soils is investigated.
The results obtained are as follows:
1. Kuroboku soil, Shirasu and Masatsuchi were divided into groups using 27 erosive parameters for principal component analysis. The properties of Shirasu and Masatsuchi were similar for those parameters.
2. The splash volume of soils from the surface was linearly related to the intensity of rainfall forraindrops, of uniform size and velocity. The slope of the line (“splash ratio”) is used to characterize the erodibility of soil materials to detachment from the surface by raindrops.
3. The prediction equation in “splash ratio” for all soils is:
e_v=a₀+a₁x₁+a₂x₁₂+a₃x₁₇+a₄x₂₆
where ev is the “splash ratio”(mm³/cc), x₁ is the “clay ratio”, x₁₂ is the coefficient of aggregation, x₁₇ is the natural logarithm of the infiltration ratio, x₂₆ is the real-specific gravity, and α₀=100, α₁=64, α₂=-0.06, α₃=-3 and α₄=-24.
4. The best single predictive parameter for the “splash ratio” was the “clay ratio”, which explained 90 percent of the variation in the “splash ratio”.

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The organic combination of friction force of the clod and sand particles forming the surface of a soil layer, and the cohesion temperately compacted with an appropriate amount of fine particle content, is closely related to the resistance of cohesive soil to the tractive force of rain water flow. The study was made on the resistance of cohesive soil to surface sheet flow, by expressing the contents of fine soil particles influencing such soil resistance as friction force, cohesion, etc. by clay ratio^* (C_r^*= clay (%)/[silt (%)+ sand (%)]) ; and soil layer compaction by dry density γ_d; and soil surface roughness by the average clod size, d_k. By letting u equal the average velocity on the surface roughness of soil layer, u_* the shear velocity, f_c the cohesion per unit of area, u/u^*=φ(h/d_k, u^*d_k/ν) equal the distribution of the velocity of surface sheet flow on the surface roughness of soil layer, and also taking into consideration the equilibrium between the tractive force of surface sheet flow and the soil resistance, the relationship of critical shear velocity was established by the formula u_*c²/gd_k=K_r(γ_d/ρ-1)+α₁f_c/ρgd_k·2cos θ/(C_Dφ_c²). The logarithms herein quoted are K_r, coefficient of friction ; α₁, area coefficient ; C_D, coefficient for flow resistance ; γ_t, wet density ; and g, gravity acceleration. For the purpose of obtaining the correlation in the respective dimensional quantity in Reynold's number of surface roughness, u_*cd_k/υ, u_*c²/(γ_d/ρ) gd_k, and C_r^*, an experiment of critical detachment was performed on 8 different native types of soil. The general susceptibility of soils to erosion was observed in the soils of different native types, and depended on the clay ratio and dry density. In most of the soils, with some exception like volcanic ash soil rich in organic matters, a considerable relation was discovered in the dimensional quantity of C_r^* for u_*c2/gd_k, u_*cd_k/υ, through which the characteristics of erosion of cohesive soil by surface sheet flow was clarified.

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To determine geometric properties and the process of development of a gully on a bare slope, the authors surveyed gully networks in longitudinal and cross section on a slope in the second Gohonmatsu district and reported results in preceding papers.
Results are as follows:
The gully networks had already been in a considerably mature stage of development (see Fig.6, 7). But the gullies, which are elements of the gully networks, developed considerably by erosion between 1976 and 1978 (see Fig.2-1, 2-2). The mechanism of the developement of the longitudinal section and cross section of the gullies can be determined by considering their properties, that is, ‘wendepunkt’, on longitudinal sections (see Fig.2-1, 2-2) and overhang on cross sections due to gully-bottom meandering (see Fig.3-1, 3-4).
Furthermore, we obtained the following result by caluculation of the gully volume. That is:
There exists a close relationship between the ayerage areas of the cross sections and their orders (see Fig. 8). Soil loss by gully erosion is far more than that by sheet erosion in a mature stage.

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Since 1950, much effort had been put forth to develop agriculture and irrigation in the Khuzistan plains. In general, the problems related to agricultural development in the plains are:
1) Limited amount of irrigation water,
2) Water quality problems,
3) Salinity and alkalinity problems of the soil in certain parts of the plains (central, south and southwest),
4) The unique climate with extremely hot summers, and
5) Pests problems which are typical of the region.
Among the above mentioned limiting factors, items (2) and (3) will be discussed in this paper, while item (4) will be implicitly referred to.
The reason for soil salinity and alkalinity problems in the Khuzistan plains can be summarized as follows:
1) Shallow and saline ground-water table,
2) Existence of salt concentrated layer in the soil profile,
3) Heaviness of soil texture, and improper soil drainage condition,
4) Salinity of irrigation water,
5) High evaporation of rainfall,
6) Destruction of vegetation and consequently cease of evaporation,
7) Frequent sand and wind storms, and
8) Salinity of soil due to human mismanagement.
The Khuzistan plains totals approximately 3.0 million hectares of which only 750, 000 hectares have the possibility of being. irrigated. The land which can be irrigated without any special soil improvement techniques lies in the northern part of the plains but the rest area needs a great amount of leaching and drainage. Leaching and drainage, even if carried out on a large scale, is extremely expensive. Also, due to the inadequate quality of the irrigation water in the region, leaching should be carefully performed due to the alkalinity existing in the area. Even after leaching, if special care is not taken, there is also the possibility thm the water will salinize again.
In this study, the data from the Shavour project, Shavour soil and water research center which are located at the central part of the plains is to be practically and theorically treated in the following papers.
In the experiment four categories of soil; light, medium, heavy, and very heavy textured, with medium to rapid hyraulic-conductivity and a very shallow to very deep saline water-table were treated in 38 experimental cylinders.

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A diversion weir for the irrigation at the field of an alluvial fan was usually constructed at the top of the fan. And the diversion from the fan river channels, not from the top of the fan, was used to be carried out without passing through any settled weirs. But recently, the weirs have beggun to be constructed also across the fan river channels, such as the Ohta Weir at the Watarase River.
Generally speaking, most of the channels at the fan rivers are in the braided condition. And these braided streams are mainly caused by the formation of imbricate sand-gravel bars, and the changes of bars settle streams unstable and accumulate sands on the weirs. Therefore, the diversion through the weirs across fan river channels seems not suitable in general.
By judging the formation and the change-of imbricate bars, have investigated two weirs across the fan river channels, One is the Ohta Weir at the Watarase River, and the other is the B Weir at the A River (its name is kept secret in the stage of planning), and clarified as follows:
a) The Ohta Weir at the Watarase River:
In this river, doublelined alternating bars are formed and move downstream, but its water routes can be kept stable by the stabilization of sand bars near the bank. Therefore, there is no need to worry about the position and the function of this weir.
b) The B Weir at the A River:
In this river, 3 or 4 lined alternating bars are usually formed and move downstream, and to stabilize the water routes seems very hard. As this result, the position of this B Weir is said unsuitable for construction. So the reconsideration of the present plan for constructing B Weir is required.

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It had been known that the ratio of the undrained shear strength to the consolidation pressure (C_u/p) of clay was a function of it's plasticity index (I_p), and the C_u/p value of clay under the natural consolidation condition was different from that obtained by laboratory tests.
The authors showed thatC_u/p was not only the function of the value of I_p, but also the function of the liquidity index and sensitivity ratio, and gave a reasonable explanation for the difference of C_u/p.
The C_u/p value of Ariake marine clay by laboratory tests was 0.3-0.4, which was certified to be a reasonable value by investigation of a natural clay layer under an embankment.

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The seepage analysis of an embankment dam by the numerical method has been performed mainly by using the finite element method (FEM) and the finite difference method (FDM), but, in this paper, a different method called the boundary element method (BEM) or the boundary integral equation method (BIEM), which only has been developed recently, will be explained in detail. This method with linear elements was first formulated and applied to both confined flow problems to discover the efficiency and simple unconfined flow problems of an embankment dam so as to compare with the FE solutions.
According to this comparison, the results of the BE analysis were very closed to that of the FE analysis and was deemed satisfactory. Afterwards, the BEM of the nonhomogeneous seepage analysis of the two regions was applied to the problem of the core-filtered system. The ratio of the permeability coefficient of the two regions, the shape of the core and width of the filter were mutually varied in this analysis and the seepage discharge, the location of the exit point. and the maximum value of the water head gradient could be obtained easily. So, although the seepage phenomena through the corefiltered region has only been clarified qualitatively due to the sharp geometrical change of the seepage region; the introduction of BEM into such seepage problems will be able to clarify the characteristics both qualitatively and quantitatively.

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In this paper, based on the preceding papers, four typical examples of the three-layer case are analyzed. The mode of failure of a three-layered sand column, and others are also considered.
The effects of a loaded filter in the three-layer case are the same as in the previous studies.
It is considered that a measurement of the force exerted on the “Yokushi-Ban” makes it possible:
(1) to measure, although indirectly, the seepage force, and
(2) to examine, by (so to say) nondestructive testing, the theoretical results obtained in this series of studies.
Thus, the “Yokushi-Ban” is very useful, and so, the effective stress diagram within a sand column with the “Yokushi-Ban” after a critical time is shown in the one-, two-, and three-layer cases.
Up to now, we have studied the failure of the one-, two-, and three-layered sand columns caused by a vertically ascending seepage flow. The treatment of the problem on the basis of internal effective stress may be more advantageous than on the basis of “ the critical equilibrium of forces” due to the followings:
(1) It is possible to predict the mode of failure of a sand column from the residual effective stress diagram (in a critical state) within the sand column.
(2) The effects of a loaded filter can be understood better.
(3) The concept of “ the stability of a sand column ” can be understood better. etc.

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