I. According to the result of our experiments in field, the durability of mole-drains under the paddy field in our country is diminished by the following causes :
(1) In time of operation, muddy water flows freely into the mole-drain hole from the slit of supporting edge plate and accumulates at the hole bottom.
(2) In time of operation, muddy water flows backward into the mole-drain hole from a tributary drainage canal.
(3) In or after operation, the hole bottom alignment of mole-drain becomes uneven.
(4) The drainage outlet or canal is managed under an undesirable condition.
(5) Livestocks or heavy farm machineries trample down the mole-drain hole from above.
(6) Crop roots infiltrate into the mole-drain hole.
(7) Crawfish (Cambarus Clarkii (Girard)) or other small animals barrow their way into the mole-drain hole.
Such items as (1) - (4) which may be eliminated by a little piece of attention occupy a large percentage of all.
II.Analytically speaking authors obtained the following results from experiments in field :
(1) Transformation of the cross-section of mole-drain hole.
Directly after operation, the ordinary cross-section of mole-drain hole becomes a vertically long ellipse, its average cross-sectional area of which is 53. 7 cm².
The cross-sectional areas directly after operation are small in the following cases : 1. deep drains, 2. small consistency index of subsoil, 3. small soil hardness.
(2) Accumulation of silt at the hole bottom.
Several years after operation, silt accumulates at the hole bottom, and the cross-section of the mole-drain hole becomes a laterally long ellipse or a crescent. A function as a drain is last after all.
Soil property of silt at hole bottom is slightly less in percentage of particle-size from 0.12 to 0.01 mm as compared with subsoil.
When the subsoil is composed of an equal ratio of clay, silt and sand, the accumulation of silt is comparatively small.
This result is quite different from the theory of Prof. Nicholson in which he says the more clay content, the longer becomes the durability of mole-drain in upland field. It is thought that this perhaps depends upon high water content of subsoil owing to the irrigation for paddy field.

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In this report the author published his researches on the character of breakers which would take place in the case the waves dashed against a dike. (1) In order to know the character of the breaking point the author first clarified the interrelationship between H₀/λ₀-h_b/λ_b, h_b/λ₀-H₀/λ₀ (breaker index) and a_1b/H_b-h_b/λ_b concerning the relation of wave profile to depth of breakers ; as a result the breaking point has been obtained much easier in the case a dike exist against waves, and (2) as for steepness and breaking condition, the author also succeeded in getting the length of breakers and the height of breakers by obtaining the relation between H_b/λ_b-h_b/H_b, h_b/H₀-H₀/λ₀ and H_b/H₀-H₀/λ₀. Moreover, in order to know the zone of breakers the author classified the height of waves changing with water depth and the change of wave crest ; and also made the relation between H₀/λ₀ and l/λ_b clear in order to decide the length of zone of breakers important to the design of dike.As an incidence wave to a dike the character of breakers in the case where a dike is non-existent has been applied up to now, but the analysis in this report shows us that the wave is of a quite different character due to the influence of partial clapotis.

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The development of a turbulent boundary layer is analyzed theoretically by solving the momentum equation for a rectangular open channel with a smooth bed on the assumption of the velocity distribution in a cross-section.
For each case of two dimensional uniform flow or non-uniform flow the process of the boundary layer development is given by the equation or by the use of the graph.
Although this graph gives an approximate value, it is easier to calculate.

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