Water and sediment discharge has been measured in a 283m² catchment at Mount Unzen on a slope composed of recently deposited pyroclastic-flow debris. Measurements began in July, 1996, one year after cessation of eruptive activity. Vegetation on these deposits has recovered rapidly, primarily because the slope has been aerially seeded twice. Revegetation has reduced sediment discharge substantially, but has had little effect on storm runoff response. Analysis suggests that the decrease in sediment discharge results from reduced soil-surface erodibility, and that the limited effect of revegetation on stormflow runoff is explained by an immaturely developed root network and by insufficient soil crust formation.

View full abstract

Hot deposits of pyroclastic flows are said to prevent surface runoff and lahar (volcanic debris flow) because they evaporate rain water. We examined this theory by calculation of the cooling process of hot deposits and concluded that : 1) The surface of deposit is rapidly cooled by radiation. The cooling of inner parts of the deposit is carried out mainly by infiltration of rain water. 2) In order to cool the hot deposit of 1m thick, 180-660mm of rainfall is required. 3) The surface runoff can occur when the surface of about 0.2m thick is cooled. Hence the surface runoff can occur after rainfall of 37-130mm.

View full abstract

This paper describes and discusses the results of the investigation at the site of the open-type steel-pipe gridded sabo dam attacked by stony debris flow. The investigation was carried out at the second dam in Ohtana-zawa constructed by Fujikawa Sabo Work Office, the Ministry of Construction. Through the investigation the following were confirmed. 1) Under normal condition the open-type steel-pipe gridded sabo dam allows ordinary sediment to flow down so that it can have sufficient space to catch the future debris flow. 2) When the debris flow attacks the dam, it can reduce the maximum discharge by trapping huge rocks. 3) There is no danger to the dam structure even when the corrosion oc-curs. 4) After debris flow attacked the dam the river flow is not dammed up completely, so the environmental merit can be kept to some extent.

View full abstract

Hillside landslides occurred in April, 1997, on the Oritatemata river, Yunotani village, Niigata prefecture. In this study, meteorological characteristics in time of occurrence of hillside landslides were examined in order to obtain informations on occurrence mechanism, prediction method and protection countermeasure, of hillside landslides caused by snowmelt. Following matters were found through this investigation. (1) There was especially higher heat of snowmelt in 1997, in comparison with an ordinary year. (2) There was precipitation more than 100 mm for the same time. Furthermore, volume of snowmelt was calculated by means of these meteorological informations. Volume of snowmelt in 1997 was twice as much as an ordinary year. The hillside landslides model applied to this collapse. In this simulation, occurrence time of collapse was explained. Hillside landslides occurred on the same day as the total of snowmelt and precipitation showed the largest volume. And hearing research about characteristics of collapse site was executed in this field. Taking results for these research into consideration, occurrence mechanism of hillside landslides caused by snowmelt was studied.

View full abstract

Recently, streamside forests have been preserved or restored in case of sediment and erosion control works for preserving the natural environment and for preventing sediment-related disasters. Many studies on streamside forests for function of sediment control have been done. However, further detailed study is required.
The purpose of this study is to improve the understanding of bed-load sediment transport rate in woody vegetated channels. Experiments were conducted about the bed-load transport rate in experimental flume in which cylinders, models of trees, with equi-spacing were settled in high density. Then, we have investigated the relation between the bed-load transport rate and the roughness concentration of models of trees. The results are summarized as follows;
(1)The dimensionless bed-load transport rate Φ_B in woody vegetated channels is calculated using by dimensionless effective tractive force τ_*e, dimensionless critical tractive force τ_*c, and tractive force τ_* using by the following equation, which is applicable in the experimental regions.
Φ_B=5⋅τ_*e^3/2(1-τ_*c/τ_*)^3/2
(2)As the roughness concentration of trees in channels increases, the dimensionless effective tractive force decreases.
(3)The dimensionless effective tractive force is estimated taking account of roughness concentration of trees by the following equations,
τ_*e=βU²_*/(σ/ρ-1)g⋅d=β×τ_*
β=(2g)^3/4n^9/10/C_D^3/4×I^3/10S^3/2/q^3/5D^3/4
where, U_*=friction velocity, σ=density of sediment, ρ=density of water, g=acceleration of gravity, d=meangrain size, β=coefficient of diminution, n=Manning's coefficient of roughness, C_D=drag coefficient, I=gradient of riverbed (energy gradient), S=space of trees, D=diameter of trees, q=discharge of water per unit width.
These empirical equations are helpful to examine sediment transport in woody vegetated channels and design of adequate arrangement of streamside forests.

View full abstract