Various trials have been performed to harmonize the sabo structures with the surrounding Nature. Mild slopes with rough masonry roughness attached to groundsels are among these structures. They look better than ordinary drop structures. What makes them better? How can we improve them? We focuses on the sound of the flow over groundsels and sabo-dams. A series of model experiments was carried out. The levels of sound and frequency distributions were measured and analyzed and the sound of the actual flow was analyzed using similar procedures. The actual flow sounds include some in a compact disk commercially recorded and our own material recorded in the field. The noise level was higher for ordinary drop-type structures than for slope-type structures. Flows with large quantities entrained air showed higher noise levels. The predominant frequency was high for small discharges and low for large discharges. The power spectrum of ordinary drop structures was similar to that of natural waterfalls in some parts. The flow on slopes had a wide spectrum. The similarity was confirmed in terms of the differences of noise levels between models and fields. The noise level increased with the drop number.
This paper presents a comprehensive estimation of the sediment-control effect by sabo dam infocussing on the slope gradient of the deposited sediment at sabo dams. The field data at five dams in the Koshibugawa, a tributary of the Tenryu River were used for the analysis. Hydraulic-model experiments were also carried out to clarify the effects of the sediment concentration in flow of water on the slope gradient of the deposited sediment at sabo dams. The following conclusions were obtained as a result of this investigation: (1) In the case of a sabo dam with relatively large capacity has been built in upstream, little or only a little degradation of the river-bed can be seen in the downstream sabo dams after floods. (2) The newly-built big sabo dam, however, gives a significant change in the period for restore the slope gradient of the deposited sediment after floods at the downstream sabo dams, because of the large impoundment of sediment and the big change in grain size at the new sabo dam. (3) When the sedimentation-form is expressed by a quadratic equation, y=ax+bx2, a linear relationship is observed between the coefficients a and b. The coefficient b reached the maximum several years after the new construction and then gradually decreased. (4) The comparison between the results of our experiments and the given values in previous literature shows that the dimensionless slope gradient of the deposited sediment, slope gradient of the deposited sediment/ original river-bed slope gradient, is an increasing function of the concentration of debris flow as a first approximation, and that the slope gradient of the deposited sediment is substantially half the original gradient of the river-bed at a lower concentration of debris flow.
A handy dynamic cone penetrometer has a cone diameter of 3cm, a weight of 5kg, and falling height of 50cm. It has been usually used to measure the soil thickness in researches on prediction of occurrence of shallow slope failure and estimation of water storage capacity of mountain watershed. Results of the penetrometer test have been expressed as Nc value that defined as the equivalent number of impact per 10 cm depth. This paper discusses on relationship between the magnitude of Nc value and physical properties of soil and saprolite. The magnitude of Nc value shows dependable logarithmic correlation to dry bulk density, field bulk density, and saturated bulk density. These results show that the magnitude of Nc value is strongly influenced on density of soil and saprolite. Fine porosity which is equivalent to water content ranged from -3 to -60cm H2O, shows no systematic correlation with Nc value. Coarse porosity, however, which is equivalent to water content ranged from -60 to -1, 000cmH2O, decreases with increasing Nc value. These correlations indicate that coarse porosity is also an important factor which influenced on the magnitude of Nc value. From the results mentioned above, the magnitude of Nc value would be a parameter indicating the soil physical properties, and it is possible to estimate some physical properties of soil and saprolite from the magnitude of Nc value. In particular, estimated saturated bulk density would be available as the parameter for slope stability analyses.