Ryoichi OHNO’s scientific contributions

It is well known that volcanic ash often generates surface flow on hillslopes as it decreases the infiltration rate of surface soil. Volcanic ash itself has a grain size of sand and has non-cohesive properties so it commonly has a high rate of water infiltration in laboratory tests. In spite of this, hillslopes covered by volcanic ash often generate surface flow, which results from the low infiltration rate of surface soil. Ordinarily, one of the reasons given for this low infiltration rate is the soil crust which develops on the surface of soil as a result of compression by drops of rainfall. This paper describes the vegetation and soil properties of Sakurajima, a volcanic island in southern Japan, where surface water and debris flow frequently occur. Laboratory tests on samples of the volcanic ash soil and in situ testing of surface flow were carried out. The test results demonstrated that slope surfaces covered by leaf litter or moss had very low infiltration rates, generating surface flow. To compound this, surface water was found to transport the ash and its deposition may have led to the formation of soil crust layers through a grading effect in the deposition process, giving the surface soil a low infiltration rate. However, if the slope has a large amount of living vegetation, such as Japanese pampas grass, surface flow rarely occurs. The presence of vegetation greatly improves the infiltration rate, limiting surface flow.

Tree roots with an underground lateral network play a very important role in slope stability. In studies such as those by Tsukamoto（1987）and Abe（1997）, the authors focused on roots that have a vertical connection between the surface soil layer and below, but they did not place much emphasis on the lateral spread of roots. Because slope failures occur in three dimensions, horizontal roots fully demonstrate their resistance against the failure. This paper aims to describe the resistance force Δ C that lateral roots provide. Two types of field investigations were made. One was the root pulling test to monitor the strength of a root pulled by a clamp. The other surveyed the lateral root distribution exposed in vertical sections excavated in the plots of Japanese cypress and cedar forests. The observed root distribution was used to calculate the resistance force Δ C. The field data show that as the distance between two trees increases, the Δ C value gets smaller. Their relationship can be described by a negative power function. It has also been proven that as the age of a tree increases so does the Δ C, but its increment gets smaller in the later stages of the tree's lifecycle. Intensive work with the field data has shown that forest thinning operations might decrease the root volume after felling and bring about an increase of Δ C that lasts for several decades. Moreover, thinning operations produce good results in slope stability when lateral roots are effectively increased. However, if the increase is insufficient, it might not contribute to slope stability.

Tree roots with an underground lateral network play a very important role in slope stability. Since failures occur in three dimensions, lateral roots fully demonstrate their resistance against failure. This paper describes the resistance force that Δ C lateral roots provide. Intensive field investigations and a model description were made. Since the field data show that the root strength Δ C varies considerably among the surveyed plots, predicting the Δ C from the plot information such as tree species, tree ages, slope and soil thickness can be difficult. To assist with prediction, this paper proposes a Δ C model to calculate the resistance force shown by trees of any age when it is given information about the species, forest operation history, and initial tree density. The Δ C model constitutes four components : a forest yield calculation table ; Karizumi's root volume regression curve ; a root distribution model proposed by Tsukamoto ; and a summation process of Δ C. The model highlights the importance of a particular parameter, which is the weight ratio of lateral roots to full roots. The model shows that the ratio of lateral roots directly affects the size of Δ C. By inspecting the field investigation data and the output of the Δ C model, it has been proved that the change of Δ C over time is well represented by a simple logarithmic curve which has only one coefficient multiplied by the logarithmic term itself. In comparing logarithmic curves to the output of the Δ C model, the logarithmic coefficient corresponds well to the weight ratio of lateral roots. The variance of Δ C shown by the field data is lumped into the coefficient. It has the potential to describe lateral root strength in a simple manner and could be a good index of Δ C. Also, when considering a forest thinning operation, the change in lateral root strength might easily be represented by the coefficient.