Haruyuki Hashimoto

Drag coefficient of unsubmerged rigid vegetation stems in open channel flows

Results of an experimental study on the drag force measurement involving a single stem kept in a channel flow stem array are presented. The data collected herein and those from literature indicate that the stem drag coefficient logarithmically increases with the areal stem density. The stem Reynolds number is noticed to have only a small effect on the stem drag coefficient which was however found to depend on the stem staggering pattern. The drag coefficient is less influenced by the Froude number in subcritical flows but it decreases with the Froude number in supercritical flows. New relationships are proposed for the stem drag coefficient which appear useful in partitioning the total flow resistance of vegetated bed streams into the stem and the bed particle resistances. The bed particle resistance applies to sediment transport through such vegetated flows for which the average flow velocity is available.

Effect of tall vegetation on sediment transport by channel flows

Results of an experimental study on sediment transport by channel flows with tall rigid stems are presented. The experiments were conducted for various stem areal densities, channel slopes and sediment sizes with uniformly distributed unsubmerged cylindrical rigid stems forming a regular square staggering pattern in plan. The rates of sediment transport in the presence of simulated vegetated surfaces were observed to be significantly smaller than those without the vegetation. A method is proposed to determine the effective (grain) shear stress in presence of vegetation under the condition that the apparent shear stress is available from easily measurable flow parameters. The present data along with literature data are used for quantification of the reduction in the rate of sediment transport by the rigid tall stems. The grain shear stress rather than the apparent shear stress is observed to better describe the variations in the rate of sediment transport by vegetated flows.

Rapid flows of sand-water mixtures at high concentration in a steep channel

Abstract Nondimensional parameters are derived for determining behavior of rapid flows of sand-water mixtures. From the result, it is found that the ratio of flow depth to sand diameter and sand concentration for the estimate of friction forces acting on a bed, and the ratio of tube diameter to sand diameter and concentration for that of drag forces on a tube are important parameters. Dependence of the drag coefficients on nondimensional tube diameter and concentration is found similar to that of the friction coefficient on relative depth and concentration. For the smaller ratio of a characteristic length to sand diameter and larger sand concentration, intergranular interactions play dominant role. For the larger ratio, the effect of turbulence of the mixture flows becomes major. For smaller concentration, interstitial water becomes important.

Effect of two succesive check dams on debris flow deposition

bovolin & mizuno, 2000; busnelli, stellinG & laRCHeR, 2001; mizuyama, kobasHi & mizuno, 1995; mizuyama, oda, nisHikawa, moRita & kasai, 2000; osti, itoH & eGasHiRa, 2007; wu & CHanG, 2003). It is generally believed that check dams can reduce sediment transport to downstream river reaches and stabilize river beds. Check dams must act to lower the peak sediment discharge and to decrease the total volume of sediment outflow to the downstream area. There are two-types of check dams; one is a closed type and the other is an open type. The former type is a traditional structural measure for controlling debris flow. However, this type of check dams has to be empty in order to trap large amounts of sediment during a debris flow event. The latter type can be subdivided into slit-check dams and beamcheck dams. Concrete slit-check dam is known as the slit type, and steel-pipe open dam is known as the beam type. An open type allows finer sediments to pass through at lower discharge and coarser sediments to be trapped at higher discharge such as debris flow. However designing their appropriate opening becomes a problem. The Hofu City in Yamaguchi Prefecture, Japan had heavy rain on July 21, 2009. The accumulated rainfall was 240.5mm and the largest hourly rainfall was 63.5mm/hour. This rainfall caused many shallow landslides on the mountainous areas of this city. Most of these landslides changed into debris flows and moved downstream in the mountainous rivers. ABSTRACT This paper describes the effect of two successive check dams on the debris flow event which occurred on 21 July, 2009 in Hofu City, Yamaguchi Prefecture, Japan. The debris flow event caused sediment deposition in the check dams in the Tsurugi and Hachimandani River. In the former river with two successive closedcheck dams, driftwood did not accumulate in the check dams but in the central region of the river bend. In the latter river with two successive open-check dams, driftwood accumulated in the opening of the check dams so that the accumulation obstructed the sediment transport to the downstream direction. The upstream check dams in these rivers have sediment deposition profile of around 2°, whereas the downstream ones have deposition profile of around 1.3°. The ratio of sediment deposition volume in the downstream check dams to that in the upstream ones is 0.3 : 1. The total amount of sediments trapped by the two successive check dams can be estimated at around 9,500 m3 in each river. Specific sediment runoff volume from the mountainous areas is qs = 6,800 (m 3/km2).

DEBRIS FLOWS PRODUCED BY HEAVY RAINS ON JULY 21, 2009 IN HOFU CITY, JAPAN

In this paper, we examine the debris flows which occurred in the mountain rivers in Hofu City, Japan on July 21, 2009. We estimate velocity and discharge of the debris flows in the rivers at Manao and Ishihara districts from field survey and numerical simulation. The numerical simulation indicates that the peak dis- charge is 181 m 3 /s in the river at Manao district and 258 m 3 /s in the river at Ishihara district. It is found that sediment yield volume by riverside erosion is larger than landslide sediment volume in the both rivers. Al- though the landslides triggered the debris flow initia - tion, they did not play the major role for the sediment outflow to the residential areas. The morphological characteristics of the river at Ishihara district increase debris flow discharge

Concentration Profile of Suspended Sediments in Open-Channel Flow at Steep Slopes

Experiments of mixture flow of fine sand and water are made in a steep open channel. Concentration profiles of suspended sand are measured. A two-layer model for the mixture flow is proposed; the lower layer of intergranular interactions and the upper layer of turbulent motion. Using the top of the lower layer as the reference level, the concentration profiles are calculated from the Rouse equation. The calculations are in good agreement with the experiments.

Flow Behaviour of Debris Flows in the Mizunashi River, Mount Unzendake

Between June and August, 1992 field measurements of debris flows were obtained at the Mizunashi River, Mt.Unzendake. The instrumented study site was a most downstream reach of the Mizunashi River, which is 25.1m wide and have an average slope of about 2°. Radio currentmeter, ultrasonic water level gauge and hand sampling of flows were used to obtain surface velocity, depth and compositional data for the several debris flows.Sediments in the debris flows were silt, while deposits from the debris flows on the study site were very coarse sand.Sediment concentration for the debris flows ranged from 6.3 to 13.5percent by volume. Nondimensional surface velocity us/u* ranged from 3 to 20.On the basis of these results, the volume of sediment runoff was estimated for the debris flows.