Norifumi Hotta

Analysis of the efficacy and cost-effectiveness of best management practices for controlling sediment yield: A case study of the Joumine watershed, Tunisia

Soil erosion can be reduced through the strategic selection and placement of best management practices (BMPs) in critical source areas (CSAs). In the present study, the Soil Water Assessment Tool (SWAT) model was used to identify CSAs and investigate the effectiveness of different BMPs in reducing sediment yield in the Joumine watershed, an agricultural river catchment located in northern Tunisia. A cost-benefit analysis (CBA) was used to evaluate the cost-effectiveness of different BMP scenarios. The objective of the present study was to determine the most cost-effective management scenario for controlling sediment yield. The model performance for the simulation of streamflow and sediment yield at the outlet of the Joumine watershed was good and satisfactory, respectively. The model indicated that most of the sediment was originated from the cultivated upland area. About 34% of the catchment area consisted of CSAs that were affected by high to very high soil erosion risk (sediment yield >10t/ha/year). Contour ridges were found to be the most effective individual BMP in terms of sediment yield reduction. At the watershed level, implementing contour ridges in the CSAs reduced sediment yield by 59%. Combinations of BMP scenarios were more cost-effective than the contour ridges alone. Combining buffer strips (5-m width) with other BMPs depending on land slope (> 20% slope: conversion to olive orchards; 10-20% slope: contour ridges; 5-10% slope: grass strip cropping) was the most effective approach in terms of sediment yield reduction and economic benefits. This approach reduced sediment yield by 61.84% with a benefit/cost ratio of 1.61. Compared with the cost of dredging, BMPs were more cost-effective for reducing sediment loads to the Joumine reservoir, located downstream of the catchment. Our findings may contribute to ensure the sustainability of future conservation programs in Tunisian regions.

Ecosystem Monitoring of Radiocesium Redistribution Dynamics in a Forested Catchment in Fukushima After the Nuclear Power Plant Accident in March 2011

The accident at the Fukushima Daiichi Nuclear Power Plant in March 2011 emitted 1.2 × 1016 Bq of cesium-137 (137Cs) into the surrounding environment. Radioactive substances, including 137Cs, were deposited onto forested areas in the northeastern region of Japan. 137Cs is easily adsorbed onto clay minerals in the soil; thus, a major portion of 137Cs can be transported as eroding soil and particulate organic matter in water discharge. Dissolved 137Cs can be taken up by microbes, algae, and plants in soil and aquatic systems. Eventually, 137Cs is introduced into insects, worms, fishes, and birds through the food web. To clarify the mechanisms of dispersion and export of 137Cs, within and from a forested ecosystem, we conducted intensive monitoring on the 137Cs movement and storage in a forested headwater catchment in an area approximately 50 km from the Nuclear Power Plant. Two major pathways of 137Cs transport are as follows: (1) by moving water via dissolved and particulate or colloidal forms and (2) by dispersion through the food web in the forest-stream ecological continuum. The 137Cs concentrations of stream waters were monitored. Various aquatic and terrestrial organisms were periodically sampled to measure their 137Cs concentrations. The results indicate that the major form of exported 137Cs is via suspended matter. Particulate organic matter may be the most important carrier of 137Cs. High water flows generated by a storm event accelerated the transportation of 137Cs from forested catchments. Estimation of 137Cs export from the forested catchments requires precise evaluation of the high water flow during storm events. The results also suggested that because the biggest pool of 137Cs in the forested ecosystem is the accumulated litter and detritus, 137Cs dispersion is quicker through the detritus food chain than through the grazing food chain.

Towards Long-Lasting Disaster Mitigation Following a Mega-landslide: High-Definition Topographic Measurements of Sediment Production by Debris Flows in a Steep Headwater Channel

Mega-landslides usually cause long-lasting subsequent sediment production, and long-term strategies for disaster mitigation are necessary in the case of such extreme events. The Ohya-kuzure landslide in central Japan is typical of sites where hillslope erosion and sediment yield have been continuously active since its formation in 1707. Sediment production is particularly active by debris flows in the headwater channels formed within the landslide. However, the dynamics of such debris flows in steep headwater channels have not been fully examined compared to those in gentler downstream reaches. To investigate the changes in headwater channel bed sediments remobilized mainly by frequent debris flows, repeated high-resolution measurements were carried out using terrestrial laser scanning. Freeze-thaw weathering in the surrounding slopes, which are composed of deformed shale and sandstone layers, delivers quantities of small particles onto the valley floor. Measurements in spring, summer, and autumn conducted over two years provided high-definition (0.1 m resolution) topographic datasets, revealing the seasonal amount of erosion and deposition to be on the order of 1000–5000 m3. Erosion and deposition along the reach also showed contrasting spatial patterns according to the sections bounded by knickpoints and valley narrows. These basic estimates of sediment production in headwater channels can be utilized for further mitigation of possible sediment-related disasters in downstream areas.

Interaction between topographic conditions and entrainment rate in numerical simulations of debris flow

Debris flow simulations are useful for predicting the sediment supplied to watersheds from upstream areas. However, the topographic conditions upstream are more complicated than those downstream and the relationship between the topographic conditions and debris flow initiation is not well understood. This study compared the use of several entrainment rate equations in numerical simulations of debris flows to examine the effect of topographic conditions on the flow. One-dimensional numerical simulations were performed based on the shallow water equations and three entrainment rate equations were tested. These entrainment rate equations were based on the same idea that erosion and the deposition of debris flows occur via the difference between the equilibrium and current conditions of debris flows, while they differed in the expression of the concentration, channel angle, and sediment amount. The comparison was performed using a straight channel with various channel angles and a channel with a periodically undulating surface. The three entrainment rate equations gave different amounts of channel bed degradation and hydrographs for a straight channel with a channel angle greater than 21° when water was supplied from upstream at a steady rate. The difference was caused by the expression of the entrainment rate equations. For channels with little undulation, the numerical simulations gave results almost identical to those for straight channels with the same channel angle. However, for channels with large undulations, the hydrographs differed from those for straight channels with the same channel angle when the channel angle was less than 21°. Rapid erosion occurred and the hydrograph showed a significant peak, especially in cases using the entrainment equation expressed by channel angle. This was caused by the effects of the steep undulating sections, since the effect increased with the magnitude of the undulation, suggesting that a debris flow in an upstream area develops differently according to the topographic conditions. These results also inferred that numerical simulations of debris flow can differ depending on the spatial resolution of the simulation domain, as the resolution determines the reproducibility of the undulations.

Influence of fine sediment on the fluidity of debris flows

Debris flows include a great diversity of grain sizes with inherent features such as inverse grading, particle size segregation, and liquefaction of fine sediment. The liquefaction of fine sediment affects the fluidity of debris flows, although the behavior and influence of fine sediment in debris flows have not been examined sufficiently. This study used flume tests to detect the effect of fine sediment on the fluidity of laboratory debris flows consisting of particles with various diameters. From the experiments, the greatest sediment concentration and flow depth were observed in the debris flows mixed with fine sediment indicating increased flow resistance. The experimental friction coefficient was then compared with the theoretical friction coefficient derived by substituting the experimental values into the constitutive equations for debris flow. The theoretical friction coefficient was obtained from two models with different fine-sediment treatments: assuming that all of the fine sediments were solid particles or that the particles consisted of a fluid phase involving pore water liquefaction. From the comparison of the friction coefficients, a fully liquefaction state was detected for the fine particle mixture. When the mixing ratio and particle size of the fine sediment were different, some other cases were considered to be in a partially liquefied transition state. These results imply that the liquefaction of fine sediment in debris flows was induced not only by the geometric conditions such as particle sizes, but also by the flow conditions.

Re-Estimation Method of Landslide-Triggering Rainfall Thresholds After an Earthquake with the Two Conceptual Models

This study used two conceptual models to examine the effects of rainstorms on post-earthquake landslides: a tank model to calculate the soil water index and a process-based model employing a digital terrain model with 10-m resolution to calculate the regional potential for shallow landslides, based on the distribution of shallow infiltration water, Darcy’s law, and a safety factor estimated by infinite-slope stability analysis. The two models were applied to the Funyu Experimental Forest of Utsunomiya University, Tochigi Prefecture, Japan. In August 1998, a heavy rainfall event caused many shallow landslides in the study area, whereas other heavy rainfall events in 1994–2003 did not cause severe landslides. The two indices were closely correlated, confirming the validity of both approaches for the estimation of rainfall properties. Response analysis of the effect of earthquakes on soil strength parameters indicated that landslide-triggering rainfall thresholds varied with decreasing effective soil cohesion, implying that the relationships between effective soil cohesion and the soil water index derived from the two conceptual models were valid for the re-estimation of the influence of rainfall properties on post-earthquake landslide occurrence.