STUDY OF SLOPE PROTECTION METHODS THAT CONSIDER THE GROWTH OF VEGETATION

  On embankment slopes or earth cuts along roads or levees, slope protection methods that make use of the growth of roots of vegetation (vegetation work) are often used to prevent soil on the slope surface from being washed away by the runoff of rainwater. Using vegetation preserves the environment, but presents one challenge: surface soil runs off when the roots of the vegetation have not grown long enough as shown in Photo 1.

Root growth and soil retention effects.

  But it is not known how long it takes for the vegetation's roots to grow long enough to prevent the problem shown by Photo 1? So, a simple model experiment using sod was performed to find out how long roots must grow to be able to retain soil. Several models were made by placing sod on top of a fixed quantity of soil packed in a box with dimensions, 30 cm * 60 cm * 30 cm. Assuming that the growing season of grass on the Kanto Plain (period when roots, leaves, and stems etc. grow) is a period from April to October, three times? before growth started, after 3 months of the growing season had passed, and after 7 months of the growing season had passed?using two models each time, 50 mm/hour of artificial rain was dropped for 4 hours at an angle of 30 degrees through a 15 cm opening from above the short side (total of 200 mm of rainfall), the quantity of soil run off from the opening was measured, and after each test, the model's soil was removed to investigate the growth of the roots. The results are shown in Photo 2 and Figure 1. When the roots had not yet grown, about 25% of the soil in the box was run off, but after three months had passed, this quantity was reduced to about 5%, and at 7 months when the first year's growing season was over, only about 1% was runoff. Examining root length showed that after 3 months, only a few roots were 20 cm long, but after 7 months had passed, many of the roots were intertwined.

Photo 2 Experiment model and state of
growth of roots

Fig. 1 Growing period and soil
runoff quantity relationship

Study of a method for use at the stage of insufficient vegetation growth.

  During the period when the roots of the vegetation have not grown sufficiently (until several months of growth had occurred during the above experiment), it is necessary to prevent soil from running off as it did in Figure 1. Therefore, a study of simple supplementary methods that do not prevent the growth of roots has been performed. A 2 m high embankment was constructed, and on 30 cm of a slope where it was necessary for the roots of vegetation to grow, the effectiveness against rainfall of supplementary methods such as mixing in rubble or building a drainage layer using rubble were verified. Photo 3 shows an example of this experiment. On the left side where a supplementary method was not executed, soil ran off after artificial rainfall of 20 mm / hour was applied for 20 minutes. On the right side, however, rubble was mixed in the material, and when 100 mm/hour of artificial rainfall was applied for 4 hours, the surface flowed a little, but the method was shown to be very effective.

  In addition, the Soil Mechanics and Dynamic Research Team is undertaking a variety of studies of slope disasters on road embankments and river levees etc.

(Contact: Soil Mechanics and Dynamics Research Team)

INTRODUCING RESEARCH ON THE USE OF ARTIFICIAL SATELLITES TO REDUCE DEBRIS FLOW DAMAGE FOLLOWING A VOLCANIC ERUPTION
- CABINET OFFICE, STRATEGIC INNOVATION PROGRAM (SIP) -

  The Strategic Innovation Program (SIP) is a national project initiated in 2014 to realize scientific and technological innovations. Since 2018, the Cabinet Office has focused on 12 phase 2 challenges. To tackle one of the 12 challenges, National Resilience (Strengthened disaster prevention and disaster amelioration), researchers and technologists in a number of industrial, administrative, and academic institutions within Japan are conducting research on methods of preventing and ameliorating earthquake, flood, sediment and volcanic disasters and research to develop technologies to clarify the state of disaster damage.

  As members of the Volcano and Debris Flow Research Team, we are in charge of part of the associated research, "analysis and prediction of the state of disaster damage when responding to a large-scale disaster." In cooperation with researchers from other organizations we are studying a number of categories of technology.

  Researchers conducting joint research are developing systems to immediately share data observed by Earth Observation Satellites (EOS) (Fig. 1) and to analyze these satellite data so that they can be applied to prevent and ameliorate disaster damage. As research concerning volcanic eruptions, researchers at the (National Research and Development Agency (NRDA)) National Research Institute for Earth Science and Disaster Resilience (NIED), Kagoshima University, and the (General Incorporated Foundation) Japan Weather Association (JWA) are conducting research on technologies to analyze data obtained by sensors called Synthetic Aperture Radar (SAR) mounted on the (NRDA) Japan Aerospace Exploration Agency (JAXA)earth observation satellite ALOS-2 and to measure the range and thickness of volcanic ash ejected and deposited (called "ashfall thickness") during volcanic eruptions. In this way, we are developing technology that collects data concerning the range of ashfall thickness to predict the scale of debris flows that will be triggered by rain falling on mountains where this fallen ash has accumulated and the locations where these debris flows will cause damage (Fig. 2).

Figure 2. Calculation technology to predict flow
of water and flow of debris flow after rainfall that we
are now developing.

  The occurrence of typhoons or other types of torrential rainfall trigger debris flows almost every year. How are volcanic eruptions and debris flows related? Materials ejected from volcanic craters during eruptions include large lapilli and fine volcanic ash, and the finer ash particles are almost entirely blown by the wind so they are deposited far from the volcano. Slopes covered with fine volcanic ash are resistant to permeation by water, so that when rain falls, large quantities of this rainwater flow down surfaces (slopes) covered with volcanic ash. When this occurs, this water tends to pick up volcanic ash and other unstable sediment as it flows to gradually become a debris flow. Even light rainfall easily causes debris flows (Fig. 3). In order for residents to evacuate before a debris flow occurs, the range where it can flow must be clarified in advance.

Figure 3. View of fallen ash before and after a volcanic eruption (left),
change of flow of water resulting from ash fall (right) from Pierson and Major,
Annu.Rev. Earth Planet. Sci. 2014, 42:469-507)

  We are, therefore, working to develop technology to ameliorate damage caused by debris flows following a volcanic eruption.

(Contact: Volcano and Debris Flow Research Team)

Research on the Growth Trends of Roadside Snow Piles for Effective Snow Hauling Operations

1. Background

  In order to ensure smooth winter road traffic, road managers are required to efficiently haul and clear snow from road shoulders. Snow hauling is the work of clearing roadside snow piles accumulated due to snowfall or snow removal operations in order to secure sufficient road width for vehicles to pass (Photo 1).

  If the growth trends (changes in the size) of roadside snow piles can be determined, they can be used as basic data to help the design of an effective snow hauling plan with an optimal snow hauling method and timing.

  Specifically, we have been working on the study of simulation based on the predicted growth trends of roadside snow piles to determine the optimal timing of hauling and whether the entire snow piles should be removed in one operation or they should be cleared in smaller amounts in multiple operations.

2. Methodology and Results

  In order to predict changes in the size of roadside snow piles, we analyzed the relationship between the measurements of cross-sectional areas of roadside snow piles and different factors considered to impact the growth of roadside snow piles (Fig. 1). There are two major techniques for snow hauling: one is to completely remove roadside snow piles, and the other is to partly remove the snow, which is referred to as the "widening" of a road (Fig. 2).

  We focused on the cross-sectional areas of snow piles as an indicator of the sizes of the snow piles. Using this indicator is helpful in that we can determine the amount of snow to be removed (workload) by multiplying the length of the concerned section of the road by the indicator, while we can also predict the snow hauling speed based on another analysis using the indicator.

  The analysis results show that the growth of roadside snow piles is greatly influenced by the maximum snow depth and the number of snow hauling operations (complete removal and/or widening).

  Using these results,it is now possible to predict the cross-sectional areas of snow piles in the current and future seasons based on real-time meteorological observations or a selected annual snowfall pattern.

  

3. Application of Research Results

  Based on the analysis results, we developed a system for predicting the growth trends of snow pile cross-sectional areas (Fig. 3). The system is capable of simulating different snow hauling timings and methods and is intended for use by road managers and snow hauling contractors.

  We are now working on improvement of the system and the accuracy of prediction in order to support effective snow hauling operations.

(Contact: Machinery Technology Research Team, CERI)

25th Study Session on Soils and Foundations Held

Ground in Hokkaido and Research by the Geotechnical Research Team

  Ground in Hokkaido Prefecture is characterized by a wide distribution of uncommon soils, such as peat and volcanic ash. Peat is made of decayed reeds, sedges, and other plants accumulated in wetlands, such a swamp or a lake. Since it is very soft, it can cause various problems such as ground slide failure and subsidence in roads and river banks constructed on it. Volcanic ash is a soil consisting of accumulated ejecta, which is said to be present in about 40% of the total area of Hokkaido. It is known that the liquefaction of embankment soil containing volcanic ash caused great damage to a residential area in Satozuka District, Sapporo City during the Hokkaido Eastern Iburi Earthquake on September 6, 2018 (Photo 1).

  It should also be noted that Hokkaido is not only an earthquake-prone area, but also a cold climate area, which is prone to ground disasters caused by freezing and thawing of soil. In summary, Hokkaido is a very difficult geotechnical environment to efficiently develop and properly maintain social infrastructure in. To address this challenge, the Geotechnical Research Team has continuously promoted research and development to find solutions.

  The results of our research and development are presented and disseminated to the public on various opportunities so that they are widely recognized and used. This Web Magazine reports on one of such activities, the Study Session on Soils and Foundations.

Study Session on Soils and Foundations

  The Geotechnical Research Team held the 25th Study Session on Soils and Foundations from October 15 to 16, 2020. This Study Session has been held almost every year since it was first held in 1993 for technical officials of the Hokkaido Regional Development Bureau of the Ministry of Land, Infrastructure and Transport (hereinafter the "HRDB"). This year's session was themed on embankment structures and was participated in by 18 officials from the development and construction departments of the HRDB (Photo 2).

  Unlike our seminar whose purpose is to give a presentation of the team's research outcomes, this Study Session is an interactive session where individual participants are invited to share examples from their construction projects, challenges, and solutions, and exchange opinions with others. Our research results are referred to in the context of this opinion exchange, which we believe helps the technologies we developed to be recognized as potential solutions to challenges in real settings. It is also an important opportunity for the team members to learn challenges that technical officials are facing and find new research topics.

  We plan to continue our activities to actively share and promote our research results.

(Contact: Geotechnical Research Team, CERI)

Research on Fish Monitoring Using Low-Cost ROVs around Coastal Structures

  Coastal structures such as fishing ports not only support stable supply of fishery products, which is their primary function, but also protect and nurture fishery living organisms with their reef-like structures and calm water areas (Fig. 1). With the recent decline in fishery resources, coastal structures are desired to enhance these secondary functions. To achieve this, it is necessary to obtain basic information on fish habitats in the vicinity of such structures continuously and widely. Conventionally, visual surveys by divers have been used to this end. A more efficient quantitative monitoring method that requires less labor and time is needed for future surveys. Until recently, remotely operated vehicles (ROVs) equipped with optical cameras had been massive and expensive. Today, we are seeing more and more small, affordable, and high-performance ROVs appearing on the market. These low-cost ROVs may be useful for improving the efficiency of underwater surveys.

  In this study, experiments with low-cost ROVs were conducted in tanks and in the fishing port and the results were compared to that of conventional visual surveys by divers, in order to explore an inexpensive quantitative fish monitoring method that can be used to determine the characteristics of fish habitats around coastal structures.

  The results of an experiment in a large circulatory tank showed that the low-cost ROVs used in this study were capable of stable maneuvering in a current at the same velocity as real currents in the fishing port. In addition, a test using an analysis camera was conducted to see how an increase in water turbidity affects the visibility of fish in the tank. As a result, it was found that it would be possible to identify fish species in many fishing ports on the Japan Sea coast (Table 1).

  In addition, an experiment on the distance between the ROVs and fish and the visibility of fish revealed that the ROVs needed to be within a 1.5 m radius in order to identify the species of small fish (about 10 cm in length), which are common in the fishing port. Finally, in a fish monitoring experiment in the fishing port, the ROV survey was capable of observing fish populations and taxonomic groups to a similar extent as the visual survey by divers (Photo 1). These results indicate that low-cost ROVs have a high potential for use in fish monitoring around coastal structures. Since the number of field tests is still small, we plan to conduct more tests in different locations and seasons so that we can develop an easy-to-use low-cost ROV fish monitoring method.

Table 1 Difference in the Visibility of Fish in
Accordance with Water Turbidity
The photos in the "fish identification" column are
enlarged images of the part of the tank in the red circle.

Photo 1 Fish Monitoring with a Low-Cost ROV

(Contact: Fisheries Engineering Research Team, CERI)