Abstract of Special Research Report (SRR-No.25)

National Institute of Occupational Safety and Health, Japan

Specific Research on Preventional Countermeasures of Labor Accidents due to Debris Flow


Noriyuki HORII

: Since Japan has a lot of steep terrain, local heavy rainfalls may sometimes produce dangerous debris flows. In December 1996, a large debris flow occurred at Gamaharazawa located at the border between Nagano and Niigata prefectures. The debris flow, which involved construction workers working in the drainage basin, resulted in a serious disaster with 14 persons dead and 9 persons injured. In this case, once a debris flow occurs not only many workers in the basin but also nearby residents might be involved and then the disaster becomes expanded widely.
    From these background mentioned above, the establishment of the effective countermeasures for preventing labor accident due to the debris flow is an urgent problem because it is thought that a lot of erosion control structure such as sabo dams. etc. may construct in the future from a geographical characteristic of our country.
    To defend the worker from the debris flow with high progressing probability, the method of the adequate foresight the occurrence of debris flow is established. And, also it is indispensable to construct alarm-evacuation system, which can ensure effective evacuation. Moreover, it is necessary to understand the flow characteristic of the debris flow for the sake of establishing the foresight method and the evacuation system.
    In this special research project, following comprehensive research have been carried out.
  1. A questionnaire survey to on-site construction safety management of debris flow.
  2. An experiment study on the flow characteristics of a debris flow.
  3. Evaluation of evacuation time in case of debris flow by on-site experiments.
  4. Detection method for debris flow with using visual information.
  5. Study and development of monitor and alarm system for debris flow.
  6. Construction of a support system for optimum arrangement of debris flow detection sensors.

Questionnaire Survey regarding On-site Construction Management of Debris Flow

Yasuo TOYOSAWA and Noriyuki HORII

: In order to establish countermeasures against accidents due to debris flow in construction work sites, a questionnaire survey of construction sites was carried out. To identify the factors concerning accidents in construction work sites, the questionnaire focused on the followings: the contents and scale of construction, site conditions, alarm and evacuation equipment, and evacuation procedure at the time of debris flow, etc. Statistical analyses were made on 226 responses to this questionnaire survey.
    The results can be summarized as follows:
  1) About 80% of the construction sites that responded are at risk of debris flow. Among these, about 40% of the sites have not taken measures to prevent from debris flow such as monitoring and alarm systems. On construction sites with fewer than 15 workers, about 80% of the sites have not taken measures to prevent from debris flow; thus, small-scale construction sites tend to lag behind in terms of safety.
  2) The following problems were identified by this survey regarding keeping the monitoring and alarm system effective: (1) No maintenance for 10% of construction sites, (2) Where maintenance is infrequent, faults or failures of the system between maintenance checks would be overlooked until the next maintenance, (3) A failure in an electric circuit in the monitoring and alarm system would not be noticed, since the inside of the system cannot be checked, neither in a brief check nor peripheral inspection.
  3) The monitoring and alarm system for debris flow would be widely used if the following issues were addressed: (1) lower cost, (2) easy maintenance and (3) high reliability without incorrect action.
  4) Some 30% of construction sites have no alarm installed, so the alarm system for evacuation is inadequate in one third of construction sites. Furthermore, maintenance procedures are not always suitable in most of the sites where alarms have been installed.

An Experimental Study on the Flow Characteristics of a Debris Flow --Influence of Particle Size and Structure Rigidity on Impact Stress in Model Debris Flow--

Noriyuki HORII, Yasuo TOYOSAWA, Satoshi TAMATE and Hideo HASHIZUME

: Since Japan has a lot of steep terrain, local heavy rainfalls may sometimes produce dangerous debris flows. In December 1996, a large debris flow occurred at Gamaharazawa located a the border between Nagano and Niigata prefectures. The debris flow, which involved construction workers working in the drainage basin, resulted in a serious disaster with 14 persons dead and 9 persons injured.
    In general, as a method to reduce damage caused by a debris flow, sabo dams are often constructed But many factors regarding the traits of these flows remain to be examined and design methods of sabo dams intended to fend off the impact of debris flows have not been studied yet.
    The aim of this study is to obtain data that can help to develop some sort of temporary construction capable of resisting debris flow. The present study investigated the influence of particle size contained in the debris flow and the rigidity of structure which prevent debris flow such as sabo dams on the impact stress due to debris flow. Centrifuge model tests were conducted to determine how particle size of debris flow and structure rigidity affect impact stress.
    From the centrifuge model tests, the main results were obtained as follows:
  (1) The velocity of the debris flow becomes slower as the particle size of model debris flow becomes larger.
  (2) The impact stress of debris flow increases as the particle size of model debris flow becomes larger.
  (3) When gravel is included in the debris flow, impact stress intends to increase in proportion to the intensification of structure rigidity.

Evaluation of Evacuation Time from Debris Flow by On-site Experiments

Yasuo TOYOSAWA and Noriyuki HORII

: It is important to predict the time required to evacuate from a debris flow at the planning stage to take effective countermeasures against debris flow such as installing a detection system consisting of monitors, alarms, etc. To prevent accidents caused by debris flow, the evacuation time should be shorter than that of taken from when an alarm sounds until the debris flow hits the construction site. The evacuation time depend on several factors such as the evacuation route, composition of workers, and geographical conditions of the site. Therefore, on-site evacuation experiments were carried out to evaluate the influence of those factors on the evacuation time.
    The following values of evacuation speed were derived through the on-site evacuation experiments.(Evacuation for one worker and route width is 1 m.)
  (1) Evacuation speed of 1.5 m/s on a flat smooth surface and 1.3 m/s on a flat surface covered with stones.
  (2) Evacuation speed of 0.7 m/s on a slope of 30 degrees and 1.1 m/s on a slope of 10 degrees.
  (3) On a ladder, the evacuation speed is 0.4 m/s for going up and 0.35 m/s for going down.
    The following relations between evacuation time and number of workers, width of evacuation route, etc. were also derived through the experiments. The evacuation speed needs to be modified according to the following factors.
  (1) On a flat surface, when the number of workers increases, the evacuation speed decreases, and when the width of route or distance increases, the evacuation speed increases. Regarding the surface conditions of the route, the evacuation speed is slower for a route covered with stones compared to a smooth flat surface.
  (2) On a slope, the evacuation speed decreases as the slope angle and number of workers increase.
    On the contrary, the evacuation speed increases as the width of the route increases.
  (3) On a ladder, the evacuation speed (individual evacuation speed) increases as the number of workers increases. The evacuation speed (individual evacuation speed) does not change even if the number of ladders increases.

Detection Method for Debris Flow using Visual Information

Kyoko HAMAJIMA, Noriyuki HORII, Yasuo TOYOSAWA and Satoshi TAMATE

: There is an extremely high possibility of encountering a landslide due to debris flow in erosion control work and disaster relief work conducted on site where the ground may essentially collapse with a high degree of danger. Due to its large destructive force in particular, there is a large possibility that the landslide may develop into a labor accident with many deaths or injuries. To avoid a disaster due to this sudden debris flow, the key points are how quickly the occurrence is detected and whether it is possible to flee to safe shelter before the debris flow arrives. To achieve these goals, a system must be developed at the work site with placing observers or detection systems to ensure detection of the debris flow, an evacuation alarm that is certain to reach workers, and shelter that workers can move to within a short amount of time. Wire sensors that are frequently used as a detection system because of their highly reliable detection, have several problems: (1) workers must enter areas where there is a danger of debris flow during placement of the system, and (2) continual detection is not possible. Therefore, supplementary measures are needed to ensure further safety. Currently, an image detection sensor has been a focus of constant attention for its use as a detection system to compensate for these problems. The characteristics of this image detection sensor are: (i) easy placement of the system with no need for workers to enter dangerous areas and (ii) there is no system damage due to the debris flow and continual detection is possible. Thus, improvement of the system's detection capability as a whole is possible if it is used in combination with existing detection systems. However, there are few image detection sensors among those currently being developed that are designed to take safety theories into consideration. In order to create a safe work site, a system must ensure and maintain safety. There are theories for safety that are defined by ISO and the like in the field of mechanical safety.
    This report will examine misdetection problems and image detection sensors that are pivotal to a Safety Reporting System that guarantees "definite safety" and offer suggestions for the determination of safe conditions with image detection sensors. In addition, this report will describe the creation of an image detection program to run on a machine with Personal Computer (PC) and the results of detection experiments that were performed with the program.

Developmeny of Monitoring and Alarm System for Debris Flow

asuo TOYOSAWA, Shigeo UMEZAKI and Noriyuki HORII

:Various measures, such as weather forecasting, prediction of debris flows, evacuation drills, etc. were taken to avert accidents at construction site caused by debris flow. In doing so, factors found indispensable to preclude debris flow accidents are as follows; (1) Detecting a flow without fail, (2) Issuing evacuation warnings, and (3) Safe evacuation.
    This research examined the requirements for (1) and (2), and developed a new monitoring and alarm system for the debris flow which satisfies the need for "definite" safety.
    Almost all debris flow detecting systems at construction sites are of the hazard detecting variety whereby an alarm trips only after a flow is detected. Evidently the hazard detecting system does not work while in safe mode (a debris flow has not occurred) or indefinite mode (a system failure). Only when the system does not in itself fail will the censor detect a debris flow and trip an alarm. Should the system malfunction, the alarm will not register even when a debris flow actually occurs. A salient problem with the hazard detecting system is that no one can notice a detecting system failure in indefinite mode.
    On the other hand, the "Safety Confirmation Type" system invariably functions even in safe state and performs safety self-checks. It continues to report "safe state" with a green light, etc., and trips an alarm when safe mode shifts to indefinite or hazardous mode. Accordingly, the "Safety Confirmation Type" system is indispensable for accident prevention since it can detect a debris flow and trip an alarm without fail. In other words, the "Safety Confirmation Type" guarantees "definite" safety.
    In this study, a monitoring and alarm system for debris flow, which satisfies the needs of a "Safety Confirmation Type" as fail-safe, was developed, and continues to confirm its principal factors as followings:
  (1) Detecting wire,
  (2) Alarm effectiveness,
  (3) Battery power for backup,
  (4) Fail-safe controller and electrical wiring.
    The system reports its "safe state" with green light when all the above factors are verified as "YES". It can also indicate a possible failure of any of the above four with an individual color light, enabling immediate remedial action.

Construction of a Support System for Optimum Arrangment of Debris Flow Detection Sensors

Noriyuki HORII, Yasuo TOYOSAWA, Satoshi TAMATE and Kyoko HAMAJIMA

: In December 1996, a large debris flow occurred at Gamaharazawa located at the border between Nagano and Niigata prefectures. The debris flow, which involved construction workers working in the drainage basin, resulted in a serious disaster with 14 persons dead and 9 persons injured. In this case, once a debris flow occurs not only many workers were in the basin but also nearby residents were involved and then the disaster becomes expanded. For this reason, "the guideline for the prevention of industrial accidents due to the debris flow" is published in accordance with the revision of a regulation of Occupational safety and health at Ministry of Labor.
    In this guideline, an installation of a debris flow detection sensor is recommended as part of the alarm-evacuation system and the following important matter is suggested about the arrangement of these detection sensors.
  1. capable of monitoring debris flows generated in any tributaries
  2. capable of evacuating laborers within the time span between debris flow generation
    In order to cope with these important matter, it is indispensable to understand the generation place, magnitude, arrival time and arrival area of a presumed debris flow, and it is necessary to build the alarm system where effective evacuation can be taken based on this information. In this paper, a support system for optimum arrangement of the debris flow detection sensors is described, which can provide required information to construct a reliable debris flow detection system.
    Former arrangement methods of debris flow detection sensors are rather subjective. They tend to decide the arrangement empirically, only considering past debris flow generation conditions and topographic features of the basin. The present method using DEM data proposes a new system to observe dynamic conditions of debris flows objectively and sequentially. The main procedure is as follows.
  (1) divide a drainage basin into some small sub-basins and obtain gradient distribution for each sub-basin
  (2) extract meshes of gradients greater than 20-22° and weight points according to sediment depth and the steepest gradient value
  (3) accumulate indices of meshes obtained in (2) on the basis of each sub-basin, and then calculate the area ratio
  (4) specify the sub-basin of the highest area ratio obtained in (3) as a danger area susceptible to slope failures
  (5) execute one-dimension river-bed fluctuation analysis in a transport reach for the debris flow presumed to be generated at the danger area
  (6) execute two-dimension inundation analysis in the downstream extent from the initial inundation point with the boundary conditions obtained from calculation results of (5)
  (7) estimate the arrival time of the debris flow coming from the generation point to residential areas based on the previous analyses
    These analyses enable us to understand dynamic features of debris flows from generation, transport to accumulation sequentially, and arrange debris flow detection sensors on a computer display.
    The present system utilizes a GIS technology in order to manage DEM data and map information simultaneously. Because debris flow analyses require complicated numerical simulation and enormous amounts of data management, visual map information is necessary to examine the final arrangement of the sensors. The present results show one example of analytical GIS applications.

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