5.1.1. Calculation of Road Blockage Probability and Its Addition to the Road Network
Road blockage probability was added to the road network data shown in
Figure 3 as an attribute. The present study takes into account two types of blockage: (1) Single blockage, defined as being when a road is blocked by the collapse of a wooden building on only one side of a road; and (2) Combined blockage, defined as being when a road is blocked by the collapse of wooden buildings on both sides of a road. Here, in the case of (1) single blockage, when the debris width produced by a collapsed building intersects with a building shape located on the other side of the road, the road located between these buildings is regarded as being blocked. Further, in the case of (2) combined blockage, when the debris widths produced by collapsed buildings on both sides of the road intersect, the road located between these collapsed buildings is regarded as being blocked.
Regarding road blockage probability for the case of (1) single blockage, since the road will be blocked by the collapse of one wooden building on one side of the road, the blockage probability is regarded as being the same as the probability that a wooden building will collapse. Meanwhile, in the case of (2) combined blockage, since the road is blocked when buildings on both the left and right sides of the road collapse, the blockage probability is found by multiplying the collapse probabilities for buildings on the left and right sides of the road. Below, methods for calculating road blockage probability and adding it to the road network are described.
(1) Roadside building extraction and classification
Roadside buildings, which are a primary factor in road blockage, were extracted. First, a buffer from the road centerline was created on the right side only, and the spatial search function of the GIS was used to extract buildings which intersected with the buffer. Similarly, next a buffer from the road centerline was created only on the left side, and buildings which intersected with the buffer were extracted. By following these steps, buildings were classified into those located on the right side of the road and those located on the left side of the road.
(2) Extraction of roadside wooden buildings and addition of collapse probability
Using the attribute search function of the GIS, wooden buildings were extracted from both the buildings on the right side and the buildings on the left side. Further, probability of collapse was added as an attribute to each of the wooden buildings.
(3) Extraction of pairs of buildings which would cause blockage
(a) Case of single blockage
A buffer with a radius equal to the debris width which would be produced from a wooden building on the right side of the road if it collapsed was created, and the spatial search function of the GIS was used to extract any building on the left side of the road which intersected with the buffer. The same was done for the left side of the road, and pairs of buildings which would cause single blockage were extracted. Regarding the outflow of debris that occurs when a building collapses, the case of maximum damage was imagined, and it was assumed that debris would flow out evenly in all directions. Further, regarding the debris width, with reference to
Ichikawa et al. (
2004), the debris width was found using Equation (5), given below. Regarding the number of floors in buildings, reference was made to data on the present building use situation. As the average floor height, a uniform average floor height of 3 m was used.
(b) Case of combined blockage
Debris width buffers were created for wooden buildings on both the left and right sides of the road. The spatial search function of the GIS was used to extract left-side debris buffers which intersected with right-side debris buffers and right-side debris buffers which intersected with left-side debris buffers. These steps were performed to extract buildings on the right side and buildings on the left side which would cause combined blockage.
(4) Generation of road section lines
With reference to
Yoshikawa et al. (
2003), section lines (lines which connected the center points of buildings which would cause road blockage) were created. Using the XY To Line tool of the GIS, line data was generated for lines which connected the center points of buildings which would cause road blockage (which were extracted in (3) above), and this data was used as section lines. XY coordinates which were the center points of buildings were found using the geometry arithmetic function of the GIS. Section lines were generated to suit three types of case - the case of single blockage caused by the collapse of wooden buildings on the right side of the road, the case of single blockage caused by the collapse of wooden buildings on the left side of the road, and the case of combined blockage. Further, blockage probability which suited the type of blockage was added for each case.
(5) Addition of blockage probability to road network
Using the spatial coupling tool of the GIS, to each road link which intersected with the three types of section line generated in (4) above, blockage probability (indicated by pk) for each of the points on section lines (indicated by k) was added.
(6) Calculation of road blockage probability
Through the above operations, to each road link, blockage probabilities (indicated by
pk) for points on section lines (these points were indicated by
k) were added. The probability
Ps that all points
k on the section lines of each road link would be passable was calculated using the blockage probability
pk, and is expressed by Equation (6) (
Takeuchi and Kondo 2002).
Therefore, the road link blockage probability
Pb was found using the Equation (7) below:
Using the field arithmetic function of the GIS, the above-described calculations were performed on the attribute table of road link data, and blockage probability was calculated for each road link.