Detection of the windfall damage to forests caused by the Typhoons 9117 and 9119
Gen Takao
Forestry and Forestry Products Research Institute of Japan
P.O. Box 16, Tsukuba Norin Kenkyu Danchi, Ibaraki 305, Japan
Abstract
The typhoons 9117 and 9119 attacked successively northern Kyushu with exceptionally strong wind in September of the last year. it left severe damages to the agriculture and forestry in those prefectures.
The author carried out the windfall-damage area detection on the forest in northern Kyushu using MOS MESSR images. The damage appeared as the inorement of the CCT value of the Band 2 on the images. Thus, the damage areas are detected using the two dimensional histograms of Band 2s of before and after the typhoons. The atmospheric effects to the images are considered locally.
Using the result of the detection, the incidence of the damage in relation to topographical factors were analysed with DTMs.
Introduction
In September of 1991, the typhoons 9117 and 9119 successively struck northern Kyushu ( the most west of Japan’s main islands ) with extremely strong wind. Among them, the typhoon 9119 was especially strong that it broke records of the maximum instantaneous wind speed at many observatories; e.g. 60. 9 m.s at Mt. Aso, 52.9 m/s at Kumamoto city or 44.4 m/s at Hita city ( Meteorological Agency, 1991 ( Figure 1)
Fiture 1. The locus of the typhoons (yamamoto, 1992)
From those typhoons, forest productions in northern Kyushu suffered considerable damages ( Figure 2) . For examples, in Ooita pref., where suffered the most damaged in the prefectures of this area, the damage amounted about 430 million dollars and the suffered area reached 220 km2, which is about one-tenth of whole forest area of the prefecture ( Forest Agency, 1992). The most part of the damage was caused by windfalls on coniferous plantations. Windfall surveys were carried out in every towns and villages by the prefectures using aerial photographs and / or ground observations.
Figure 2. The windful damage in a coniferous stand (Forest Agency, 1992)
To understand the distribution and tendency of damages to the topography in this area, the overall survey with a criteriaon is necessary instead of one as the sum of small scale surveys among which the cirterion may vary. In this paper, a detection of wide – speeded windfall damages of forests over northern kyushu is attempted using satellite imagery.
Data and Preprocessing
Two sets of MOS MESSR data taken before and after the typhoon are used. Each set has contiguous two scenes on an orbit. One set was observed in Apr. 15, 1991 ( hereafter referred to as the “April image” ) and the other was in Nov. 5, 1991 ( hereafter referred to as the
“November image”) These scenes cover the most severely damaged area, i.e. whole Ooita pref., western Fukuoka pref. and northern Kumamoto pref., No clouds can be seen on the April image but some clouds on mountains on the November image.
a
The DEM and the municipality map of the Digital National Land Information are prepared to classify the slope direction of damages and to round off the damage for each municipality. The unit of the data is the ¼ mesh’ of the Digital National Land Information, which has 45 sec. width in longitude and 30 sec. width in latitude.
Two scenes of each set of images are merged into an image and the consequent images are projected on the longitude – latitude coordinate. The size of a pixel is equivalent to one-fifth in either direction of a ¼ mesh, and is about 58 meters along longitude and 46 meters along latitude around Hita city, Ooita pref. ( N320 20).
Forest areas are extracted from the April image by clustering method. Decidous forests are excluded because the leaves had not opened yet at the time observed. The other forest types, such as coniferous plantations, pine forests, evergreen or mixed broad – leaved forests are not separated from each other in the forest classes. The optical characteristics of forested areas are rather stable except for deciduous trees in winter ( Kaufman, 1989). Then, the change detection is considered to be possible within the forest classes. Clouds and their shadows on the November image are extracted and the corresponding part of the forest areas are reduced. Then, the edge of the forest areas are removed from the further analyses to avoid detecting errors on overlaying . Finally, a forest mask on the images is prepared for following analyses.
For the most part of processing and analyses, the International Imaging System’ the System 600 on a Sun-3 and a Sun Sparc Station 2 are used, and some detailed analyses are carried out with programs coded in Sun Fortran,
