Condition of gre 
  
Fig.5 Condition of groundwater table 
Soil and Geology 
k map of 
the total thickness above bed ruck 
  
Fig.6 Soil and Geology 
to be the vegetation area, which is rated from 
1 to 
Rainfall 
Guidicini and Iwasa(1977) presented a study on 
the occurrence of landslide relation to the 
cumulative precipitation record in Brazil. Based 
on 46-year record of mean annual 
precipitation, they also established a danger 
level chart which is the relationship between 
rainfall and landslides. Their methodologies are 
applied for this study. 
Rainfall has to be analysed in three important 
aspects; climate cycles over a period of year, 
rainfall accumulation in a given year, intensities 
of given storms. Fortunately, rainfall records 
of 10-year  period(1978-1987) were avaliable 
from Suwon Meteorological Observatory Station 
(Environmental Impact Assessment Report, 
1989). The data reveals that the average 
annual rainfall is about 1200 to 1300 mm. In 
general, rainfall is concentrated between June 
and September, recording about 820-830 mm. 
One of the heaviest storm on Sept.1, 1984 
marked 190.5 mm of precipitation during 24 
hours. On the basis of the chart of Guidicini 
and Iwasa, mean annual precipitation were 
divided into four categories; above 2500mm(4), 
2000-2500mm(3), 1200-2000mm(2), below 1200 
mm(1). According to the chart, the mean 
678 
OVERLAY PROCESS(Weight & Rating) 
Hoc rnm HIT TE 0 
TLR ed EY 491; 89-3 
  
Fig.7 Overlay process 
Production of the final map 
iB) Piüal map 
  
Fig.8 Production of final map 
annual precipitation of the study area belongs 
to the rating(2). 
Soil 
A soil classification map(Fig.6-A) was produced 
on the basis of agricultural soil maps(Institute 
of Agricultural Sciences, 1977) and laboratory 
tests. The classification was made according to 
the Unified Soil Classification System. Important 
factors for rating system of a given soil are: 
soil texture, permeability, shear strength and 
weight ratio of sand and clay. 
Silty sands(SM) and clayey sands(SC) occupy 
in most of the study area. Laboratory tests 
showed that the contents of silt were about 
29-43% and those of clay were about 6-15%. 
Flow type of landslide is common in the fine- 
grained granular soil zone. Soil formation is 
considerably thick(about 5-10m). Considering the 
soil characteristics of the study area, the soils 
are classified in three groups: SM(1), SC(2), 
ML(3:silis and very fine sands). Although 
gravelly sands are highly permeable(estimated 
permeability: about 10? m/s), they fall on the 
lowest rating. They are mostly located on the 
very gentle slopes of less than 5%, therefore 
they do not affect on landslide occurrences in 
the study area(especially along the side of 
streams).