| Loess distributes in Shaanxi, Gansu, Ningxia province, etc. and covers about 6.6% of total land area in China. Loess is easily softened and invaded for its large pore. The geological environment is very fragile. On account of dry arid climate and concentrated rainfall, there are a large number of hazard in loess area, which brings large damage to the local human life and property. According to figures, about 1/3 of total hazard happened in loess area. Therefor, the risk assessment of geological harzard in loess county is significant to the county planning and conductive to preventing and reducing disaster.Huachi county locates in Qingyang, east of Gansu province loess plateau, with many ravines, severious water and soil loss, and frequent human engineering activeity. A lot of slide, collapse, debris flow occured in study area. These disasters bring much impact to urbanization of Rouyuan county. This paper is based on 1:10000 scale contour map and 0.5m remote sensing map to discuss the hazard development mechanism and distribution character by remote sensing interpretation and field survey. Under the guidance of Geological Hazard Survey And Risk Assessment Of Important Counties in Gansu, the work is done with the aid of GIS in Rouyuan. Main content of this paper is as the following:(1) Based on data collection, field survey, and remote sensing interpretation, 25 slide, 41 unsteady slopes, 2 collapse and 7 debris flow were found in Rouyuan. These hazard threatened 18679 persons and ￥ 84746.88 property. The relationship between hazard and lithology, topography, hydrogeological condition, vegetation, and human engineering activities was intimate. The hazard was inclined to happen in the area where is closed to loess unconformity surface, in high relief, closed to valley, with few vegetation and intense human activity.(2) The altitude of loess undersurface was figured out by layers boundry in geological map and boundry point by situ investigation with the aid of interpolation tools in ArcGIS. The loess thickness map was got by subtracting loess undersurface from initial DEM. Hence, the relationship between loess layer thickness and hazard was disussed. The result showed that the most slide happened in area where the loess layer thickness is less than 40 m.(3) Change point method was used to get best unit size of average relief in loess area in large scale based on the spatial analysis tools in ArcGIS. The difference value between the highest point and lowest point in each unit which range from 10m×10m, 15m×15m, 20m×20m, …, 125×125m is relief. Curve fitting method and the mean change point method were used to find the optimum unit. The result was 65m×65m.(4) The hazard assessment model in loess area was established including history hazard intensity and potential slide hazard. The former was expressed by areal density, and the later was expressed by formation lithology, geomorphology, hydrogeology, vegetation, and human activity. The weight of factors were got by AHP and hazard assessment was completed. The vulnerability assessment of Rouyuan was completed according to the difference between landcover after field survery and remote sensing interpretation.(5) The risk assessment map was got from hazard assessment and vulnerability assessment by raster overlaying tool in ArcGIS. The study area was devided into high risk, middle risk, low risk, very low risk area, which covers 4.6%, 11.7%, 30.2%, 53.5% of total area. The risk of local people living at Military Office unsteady slope was assessed by engineering geological experience. The result showed that the risk can not be accepted following the international standard. |
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