Analysis on formation conditions of debris flows in Longde county of Ningxia
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Abstract:
Longde county suffered a heavy rain of once every 50 years，resulting in 17 debris flows. Based on the detailed field investigation and geological environmental conditions of this region，the formation conditions of the debris flow were analyzed from 3 aspects as the gradient of gully and slope，material sources and water conditions. The results indicated that the debris flows of Longde county were distributed in the hilly region, where the slope was 20-50°and the height difference was 50-100m. These were favorable geological and geomorphological conditions for debris flow. In the basin the residual soil of orange sandy mudstone of Neogene (Ngn) has the weak dilatability. A certain degree of degradation and softening will occur after immersed by water，resulting in the connection of structure and strength is weakened. Therefore，it was the material source of debris flow. Rainfall was the most important trigger factor of the debris flow. Depending on field investigation and data analysis，the formation of debris flow in Longde county was mainly affected by antecedent precipitation. They belonged to the antecedent precipitation control pattern. So the debris flows in Longde county could be warned by monitoring the antecedent precipitation，especially the indirect antecedent precipitation.
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