Potential Debris Flow Hazard Assessment After Earthquake In Wenchuan City Based On Watershed Scale

Wenchuan County is located between Longmen mountains and Qiong Laimountains, in Jiuding Mountain Neocathaysian structural belt and is an alpine valleyregion. In the region, the geological structure is complex, new tectonic movement isstrong, rock is cracked, joints and fissures are developed. Widely distributed brokenloose rocks provide the material basis for geological disasters. The5·12earthquakeinduced a lot of collapse, landslides and debris flows in mountain and steep slopeareas along the Wenchuan-Yingxiu fracture and Maowen fracture which destroyedroad, blocked river, destroys houses and mining facilities. In Minjiang River basin, anumber of debris flows were triggered by heavy rain which brought great loss andserious influence to local people’s life and property security. According to researchresult, these disasters will last20years or so, therefore, the potential debris flowhazard assessment of the region will afford basis to disaster loss evaluation andDisaster warning forecast.Research of debris flow hazard assessment at home and abroad is a lot, but manyof which is on a cell basis, through the study of influence factor and evaluation modalto conduct the evaluation work, yet research on a basin basis to evaluate debris flowhazard is less. Ministry key laboratory of land and resources information technologyof Chengdu university of technology in2009-2010interpreted the potential debrisflows in Wenchuan area and on which basis divided the region to hundreds ofwatersheds with a threshold of100000and500000on the data base of DEM before5.12earthquake, then introduced a geomorphic information entropy and evaluated thedanger of potential debris flows. The result shows that evaluate result is valid, andwhat is preferable is basin unit portrayed integrity of debris flow activities unit and ismuch more practically. On all that basis, the author goes into a further exploration, divides more watersheds with smaller thresholds and chooses key factors to evaluatethe danger of debris flows in order to get more accurate result.Taking the DEM of2009as basic data, this paper conducts potential debris flowshazard assessment of Wenchuan County based on watershed scale, the main workincludes the following aspects:(1) The formation condition of debris flow in Wenchuan County is expoundedand the debris flow impact factor system is constructed through the analysis of thenatural geography, regional geological environment, human economic activitiescharacteristics. The influence factors are divided into four categories: geologicalfactors, topography factors, the hydrological and meteorological factors, and humanactivity factors. Geological factors include tectonic movement (geomorphicinformation entropy), stratum lithology. Topography factors include altitudedifference, slope, surface roughness. The hydrological and meteorological factorsinclude distance away from the river, precipitation. Human activity factors includevegetation coverage, land use. The nine factors are the factors to participate in debrisflow hazard assessment.(2) Based on DEM of2009, in the hydrologic analysis module of ArcGIS9.3,through the steps of depressions filling, the ground uplift, flow direction extraction,confluence cumulant calculation and watershed dividing, stream net are respectivelyextracted based on the water threshold of10000and5000, follow on which theresearch area are divided to277and534watersheds according to the twothreshold-based stream net. Based on watershed, according to the geomorphicinformation entropy principle, each watershed’s Strahler area-elevation integral valueand geomorphic information entropy value is calculated to estimate the strength of thebasin’s tectonic activity. From the geomorphic information entropy thematic map,when threshold is10000, there are34basins are in infancy stage and have thestrongest tectonic activities;117basins are in prime initial stage and have lessstronger tectonic activities; while only27basins are in prime late stage and old agestage. When threshold is5000, there are88basins are in infancy stage and have thestrongest tectonic activities;205basins are in prime initial stage and have lessstronger tectonic activities; while only57basins are in prime late stage and old agestage. The statistics indicates that tectonic activities are dynamic as a whole inWenchuan County and the internal power conditions to induce debris flow is equippedin the area.(3) The weight of each factor is given through Analytic Hierarchy Process. Combining with the factor scoring method, the potential debris flow hazardassessment model is constructed to calculate the risk value of each basin, thenpotential debris flow hazard assessment thematic map of the study area is obtainedthrough re-classification of the risk value according to certain standards. Whenthreshold is10000, there are21basins in high danger while25basins in low danger.When threshold is5000, there are54basins in high danger while56basins in lowdanger. From space distribution, the southeast of the county is in high danger, overallfrom southeast to northwest the risk gradually reduced. From the perspective ofadministrative division, Shuimo Town, Xuankou Town, Baihua Town, Yingxiu Town,Yinxing Town and southeast of Sanjiang Town belong to debris flow highly dangerareas.(4) This article makes a contrast with Li Yahui’s article “the assessment of debrisflow based on watershed scale”（2011）in which the potential debris flow riskassessment is conducted based on a threshold of100000. The contrast shows thatoverall trend of both for the potential debris flow hazard zonation is similar, but alsohas many differences, especially come to the distribution of high risk debris flowgully. The main reasons are as follows: first, the latter took the DEM before5.12earthquake as basic data while the former takes the DEM of2009which changed dueto the effect of earthquake. Second, the watershed division of the former is basedon the10000and5000catchment threshold, while the latter is based on100000orgreater threshold. Third, seismic area exist quantity of loose solid material afterearthquake, so the critical rainfall stimulate condition to debris flow is dropped.Therefore, the former takes precipitation factor as an important influence factor toinvolve in risk calculation while the latter did not consider the effect of rainfall factor.