基于NDVI的汶川大地震前后北川县次生地质灾害区植被破坏评估

摘要

选取北川县“5·12”大地震前后2个时相的遥感数据,利用归一化植被指数(NDVI)提取2期影像的植被信息,以此为基础,反演植被覆盖度,采用破坏指数DDI(the damage degree index)表示3种灾害类型区域震后植被破坏情况.结果表明:1)研究区内共解译滑坡103处,崩塌122处,泥石流10处,灾害面积共计17.5 km2,震前85％以上区域处于中植被覆盖度以上级别,震后中植被覆盖度以上级别土地面积减少8.01 km2,占灾害区域总面积的45.77％;2)滑坡区域植被破坏程度相对较低,中度及重度破坏的面积占总灾害面积的68.66％,崩塌区域植被破坏程度次之,中度及重度破坏的面积占总灾害面积的88.15％,泥石流区域的植被破坏最彻底,中度及重度破坏的面积占总灾害面积的99.74％;3)植被破坏与海拔、坡度有一定关系,破坏较严重的地区主要集中在海拔611～1 543 m、坡度25.～45.范围内,植被破坏主要集中在重度破坏这一级别,并随海拔、坡度增加而增加,植被破坏与坡向关系不大.%This study selected two phases remote sensing images before and after Wenchuan Earthquake on May 12th,2008 in Beichuan County.The two images before and after the earthquake was obtained to extract vegetation covering information using normalization difference vegetation index(NDVI),and NDVI was used to estimate vegetation coverage.Then the damage degree index (DDI) of the vegetation was defined,indicating the difference of the normalized difference vegetation index before and after earthquake.The results showed that:1) 103 landslides,122 collapses and 10 mudslides were interpreted from Formosat-2 image taken after Wenchuan Earthquake,and the area of disaster was 17.5 km2.The area of medium vegetation coverage status was 85％ before the earthquake,and it reduced by 8.01 km2 after the earthquake,which accounted for 45.77％ of the whole area of disaster.2) The vegetation damage was relative less serious in landslides regions,where moderate and severe damage vegetation occupied 68.66％ of the whole area.The vegetation damage was modest in collapses regions,where moderate and severe damage vegetation occupied 88.15％ of the whole area.Most vegetation damage occurred in mudslides regions,where moderate and severe damage vegetation occupied 99.74％ of the whole area.3) The vegetation damage was closely related to the elevation and slope grade,which was prone to occur at the elevation of 611-1 543 m and slope grade of 25°-45°regions.The vegetation damage was worse,most of which were severe damage with a decreasing trend along with elevation and slope grade increasing,but weakly related to the aspect factor.