Spatial and temporal variations of the Longxi river channel in Sichuan province after the 2008 Wenchuan earthquake, China

Abstract:

Biyun Guo^1,2

¹College of Marine Science and Technology, Zhejiang Ocean University, Zhoushan 316004,China;

²State Key Laboratory of Hydroscience and Engineering, Tsinghua University, Beijing 100084, China

 Abstract: The great Wenchuan earthquake(Ms=8.0) occurred on May 12, 2008 in the southwestern China which triggerednumerous landslides and collapses that providedflows of an abundant unconsolidated material of debris. Landslides and debris flows were very active and caused river channel change over the affected mountain area. River channel changes due to the earthquake.Many potentially dangerous debris filledgullies have yet to be identified. In this paper,we present a study in the Longxi river watershed of the upper Min River located in Dujiangyan, discussingthe unconsolidated deposits and debris flows and their relationship with the riverbed changes. The secondary geological disasters triggered by this earthquake is delineated across the Longxi Rvier basin. Based on remote sensing and image interpretation of two QuickBird panchromatic images and comparison between two DEM data of different time periods, wevalidatedthe data with field survey by a differential GPS and laser rangefinder. Several variables were systematically measured (width, slope, bed level change, sediment size) along an18.22 km stretch of the river for specific sites.

We have found from the study that: (1) the significant feature of geological disaster was near the Longxi river, where it had the mostintensity; (2) the longitudinal profile of channel bed was changed; (3) channel changes were associated with the land use and cover change, especially with the forest degradation and reforest, and post-quake reconstruction;(4)the landslide and debris flows caused the morphology changes in the Longxi River watershed, leading to sharp changes of the riverbed profiles and the slope in the upper and lower reaches.

Keywords: Earthquake ;landslides; debris flows; channel change; riverbed evolution