Landslide-induced levee failure by high concentrated sediment flow — A case of Shan-An levee at Chenyulan River, Taiwan

Abstract

Typhoon Mindulle struck northeast Taiwan on July 2, 2004, and brought an abundant air current in the following days that caused serious flood disasters to the mountainous areas in central Taiwan. One of sever flood events during Typhoon Mindulle was about hundreds of hectares of farmland loss induced by an 1620 m break of Shan-An levee at the Chenyulan River. This paper examines the nature and causes of Shan-An flood by integrating catastrophic investigation and scenario simulation. The catastrophic investigation of the disaster scene, including field survey, outcrop investigation, and remote sensing image interpretation, was executed to reveal the inundation for disaster response and recovery. SPOT-IV and FORSAT-II satellite images and airborne images provided temporal and spatial information for inundation investigation. The HEC-RAS hydraulic model was applied to estimate the water surface profiles of Shan-An levee around the lower reach of Chenyulan River. Also, the impacts of debris and water in the hyper-concentrated sediment flow were estimated to illustrate the effects of sediment yield on Shan-An levee. The result shows that the levee was designed to sustain the heavy rainfall bought by Typhoon Mindulle but failed to the impact of high concentrated sediment flow induced by sediment yield from the landslide. The seismic-triggered landslide on the left riverside was reactivated by intensive rainfalls to deposit a great amount of sediments on the riverbed that narrowed down the river width, increased the impact force, and diverted water toward Shan-An levee. The levee failure should draw more attention from engineers while designing a new flood control structure in the waterway of Chenyulan River due to the occurrence of the hyper-concentrated sediment flow and landslide residuals after the Chi-Chi earthquake (M L 7.3). In addition, the perceived investigation and analysis of an earthen levee failure provides essential information to setup a proper recovery plan of Shan-An levee and a basin management strategy of Chenyulan River in the future.