Dramatic undercutting of piedmont rivers after the 2008 Wenchuan Ms 8.0 Earthquake

Abstract

Changes in river channel erosion or deposition affect the geomorphic evolution, aquatic ecosystems, and river regulation strategies. Fluvial processes are determined by the flow, sediment and boundary conditions, and it has long been expected that increasing sediment supply will induce aggradation. Here, based on thorough field surveys, we show the unexpected undercutting of the piedmont rivers influenced by the 2008 Wenchuan (Ms 8.0) Earthquake. The rivers flow from the Longmen Mountain with significant topographic relief to the flat Chengdu plain. In the upstreams, sediment supply increased because of the landslides triggered by the earthquake, causing deposition in the upstream mountain reaches. However, the downstream plain reaches suffered undercutting instead of deposition, and among those rivers, Shiting River was the most seriously affected, with the largest undercutting depth exceeding 20 m. The reasons for this unexpected undercutting are proposed herein and relate to both natural and anthropogenic causes. In addition, we also demonstrate, at least for certain conditions, such as rivers flowing from large-gradient mountain regions to low-gradient plain regions, that upstream sediment pulses may induce aggradation in upstream and degradation in downstream, causing the longitudinal profile to steepen to accommodate the increasing sediment flux.

Analysis of Ice Storm Impact on and Post-Disaster Recovery of Typical Subtropical Forests in Southeast China

“…Natural disasters, such as snowstorms, ice storms, earthquakes, landslides, tornadoes, volcanoes, hurricanes, and other types of disasters, affect natural ecosystems in complex and profound ways [1][2][3][4]. Forest ecosystems are particularly disturbed by such disasters, with the effects including the decline in tree density, loss of forest cover, and the change of biodiversity [5,6].…”

Case Study: Model Test on the Effects of Grade Control Datum Drop on the Upstream Bed Morphology in Shiting River

“…Thus, the impacts of stream curve and other instream structures on the bed morphology are negligible. As mentioned previously, significant bed degradation occurred in the piedmont reach of Shiting River after 2008 [39,40]. Thus, an 18 m high grade control structure was built upstream of the studied reach (2 km upstream of the 105th Provincial Highway Bridge) for stabilizing the upstream riverbed.…”

“…The gradient of the river reach downstream of the mountain decreases gradually and ranges between 12 and 2.5% before it joins the Tuo River. Since the Wenchuan Earthquake (Ms 8.0) in 2008, serious bed degradation occurred in the piedmont reach of Shiting River, significantly exposing and endangering the foundations of instream infrastructures [39,40]. Thus, grade control structures (GCSs) were extensively used in this river reach as countermeasures for bed degradation.…”

Bed Morphology around Various Solid and Flexible Grade Control Structures in an Unstable Gravel-Bed River

“…The study area is the plain reach of the Jianjiang River. The Jianjiang River originates from Longmen Mountain and flows to the Chengdu Plain, which converges with the Shiting and Mianyuan Rivers to form the Tuojiang River, which is a tributary of the Yangtze River [10]. The Jianjiang River covers a catchment area of 2808 km 2 and has a length of approximately 120 km.…”

“…Both natural processes such as landslide damming [1,2] and anthropogenic causes such as dam construction [3,4], bed mining [5], land-use changes [6,7] and channelization [8] could destroy the relatively steady equilibrium states of rivers. In particular, gravel-bed rivers that flow from large-gradient mountain regions to low-gradient plains regions would become extremely unstable [9,10]. The instability of streambeds causes not only damage in riverbanks and other infrastructure but also failure of cross-river structures, such as bridges and underwater pipelines.…”