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.
Highlights
The devastating Ms 8.0 Wenchuan Earthquake occurred on May 12, 2008, directly triggered tens of thousands of landslides and rock falls[1] with a total volume of 2.8 × 109 m3 2
The sediment supply increased sharply compared to the pre-earthquake state because of the large amounts of loose materials from the landslides and rock falls in the mountain regions
We conducted more than 40 extensive qualitative field surveys of the rivers in the earthquake shock regions between May 2008 and March 2016, revealing that all of the rivers aggraded to some extent in the mountain reaches, due to large amounts of particles produced by Wenchuan Earthquake excessed the transport capacity by the hydrodynamic condition, which was accordance with previous studies[7,8,9]
Summary
Basin area of 701 km[2] and a length of 52.7 km. The measured average flow is 20.1 m3/s, and floods with flows exceeding a hundred times the annual average flow can be induced by the summer and autumn monsoons. Note that some of the measured points had no sills (Fig. 2b), whereas others had sills to prevent further undercutting (Fig. 2a) These sills could protect the reaches upstream, but at the toe of each sill, scarp undercutting appeared due to local sour. In these cases, we measured each undercutting depth both above and below the sill’s crest. The undercutting volume was calculated to be 1.0 × 108 m3 (see Methods), by comparison, the aforementioned loose material triggered by the Wenchuan Earthquake in the mountain regions of Shiting River was 2.7 × 108 m3 2
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