EGU25-14540, updated on 15 Mar 2025
https://doi.org/10.5194/egusphere-egu25-14540
EGU General Assembly 2025
© Author(s) 2025. This work is distributed under
the Creative Commons Attribution 4.0 License.
Large-scale alluvial fan-river system of the middle Yangtze River: morphological diversity, grain size discontinuity, and sediment dynamics complexity
Zican He1, Zhaohua Sun2, Yitian Li2, Hualong Luan1, and Geng Qu1
Zican He et al.
  • 1Changjiang River Scientific Research Institute, River Research Department, China (zican_he@163.com)
  • 2State Key Laboratory of Water Resources Engineering and Management, Wuhan University, Wuhan, China (zhsun@whu.edu.cn)

Alluvial fans develop at the base of mountain fronts, where rivers emerge from the constrained mountain area onto the plain. Acting as a transition zone between mountain streams and alluvial rivers, the fan-river system is typically characterized by a slope break in the bed profile, a significant discontinuity in bed surface sediment fining from gravel-sized to sand-sized, and a sudden increase in channel width. In large rivers with great morphological diversity and strong human interference, the shift between upstream and downstream river morphology and sediment dynamics within the alluvial fan-river system exhibits a more complex process. However, this phenomenon remains insufficiently documented and lacks comprehensive analysis.

Here, we take the middle Yangtze alluvial fan as an example and use field observations and numerical modeling to improve the understanding of the large-scale alluvial fan-river system. The result shows that, in contrast to other alluvial fan-river systems, the Yangtze alluvial fan downstream of the Three Gorges Valley had no obvious breaks in the recent bed profile. In addition, the channel width showed an abrupt increase at Zhicheng, followed by a narrowing trend beginning at Chenjiawan. After the Three Gorges Dam (TGD) operation in 2003, the erosive water released from the TGD induced significant erosion, however, the spatial pattern of the bankful width remained stable. The bed profile exhibited increasing variability but continued to lack a distinct slope break; The transition in surface material from gravel to sand was observed throughout approximately 60 kilometers and the location migrated 40 kilometers downstream in the post-TGD period, with gravel and sand patches alternating randomly; Zhicheng and Chenjiawan are two characteristic locations marking the shifts in the mode of sediment transport in the middle Yangtze alluvial fan-river system. For sand transport mode, the reach upstream of Zhicheng had sand transported in suspension, whereas the downstream reaches were dominated by mixed-load transport. For gravel transport mode, gravel from upstream, mostly in the 25–50 mm grain size range, was selectively transported downstream of Zhicheng and deposited at Chenjiawan; The sediment dynamics in the Yangtze alluvial fan-river system were controlled by the width variability and distributary streams. The deposition of fine sand upstream of the gravel smoothed the previously deposited gravel fan profile, resulting in the absence of a slope break in the bed profile. Since 2003, the pattern of the sediment transport mode remained stable despite some local adjustments. This stability is attributed to the stable fan-river morphology induced by the strong resistance of riverbank lithologies and the Jingjiang Great Levee constraints.

How to cite: He, Z., Sun, Z., Li, Y., Luan, H., and Qu, G.: Large-scale alluvial fan-river system of the middle Yangtze River: morphological diversity, grain size discontinuity, and sediment dynamics complexity, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14540, https://doi.org/10.5194/egusphere-egu25-14540, 2025.