## 5 Discussion
### Stability Mechanism
Meanwhile, the consolidated fine-grained layer in the lower dam reduces seepage flow, thereby lowering the downstream groundwater table. Under these conditions, even in the event of dam overtopping, the downstream fine-grained materials may not saturate rapidly, thus preventing breaching failure. The massive boulders located in the upper dam body and spillway (Figure 18A) are critical for preventing erosion of the lower fine-grained layer by lake water. On one hand, as lake water flows over the boulder surfaces, it generates significant drag forces on the spillway bed. However, the densely packed boulders within the spillway bed exhibit high erosion resistance, thereby shielding the underlying fine-grained layer from hydraulic erosion. On the other hand, these boulders facilitate the energy dissipation from the lake water flow, substantially reducing its erosive capacity. In the 2D flow field distribution, the significantly more pronounced flow field within the coarse-grained layer compared to the fine-grained layer provides clear validation of this assertion (Figure 13). In addition, studies of failed dams (e.g., Baige) consistently document fine-dominated compositions, corroborating the critical stabilizing function of coarse blocks through empirical counterevidence. Figure 17: Formation process of the inverse grading structure in the Conaxue Co landslide dam. (**A**)—Process of vibrational sieving; (**B**)—impact, ejection, and compaction processes; (**C**)—formation of the landslide dam. Regarding energy dissipation in the coarse-grained layer, we identified a step-pool system formed by long-term fluvial transport of boulders within the spillway (Figure 18B). This system features an alternating sequence of steep slopes, gentle slopes, and plunge pools arranged in step-like morphology [33]. It has been widely documented to generate exceptionally high flow resistance. This behavior effectively helps to dissipate stream power, thereby mitigating bed incision in the spillway channel [34; 35]. The energy dissipation mechanisms of the step-pool system in the spillway of the Conaxue Co landslide dam primarily include step-induced dissipation and pool-induced dissipation. For step-induced dissipation, the dominant modes are skin friction and hydraulic jumps downstream of the steps. When the lake water flows over the steps, intense friction and form resistance generated by the massive boulders dissipate energy. Subsequently, as the flow is deflected by the steps, it entrains air and generates localized hydraulic jumps with vortices (Figure 19A). These hydraulic jumps differ from typical pool hydraulic jumps; they occur as smaller-scale phenomena directly on step surfaces. For pool-induced dissipation, energy is first dissipated through hydraulic jumps in the main flow zone. Subsequently, turbulent kinetic energy is generated at the interface between the main flow and surrounding water, converting bulk kinetic energy into turbulence. Finally, large-scale vortex zones along the main flow margins transfer energy to smaller-scale turbulent eddies (Figure 19B). Consequently, the stream power of the Conaxue Co dammed lake flow is substantially reduced after passing through the step-pool system, thereby maintaining the long-term stability of the dam. This study employed the energy dissipation rate calculation formulas for step-pool systems derived from experimental and field data by [PERSON] [36] and [PERSON] and [PERSON] [37] to quantitatively assess the energy dissipation rate of the step-pool system in the Conaxue Co landslide dam. The specific calculation formula is as follows  
\[\eta=0.226\ (\frac{h_{c}}{H_{s}})^{-0.584} \tag{2}\]  
\[h_{c}=\sqrt[3]{Q^{2}/W^{2}g} \tag{3}\]  
where \(\eta\) is the energy dissipation rate of the step-pool system, hc is the critical water depth (m), Hs is the step height (m), Q is the discharge (m\({}^{3}\)/s), W is the channel width (m), and g is the gravitational acceleration (m/s\({}^{2}\)).