Equilibrium sediment transport, grade and discharge for suspended-load-dominated flows on Earth, Mars and Titan
Equilibrium sediment transport, grade and discharge for suspended-load-dominated flows on Earth, Mars and Titan
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DOI：
10.1016/j.icarus.2020.114243
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发表时间：
2021-02-21
期刊：
影响因子：
3.2
通讯作者：
Dorrell, Robert
中科院分区：
文献类型：
--
作者：
Amy, Lawrence;Dorrell, Robert
最近的地表观测强调了火星和土卫六上河流和溪流输沙的重要性。然而，以前的“水力”分析也表明，与地球相比，在重力降低的情况下，如果不是由于临界剪应力较低而更普遍地作为悬沙输送，那么应该可以作为悬沙输送；在那里，河流泥沙通量的主要模式是作为悬沙输送。利用流量平衡模型，提出了一种新的悬沙平衡条件(零净冲淤)约束条件，该模型适用于输沙能力有限的冲积河道。该理论将Gilbert的坡度和水流能力模型关系与Shields的起动准则统一起来。盾牌应力分析表明，地球、火星和土卫六的泥沙运移阈值在动力学上是相似的。因此，尽管各向剪应力差异很大，但阈值和平衡条件所需的临界坡度和水流深度的变化相对较小。在重力降低的条件下，泥沙作为悬浮物的运移潜力略高；但具有高颗粒-流体密度比的系统除外，这可能适用于土卫六上的有机颗粒运移。重要的是，我们表明，在重力降低的情况下，有坡度的悬移质河道应该形成较平缓的坡度，而不是以前根据陆上河道和海底河道的广义水力几何关系所推断的较陡峭的坡度。我们进一步认为，使用海底航道作为减轻重力下形成的航道的类似物，在物理上是不合理的。最后，与推移质渠道相比，悬移质渠道也应该有明显更高的流量，这对基于机械的流量预报和降水模型是有意义的。
Recent surface observations have emphasised the importance of bedload transport by rivers and streams on Mars and Titan. Previous "hydraulic" analysis, however, has also shown that transport as suspended load should be possible, if not more prevalent due to lower critical shear stresses, under reduced gravity conditions compared with Earth; where the dominant mode of sediment flux by rivers is as suspended load. A new suspended-load equilibrium condition (zero net erosion and deposition) constraint is advanced, using a flux-balance model applicable to transport (capacity) limited alluvial channels. The theory unifies Gilbert's relation for slope and flow capacity model with the Shields criterion for incipient motion. Shields-stress analysis shows that sediment transport thresholds on Earth, Mars and Titan are dynamically similar. Thus, despite large differences in dimensional shear stresses, variations in the critical slopes and flow depths required for threshold and equilibrium conditions are comparatively modest. Under reduced gravity conditions, there exists a slightly higher potential for sediment transport as suspended load; except for systems involving high particle-fluid density ratios, as may apply to the transport of organic particles on Titan. Critically we show that graded suspended-load channels should develop gentler slopes under reduced gravity, and not steeper slopes as previously inferred based on generalized hydraulic geometry relationships of terrestrial river and submarine channels. We further argue that the use of submarine channels as analogues for channels formed under reduced gravity is not physically justified. Finally, compared with bedload channels, suspended-load channels should also have markedly higher discharges, with implications for mechanistic-based discharge predictions and precipitation models.