Experimental landform development by rainfall erosion with uplift at various rates.
Experimental landform development by rainfall erosion with uplift at various rates.
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DOI：
10.1016/j.geomorph.2015.03.001
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发表时间：
2015-06
期刊：
影响因子：
3.9
通讯作者：
S. Ouchi
中科院分区：
文献类型：
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作者：
S. Ouchi
研究人员进行了四次物理模拟模型实验（1、2、3和4次），在约38毫米/小时的人工降雨条件下，一个正方形（约60 × 60厘米）的细砂和高岭石混合物柱以不同的速率（分别约5.1、1.3、0.5、0.2毫米/小时）缓慢上升，观察实验地形与这些抬升速率的关系。随着方形土丘逐渐从地面隆起，河流侵蚀从土丘边缘开始，发展成河谷系统。这一河流侵蚀过程以坡度（最大高度-最小高度）和平均单元坡度（1cm × 1cm栅格中最高坡度的平均值）之间的线性关系表示，在坡度达到约60mm之前占主导地位，大约是边坡破坏（滑坡）开始占主导地位的时候。如果河流侵蚀在整个坡度中占主导地位（坡度保持在60毫米以下），则抬升速率被认为低于下限阈值，地形发育处于“特征坡度阶段”。在所有四次运行中，起伏大于60 mm，随着山体的增长，滑坡变得明显，表明该系列的隆升率高于下限阈值。在运行1中，高速率的隆起压倒了侵蚀，尽管出现了大的滑坡，但仍形成了巨大的山状地形。1道抬升速率高于上阈值，地貌发育处于“造山期”。当山的生长速度超过由非隆升速率因素决定的山的生长极限时，山就有可能坍塌。在第2、3和4次滑道中，当山谷系统在地表发育后，丘陵随着滑坡的发生而生长，河道剖面似乎在与隆升速率相对应的梯度上变得稳定。当斜坡的坡度超过一定的“临界坡度”（可能是干土丘形成物质的休止角）时，它们就容易受到滑坡的影响。然而，在没有触发事件的情况下，含有水和粘土的物质的斜坡会变得比这个梯度更陡。单元斜率的频率分布变为双峰分布，表明两种类型的斜率占主导地位，一种低于另一种高于临界梯度。前者代表主要由河流作用形成的表面，后者代表由和/或等待滑坡形成的表面。滑坡减少了坡度，但平均高度（平均高度）不会下降，除非滑坡产生的沉积物被河流作用带走。边坡破坏、河流作用和抬升的结合最终使海平面在一定高度附近保持稳定，这取决于抬升速率，而抬升频率则会减少或增加。第2、3、4道的实验地貌似乎已经达到了某种稳定的隆升和侵蚀状态。这些地层的地貌发育被认为处于“稳态阶段”，抬升速率介于下限和上限之间。这些观察和解释与对真实地貌的研究惊人地一致，对解释它们的发展具有价值。
Four runs (1, 2, 3, and 4) of physical analog model experiments, in which a square (ca. 60 × 60 cm) column of a mixture of fine sand and kaolinite is slowly raised at different rates (ca. 5.1, 1.3, 0.5, 0.2 mm/h, respectively) under artificial rainfall of about 38 mm/h, were conducted to observe how experimental landforms develop in relation to these uplift rates. As a square mound gradually emerges from ground level, fluvial erosion starts at the mound edges and develops into valley systems. This process of fluvial erosion, expressed in a linear relationship betweenrelief(maximum height–minimum height) andmean cell slope(mean value of the highest slope gradient in a 1 cm × 1 cm grid cell), dominates untilreliefreaches about 60 mm, around the time when slope failures (slumps) start to dominate. If fluvial erosion dominates throughout the run (reliefstays below 60 mm), the uplift rate is considered to be below the lower threshold and landform development is in the “characteristic relief phase.” In all four runs,reliefincreases above 60 mm and slumps become significant as hills grow, indicating that uplift rates in this series are above the lower threshold. In run 1, the uplift at a high rate overwhelmed erosion and a massive mountain-like topography formed despite the occurrence of large slumps. The uplift rate in run 1 is thus above the upper threshold and landform development is in the “mountain building phase.” The mountain is likely to collapse when it grows higher than the limit of mountain growth determined by factors other than uplift rates. In runs 2, 3, and 4, after valley systems develop over the surface, hills grow with the occurrence of slumps and channel profiles seem to become stable at gradients corresponding to the uplift rates. As slopes grow steeper than a certain “critical gradient,” which is possibly the angle of repose of dry mound-forming material, they become vulnerable to slumps. However, slopes of material containing water and clay can grow steeper than this gradient in the absence of triggering events. The frequency distribution ofcell slopesbecomes bimodal, indicating the dominance of two types of slopes divided by the critical gradient, one below and another above. The former represents surfaces formed mainly by fluvial processes, and the latter surfaces formed by and/or waiting for slumps. Slumps reducerelief, but average height (zmean) does not fall unless sediments produced by slumps are carried away by fluvial processes. The combination of slope failure, fluvial processes, and uplift eventually works to keep zmeanstable around a certain height depending on uplift rates, whilereliefrepeats decrease and increase. The experimental landforms in runs 2, 3, and 4 seem to have achieved a certain steady state with uplift and erosion. The landform development in these runs is considered to be in the “steady-state phase,” with uplift rates between the lower and upper thresholds. These observations and interpretations are surprisingly consistent with studies on real landforms and can be of value in interpreting their development.