An Experimental Study of River Incision Into Bedrock: The Role of Sediment Supply

9706082迪特里希河切入基岩是连接构造和地貌过程的关键过程，但我们对切削率的控制了解很有限。很少进行实地研究，也没有系统地收集实验观测数据。大多数研究假设切割率与水流功率或边界剪切力成正比，并隐含地假设泥沙供应对切割率的影响简单地与流域面积成正比。然而，Gilbert(1877)提出，随着泥沙供应量的增加，对于给定的输沙量，存在最优的侵蚀，超过该输沙量，由于颗粒对床面的屏蔽增加，切割速率会下降。我们提出了一个跳跃磨损的准力学模型，该模型支持Gilbert假设，但也确定了另一种最优侵蚀。随着输沙量的增加，颗粒的运动方式向悬浮方向转变。如果床层颗粒足够细，高输送能力可能会减少与床层的接触，从而减少切割。我们建议使用两个独特的水槽进行一组三个实验，目的是探索磨损对河流切割的作用。前两个实验将使用一个8.5米长、12厘米宽、1.3米深的水槽，里面填满了由弱混凝土制成的高度可擦除的人造基岩。第一个实验将解决的问题是，是什么控制着基岩下的河道上冲积覆盖的范围。将在不可擦除和不可擦除的河床上进行一系列试验，以确定在何种泥沙负荷和输送能力的组合下，河床完全或部分变成冲积层。为了了解泥沙供应对切割速率的影响，这是一个必要的第一次实验，但它也应该提供关于是什么决定了河流中基岩、混合基岩和冲积以及完全冲积河床的模式的关键洞察。第二个试验将检验吉尔伯特侵蚀最优假说和最优通过能力假说。将进行大量的小型试验，系统地改变供沙量、流量、坡度、粒度和河床耐磨性，并量化基岩侵蚀速率。由于给定输沙能力的侵蚀速率和输沙能力之间以及给定的泥沙供应的侵蚀速率和输沙能力之间存在预期的抛物线关系，因此有必要进行多次运行。第三个试验将与另外两个试验同时进行，将测试天然和人工河床的磨损率是否随着抗压强度的变化而变化。我们将使用两台直径35厘米的圆柱形磨床来测试至少24个从现场和前两个实验室实验中获得的样品。如果磨损与抗压强度在很大范围内相关，那么我们可以更成功地将我们的实验结果外推到现场尺度，并且现场测量的抗压强度可以与基岩磨损率进行定量比较。如果这些实验被证明是成功的，它们将为进一步的实验奠定基础，这些实验针对的是河道宽度、支流泥沙负荷的增加以及基准面下降的可变速率对基岩河道纵向发育的影响。更重要的是，它们将为泥沙供应在河流切割中的作用提供第一个定量证据，为更具机械性的侵蚀规律提供洞察，为河流切割的定量野外研究提供指导，并为现场岩石强度测量与切割速率之间的关联提供工具。

9706082 Dietrich River incision into bedrock is the crucial process linking tectonic and geomorphic processes, yet our understanding of the controls on incision rates is very limited. Few field studies have been conducted and no experimental observations have been systematically collected. Most studies assume that incision rate is proportional to a stream power or boundary shear stress, and implicitly assume that sediment supply influence on incision rate is simply proportional to drainage area. Gilbert (1877) proposed, however, that with increasing sediment supply there is an erosion optimum for a given transport capacity above which incision rates decline due to increasing shielding of the bed by particles. We present a quasi-mechanistic model for abrasion by saltation which supports the Gilbert hypothesis, but which also identifies another erosion optimum. As the transport capacity increases for a given sediment supply, the mode of particle movement shifts towards suspension. If bed particles are sufficiently fine, high transport capacity may lead to reduced incision due to reduced contact with the bed. We propose to conduct a set of three experiments using two unique flumes built specifically for the purpose of exploring the role of abrasion on river incision. The first two experiments will use a 8.5 m long, 12cm wide, 1.3 m deep flume filled with highly erodable artificial bedrock made of weak concrete. The first experiment will address the question of what controls the extent of alluvial cover over a channel underlain by bedrock. A series of experimental runs will be conducted with unerodable and erodable beds to identify under what combinations of sediment load and transport capacity the bed turns fully or partially alluvial. This is a necessary first experiment to understand sediment supply effects on incision rate, but it also should provide critical insight about what sets the pattern of bedrock, mixed bedrock and alluvial, and fully alluvial beds in rivers. The secon d experiment will test the Gilbert erosion optimum hypothesis and the optimum transport capacity hypothesis. A large number of short experimental runs will be conducted in which sediment supply, discharge, slope, particle size and bed resistance to abrasion are systematically varied and bedrock erosion rate is quantified. Numerous runs are necessary because of the expected parabolic relationships between erosion rate and sediment transport for a given transport capacity and between erosion rate and transport capacity for a given sediment supply. The third experiment, which will run simultaneously with the other two, will test whether abrasion rate of natural and artificial river beds varies with compressive strength. We will use two 35 cm diameter cylindrical abrasion mills to test at least 24 samples obtained from the field and from the first two laboratory experiments. If abrasion correlates with compressive strength across a broad range of values then we can extrapolate our experimental results to field scales more successfully, and field measurements of compressive strength can be compared quantitatively to rates of bedrock abrasion. If these experiments prove successful they will form the foundation for further experiments directed at the role of channel width, tributary additions of sediment load, and variable rates of base-level lowering on bedrock channel longitudinal development. More importantly, they would provide the first quantitative evidence for the role of sediment supply in river incision, give insight for a more mechanistic erosion law, provide guidance for quantitative field studies of river incision, and offer a tool for relating field measurements of rock strength to rates of incision.