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

Dietrich河切割成基岩是连接构造和地貌过程的关键过程，但我们对切割速率的控制作用的认识非常有限。很少进行实地研究，也没有系统地收集实验观察结果。大多数研究假设切割速率与水流功率或边界剪应力成正比，并隐含地假设泥沙供应对切割速率的影响与流域面积成正比。然而，Gilbert（1877）提出，随着沉积物供应的增加，对于给定的输送能力存在一个最佳侵蚀，超过该能力后，由于颗粒对河床的屏蔽作用增加，切割率下降。我们提出了一个准机制模型，支持吉尔伯特假说，但也确定了另一个最佳侵蚀。随着输沙量的增加，泥沙运动向悬浮模式转变。如果床层颗粒足够细，由于与床层的接触减少，高输运能力可能导致切口减少。我们建议使用两个专门建造的独特水槽进行一组三个实验，以探索磨损对河流切口的作用。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，填充由弱混凝土制成的高度易侵蚀的人造基岩。第一个实验将解决是什么控制了基岩下河道的冲积覆盖范围的问题。将对不可侵蚀和可侵蚀的河床进行一系列试验，以确定在泥沙负荷和输送能力的哪种组合下，河床完全或部分变成冲积物。这是了解沉积物供应对切口率影响的必要第一次实验，但它也应该为了解是什么决定了基岩、混合基岩和冲积层以及河流中完全冲积层的模式提供关键的见解。第二个实验将检验吉尔伯特侵蚀最优假设和最优输送能力假设。将进行大量的短期试验运行，系统地改变泥沙供应、流量、坡度、粒度和床的耐磨性，并量化基岩侵蚀速率。由于在给定的输沙能力下，侵蚀速率和输沙能力之间，以及在给定的泥沙供应下，侵蚀速率和输沙能力之间预期存在抛物线关系，因此需要进行大量的运行。第三个实验将与前两个实验同时进行，将测试天然河床和人工河床的磨损率是否随抗压强度的变化而变化。我们将使用两个直径35厘米的圆柱形研磨机来测试从现场和前两个实验室实验中获得的至少24个样品。如果磨损与抗压强度在一个广泛的数值范围内相关，那么我们可以更成功地将实验结果外推到现场尺度，并且可以将现场抗压强度测量值与基岩磨损率进行定量比较。如果这些实验证明是成功的，它们将为进一步的实验奠定基础，这些实验旨在研究河道宽度、支流泥沙负荷的增加以及基岩河道纵向发展中基准面下降的可变速率的作用。更重要的是，它们将为沉积物供应在河流切割中的作用提供第一个定量证据，为更机械的侵蚀规律提供见解，为河流切割的定量现场研究提供指导，并为将岩石强度的现场测量与切割速率联系起来提供工具。

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.