How concave are river channels?
How concave are river channels?
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DOI：
10.5194/esurf-6-505-2018
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发表时间：
2018-06
影响因子：
3.4
通讯作者：
S. Mudd;F. Clubb;B. Gailleton;M. Hurst
中科院分区：
文献类型：
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作者：
S. Mudd;F. Clubb;B. Gailleton;M. Hurst
摘要。一个多世纪以来，地貌学家一直试图利用河流剖面来揭示有关地貌演化以及驱动其演化的过程的信息。许多研究将新的地形数据集与河道下切的理论模型相结合，以推断侵蚀速率、识别抗侵蚀能力不同的岩石类型以及探测构造活动的潜在区域。用于分析河流剖面几何形状的最常见指标是河道坡度，即ks。然而，河道坡度的计算需要根据流域面积对河道梯度进行归一化。这种归一化需要一个幂律指数，即河道凹度指数。尽管凹度指数在确定河道坡度方面至关重要，但它很难确定。在本文中，我们比较了计算凹度指数的坡度 - 面积方法和基于沿河道长度对流域面积进行积分的方法，即所谓的“χ”分析。我们提出了一种新的基于χ的方法，该方法直接将支流节点的χ值与干流上的χ值进行比较；这种方法使我们能够在不假设χ与高程之间存在线性关系的情况下确定过渡地貌中的凹度指数。凹度指数的模式与河流功率下切模型的面积和坡度指数之比（m∕n）有关；因此，我们构建了遵循分离限制型河流功率的简单数值模型，并针对设定了m和n的模拟对不同方法进行测试。我们发现，当我们的数值地貌受到瞬时隆升或空间变化的隆升以及河流可蚀性影响时，基于χ的方法在重现设定的m∕n比值方面优于坡度 - 面积方法。我们还在几个真实地貌上测试了我们的方法，包括具有岩性和结构异质性的地点，以提供这些方法的性能和局限性的实例。这些方法在一个新的软件包中可用，以便其他研究人员能够探索凹度指数在不同地貌中的变化情况，目的是提高我们对基岩河道下切背后的物理原理的理解。
Abstract. For over a century, geomorphologists have attempted to unravel information about landscape evolution, and processes that drive it, using river profiles. Many studies have combined new topographic datasets with theoretical models of channel incision to infer erosion rates, identify rock types with different resistance to erosion, and detect potential regions of tectonic activity. The most common metric used to analyse river profile geometry is channel steepness, or ks. However, the calculation of channel steepness requires the normalisation of channel gradient by drainage area. This normalisation requires a power law exponent that is referred to as the channel concavity index. Despite the concavity index being crucial in determining channel steepness, it is challenging to constrain. In this contribution, we compare both slope–area methods for calculating the concavity index and methods based on integrating drainage area along the length of the channel, using so-called “chi” (χ) analysis. We present a new χ-based method which directly compares χ values of tributary nodes to those on the main stem; this method allows us to constrain the concavity index in transient landscapes without assuming a linear relationship between χ and elevation. Patterns of the concavity index have been linked to the ratio of the area and slope exponents of the stream power incision model (m∕n); we therefore construct simple numerical models obeying detachment-limited stream power and test the different methods against simulations with imposed m and n. We find that χ-based methods are better than slope–area methods at reproducing imposed m∕n ratios when our numerical landscapes are subject to either transient uplift or spatially varying uplift and fluvial erodibility. We also test our methods on several real landscapes, including sites with both lithological and structural heterogeneity, to provide examples of the methods' performance and limitations. These methods are made available in a new software package so that other workers can explore how the concavity index varies across diverse landscapes, with the aim to improve our understanding of the physics behind bedrock channel incision.