Doctoral Dissertation Research: Downstream Channel Adjustment to Abrupt Changes in Hydrology and Lithology: Llano River Basin, Texas Hill Country, USA

河流是复杂的实体，呈现出各种形状和图案。航道调整是一个复杂的过程，涉及不同时空尺度的多个过程。航道整治的两个主要维度是平面形态和断面形态。控水控沙航道调整。最常见的径流指标是岸滩流量，它与河道地貌的大小有关，通常等同于预计每一到两年出现的最大流量。最近的研究表明，控制河道形态的指数在区域上是不同的，特别是在以可变流型为主的环境中。当考虑到沿水文和岩性过渡带的河道调整的空间变异性时，这一点尤其重要，美国得克萨斯州山村就是一个例子。本博士论文研究项目的目标是确定拉诺河流域内河道形态和形态相互调节的控制因素。该区域气候的特点是从半干旱条件向半湿润条件过渡，该区域的水文状况以常年流量低而不时发生极端洪水而闻名。此外，河流沉积物由于两种不同的岩性而突然变化，一种是与爱德华兹高原有关的碳酸盐岩，另一种是与拉诺隆起有关的火成岩和沉积岩。浮华的水文状况对该地区的航道调整具有重要影响，特别是在检查岸滩泄流的有效性方面。该项目提出了以下问题：1)是什么流量控制着以浮华、洪水为主的水文状况为特征的河流的水力几何形状？2)泥沙的变化如何在不同大小的河流的河道几何形状中表现出来？3)河床材料或河岸材料对河道几何形状更重要？这些问题将通过现场分析和实验室分析来解决，这些分析涉及以下目标：统计洪水和溪流功率变异性；表征河床物质和岩性的下游变异性；根据内聚力、剪切强度、压实和方解石胶结来表征河岸强度；表征河床形态和形状；估计河床泥沙夹带频率；以及使用统计程序将水流和泥沙参数与下游河道调整联系起来。这项研究通过研究文献中未见报道的半干旱河流的河道几何控制，有助于从理论上理解河流地貌。从应用的角度来看，这些发现对工程师、水文学家和河流生态学家的受众是有用的。政府机构的生物学家和工程师关心与水生栖息地和河岸恢复工作有关的交通基础设施、水库沉积和河道稳定性的结构完整性。这一博士论文研究改进奖为有前途的学生作为一名独立学者建立强大的研究轨迹提供了关键支持。

Rivers are complex entities that exhibit a variety of shapes and patterns. The process of channel adjustment is complex and involves a multitude of processes at varying spatial and temporal scales. Two primary dimensions of channel adjustment are planform pattern and cross-sectional shape. Streamflow and sediment control channel adjustment. The most common index of streamflow is bankfull discharge, related to the size of channel features and usually equated to the maximum flow expected to occur every one to two years. Recent studies have shown that indices controlling channel morphology vary regionally, especially in settings dominated by variable flow regimes. This is particularly important when considering spatial variability in channel adjustment along transition zones in hydrology and lithology, exemplified by the Texas Hill Country, USA. The goal of this doctoral dissertation research project is to identify the controls on mutual adjustment of channel pattern and shape within the Llano River watershed. The regional climate is characterized by a transition from semiarid to subhumid conditions, and the hydrologic regime of the region is noted for low perennial flows punctuated by extreme floods. Additionally, river sediment abruptly varies as a result of two distinct lithologies, carbonate rocks associated with the Edwards Plateau and igneous and sedimentary rocks associated with the Llano Uplift. The flashy hydrologic regime has important implications to channel adjustment in the region, particularly in examining the validity of bankfull discharge. The project asks the following questions: 1) What discharge controls hydraulic geometry for rivers characterized by flashy, flood-dominated hydrologic regimes? 2) How does the change in sediment manifest itself spatially in channel geometry for rivers of different size? 3) Is bed material or bank material more important to channel geometry? These questions will be examined through field and laboratory analyses that address the following objectives: statistically characterize flood and stream power variability; characterize downstream variability in bed material and lithology; characterize channel bank strength according to cohesion, shear strength, compaction, and calcite cementation; characterize channel pattern and shape; estimate bed-material entrainment frequency; and use statistical procedures to relate flow and sediment parameters to downstream channel adjustment.The study contributes to a theoretical understanding of fluvial geomorphology by examining channel geometry controls for semi-arid rivers, which are underrepresented in the literature. From an applied perspective the findings are useful to an audience of engineers, hydrologists, and stream ecologists. Biologists and engineers in government agencies are concerned with the structural integrity of transportation infrastructure, reservoir sedimentation, and channel stability as related to aquatic habitat and riparian restoration efforts. This Doctoral Dissertation Research Improvement award provides critical support for a promising student to establish a strong research trajectory as an independent scholar.