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，呈现出各种各样的形状和模式。航道调整过程复杂，涉及多种不同时空尺度的过程。渠道调整的两个主要维度是平台形状和断面形状。控流控沙河道调整。最常见的流量指标是河岸流量，与河道特征的大小有关，通常等同于每一到两年预计发生的最大流量。最近的研究表明，控制河道形态的指数因区域而异，特别是在由可变水流状态主导的环境中。当考虑到水文和岩性过渡带的河道调整的空间变异性时，这一点尤为重要，例如美国德克萨斯丘陵地区。本博士论文研究项目的目标是确定拉诺河流域河道形态和形状相互调节的控制因素。区域气候的特点是从半干旱到半湿润的过渡条件，该地区的水文制度是注意到低的常年流量被极端洪水打断。此外，由于两种不同的岩性，与爱德华兹高原有关的碳酸盐岩和与拉诺隆起有关的火成岩和沉积岩，河流沉积物突然发生变化。浮华的水文制度对该地区的河道调整具有重要意义，特别是在检查河岸排放的有效性方面。该项目提出了以下问题：1)对于以浮华、洪水为主的水文系统为特征的河流，是什么流量控制着水力几何形状？2)不同大小河流的泥沙变化在空间几何上是如何表现的？3)河床材料还是河岸材料对河道几何形状更重要？这些问题将通过现场和实验室分析来检验，以解决以下目标：统计表征洪水和溪流功率变化；描述河床物质和岩性的下游变异性；根据内聚力、抗剪强度、压实和方解石胶结来表征河道岸强度；表征河道形态和形状；估计床质夹带频率；并利用统计程序将水流和泥沙参数与下游河道调整联系起来。该研究通过检查文献中代表性不足的半干旱河流的河道几何控制，有助于对河流地貌学的理论理解。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很有用。政府机构的生物学家和工程师关注交通基础设施的结构完整性，水库沉积，以及与水生栖息地和河岸恢复工作相关的渠道稳定性。这个博士论文研究进步奖为有前途的学生提供了重要的支持，以建立一个强大的研究轨迹作为一个独立的学者。

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.