Collaborative Research: Quantifying Sediment Storage Transit Times For Geomorphically Explicit, Watershed Scale Suspended Sediment Routing

合作研究：量化地貌明确的流域规模悬浮沉积物路线的沉积物储存运输时间

• 批准号: 1424127
• 负责人: Diana Karwan
• 金额: $ 9.8万
• 依托单位: University of Minnesota-Twin Cities
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2014
• 资助国家: 美国
• 起止时间: 2014-09-01 至 2017-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1424127&HistoricalAwards=false
• 关键词: Collaborative; Research; Quantifying; Sediment; Storage

Nontechnical Explanation of the Project's Broader Significance Suspended sediment and associated contaminants and nutrients are leading causes of environmental degradation in rivers, but predicting the movement of fine-grained material through watersheds remains difficult. Most watershed management planning treats fine-grained suspended sediment as "washload" that is rapidly transported to receiving waters once particles enter stream channels. However, sediment particles actually move downstream by a series of discrete jumps, with each jump punctuated by a period of resting in floodplains and other alluvial deposits. According for this view, particle travel times from upland sources to estuarine sinks may be very long (up to 1000 years for the Chesapeake Bay watershed, for example). The disparity between these two views is most critical where Best Management Practices are used to reduce the contribution of sediment and other contaminants from upland watershed sources to downstream receiving waters, such as the Chesapeake Bay. Sediment travel times are not currently incorporated into existing watershed modeling schemes used to evaluate and design Best Management Practices, so the time required for them to achieve their maximum effectiveness is unknown. During this project, particle waiting times will be determined for a sub-basin of the Chesapeake Bay watershed, and these results will be incorporated into a new modeling framework for predicting the movement of particles through watersheds that explicitly incorporates measured particle waiting times. These results will help evaluate restoration schemes designed to reduce loading of sediment and other particles from watersheds to ecologically damaged receiving waters. Technical Description of the ProjectGeomorphologists widely acknowledge that suspended sediment routing through large (100-1000 km spatial scales) watersheds must account for the 100-10,000 year timescales associated with alluvial storage, but this knowledge is rarely reflected in watershed models. A few recent modeling schemes include both sediment transport and storage, but the necessary reach averaging introduces a new variable, the sediment-storage (waiting) time distribution that is virtually undocumented. This study will use pollen analyis, optically-stimulated luminescence, radiocarbon dating, and fallout radionuclide concentrations to measure contemporary waiting time distributions for all significant alluvial storage reservoirs in a mid-Atlantic Piedmont watershed where rates of erosion, deposition, and transport are well-constrained. The measured waiting time distributions will be used with a suspended-sediment routing model in a series of scenarios designed to illustrate timescales required for suspended sediment to move from upland sources to basin outlet in the Chesapeake Bay watershed. The results will improve existing theory concerning sediment waiting time distributions and sediment budgets within fluvial systems. The results will also help interpret the complex histories of watershed disturbance amd provide a means for a more accurate prediction of future changes. This study is designed to test the following hypotheses: 1) sediment storage or waiting time distributions can be quantified using measured erosion rates, geomorphic mapping, radiometric dating, and reservoir theory; 2) sediment waiting times for mid-Atlantic upland watersheds range from months to millennia, with characteristic values on the order of 1000 years, implying very long timescales to transport most suspended particles from upland sources to basin outlets; 3) observed waiting time distributions are not exponential, suggesting that valley storage reservoirs are not well-mixed.

对该项目更广泛意义的非技术性解释悬浮泥沙及相关污染物和营养物质是河流环境退化的主要原因，但预测细粒物质在流域中的移动仍然困难。大多数流域管理规划都将细粒悬浮泥沙视为“冲刷负荷”，一旦颗粒进入河道，这些泥沙就会被迅速输送到受纳水域。然而，沉积物颗粒实际上通过一系列离散的跳跃向下游移动，每一次跳跃都被泛滥平原和其他冲积沉积物中的一段时间打断。根据这一观点，颗粒从高地来源到河口汇的旅行时间可能会很长(例如，切萨皮克湾分水岭的时间长达1000年)。这两种观点之间的差异是最关键的，因为最佳管理做法被用来减少来自高地分水岭来源的沉积物和其他污染物对下游接收水域(如切萨皮克湾)的贡献。目前，用于评估和设计最佳管理做法的现有流域建模方案中没有包含泥沙行程时间，因此，这些方案实现最大有效性所需的时间是未知的。在这个项目中，将确定切萨皮克湾分水岭的一个子流域的颗粒等待时间，这些结果将被纳入一个新的建模框架，用于预测颗粒在分水岭中的运动，该框架明确包含测量的颗粒等待时间。这些结果将有助于评估旨在减少来自流域的泥沙和其他颗粒向生态破坏的接收水域的负荷的恢复方案。项目技术说明地貌学家广泛承认，悬沙通过大流域(100-1000公里空间尺度)必须考虑与冲积储存相关的100-10000年时间尺度，但这一知识很少反映在流域模型中。最近的一些模拟方案既包括泥沙输送，也包括泥沙储存，但必要的平均河段引入了一个新的变量，即泥沙储存(等待)时间分布，几乎没有文献记载。这项研究将使用花粉分析、光激发发光、放射性碳测年和放射性核素沉降物浓度来测量大西洋中部山前分水岭中所有重要的冲积水库的当代等待时间分布，那里的侵蚀、沉积和运输速度受到很好的限制。在切萨皮克湾流域，测量的等待时间分布将与悬浮泥沙路线模型一起用于一系列情景，以说明悬浮泥沙从高地源头移动到流域出口所需的时间尺度。这一结果将改进现有的河流系统泥沙等待时间分布和泥沙收支的理论。研究结果还将有助于解释流域扰动的复杂历史，并为更准确地预测未来变化提供手段。这项研究旨在检验以下假设：1)泥沙储存或等待时间分布可以通过测量的侵蚀速率、地貌制图、辐射测年和水库理论来量化；2)大西洋中部高地分水岭的泥沙等待时间从几个月到几千年不等，特征值在1000年左右，这意味着将大多数悬浮颗粒从高地来源输送到流域出口需要很长的时间尺度；3)观测到的等待时间分布不是指数分布，表明山谷水库的混合不好。