SGER: Analysis and Paleohydrologic Application of Sedimentary Deposits of the Record Breaking 1993 Mississippi River Flood

这个项目的总体目标是定量分析1993年在爱荷华州Keokuk的密西西比河上游中游和下游破纪录的洪水的现场地层记录。基奥库克河水位的洪水记录分析表明，1993年的洪水在这条河的这一段只有500年一次的预期复发频率。因此，对1993年洪水产生的地层记录的分析提供了一个机会，可以建立历史记录中很少见到的罕见洪水事件的沉积学和地球化学性质的定量文件。具体的项目目标包括：(1)工程调查，定量确定沉积物沉积深度和体积之间的关系，以及相对于主要水流的沉积表面的相对高度；(2)洪水沉积单元的沉积学性质分析，包括粒度特征和沉积构造；(3)洪水沉积层序有机碳、碳酸钙、碳酸镁垂直变化特征分析；(4)边谷支流的静水沉积分析，评价受以下因素影响的沉积特征：支流规模和坡度、1993年密西西比河洪水期间和以后的支流洪水历史以及支流与密西西比河干流的交汇角。必须尽快对1993年的洪水沉积物进行分析，因为大多数沉积物位于农业耕种的土地上。因此，许多优良的沉积序列不太可能在即将到来的生长季节开始之后才能得到，那时作物将进行春季种植。由于以密西西比河上游河谷为代表的湿润气候区沉积物的自然生物扰动和地球化学变化通常使地质记录中的细粒度岸上地层记录难以解释，因此研究结果将大大改进基于洪水事件地质记录的古洪水水文方法。因此，1993年罕见的大震级洪水中新鲜的、未风化的和相对未受干扰的沉积物的记录将特别有利于提高对地质记录中应寻求的关键物理证据的理解，因为地质记录的重点是重建洪水震级和频率的过去历史。最后，考虑到河流沉积物在地球化学循环中的重要作用，研究结果也将有助于提高对冲积系统中沉积物长期流动和储存的理解。

9409778 Knox The general objective of this project is to quantitatively analyze the field stratigraphic record of the record-breaking 1993 flood on the middle reach of the upper Mississippi River upstream and downstream of Keokuk, Iowa. Analysis of flood records for the Keokuk River gage indicates that the 1993 flood had an expected recurrence frequency of only once in 500 years for this reach of the river. Analysis of the stratigraphic record resulting from this 1993 flood therefore provides an opportunity to establish quantitative documentation of sedimentological and geochemical properties of a rare flood event that is seldom witnessed in historical records. Project-specific objectives include (1) engineering surveys to quantitatively determine relationships between depths and volumes of sediment deposition in relation to relative heights of depositional surfaces with respect to main currents of flood flow; (2) analysis of sedimentological properties of flood depositional units, including grain size characteristics and sedimentary structures; (3) analysis of vertical variations of organic carbon and calcium and magnesium carbonate in flood depositional sequences; and (4) analysis of slackwater sedimentation in sidevalley tributaries to evaluate sedimentation characteristics as influenced by: tributary size and gradient, tributary flooding history during and since the 1993 Mississippi flood, and tributary junction angle with mainstream Mississippi Valley. Analyses of the 1993 flood deposits need to be undertaken as soon as possible because most are located on land that is under agricultural cultivation. Therefore, it is unlikely that many of the excellent depositional sequences will be available beyond the beginning of the coming growing season when cultivation for spring planting of crops will take place. Results of the search will significantly improve the methods of paleoflood hydrology which are based on the geological record of flood events bec ause natural bioturbation and geochemical alterations of sediments in humid climate regions, such as represented by the upper Mississippi Valley, normally make the fine-grained overbank stratigraphic record difficult to interpret in the geological record. Hence, documentation of fresh, unweathered, and relatively undisturbed sediments from the rare, large-magnitude 1993 flood will be particularly beneficial for improved understanding of key physical evidence that should be sought in the geologic record where emphasis is on reconstructing past histories of flood magnitudes and frequencies. Finally, given the important role that river sediments play in geochemical cycles, the results of the research will also contribute significantly to improved understanding of long-term sediment mobility and storage in alluvial systems.