The Role of Large Wood in Promoting Channel-Floodplain Connectivity for River Restoration

河流恢复最初侧重于河道形态，后来逐渐扩大到包括重新连接河道和泛滥平原。尽管河流生态学家几十年来一直认识到泛滥平原的关键生态作用，水文、地貌和生物地球化学研究现在也越来越强调泛滥平原的重要性，但泛滥平原没有得到通航水的法律保护，在美国各地都发生了广泛的变化。因此，河流管理和恢复的首要问题之一是如何有效地重新连接改造后的河道和泛滥平原。重新连接需要在特定设置中进行多个不同重要性的更改。大型木材(LW)的重新引入或保留属于这一范围，但尚未制定关于重新引入多少LW、在什么配置下以及在哪里重新引入的指导方针。这项研究将解决关于低水位堵塞对河道-洪泛区水文连通性的定量影响以及这些影响如何为河流恢复设计提供信息方面的一个重要知识空白。大型木材可能是河道和泛滥平原复杂性的主要驱动因素。LW可以改变当地水动力的分布、泥沙动力学、地下水流、河道横断面和平面几何形状，以及河道与滩地的连通性。这些效应以一种非线性的方式运作，因为LW在堵塞中的积累，以及在逐渐变大和更紧密的堵塞中，在这些响应变量中产生了紧急模式。然而，对于响应变量相对于LW数量或空间分布的变化幅度，阈值还不能被预测。这在河流恢复的背景下很重要，尽管无法预测水力、地貌或生态响应，但河流恢复仍会在河道中重新引入LW。在定量预测低水位对河道-泛滥平原水文连通性的物理影响方面，这一知识鸿沟将通过以下方式加以解决：(1)测量不同野外环境中的低水位及其相关连通性；(2)评估河流地貌和低水位特征引起的连通性的阈值响应；(3)制定在数值模型中表示低水位特征和相关水力响应的准则。在此背景下，该项目将河道-泛滥平原的水文连通性定义为当水流越过形态的河道堤岸并扩散到邻近的泛滥平原时存在。该项目的目标是：(1)从天然河道收集关于低水位堵塞特征及其对河道-泛滥平原连通性的相关影响的数据；(2)利用这些数据为校准低水位特征和数值水力模型中的水力响应制定准则；以及(3)与河流修复从业者共同产生知识和最佳做法，以长期监测低水位堵塞的效益。该项目将把有关低水位堵塞如何影响河道-泛滥平原相互作用的新知识与目前和未来对河流恢复项目的投资结合起来，计划开展的活动旨在促进河流恢复设计的可持续性，促进对不同利益攸关方和恢复从业者的恢复监测和公民科学学习，并改进注重可持续和有弹性的自然系统的STEM教育。该奖项由CBET/ENG中的环境可持续发展项目和EAR/GEO中的水文科学项目共同资助。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

River restoration has initially focused on channel morphology and subsequently gradually expanded to include reconnection of channels and floodplains. Although stream ecologists have recognized the critical ecological role of floodplains for decades, and hydrological, geomorphic, and biogeochemical research now increasingly emphasizes the importance of floodplains, floodplains do not receive the legal protection of navigable water and have been extensively altered throughout the US. Consequently, one of the issues now at the forefront of river management and restoration is how to effectively re-connect altered channels and floodplains. Reconnection requires multiple changes of differing importance in specific settings. Large wood (LW) reintroduction or retention falls within this scope, but guidelines have not been developed regarding how much LW to reintroduce, in what configuration, and where. This research will address an important gap in knowledge regarding the quantitative impacts of LW jams on channel-floodplain hydrologic connectivity and how these impacts can inform river restoration design. Large wood can be a dominant driver of channel and floodplain complexity. LW can alter the local distribution of hydraulic forces, sediment dynamics, hyporheic exchange flows, channel cross-sectional and planform geometry, and channel-floodplain connectivity. These effects operate in a nonlinear fashion as accumulations of LW in a logjam and in progressively larger and more closely spaced logjams create emergent patterns in these response variables. However, thresholds can not yet be predicted for the magnitude of change in response variables with respect to LW quantity or spatial distribution. This is important in the context of river restoration, which reintroduces LW in channels despite inability to predict hydraulic, geomorphic, or ecological responses. This knowledge gap of quantitatively predicting the physical effects of LW on channel-floodplain hydrologic connectivity will be addressed by by (i) measuring LW and associated connectivity in diverse field settings; (ii) assessing threshold responses in connectivity due to river morphologic and LW characteristics; and (iii) developing guidelines for representing LW characteristics and associated hydraulic responses in numerical models. In this context, the project defines channel-floodplain hydrologic connectivity as being present when flow overtops the morphological channel banks and spreads onto the adjacent floodplain. The project objectives are to: (1) collect data from natural channels on LW jam characteristics and associated influences on channel-floodplain connectivity; (2) use these data to develop guidelines for calibrating LW characteristics and hydraulic responses in numerical hydraulic models; and (3) co-generate knowledge and best practices with river restoration practitioners for long-term monitoring of LW jam benefits. The project will integrate new knowledge of how LW jams influence channel-floodplain interactions with ongoing and future investments in river restoration projects, and planned activities are aimed to advance the sustainability of river restoration design, promote restoration monitoring and citizen-science learning to diverse stakeholders and restoration practitioners, and improve STEM education focused on sustainable and resilient natural systems. This award is co-funded by the Environmental Sustainability program in CBET/ENG and the Hydologic Sciences program in EAR/GEO.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.