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，随后逐渐扩大到包括河道和洪泛平原的重新连接。尽管河流生态学家几十年来已经认识到洪泛平原的重要生态作用，并且水文学、地貌学和生物地球化学研究现在越来越强调洪泛平原的重要性，但洪泛平原没有受到通航水域的法律保护，并且在整个美国已经被广泛改变。因此，目前河流管理和恢复的前沿问题之一是如何有效地重新连接改变的河道和洪泛平原。重新连接需要在特定环境中进行不同重要性的多次更改。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，以何种配置以及在何处重新引入的指导方针。这项研究将解决一个重要的知识缺口，即LW堵塞对河道-洪泛平原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。大型木材可能是河道和洪泛平原复杂性的主要驱动因素。LW可以改变水力学的局部分布、泥沙动力学、浅流交换流、河道横截面和平台几何形状以及河道与洪泛平原的连通性。这些效应以一种非线性的方式起作用，因为在阻塞中LW的累积，以及在越来越大、间隔更近的阻塞中，这些响应变量会产生紧急模式。然而，还不能预测响应变量在LW数量或空间分布方面的变化幅度的阈值。这在河流恢复的背景下很重要，尽管无法预测水力、地貌或生态反应，但在河道中重新引入LW。通过以下方式解决定量预测低水位对河道-洪泛平原水文连通性的物理影响的知识差距：(i)在不同的野外环境中测量低水位和相关的连通性；（ii）评估因河流形态和河流水位特征而导致的连通性阈值反应；（iii）制定在数值模型中表示LW特征和相关水力响应的准则。在这种情况下，该项目将河道与洪泛区的水文连通性定义为水流超过河道河岸并扩散到邻近的洪泛区。该项目的目标是：(1)收集有关天然水道淤塞特征及其对水道与洪泛平原连通性的相关影响的数据；(2)利用这些数据制定在数值水力模型中校准LW特性和水力响应的指南；(3)与河流恢复从业者共同创造知识和最佳实践，以长期监测河流淤塞的效益。该项目将整合关于河流淤塞如何影响河道-洪泛平原相互作用的新知识，以及正在进行和未来的河流恢复项目投资，计划的活动旨在推进河流恢复设计的可持续性，促进恢复监测和向不同利益相关者和恢复从业者提供公民科学学习，并改善STEM教育，重点关注可持续和有弹性的自然系统。该奖项由CBET/ENG的环境可持续性项目和EAR/GEO的水文科学项目共同资助。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值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.