Collaborative Research: Near-bed flow, turbulence, and emergent hydrodynamics of biologically-conditioned labile river channels

这项拟议的研究旨在研究河流中的水流和沉积物运输过程如何与钻入河床的淡水贻贝（Unionidae）相互作用。淡水贻贝是北美最受威胁的水生生物之一，人们已经开展了许多工作来了解和保护它们。这里的假设是，淡水贻贝可以在给定的河流位置存活相对较长的时间（长达几十年），即使在高流量事件和大量沉积物的运动中。为了做到这一点，贻贝可能具有特殊的适应性，使它们能够在高流量阶段留在河床上。实地调查、数值模拟和实验设施将证明河床上的贻贝可以在原地停留相对较长的时间，表明将贻贝移出洞穴所需的力高于预期值，并说明贻贝可以改变河流的流量，以帮助它们长期生存。这项工作旨在为贻贝在河流中茁壮成长的过程提供关键信息，然后可以用来帮助保护工作。研究生和本科生将在河流和水生生态学的实地、实验和数值方法方面进行培训，为他们未来的职业生涯提供必要的技能。最后，将组建一个创新的跨学科研究团队，以推进对水生生物如何与河流水流相互作用的基本理解。淡水贻贝（Unionidae）是北美河流中最濒危的水生生物之一，人们花了很多努力来了解它们的急剧减少。目前的范式是，河床在很长一段时间（几十年）内的相对稳定性（不动性）是贻贝生态成功和弹性的关键组成部分。然而，这些信息与对河流自我形成的核心认识形成鲜明对比。本研究计划的目的是研究近河床湍流和沉积物运输之间的基本相互作用，在砂和砾石河床与底栖生物居住的不稳定河床。这项研究的重点是一种危险的淡水贻贝，它可以被认为是一个“生态工程师”。假设：(1)底栖生物，如淡水贻贝，可以在具有不稳定河床的动态、自制河流系统的一个站点上相对长时间（约几十年）繁荣，并且这些相同的河段经常经历河道形成的排放；(2)由于生物物理适应（生态工程），在不稳定河床中，选择的活底栖生物的携带阈值明显大于完全等效的非粘性沉积物颗粒。(3)近河床水流、湍流、粗糙度和生态工程适应的建设性干涉创造了一种水动力涌现现象，即“临界点”，它在相对高流量阶段增加了不稳定河床的稳定性。首先，实地调查和数值模拟将在具有历史和生态意义的贻贝种群的两条河流的选定河段进行，以明确地证明贻贝床在不稳定的河道中的持久性。其次，在数值模拟的支持下，实验室实验将直接测量作用在有和没有主动过滤的穴居贻贝上的升力和阻力，以评估夹带阈值和近床流体动力学。第三，结合实验和数值模拟活动将量化流体动力学“临界点”的出现，其中作用于贻贝覆盖的床上的流体阻力通过可变边界条件的建设性干涉而减小。这项工作旨在证明，进化适应促进有利于贻贝在河流中的持久性和生存的水动力条件。

This proposed research is designed to examine how flow and sediment transport processes in rivers interact with freshwater mussels (Unionidae) burrowed into the river's bed. Freshwater mussels are one of the most threatened aquatic organisms in North America, and much work has been conducted to understand and conserve them. It is hypothesized here that freshwater mussels can survive for relatively long time periods (up to several decades) at a given river location even during high flow events with much sediment in motion. To do this, mussels may have special adaptations that enable them to remain in place on a river bed during high flow stages. Field surveys, numerical modeling, and experimental facilities will demonstrate that mussels on river beds can remain in place for relatively long time periods, to show that the forces required to move mussels out of their burrows are higher than expected values, and to illustrate that mussels can alter the flow within a river to aid in their long-term survival. This work seeks to provide critical information on those processes that enable mussels to thrive in rivers, which then could be used to aid conservation efforts. Graduate and undergraduate students will be trained in field, experimental, and numerical methods employed in rivers and aquatic ecology, providing them the necessary skills for their future careers. Lastly, an innovative interdisciplinary research team will be assembled to advance the fundamental understanding of how aquatic organisms interact with river flow.Freshwater mussels (Unionidae) are one of the most imperiled aquatic organisms in North American rivers and much effort has been expended to understand their precipitous decline. The current paradigm is that the relative stability (immobility) of river beds over long time periods (decades) is a critical component to the ecological success and resiliency of mussels. Yet such information is in stark contrast to the central understanding of self-formed rivers. This research program is designed to examine the fundamental interactions between near-bed turbulent flow and sediment transport within sand- and gravel-bedded rivers with labile beds populated by benthic organisms. The focus for this study is an imperiled freshwater mussel, which can be considered an "ecologic engineer." It is hypothesized that (1) benthic organisms like freshwater mussels can thrive for relatively long time periods (ca. decades) at-a-station in dynamic, self-made river systems with labile channel beds, and that these same reaches often experience channel-forming discharges, (2) entrainment thresholds for selected live benthic organisms in labile river beds are measurably greater than fully-equivalent non-cohesive sediment particles due to biophysical adaptations (ecological engineering), and (3) the constructive interference of near-bed flow, turbulence, roughness, and ecological engineering adaptations create a hydrodynamic emergent phenomenon, a "tipping-point," that increases the stability of labile river beds at relatively high flow stages. First, field surveys and numerical modeling will be employed at selected reaches of two streams with historically- and ecologically-significant mussel populations to demonstrate unequivocally the persistence of mussel beds in labile river channels. Second, laboratory experiments, also supported by numerical modeling, will directly measure the lift and drag forces acting on burrowed mussels, with and without active filtering, to assess entrainment thresholds and near-bed hydrodynamics. Third, a combined experimental and numerical modeling campaign will quantify the emergence of a hydrodynamic "tipping-point" wherein the fluid drag acting on a mussel-covered bed becomes reduced through the constructive interference of variable boundary conditions. This work seeks to demonstrate that evolutionary adaptations promote hydrodynamic conditions beneficial to the persistence and survival of mussels in rivers.