In 2009, work will continue on a broad range of stream restoration issues, including erosion, water quality, environmental engineering, and aquatic habitat. In future years, the meandering stream in the Riparian Basin may be replaced by multiple separate stream channels, a braided river, impoundments, or other configurations. Pending funding resources, future plans also include reviving an adjacent longer flood bypass channel, which will become the Riverine Corridor.
Projects within the OSL in 2009 will likely include the following:
Improving the prediction of meandering river migration rates by characterizing the effect of vegetated slump blocks
River migration is a long-term process that unfolds over centuries, yet it results from erosion and deposition processes during individual flood events. Previous work has shown that bank vegetation can stabilize banks and limit migration rates. Even in areas where root depth is shallow relative to the stream depth, it is hypothesized that blocks of sediment dislodged from the bank, called slump blocks, that lodge on the bank toe can substantially reduce bank erosion. In this project, we will couple field and field-scale laboratory investigations into the geotechnical properties of slump blocks and their effect on near-bank shear stresses. Field-scale laboratory work at the St. Anthony Falls Laboratory Outdoor StreamLab will focus on the distribution of velocity and shear stresses around slump blocks harvested from a local field site. Companion field studies will characterize slump block properties in the field with the goal of predicting how long it takes for them to break down. This work is also of interest to ecologists trying to understand and manage in-stream habitat for fish and macroinvertebrates.
The geomorphic signature of life: biotic feedbacks on physical systems
Biotic feedbacks on local-scale fluid and sediment dynamics influence floodplain patterns of channel dynamics and subsequent controls on species diversity. Understanding and predicting these feedbacks, however, entails integrating across disciplines and methodologies as well as across spatial and temporal scales. We will investigate the feedbacks between vegetation growth, flow hydraulics, sediment transport rates, and channel topography to identify the influence of vegetation on channel bar and floodplain sedimentation. We hypothesize that vegetation on channel banks may locally increase flow roughness, and reduce the average flow velocity and sediment transport rates. Conversely, the growth of vegetation within channel bars may significantly increase the average shear stress, near-bed turbulence intensities (for the same flow velocity), and sediment fluxes. Thus, the impact of vegetation on channel topography will vary with the position of vegetation in the channel and floodplain. We also hypothesize that the vegetation density (amount in an area) and species (i.e. rigid or flexible) may determine local and reach-scale patterns of deposition and erosion.
Development of design methods for in-stream flow control structures
The lack of engineering standards for stream restoration techniques is underscored in the design and installation of shallow, in-stream, low-flow structures. These structures are preferred by many federal, state, and local governmental agencies to assist in stabilizing beds and banks in stream restoration projects while enhancing aquatic habitat. The complexity of flows around in-stream structures makes it difficult to obtain much needed design criteria from any single research approach. We here use a comprehensive research strategy to develop comprehensive quantitative engineering guidelines, design methods, and recommended specifications for in-stream structure installation, monitoring, and maintenance. To accomplish this goal, we are coupling an in-depth review of the current use of in-stream structures to a comprehensive study using physical and numerical models to explore the most promising structures. A key component of this project is an examination of the performance and stability of in-stream structures within a sinuous channel within the Outdoor StreamLab in order to quantify potential for failure as function of flow rate and stream geomorphology as well as the impact of the structures on erosion rates, stream stability, and lateral channel migration. Results from these measurements will be extended to a wide range of other stream morphologies and types using SAFL's advanced numerical model, the Virtual StreamLab, which will provide the breadth of information necessary to develop guidelines for the successful construction of stable in-stream structures in a wide range of streams. Model predictions will be verified against measurements of changes in bed and bank topography obtained from several existing field installations of in-stream structures. These results will be combined into comprehensive quantitative engineering guidelines.
Determination of the effect of engineered structures on residence time, nutrient processing, and habitat within benthic and hyporheic environments
Understanding interactions that cross multiple trophic levels is necessary to develop models that can predict stream ecosystem response to disturbance and restoration. By altering stage, slope, and velocity distribution within the channel, in-stream structures such as J-hooks and W-weirs alter both in-stream and subsurface ecogeomorphic function. Although a range of ecosystem benefits are claimed for such structures, including an increase in nutrient removal, there is little reliable information to predict the nature of these changes. Here, we examine the effect of structures on both in-channel and subsurface ecogeomorphic function. Our major research goals are to determine the effect of in-stream structures and the resulting in-stream velocity distribution, sediment transport, bed topography, and hyporheic exchange on (i) in-channel oxygen, temperature, nutrients, photosynthetically active radiation (PAR), periphyton, and grazers and (ii) oxygen distributions and nutrient processing within bed and banks. We seek generalization through numerical models, coupling experimental measurements of fluid mechanics, sediment transport, metabolism, and nutrient processing with high-resolution numerical simulations that resolve the unsteady, three-dimensional flow characteristics of in-stream flow and a coupled channel/subsurface model to calculate exchange and nutrient processing rates. The widespread use of in-stream structures makes these experiments directly relevant to stream restoration practice. Our long-term goal is to develop predictive guidance on the effect of physical channel structure and disturbance on stream restoration objectives related to primary productivity and nutrient processing.
Enhancing quantitative prediction of fish habitat characteristics
At least one third of restoration projects are designed to increase or manage available stream habitat, with many of them specifically targeting fish species. However, our current understanding of both the mechanics and neural control of fish swimming is insufficient to predict how fish make use of field environments. Understanding how organisms interact with turbulent flows in aquatic ecosystems is a critical prerequisite for developing a science-based aquatic restoration framework. Here, we will perform measurements to examine how several species of fishes with different body shapes respond to environments with different physical characteristics. The Outdoor StreamLab provides realistic environmental conditions yet still allows experimental control and detailed data collection from fishes in localized environments. Simultaneous observations of fish position, simple kinematic parameters such as tail beat frequency, and flow characteristics will focus on three localized environments within the OSL: a deep quiescent pool, a shallow high-gradient riffle, and the region directly adjacent to a boulder or habitat structure. These results will be compared to multi-day records of fish position within the OSL channel to provide additional insight into specific habitat types preferred and avoided by various fish species to determine how turbulence characteristics and habitat heterogeneity affect the diversity of ecological niches present within the stream. |
