Geomorphic and hydrodynamic responses of experimental alluvial channels to rigid vegetation
Vegetation such as trees and woody debris remains a key component of bank stabilization and stream restoration programs because of the beneficial ecologic and hydraulic effects and attributes it brings to river corridors. Yet few design criteria currently are available to guide the use and application of these activities, and part of this problem may lie in not knowing precisely how river corridors respond to the newly introduced vegetation. Physical experiments provide the unambiguous quantification of the geomorphic, hydraulic, and hydrodynamic responses of alluvial channels to the introduction of vegetation. To this end, a range of physical experiments have been conducted using simulated stands of rigid, emergent vegetation and submerged large woody debris in both fixed- and mobile-bed channels to identify these geomorphic responses to and hydrodynamic effects of the introduced vegetation. Experimental results will focus on flow resistance, secondary flow and meander development, coherent flow structures and turbulent mixing, localized erosion and deposition, and the potential for nutrient retention, all in relation to the size, shape, orientation, and density of managed plantings of vegetation or the placement of large woody debris. These experimental results will be compared to both theory and numerical models, and will be discussed within the context of stream restoration design.
The Effects of Vegetation on Turbulent Boundary Layer Flows Revealed through LES
Fully developed turbulent flows with submerged vegetation are investigated using Large Eddy Simulation (LES), with a focus on understanding the role of the coherent structures on the momentum transfer across the water-plant interface. LES model results are compared with laboratory measurements reported in the literature. As with Reynolds-Averaged Navier-Stokes models, LES models effectively simulate the effects of submerged vegetation on the mean flow field, but they also account for the anisotropy of the Reynolds stresses due to the vegetation layer, and resolve coherent structures observed in the instantaneous flow field. Comparisons with fully developed turbulent boundary layer flows in unobstructed (non-vegetated) channels are made to show how the vegetation significantly changes the mean flow, Reynolds shear stress, turbulence intensities, turbulence event frequencies and the energy budget within and above the vegetation layer. LES provides direct visual evidence that coherent structures, namely spanwise vortices (rolls) and streamwise vortices (ribs), develop at the water-plant interface at the top of the vegetation due to the well-known Kelvin-Helmholtz instability. Implications for sediment transport are also discussed.
Assessing Success of Instream Structures for Salmonid Stream Restoration
Stream restoration is a billion dollar industry in North America; despite this expenditure there remain questions regarding the effectiveness of current techniques such as the installation of instream structures. Assessing the effect that such structures have on physical habitat and on salmonid density are key ways of determining project success. The objectives of this research were to assess the impact of instream structures on physical habitat in the Nicolet River (Quebec) and to analyze physical habitat and fish density data from many stream restoration projects in North America. Results of intensive surveys of the Nicolet River show that the installation of weirs and deflectors results in a greater frequency of pools. These pools have significantly greater depths, lower velocities, larger sediment size and higher percent cover than those without structures. Meta analysis of data from 187 stream restoration projects in North America also show significant increases in percent pool area, average depth, and percent cover as well as decreases in channel width following the installation of structures. The physical changes observed in the Nicolet River resulted in improved trout habitat, as measured by applying habitat preference curves, but uneven stocking practices and fishing pressure confounded attempts to verify differences in trout density based on presence or absence of structures. The meta analysis, however, shows significant increases in salmonid density, measured as fish/m2, following the installation of structures. On average, density increased by 161%. Different structure types result in significantly different changes in physical habitat, with weir structures providing the largest density increase. Multiple linear regression analysis reveals that the combination of change in relative pool area and in width is the best predictor of change in salmonid density (r2=0.511). Instream structures are significantly more successful at increasing brook trout density than cutthroat and steelhead trout or coho salmon. Furthermore, salmonids over 15cm in length show significantly higher increases in density than smaller fish. These results highlight that restoration structures can play an important role in creating better habitat for salmonids and increasing their densities, but much work is needed to determine the best way to rehabilitate disturbed streams for various species.
DEM Mapping of Stream Power for Southern Ontario Streams
Mapping of stream power along a stream system, a known determinant of channel form and dynamics, is a valuable component of geomorphic stream assessment procedures that, unlike current methods, is physically-based, time- and cost-effective, objective and repeatable. Continuous maps of tream power can be obtained by extracting channel slope from DEMs and combining them with a discharge-drainage area function. Using the case of Highland Creek, a highly urbanized basin in Scarborough Ontario for which extensive data and background information is available, it is shown that reliable and precise stream power maps can be obtained from the Ontario provincial DEM. Local stream power variation can be seen to match known features of the channel and both reach-scale and overall trends in stream power match those from a 1D computational model (HEC-RAS). Stream power maxima and minima also coincide with known areas of channel instability and deposition.
Productive Capacity of Semi-Alluvial Streams in Ontario: The Importance of Alluvial Material for Fish, Benthic Invertebrates, Periphyton and Organic Matter.
The natural flow regime is a key component of creating and maintaining in-channel and floodplain conditions
critical for aquatic and riparian life. Changes in land-use and climate (e.g., urbanization, agriculture, damming)
are expected to result in modified flow regimes leading to flashier and more powerful and erosive flows.
Armouring streambanks to reduce erosion and damming leads to lower recruitment of gravels into streams
while larger peak flows increases transport capacity leading to a loss of gravels: critical habitat for aquatic
biota. Semi-alluvial streams are characterized by having only a thin veneer of alluvium on top of a non-erodable
base of bedrock or clays. Under this new flow and sediment regime we will see more sections of exposed
clay or bedrock. What does this loss of gravel mean for benthic invertebrates, fish and productive capacity?
We sampled streams with varying degrees of exposed clay and bedrock for fish, benthic invertebrates,
periphyton and coarse particulate organic matter. Our results indicate that gravel substrates are more
productive than clay, containing a higher biomass of benthic invertebrates, periphyton and coarse particulate
organic matter. Bedrock substrates were more productive in some cases and less in others, relating to the
nature of the flow regime, bed transport, and bed scour. Fish density and biomass were not different among
sites. Our study shows the importance of substrate in the productive capacity of streams. Further research
should focus on other substrate types and watershed-scale substrate modeling to allow quantification of gains
and losses of fish habitat.
Tightening the River Meander-Belt: Application of a Topographic Erodible Corridor Concept Using DEM Raster Analysis. A Case Study of Highland Creek, Ontario.
Planimetric river hazard assessments, typically delineated as meander-belts, are complicated in southern Ontario by rivers which are incised into thick glacial sediments. Active and relic floodplain surfaces are topographically diverse, with river terraces commonly observed in the valleys due to deglacial and Holocene incision. Consequently, channels are often in contact with a mixed boundary of alluvial and glaciogenic sediments. Accepted meander-belt delineation procedures and protocols vary between intra-national and international jurisdictions; however, a focus on planimetric mapping and historical techniques is common place. In the southern Ontario context, this type of reach-scale river hazard assessment is important for protection of upland property, erosion risks to valley bottom infrastructure, and delineation of new development limits. Given the ecological and public safety benefits, there is growing acceptance and expectation that river bank erosion processes should be preserved within an erodible corridor, with a decreased emphasis on channel intervention and engineering approaches where possible. However, the use of planimetric meander- belt delineation techniques for incised valley settings frequently meets both practical and conceptual challenges. This study explores the potential for a Topographic Erodible Corridor Concept (TECC) as an improved representation of river evolution compared to the traditional planimetric techniques, particularly in previously glaciated regions. Such a concept would account for differences in erodible volumes of sediment associated with topographic variations within incised river valleys. Application of this concept is investigated using raster analysis of a high resolution digital elevation model (DEM), within widely available GIS software. Initial results from a case study on Highland Creek (Ontario) confirm that the corridor alignment and diverse topography of the incised valley morphology are well represented by a TECC model, which can be translated into a detailed assessment of erosion risks. Erosion rate estimates may still require historical overlay techniques to characterize channel migration rates. However, it is proposed that development of a regional-scale probability distribution of erosion rates may later be integrated into the TECC model to constrain the erodible corridor envelope within typical planning horizons. Such probability distributions may be indexed to reach-scale characteristics such as stream power, boundary material, or basin landuse, for example. Sensitivity of local bank erosion rates to particular channel alignments, processes, and boundary materials may be considered in future TECC models to improve representation of lateral migration processes and avulsion potential. The TECC model also provides a solid framework to integrate geotechnical stable-slope analyses to erodible valley walls for confined valley settings.
Developing a Master Plan for Restoring/Stabilizing an Urban Watercourse: Highland Creek
Highland Creek is a fully urbanized watershed (104 km2 ) in Toronto, Ontario. Through the process of urbanization and placement of sanitary, storm and transportation infrastructure within the channel corridor, the length of channel within the drainage network has been reduced. Of the remaining length (38 km), more than half the channel is protected by engineering counter measures along channel bank and/or bed, many of which are failing. In addition, through the processes of channel adjustment (i.e., primarily degradation and widening) in response to urban hydromodification, 17 % of the 143 subsurface sanitary sewer crossings are currently exposed and at risk of failure. Indeed, a major storm event in 2005 caused substantial channel movement, failure of a manhole and underlying sanitary sewer, leading to sewage discharge into Highland Creek. A consequence of all of these modifications has been the creation of numerous fish barriers, loss of all but the most tolerant fish species and degradation of both the physical and chemical habitat conditions. The City of Toronto has initiated a study to develop a Geomorphic Systems Master Plan to stabilize/restore Highland Creek with the primary intent of protecting infrastructure. The study is following the Nine Step Analysis Procedure of the Adaptive Management Methodology (Ontario Ministry of Natural Resources) which includes three stages of analyses (i.e., Watershed Issue Assessment, Planning and Environmental Assessment, and Design Process). The study is multi-disciplinary and has included a thorough inventory of existing channel conditions and characteristics (biologic, geomorphic); a comprehensive risk assessment that considers implications of historic channel change and existing conditions exposure/failure of infrastructure/counter measures; assessment of further anticipated channel responses (cross-section, profile, planform) to urban hydromodification have been undertaken. Analyses to assess the effectiveness of various stormwater management strategies in reducing impacts on the channel has been completed. The intent of all analyses is to develop a plan (spatial and temporal) for stabilizing the watercourse, reducing risk to infrastructure and private property, and enhancing fish habitat.
Stream Corridor Lowering for Servicing: Considerations and Approaches to Natural Channel Design in Southern Ontario, Canada
Although there are numerous approaches to natural channel design, all approaches generally advocate application of geomorphic principles to develop stable watercourses with improved habitat function. In southern Ontario, natural channel design approaches are increasingly utilized in stream corridor management. In numerous greenfield developments within southern Ontario, creek corridors are lowered and relocated to address potential hazards and facilitate development. These projects usually utilize natural channel design approaches. Although lowering for servicing can be a controversial technique, this approach has resulted in the maintenance of channels that may have previously been enclosed and lost. In the southern Ontario context mimicking natural corridor form and function is complicated by a surficial geology dominated by glacial sediments. Approaches to natural channel design have evolved over time to address this encumbrance. This presentation examines the geomorphology of streams and stream corridors within southern Ontario. Case studies from southern Ontario are provided to illustrate many of the impediments to, and innovations in, natural channel design. This lays the foundation for illustrating how these design approaches address potential hazards, provide for stream form and function, and mimic much of the physical and biological interactions found within natural stream corridors.
Coupling Stream Restoration and Environmental Flow Initiatives
Restoring environmental, or ecological, flows may be considered a means to restore streams. Extreme low flows, baseflows, high flow pulses, and floods with associated timing, duration and frequency, may be specified to restore ecological functions. But the sediment regime also plays a role in geomorphic functions and habitat suitability. If the sediment regime and the stream have been altered, restoring flows is not likely to have the desired effect, particularly where target flows are determined as an acceptable departure from a reference (e.g pre-development) flow regime. In some cases, restoring ecological function (e.g. connection between the channel and its floodplain) may require other stream restoration activities (e.g. addressing channel incision) before an environmental flow allocation would be effective. On the other hand, the long-term success of some stream restoration projects depends upon flow regimes and may benefit from specification of environmental flows. The presentation will explore the need for more explicit coupling of stream restoration and environmental flow initiatives.
Interdisciplinary education, research, and training for ecosystem restoration
The restoration of impaired ecosystems is inherently an interdisciplinary enterprise. However, the need for interdisciplinary education and training of researchers and practitioners has only recently been fully recognized. Furthermore, while grounded in established disciplines such as geomorphology, conservation biology, and hydraulic engineering, professional practice can vary considerably according to the particulars of local projects. The University at Buffalo has recently initiated a multi-department doctoral program that focuses on Ecosystem Restoration through Interdisciplinary Exchange (ERIE). Supported by the U.S. National Science Foundation, ERIE brings together scientists, engineers, and policy scholars to provide an integrated program of study that is grounded in place-based considerations relevant to the lower Great Lakes region. The presentation provides an overview of the ERIE program with an emphasis on several of its unique features, including interdisciplinary research experiences, field-oriented training by a diverse group of professional partners, focused consideration of Native American ecological and cultural perspectives, cross-border research exchanges with Canadian scholars, and formalized case study methods for disseminating lessons learned from local restoration projects.