River Restoration From Boundary Layer to Watershed: Integrating Physical and Biological Science Over Space and Time III
Presiding: P Wilcock, Applied Physics Laboratory, Johns Hopkins University; M Power, University of California, Berkeley
NB22G-01 10:30h
The Missing Link: the Role of Floodplain Tie Channels in Connecting Off River Water Bodies to Lowland Rivers
Along lowland river systems across the globe the exchange of water, sediment, carbon, nutrients and biota between main stem rivers and off-river water bodies (ORWB) is facilitated by the presence of stable secondary channels referred to here as tie channels. Sixty five percent of the ORWB along the middle Fly River in Papua New Guinea connect to the river through such channels. A similar percentage of the 37 ORWB located between Baton Rouge and Memphis on the lower Mississippi River at one time were linked to the river by tie or batture (as they are locally known) channels. Levee construction and other alterations aimed at flood control or navigation on the Mississippi have left only a handful of lakes connected to the river, of these, most are heavily altered by dredging or other modifications. Tie channels were also once common along major tributaries to the Mississippi, such as the Red River. In the much less disturbed Alaskan environment, tie channels are still common, especially along Birch Creek and the Koyukuk and Black rivers. Our studies on the Mississippi River, in Alaska and in Papua New Guinea indicate that tie channels possess a common channel form that is stable and self-maintaining for hundreds to possibly a thousand years. Tie channels exhibit narrow width to depth ratios (~ 5.5) and consistently scale in cross-sectional dimensions to the size of the lake into which they flow. Variations in river and lake stage drive flow bi-directionally through tie channels. A local high or sill in the bed of tie channels controls the degree and duration of connection between the river and ORWB, with many lakes becoming isolated during periods of low stage. The life-span of a tie channel depends on the rate of sediment loading to the ORWB. Our research indicates that this rate directly corresponds to the sediment loading in the main stem river. Along the Fly River, for example, a 5 to 7 fold increase in the river sediment load has resulted increases of 6 to 17 times in tie channel progradation rates. In a few instances Fly River tie channels have become filled with sediment following the increase in sediment loading. The precise role of tie channels in the ecology of lowland river systems has yet to be quantified, but given their critical role in connecting rivers with floodplain habitats it is likely they provide an important source of refuge, breeding habitat, and biomass production for many aquatic organisms. As restoration efforts increasingly focus on the improving or reestablishing connectivity between lowland rivers and their floodplains, consideration should be given as to whether tie channels are an important missing component of such systems.
NB22G-02 10:45h
Effective Suspended Sediment Discharges of Illinois Streams
Dominant discharge theory is often utilized in stream restoration design. Dominant discharge theory assumes that there exists a single discharge that, if constantly maintained within a channel, will create the same average channel dimensions as those produced by a stable channel's entire hydrologic regime. If such a discharge exists, one can theoretically use this discharge as a basis for designing stable channel configurations in stream restoration projects. A stream's bankfull discharge, 1.5 year flow event, and effective discharge are often used as dominant discharge estimates. However, estimating such values within ungaged and incised streams, where stream restoration projects typically occur, is problematic. Moreover, prior research has shown that the recurrence intervals of bankfull and effective discharges are not always equal to the 1.5 year recurrence interval (as commonly assumed), but vary with watershed properties (e.g. size, hydrologic regime, and geology). Subsequently, there exists a desire to develop regionalized dominant discharge values which can be applied at ungaged locations. Two effective discharge values were computed at 20 sediment-discharge monitoring stations throughout Illinois. The first effective discharge value at each location was computed using a power curve to derive that station's sediment-discharge relationship. The second effective discharge value was computed using a "class-interval mean" approach to derive the monitoring station's sediment-discharge relationship. Recurrence intervals for both sets of effective discharge estimates were generally greater than a stream's mean discharge, but less than the 1.1 year recurrence interval. However, effective discharge estimates computed with the power curve were, on average, smaller than those estimated using the class-interval mean approach. Effective discharge results are difficult to validate and are sensitive to sediment-discharge relationships, the amount and quality of sediment data, and class-interval size. Consequently, channel design, based on effective discharge estimates, remains problematic. Suggestions for improving effective discharge estimates and regional dominant discharge studies are provided.
NB22G-03 11:00h
Two case studies in river naturalization: planform migration and bank erosion control
A sound understanding of river planform evolution and bank erosion control, along with integration of expertise from several disciplines is required for the development of predictive models for river naturalization. Over the last few years, several methodologies have been presented for naturalization projects, from purely heuristic to more advanced methods. Since the time and space scales of concern in naturalization vary widely, there is a need for appropriate tools at a variety of time and space scales. This study presents two case studies at different scales. The first case study describes the prediction of river planform evolution for a remeandering project based on a simplified two-dimensional hydrodynamic model. The second case study describes the applicability of a Computational Fluid Dynamics (CFD) model for evaluating the effectiveness of bank-erosion control structures in individual meander bends. Understanding the hydrodynamic influence of control structures on flow through bends allows accurate prediction of depositional and erosional distribution patterns, resulting in better assessment on river planform stability, especially for the case of natural complex systems. The first case study introduces a mathematical model for evolution of meandering rivers that can be used in remeandering projects. In United States in particular, several rivers have been channelized in the past causing environmental and ecological problems. Following Newton's third law, "for every action, there is a reaction", naturalization techniques evolve as natural reactive solutions to channelization. This model (herein referred as RVR Meander) can be used as a stand-alone Windows application or as module in a Geographic Information System. The model was applied to the Poplar Creek re-meanderization project and used to evaluate re-meandering alternatives for an approximately 800-meter long reach of Poplar Creek that was straightened in 1938. The second case study describes a streambank protection project using bendway weirs. In the State of Illinois, bendway weirs constructed of rock have been installed at hundreds of sites, especially on small streams, to control streambank erosion. Bendway weirs are low hard structures installed in the concave bank of a meander bend. Design criteria for these weirs are approximate and have not been rigorously evaluated for overall effectiveness at low-, medium- and high flows. This initial step of the study attempted to describe the hydrodynamics around the weirs and the influence of the hydrodynamic patterns on sediment transport (near-field and far-field). To do that, a state-of-the-art three-dimensional CFD model was used to simulate flow through meander bends where 3D velocity measurements have been obtained to validate model predictions at low stages. Results indicate that the weirs produce highly complex patterns of flow around the weirs, which in some cases may actually increase erosional potential near the outer bank. These two case studies represent components of an emerging initiative to develop predictive tools for naturalization over a range of spatial and temporal scales
NB22G-04 11:15h
Use of a Fiber-Bundle Model to Quantify Root Reinforcement in Streambanks
Streambank instability poses a number of economic and ecological problems for river managers and users. Riparian vegetation plays a number of roles in protecting stream banks from erosion by particle entrainment and mass wasting, with the impacts generally separated into those that are mechanical and those that are hydrological. This paper investigates ways of quantifying the mechanical reinforcement provided to the soil matrix of the banks by riparian plant-root networks. Established perpendicular models used to quantify root reinforcement sum the ultimate tensile strengths of the roots in the soil matrix and add these to the ultimate soil strength. However, in reality the roots do not all break at once, leading to overestimation by these root-reinforcement models. First, the assumptions made by established perpendicular root-reinforcement models were investigated, and the results compared to a newly constructed root-reinforcement model, RipRoot, which is based on the fiber-bindle models used in materials science. Results of the two root models were compared to direct-shear tests of root-permeated and non-root-permeated soil samples. Second the root reinforcement values obtained from the two root-reinforcement models were input to a streambank stability model in order to assess the impact of the differences between the root models on streambank factor of safety values. The results show that the simple perpendicular root-reinforcement model overestimated the actual root reinforcement provided to a streambank. The new fiber-bundle model, RipRoot, provided more accurate estimates of root reinforcement through its inclusion of progressive root-breaking during mass-failure of a streambank. In cases where bank driving forces were great to enough to break all of the roots, the perpendicular root-model overestimated root reinforcement by up to 50 %, with overestimation increasing an order of magnitude in model runs where streambank driving forces did not exceed root strength. The difference between the root reinforcement estimates from the perpendicular root model and RipRoot had a considerable impact on streambank factor of safety values. For the highest banks modelled (3 meters) the difference in factor of safety values between the runs with the two models varied from 0.13 to 2.39 depending on the riparian species considered.
NB22G-05 11:30h
Stream Restoration Toolbox of the National Center for Earth-surface Dynamics
The design of stream restoration projects can be greatly aided by the availability of the latest results from basic research on river science and engineering. These results, however, need to be cast in a form that can be directly used by the practitioner. Academics have from time to time engaged in vigorous criticism of the scientific and engineering basis for the practice of stream restoration, but without necessarily offering easily accessible alternatives. The National Center for Earth-surface Dynamics is attempting to fill the gap between basic research and practice by developing the Stream Restoration Toolbox. Several of the tools under development are presented. These include: a) a predictor for bankfull geometry in gravel-bed streams, b) a designer for threshold channels, c) a predictor for the response of a river to the restoration of flood flows and d) a discriminator for braided and meandering streams. Each tool in the toolbox includes a central PowerPoint presentation, and may include Excel files to allow for direct calculation. The tools will be downloadable from the NCED web site at no cost. Each tool will be beta-tested by a group of practitioners.
NB22G-06 11:45h
Development and Application of a Geostatistical Framework for Quantifying the Spatial Variability of River Channel Morphology and In-stream Habitat
Current research at the interface between geomorphology and ecology emphasizes linkages between geomorphic variability, habitat complexity, and the diversity and productivity of aquatic ecosystems. The goal of river restoration is to reestablish these connections by reintroducing the natural fluvial processes that create and maintain the habitat needed to sustain viable populations of critical species. Achieving this objective will require a novel, quantitative approach to the description of channel morphology and associated habitat conditions. Geostatistics provides a spatially explicit, stochastic framework for this characterization by summarizing variance at different scales, based on differences between pairs of observations separated by various lag distances. Although Euclidean distance is not a valid, in-water metric for the non-convex geometry of meandering channels, a more appropriate representation can be obtained by transforming measurement locations to a channel-centered coordinate system defined by streamwise s and normal n axes. I have developed an efficient transformation algorithm that allows for geostatistical modeling in the (s,n) space. Using field data from a pristine gravel-bed river in Wyoming and a recently restored reach of the Merced River in California, I illustrate how variogram model parameters are related to geomorphic context and ecologically relevant channel characteristics. More specifically, I show that spatial structure varies among reaches with different disturbance histories and demonstrate how cross-variograms of flow depth and velocity can be used to evaluate aquatic habitat. In addition to its utility as a tool for quantifying channel form and the evolution of simple restored reaches toward a more natural, complex state, this geostatistical approach could yield novel insight into a number of restoration-relevant issues including partial transport of mixed grain size sediment and the response of various organisms to different scales of morphologic and hydraulic variability. Extensions to spatial prediction, stochastic simulation, and decision-making under uncertainty are also discussed.