Research Outputs
The influence of pool-riffle morphological features on river mixing
Accurate prediction of pollutant concentrations in a river course is of great importance in environmental management. Mathematical dispersion models are often used to predict the spatial distribution of substances to help achieve these objectives. In practice, these models use a dispersion coefficient as a calibration parameter that is calculated through either expensive field tracer experiments or through empirical equations available in the scientific literature. The latter are based on reach-averaged values obtained from laboratory flumes or simple river reaches, which often show great variability when applied to natural streams. These equations cannot directly account for mixing that relates specifically to spatial fluctuations of channel geometry and complex bed morphology. This study isolated the influence of mixing related to bed morphology and presented a means of calculating a predictive longitudinal mixing equation that directly accounted for pool-riffle sequences. As an example, a predictive equation was developed by means of a three-dimensional numerical model based on synthetically generated pool-riffle bathymetries. The predictive equation was validated with numerical experiments and field tracer studies. The resulting equation was shown to more accurately represent mixing across complex morphology than those relations selected from the literature.
Urban stream syndrome: Quantifying topographic variation along an urban-rural gradient
Topographic variation within fluvial systems is essential for providing a mosaic of physical habitats and supporting the dynamic hydraulic, geochemical, and biological processes that determine both aquatic and riparian ecosystem function. In highly-modified rivers through both urban and rural settings, the physical heterogeneity of alluvial channels has been diminished by anthropogenic activities. As riparian areas are increasingly under pressure from agricultural and urban development, identifying the geomorphic controls on physical heterogeneity through these environments is critical. In this study, we use the bed coefficient of variation (CV) extracted from a high-resolution bathymetric LiDAR survey as a dimensionless metric for topographic variation and physical heterogeneity over 100 km of the Boise River corridor that spans an urban-rural gradient. Our CV results for both the streambed and channel demonstrate that the average topographic variation of reaches in urban areas is 22–25% lower than reaches located in rural areas along the same river. While these results initially support the application of the urban stream syndrome hypothesis, CV values had similar magnitudes in both urban and rural reaches suggesting there is a dominant control on topographic variation that was not directly related to urban land use. Analysis of CV values relative to normalized levee width indicates that the causative driver of morphologic simplification in the channel was lateral constraints from levees. In the Boise River, topographic variation increased linearly with normalized levee widths that ranged between 50% and >300% of the average channel width. Further, topographic variation was maximized in reaches where flow expansion during high discharge inundated between 1 and 2 times the average channel width (approximately 65–70% of the available floodplain). Our simple and objective watershed-scale approach leverages highresolution topography data to identify reaches of high physical heterogeneity for river conservation, as well as help guide environmental flow releases in managed rivers.
Evaluation of extremal hypotheses in an undeveloped alluvial river
Recent work in the undeveloped Rio Murta Basin, located in Chilean Patagonia, identified an evolutionary trend in the fluvial system as it progresses toward and away from dynamic equilibrium. A location-for-time-substitution model employed over the longitudinal extent of a 16 km study site assessed the performance of extremal hypotheses in identifying dynamic equilibrium conditions. Numerous extremal hypotheses were successful in identifying the spatial trend, but no means were available to discern differences between them. Thus, this study uses field measurements within the evolutionary trend to propose a new metric for evaluating extremal hypotheses. A thorough review and synthesis of the extremal approach are additionally presented. The new method compares theoretical predictions against field-measured values to determine which extremal hypothesis is most effective in identifying the condition of dynamic equilibrium in a gravel-bed river. Channel width and depth are identified as the dependent stream variables that uniquely differentiate most extremal hypotheses from one another. The results indicate that extremal hypotheses based on energy metrics of the flow are most successful, with the strongest support for minimum kinetic energy and minimum specific stream power.
Testing the effective-discharge paradigm in gravel-bed river restoration
Restoration of an alluvial wet-meadow system was conducted in the late 1990s to reestablish hydraulic interactions between the river, floodplain, and groundwater to support aquatic–riparian ecosystem function. A single-discharge approach sized the bankfull channel dimensions to the effective discharge (Qe) and three degrees of channel widening relative to the Qe design were explored to identify which design attained dynamic equilibrium in the shortest time. The three experimental channel designs were implemented with bankfull widths of 96%, 157%, and 191%, respectively, of the Qe geometry. Response trajectories were documented for channel dimensions, sediment mobility, channel morphology, floodplain connectivity, and riparian vegetation for the three channel designs, and the efficacy of a single-discharge approach for restoration was examined. Analysis of 20 years of monitoring data and hydraulic modeling revealed that each design responded differently to the imposed initial channel conditions and evolved at substantially different rates. The design with bankfull dimensions most closely approximating Qe reached dynamic equilibrium within four years of restoration, whereas the moderately over-widened channel (57% larger) exhibited slower responses toward dynamic equilibrium for some metrics and did not fully attain the Qe design dimensions within the monitoring period. The extremely-over-widened channel (91% larger) mainly induced slow rates of bed deposition that are projected to take nearly 300 years for the bankfull dimensions to narrow to the Qe width. All reaches had low bed mobility (bankfull Shields stress < 0.03) 14 years after restoration, demonstrating the challenge of reducing the drivers of channel widening, while maintaining sufficient competence for bedload transport and a sustained supply of coarse bed material for salmonid habitat. Restoration that sizes the channel to Qe can provide rapid dynamic equilibrium, but is a first-order simplification of 1) channel dynamics and 2) the range of flows needed for restoring physical and biological processes in wet-meadow systems.
Identifying dynamic equilibrium of an undeveloped alluvial stream by extremal hypotheses
The indeterminate channel problem arises from uncertainty in finding a closure relation for alluvial channels created by self-organizing erosional and depositional processes. Extremal hypotheses have been proposed as one potential approach to closing the system of governing equations for alluvial channels. Many different extremal hypotheses have been presented, but no substantive evidence has been developed to select which hypothesis may be most appropriate for natural alluvial river systems. This paper evaluates the ability of ten extremal hypotheses to identify dynamic equilibrium across a geomorphic gradient in the remote and undeveloped mid-latitude watershed of Rio Murta, Chile. This study (a) introduces extremal hypotheses, (b) describes the field site and geomorphic conditions, and (c) examines which extremal hypotheses are supported by the field data in identifying the evolutionary trend toward dynamic-equilibrium. The extremal hypotheses that identified dynamic equilibrium within the geomorphic gradient in the field are: (1) minimum kinetic energy, (2) minimum specific stream power, (3) maximum friction factor, and (4) maximum total friction factor, which collectively support minimizing kinetic energy of the system.