Impact evaluation of instream bar management using morphodynamic modelling.

The Bow River's 2013 flood was the costliest natural disaster in the City of Calgary's history. Flood-induced bar growth and subsequent riparian vegetation colonization at many locations has constricted the river channel, which increases flood risk. Although bar removal has been widely employed as a flood mitigation strategy, its effectiveness and associated impacts are still uncertain. This study employs Delft3D to develop a two-dimensional (2D) morphodynamic model in order to evaluate the impacts of a conventional plan of bar removal and a novel plan of bar realignment in terms of flood mitigation, aquatic habitat protection and river recreation realization. A hydrodynamic model was firstly developed and calibrated using post-flood spatially distributed velocimetry data. A morphodynamic model was then developed and validated using post-flood bed elevation survey data. Then, the future channel response and flood peak levels using different bar management plans were modelled and compared. Results show that appropriate bar realignment can protect aquatic habitat and provide river recreation opportunities while bar removal performs the better in terms of lowering the future flood peak level. The findings indicate that manipulation on instream bars has little morphological impact to downstream reach and creating a less obstructed channel is the fundamental strategy in flood mitigation.

Impacts of Channel Morphodynamics on Fish Habitat Utilization.

It is reasonable to expect that hydro-morphodynamic processes in fluvial systems can affect fish habitat availability, but the impacts of morphological changes in fluvial systems on fish habitat are not well studied. Herein we investigate the impact of morphological development of a cohesive meandering stream on the quality of fish habitat available for juvenile yellow perch (Perca flavescens) and white sucker (Catostomus commersonii). A three-dimensional (3D) morphodynamic model was first developed to simulate the hydro-morphodynamics of the study creek. The results of the morphodynamic model were then incorporated into a fish habitat availability assessment. The 3D hydro-morphodynamic model was successfully calibrated using an intensive acoustic Doppler current profiler (ADCP) spatial survey of the entire 3D velocity field and total station surveys of topographic changes in a meander bend in the study creek. Two fish sampling surveys were carried out at the beginning and the end of the study period to determine presence-absence of fish as an indicator of the habitat utilization of each fish species in the study reach. It was shown that morphological development of the stream was a significant factor for the observed changes in the habitat utilization of juvenile yellow perch. It is shown that juvenile yellow perch mostly utilized habitat where deposition occurred whereas they avoided areas of erosion. The results of this study and the proposed methodology could provide some insights into the potential impact of sediment transport processes on the fish occurrence, and distribution and has implications for management of small fluvial systems.

Hypoxic conditions in stormwater retention ponds: potential for hydrogen sulfide emission.

Improper design and maintenance of stormwater ponds (SWPs) may lead to hypoxic conditions, poor water quality and the production of hydrogen sulfide (H2S). The objective of this study is to develop a comprehensive understanding of hypoxic conditions of SWPs, with a focus on the potential for H2S production and emission. This study was conducted at two retention SWPs in Ottawa, Canada; a problematic pond with the propensity for H2S emission and a reference pond that did not demonstrate H2S emission. The investigation illustrated a significant impact of low dissolved oxygen (DO) concentrations, hypoxic conditions, on the concentration of total sulfides in the water column. Both ponds were shown to periodically experience hypoxic conditions at depth, especially during summer periods with less precipitation and across longer periods of winter, ice-covered conditions. The problem pond, however, was shown to experience lower DO and longer hypoxic conditions than the reference pond in both non-ice-covered and ice-covered conditions due to greater depth and a longer hydraulic retention time. Hypoxic conditions were initiated at the deepest locations in the problem pond and subsequently were spread across the entirety of the pond under winter, ice-covered conditions. Algal biomass (Chlorophyll-a) and soluble biochemical oxygen demand concentrations were shown to not likely be significant factors in the development of hypoxia in the H2S-generating pond. Algal blooms of colonial Chrysophyceae, Synura, a known mixotroph, were observed during ice-covered conditions in the problem pond possibly due to stress-coping mechanisms of algae.

Flow in bedrock canyons.

Bedrock erosion in rivers sets the pace of landscape evolution, influences the evolution of orogens and determines the size, shape and relief of mountains. A variety of models link fluid flow and sediment transport processes to bedrock incision in canyons. The model components that represent sediment transport processes are increasingly well developed. In contrast, the model components being used to represent fluid flow are largely untested because there are no observations of the flow structure in bedrock canyons. Here we present a 524-kilometre, continuous centreline, acoustic Doppler current profiler survey of the Fraser Canyon in western Canada, which includes 42 individual bedrock canyons. Our observations of three-dimensional flow structure reveal that, as water enters the canyons, a high-velocity core follows the bed surface, causing a velocity inversion (high velocities near the bed and low velocities at the surface). The plunging water then upwells along the canyon walls, resulting in counter-rotating, along-stream coherent flow structures that diverge near the bed. The resulting flow structure promotes deep scour in the bedrock channel floor and undercutting of the canyon walls. This provides a mechanism for channel widening and ensures that the base of the walls is swept clear of the debris that is often deposited there, keeping the walls nearly vertical. These observations reveal that the flow structure in bedrock canyons is more complex than assumed in the models presently used. Fluid flow models that capture the essence of the three-dimensional flow field, using simple phenomenological rules that are computationally tractable, are required to capture the dynamic coupling between flow, bedrock erosion and solid-Earth dynamics.

Stochastic calibration of riverine water quality models.

Though commonly used, the suitability of deterministic calibration criteria for stochastic model calibration and uncertainty analysis is unclear. The purpose of this paper is to examine the suitability, relative benefits, and substantial disadvantages of "deterministic-optimization" approaches, such as root mean square error (RMSE), in stochastic contexts. Three alternate calibration strategies that are suitable for stochastic modeling of water quality under uncertainty are proposed and then demonstrated. The three alternate strategies are the absolute relative error (ARE), weighted relative error, and stochastic exceedance calibration strategy. The findings suggest that potential improvements can be made to current calibration paradigms. The alternate calibration strategies, all of which are based on relative error, were found to match or exceed RMSE calibration strategies, in terms of overall performance, with the enhancement of superior model surface-response characteristics. Additionally, the application of more stringent ARE criteria offered greater improvement in the stochastic calibration response than increasing the RMSE threshold criteria. Several qualitative benefits of ARE and related metrics also were shown. Because many environmental systems and almost all water quality models are subject to substantial uncertainty, approaches such as those proposed hold substantial, widely applicable benefits.