Yuntong She

Modeling Thermal and Dynamic River Ice Processes

River ice jam formation and release both have the potential to cause flooding and present a threat to human safety. From a flood forecasting perspective, it is highly desirable to be able to predict the effects of ice on streamflow hydraulics. Although a number of models have been developed over the years that do consider ice effects, the majority of these are proprietary. This paper reports on recent developments in incorporating ice processes into the public domain River1D hydrodynamic model. The first objective in this effort was to incorporate river ice formation and melting processes and the application of these components is illustrated for the Peace River in western Canada. The model employs an Eulerian frame of reference for both the flow hydrodynamics and the ice processes and uses a Petrov-Galerkin finite element method to solve the primary equations. Model calibration and validation results with historical data are presented; these indicate that the present model adequately simulates water temperature and ice front progression. However, further enhancements are required to include certain dynamic freeze-up processes, in order to refine the ice front results. Ice jam formation and release components have also been added to the model to facilitate the ice jam flood forecasting application and these capabilities are demonstrated by modeling the large ice jam release event that occurred on the Athabasca River, AB, in 2002. Total ice and water mass and momentum equations are solved in an uncoupled sequence with ice mass conservation, with ice momentum effects considered empirically. The model is found to do a good job of modeling release wave speed and peak magnitude. Further enhancements to consider ice momentum effects more explicitly are underway.

Interrelationship Between Nutrients and Chlorophyll-a in an Urban Stormwater Lake During the Ice-covered Period

Urban stormwater lakes in cold regions are ice-covered for substantial parts of the winter. It has long been considered that the ice-covered period is the “dormant season,” during which ecological processes are inactive. However, little is known about this period due to the historical focus on the open-water season. Recent pioneering research on ice-covered natural lakes has suggested that some critical ecological processes play out on the ice. The objective of this study was to investigate the active processes in ice-covered stormwater lakes. Data collected during a two-year field measurement program at a stormwater lake located in Edmonton, Alberta, Canada were analyzed. The lake was covered by ice from November to mid-April of the following year. The mean value of chlorophyll-a during the ice-covered period was 22.09% of the mean value for the open-water season, suggesting that primary productivity under ice can be important. Nitrogen and phosphorus were remarkably higher during the ice-covered period, while dissolved organic carbon showed little seasonal variation. Under ice-covered conditions, the total phosphorus was the major nutrient controlling the ratio of total nitrogen to total phosphorus, and a significant positive correlation existed between total phosphorus and chlorophyll-a when the ratio was smaller than 10. The results provide preliminary evidence of the critical nutrient processes in the Stormwater Lake during the ice-covered period.