Modeling the Release of River Ice Jams and their Impact on River Bed Scouring.

The Great Lakes are the largest fresh water reservoir on the planet. Lakes Huron and Michigan drain into Lake Erie through the Huron Erie Corridor. The average water level in Lake Huron has been dropping. The results of hydrodynamic model simulations imply that there has been an increase in the conveyance of St. Clair River that took place in the 1980’s. 1984 was marked by a massive ice jam. The jam had a duration of 24 days and its release was accompanied by high flow velocities. It is speculated that the high water flow velocities following the release of the jam caused scouring of the river bed, that lead to an increase in conveyance. A model is developed to simulate changing river bed morphology, and is combined with a hydrodynamic model in order to simulate scouring during the release of an ice jam. The model consists of modeling the river bed morphology, finding the sediment fluxes on the bed, and updating the bed morphology when scouring occurs. The hydrodynamic model uses a step-wise approximation for the bed morphology. A geometric scheme is developed to compute the local angle of inclination. The bed elevation is updated by numerically solving the Exner equation by using a finite volume approach. A new methodology is developed in order to adapt the grid to the changing bed morphology. The ice jam is modeled as an initially stationary body of water. Water is allowed to flow freely under the body. The body of water is released in the flow, accelerating and causing flow velocities over the entire river to rise rapidly. It is found that an ice jam similar to the 1984 ice jam will cause scouring of the river bed.PHDMechanical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/99841/1/mihalis_1.pd

Bed Scouring During the Release of an Ice Jam

A model is developed for simulating changes in river bed morphology as a result of bed scouring during the release of an ice jam. The model couples a non-hydrostatic hydrodynamic model with the processes of erosion and deposition through a grid expansion technique. The actual movement of bed load is implemented by reconstructing the river bed in piecewise linear elements in order to bypass the limitations of the step-like approximation that the hydrodynamic model uses to capture the bed bathymetry. Initially, an ice jam is modeled as a rigid body of water near the free surface that constricts the flow. The ice jam does not exchange mass or momentum with the stream, but the ice body can have a realistic shape and offer resistance to the flow of water through the constriction. An ice jam release is modeled by suddenly enabling the ice to flow and exchange mass and momentum with the water. The resulting release resembles a dam break wave accelerating and causing flow velocities to rise rapidly. The model is used to simulate the 1984 ice jam in the St. Clair River, which is part of the Huron-Erie Corridor. The jam had a duration of 24 days, and its release was accompanied by high flow velocities. It is speculated that high flow velocities during the release of the jam caused scouring of the river bed. This led to an increase in the river’s conveyance that is partly responsible for the persistence of low water levels in the upper Great Lakes. The simulations confirm that an event similar to the 1984 ice jam will indeed cause scouring of the St. Clair River bed

Rogue Wave Formation in Adverse Ocean Current Gradients

Studies of the nonlinear Schrödinger (NLS) equation indicate that surface gravity waves traveling against currents of increasing strength gain energy and steepness in the process, and this can be a mechanism for rogue wave formation. Likewise, experimental studies have shown that stable wavetrains traveling against adverse currents can give rise to extreme waves. We studied this phenomenon by using computational fluid dynamics (CFD) tools, whereby the non-hydrostatic Euler equations were solved utilizing the finite volume method. Waveforms based on a JONSWAP spectrum were generated in a numerical wave tank and were made to travel against current gradients of known strength, and wave characteristics were monitored in the process. We verified that waves gain energy from the underlying flow field as they travel against current gradients, and the simulated level of energy increase was comparable to that predicted by earlier studies of the NLS equation. The computed significant wave height, H s , increased substantially, and there were strong indications that the current gradients induced nonlinear wave instabilities. The simulations were used to determine a new empirical relationship that correlates changes in the current velocity to changes in the Benjamin–Feir Index (BFI). The empirical relationship allows for seafaring entities to predict increased risk of rogue waves ahead, based on wave and current conditions