Abstract:
Bedrock channels have been considered challenging geomorphic settings for the application
of numerical models. Bedrock fluvial systems exhibit boundaries that are typically less
mobile than alluvial systems, yet they are still dynamic systems with a high degree of spatial
and temporal variability. To understand the variability of fluvial systems, numerical models
have been developed to quantify flow magnitudes and patterns as the driving force for
geomorphic change.
Two types of numerical model were assessed for their efficacy in examining the bedrock
channel system consisting of a high gradient portion of the Twenty Mile Creek in the Niagara
Region of Ontario, Canada. A one-dimensional (1-D) flow model that utilizes energy
equations, HEC RAS, was used to determine velocity distributions through the study reach
for the mean annual flood (MAF), the 100-year return flood and the 1,000-year return flood.
A two-dimensional (2-D) flow model that makes use of Navier-Stokes equations, RMA2, was
created with the same objectives. The 2-D modeling effort was not successful due to the
spatial complexity of the system (high slope and high variance). The successful 1 -D model
runs were further extended using very high resolution geospatial interpolations inherent to
the HEC RAS extension, HEC geoRAS.
The modeled velocity data then formed the basis for the creation of a geomorphological
analysis that focused upon large particles (boulders) and the forces needed to mobilize them.
Several existing boulders were examined by collecting detailed measurements to derive
three-dimensional physical models for the application of fluid and solid mechanics to predict
movement in the study reach. An imaginary unit cuboid (1 metre by 1 metre by 1 metre)
boulder was also envisioned to determine the general propensity for the movement of such a
boulder through the bedrock system.
The efforts and findings of this study provide a standardized means for the assessment of
large particle movement in a bedrock fluvial system. Further efforts may expand upon this
standardization by modeling differing boulder configurations (platy boulders, etc.) at a high
level of resolution.
