Presenter/s: Nora Boylan, PG
Symposium Session: 2024 - 01 Urban River Restoration
Topics covered: flow augmentation, instream structure (culvert/bridge/dam), and urban
ABSTRACT
Hydromodification, the change in flow regime due to land development, increases erosivity of streamflows and degrades streams. Stream degradation, in turn, diminishes habitat and exposes critical infrastructure that requires exorbitant costs to repair. Connecting these impacts to changes in erosivity is a critical linkage in understanding the problems and potential management solutions. To make this connection, we employ the concept of excess specific stream power, a measure of potential erosivity in units of kilowatt-hours – the same as your electricity bill! This “stream energy meter” integrates the magnitude and frequency of erosive events relative to established erosion thresholds. Understanding the flow events that do the most erosion, and how land use and/or climate change are changing the frequency of these events, is a crucial part of any successful urban watershed program. As in forest environments, unit stream power provides a user-friendly erosion measure because it can easily be adjusted to reflect various management and restoration regimes. Furthermore, established erosion thresholds allow us to assess potential stream degradation in varying stream environments.
Using Tryon Creek, a small, urbanized tributary in Portland, Oregon, we find that stream restoration and floodplain reconnection are essential for managing the excess energy. Our analysis shows that a simple doubling of the active channel width can reduce the excess energy of the stream by up to 80 percent. These results highlight integrated benefits of stream restoration at improving habitats, reducing risk to infrastructure, and generally offsetting the impacts of urbanization.