SEDHYD-2023, Sedimentation and Hydrologic Modeling Conference

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Proactive River Corridor Definition: Recommendations For A Process-Based Approach

In the coastal Pacific Northwest, early European settlement on the floodplains quickly led to flood control works that defined the margins in which river flows and channel migration processes were contained. In most cases the goal was to maximize floodplain use for agriculture and related development, leading to narrow, main channel-only flood management corridors. The National Flood Insurance Program led to the floodway corridor definition for regulatory controls on floodplain development. On the habitat and environment side, various river corridor protection approaches have been implemented, including riparian buffers and channel migration zones. Many approaches define river corridors, but these often focus on a single-purpose use. In addition, paradigms for floodplain landuse planning often view the needs for habitat and flood management as adversarial counterpoints in a zero-sum equation. We argue that evaluating the effects of river corridor width on multiple physical processes and how these processes influence habitat and flood management objectives, can allow the creation of an optimized and realistic river corridor. Our methodology begins based on the following: 1. review of existing approaches to define river corridors from both habitat and flood management lenses 2. review of principal river corridor processes including floodplain accretion and rejuvenation, formation of floodplain habitat, the role of large wood, and effects of constriction and confinement, and 3. quantification of key process rates and evaluation of how these rates relate to the river corridor width for an example reach of a characteristic river From this basis, we define how river corridor width affects function of three flood management and eight habitat goals for the characteristic reach. The results of this analysis show how channel migration is a vital process to sustain river- and riparian health. It allows the river channel to dynamically adjust to changing inputs of water, sediment, and wood, maintaining a corridor through which flood flows may be conveyed, produces topographic variability across the floodplain to support diverse aquatic and terrestrial habitats, is a primary mechanism by which large wood is entrained into larger rivers, and ultimately creates and maintains the complexity and diversity in channel and floodplain hydraulics that is necessary for the flourishing of aquatic organisms. We find major stepwise gains for habitat and flood protection values occur at the transition from constricted- to confined conditions, additional rapid gains in habitat and flood function through the range of river corridor widths expected to correlate to margin-controlled confinement, followed by asymptotically-reducing rate of gain in function for river corridor widths substantially exceeding the threshold for planform controlled conditions. This implies that major improvements in flood and habitat function values would be unlocked by a marginal increase in the river corridor width compared to the existing condition; these large gains occur at widths much narrower than the entire valley bottom. The implication is that the magnitude of change needed to both improve flood management goals and achieve a healthy river may be socially acceptable and represents a worthwhile medium to long-range planning goal.

Andrew Nelson
Northwest Hydraulic Consultants
United States

Jeremy Payne
Northwest Hydraulic Consultants
United States

Tim Abbe
Natural Systems Design
United States

Vaughn Collins
Northwest Hydraulic Consultants
United States

 



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