Presenter/s: Marjorie Wolfe
Symposium Session: 2020 - 03 An ecological approach to restoration
Topics covered: beavers, climate change, estuary, fish passage, floodplain, hydraulics, monitoring, risk, sediment transport, and stream
In a future featuring climate and land-use changes, enduring success in delivering ecosystem resilience relies on restoring the capacity of the river to adapt to the impacts of those changes. Yet we continue to design for stability, selecting channel dimensions, gradients and planforms suited to past and current flow and sediment regimes, and making little or no allowance for adjustment and evolution of the fluvial system. Further, we make clear the delineations between the river’s channel (or channels), its associated wetlands, and its floodplains, often to fit permitting requirements. The unintended consequences of designing for stability and rigidly defining each component of the complex channel-wetland-floodplain system are that the habitats provided by the restoration are only functional within a narrow range of hydrologic, sediment and environmental conditions.
When these conditions change, a fixed, stable channel-wetland-floodplain design cannot adapt progressively to accommodate change, and this may lead to abrupt and radical adjustment that are, in engineering terms, regarded as catastrophic failure of the project. We must recognize that it is not a matter of if an engineered log jam will be mobilized or a meander bend will shift, but when and how. The fact is that wood rots and banks erode in nature and, in a restored reach, if they don’t move when they should, this may be the worst possible outcome for habitats and biodiversity in a river. This presentation uses case studies to help us rethink restoration design approaches appropriate to a future that is not just uncertain but unknowable. Issues addressed include designing for failure, monitoring for success, and meeting the challenges posed by permitting and funding requirements. Case studies include: projects that have been stable but haven’t been resilient, due to permitting or funding restrictions, and; projects that have delivered more resilient habitat, even though they have ‘failed’ when judged against their stable design goals.