Process Units River Evaluation (PURE) Method for River Corridor Delineation
Abstract
River corridors are defined as areas adjacent to rivers that are likely to affect and be affected by river and floodplain processes. Geomorphic and hydrologic models can be used to delineate river corridors to address fundamental questions, such as which parts of the fluvial landscape are most likely to respond to climate and land-use change. Land managers are also increasingly using river corridors for applied purposes such as hazard avoidance, ecological conservation, and water quality protection. Currently, the most-used river corridor delineation methods rely on historic maps, field surveys, and/or calibrated empirical models which may be time-intensive, subjective, or require expert user input. Here we present a novel approach for delineating river corridors that utilizes widely available geospatial data (and thus can be applied uniformly across broad spatial extents) to predict areas susceptible to flooding and geomorphic change, which also includes critical areas for riparian ecology and water quality. Our approach is called the Process Units River Evaluation (PURE) River Corridor Delineation Method, and it is derived from the sum of five functional process units that together capture the river corridor: (i) the Flood Processes Unit, derived from hydraulic modeling to determine areas subject to overbank deposition and erosion, in-channel deposition and erosion, bank erosion, and channel avulsions; (ii) the Landslide and Steep Terrain Processes Unit, based on terrain slope to show locations subject to sediment delivery, bank failures, and other mass wasting proximal to the flood-prone area; (iii) Wetland Processes Unit, based on the U.S. National Wetlands Inventory to show areas where wetland processes occur; (iv) Channel Migration Processes Unit, based on channel location and migration rates to show areas susceptible to lateral channel movement; and (v) Riparian Ecologic Processes Unit. We examine this delineation approach in three river reaches in mountainous and low-relief watersheds in the northeastern U.S. and compare results with other river corridor and flood area delineation techniques. Predicted PURE river corridors are also tested against areas of recent geomorphic change, observed in the field and in historic imagery, and the PURE corridor performs very well, capturing 92% of observed landslide areas, 87% of observed floodplain deposition areas, and 100% of channel migration areas. Our river corridors also show general agreement with more time-intensive delineation approaches, and the resulting corridors are typically broader than areas of recent geomorphic change. The case studies indicate the PURE Corridor method is successful at providing both an accurate assessment of potential active hazard areas and sensitive environmental areas, and that it also includes a margin of safety that many managers desire. This work exhibits the potential for rapid, uniform, and objective river corridor delineation across large areas with transparent communication of the data inputs. Its modular nature allows for flexible weighting of different metrics to suit specific applications, and piecewise updating as new approaches become available.