When mapping fluvial erosion hazard risk zones, it may be necessary to take additional steps to modify set-back distances according to the landforms and landscape features adjacent to the channel. LiDAR imagery and aerial photography may help in the identification and inclusion or exclusion of natural features within the risk zone. Two features of particular value are valley walls and meander scrolls.
Valley walls can be defined as the edge of the erodible valley bottom at which point the risk of fluvial erosion hazard becomes greatly reduced or even negligible. In Indiana, these may occur as abrupt changes in elevation, relative to the active channel and its floodplain, caused surficial exposure of material more resistant to erosion (i.e. bedrock) or ascension onto a terrace. Valley walls may contain fluvial erosion to within the lower valley bottom. As a result, valley walls may act as natural lines that may be followed to delineate risk of fluvial erosion hazard. Additionally, their inclusion within buffers created by mapping of set-back distance can be easily seen and appropriately adjusted. In the images immediately below, one can identify a number of instances where set-back lines have been mapped, but overlap areas of higher elevation, indicated by the areas of yellow to red shading in the LiDAR image, where the risk of fluvial erosion is less likely to exist (left). With this in mind, a preliminary fluvial erosion hazard map of this area, adjusted to account for the overlap with adjacent valley walls, may appear more like the area outlined with black borders (right).
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Meander scrolls are former channel reaches that have since been severed and abandoned by the nearby active channel as it has migrated laterally. While currently not included in the active channel, meander scars are indicators of past channel positions. Comprised of alluvial sediments and often existing as depressions within the erodible corridor, meander scars can be particularly susceptible to reactivation by the active channel as it migrates laterally across the valley bottom. Unlike valley walls that require exclusion from preliminary set-back corridors, meander scars should be considered for inclusion into areas of concern. The Indiana FEH team is continuing to investigate the potential role of these and other landforms in fluvial erosion hazard mapping.