Comparison of the methods for valley bottom extraction: A case for the mountain drainage basin where colluvial and bedrock valleys dominate

Valleys associated with convergent topography are common features on the earth’s surface. Valley bottoms along which most rivers flow have been considered geomorphologically important since stream channels adjust their forms within them and, consequently, the width of the valley bottom determines the planform and type of channel reach. The headwater streams comprise transitional and colluvial channels and account for nearly 70 to 80 percent of the river network. They play a role in the movement or retention of sediments and nutrients transferred from surrounding hillslopes, and the provision of habitats for the various aquatic and riparian organisms in mountainous regions. Thus, accurate delineation of the valley bottoms, including colluvial valley portions, is fundamental for managing mountain rivers. In this study, we applied widely used methods for extracting valley bottoms into an intermontane catchment where colluvial valleys dominate (i.e., the Huengjeong basin in South Korea), verified and compared their results to suggest a practical procedure for applying these methods to the uppermost basins in a mountainous region. We compared two methods: the Valley Confinement Algorithm (VCA) which extracts valley bottoms based on flood-prone areas estimation, and the Geomorphon-based method using geomorphic units matching with the topography of valley bottoms. Our results show that the valley bottoms delineated from the VCA with flooding factor 1, smaller than the value (i.e., 3) used for extracting downstream unconfined valley bottoms, characterize well the valley bottoms in the study area. However, the valley bottom width becomes wider where low order tributary flows over flat areas such as river terraces. For the Geomorphon-based method, it turned out that the geomorphic units based on a Geomorphon calculated using a single set of parameter values (i.e., analysis radius and flatness threshold) did not properly capture both headwater colluvial valley and downstream fluvial valleys. Therefore, we extracted valley bottoms for colluvial and fluvial valley bottoms, respectively: Colluvial valley bottoms could be delineated by a set of geomorphic units (valley, hollow, slope, and spur) based on a Geomorphon calculated using a 50 m analysis radius and 0.5-degree flatness threshold; The fluvial valley bottoms could be delineated by a set of geomorphic units (valleys, depressions, foot slopes, and flats) based on 125 m analysis radius and 5-degree flatness threshold. While choosing the geomorphic units corresponding to valley bottoms for the Geomorphon-based method is highly subjective and time-consuming, the VCA could easily detect flood-prone areas below river terraces by only adjusting flooding depth. Thus, the VCA is thought to be suitable for extracting valley bottoms from colluvial valley-dominated areas.