Development of design method for the improvement of aquatic ecosystem habitat in urban stream using riffle structures

This study focused on improving aquatic ecosystem habitats through the implementation of an instream structure that mimics a riffle-pool structure in the streams with severe channelization and degradation. In order to determine the maximum height and shape of riffles suitable for the gentle urban stream with a slope of <1% such as the Mokgamcheon Stream in Korea, a new riffle design method using choking conditions on specific energy was adopted, unlike previous studies focused mainly on geomorphology. Since the target stream was an urban river where sediment transport occurs limitedly under a floodplain, the height of the riffle crest was determined to be in the range of 0.36 to 0.42 m considering the bankfull discharge conditions. The v-shaped design of the riffle openings was employed to create effective ecological habitats, catering to the seasonal preferences of the target fish species. For the Mokgamcheon, characterized by mild bed slopes below 1%, the riffle spacing was set at 15.4 times the bankfull channel width to facilitate efficient water flow. Comparing the Weighted Usable Area (WUA) before and after implementation, we observed significant enhancements in the ecological habitat for Carassius auratus and successful establishment of species diversity for Zacco platypus. Additionally, integrating the sinuosity of meandering channel in the installation of riffles effectively restored ecological habitats and improved the distribution of both Zacco platypus and Carassius auratus habitats, thereby promoting species diversity. Based on specific energy to determine riffle height, shape, and spacing at gentle slopes of <1%, this study proposed an effective design of an instream structure that mimics riffle structures for straight and meandering streams in consideration of hydraulic and ecological engineering aspects.