Assessing Geomorphic, Ecological and Social Benefits of Bankfull Flow in Onondaga Creeks Urban Channel

Naturalized river restoration often attempts to reconnect channel and floodplain by manipulating the frequency of bankfull discharge. This research identifies limits to the geomorphic and ecological benefit of floodplain building in Syracuse, NY's armored and channelized Onondaga Creek given the community risk of flooding along with the imperfect method of establishing urban bankfull. The study reach of 300 meters, classified as Rosgen G4 and departed from a reference C3, has an estimated bankfull discharge of 31 m³ s^–1 yet was channelized to contain flows up to 85 m³ s^–1. While bankfull flow in rural areas builds and colonizes floodplain, such work is in conflict with, and undermined by, urban property, infrastructure, and emergency management objectives and activities. Urban restoration that abandons lateral migration must still use bankfull estimates to address sediment transport and fish passage, as well as avoid aggradation due to Onondaga Creek's upstream mud boils that can generate up to 27E3 kg of daily sediment loads. Unfortunately, urban geomorphic assessment of bankfull flow, the upper limit to prevent flooding, was difficult due to the absence of traditional indicators. In short, urban incised rivers need excavated floodplains in which to satisfy the ecological, geomorphic, and social functions. This research explores four methods for setting floodplain excavation targets, which are measuring available floodplain, stream shear stress, bank collapse, and channel effective discharge.