Multiscale characterization of splays produced by a historic, rain-on-snow flood on a large braided stream (Platte River, Central USA)

Abstract

Splays—fan-shaped depositional landforms produced by overbank deposition by unconfined flows—can damage structures, degrade arable land and incur substantial mitigation costs. Splay-related hazards along many rivers are likely to worsen with the increasing magnitude and frequency of major floods. The highly incomplete understanding of splays on braided streams is a conspicuous knowledge gap in a changing world with more frequent and intense floods. The largest recorded flood on the braided, sand-dominated lower Platte River (eastern Nebraska, USA) in March 2019 resulted from the rapid melting of a deep, moist snowpack during an extreme rain-on-snow, bomb-cyclone event. This flood produced 32 large (as much as 234 ha) splays that buried structures and cropland under sand. A total of 1,438 ha of row crop was buried, equating to 1.2 million dollars in lost revenue. These splays diverged from the channel by 14° to 104° along a 122 km reach. The topography of preexisting abandoned channels strongly controlled the shape and orientation of most splays, although forested areas tended to trap or divert sediment. The flood eroded 2.2 to 202 m² m⁻¹ of the streambank at 11 of the splays. The five largest splays (>100 ha) deposited as much as 2.4 m of sand. Ground-penetrating radar profiles of the largest splay indicate that it consisted almost entirely of overbank deposits exhibiting simple downstream accretion that buried the pre-flood soil under ≤ 1 m or less of sand. Locally, however, this soil was eroded during the flood. Climate models predict increasing winter precipitation in the Platte River basin; therefore, the frequency of major floods should increase, making splays recurrent hazards. Our geomorphic assessment of the splays on the lower Platte River illustrates the need for future hazard and mitigation planning.