DIFFERENTIATING FLASH FLOOD-BORNE SEDIMENTS IN A SMALL AGRICULTURAL HEADWATER SYSTEM USING ISOTOPIC TRACERS

University Of Iowa, Iowa City IA

Investigators

Abstract

Flash flood events are geomorphically critical because they affect stream morphology and floodplain sediment redistribution, as well as lead to the formation of ephemeral gullies, which become key conduits of soils derived from the uplands and floodplain. The June 19th, 2009 flash flood event in the University of Iowa Clear Creek NSF testbed offers a unique opportunity to derive sediment rating data points that, to our best knowledge, do not exist in the literature. In addition, the pre-existing infrastructure in the NSF test bed allowed us to capture soils/ sediments of different origin, i.e., sediments that originate from hills, ditches, riparian zone, stream banks and bed. These unexpected measurements along with the availability of detailed hydrological measurements from radar and preexisting LiDAR data will help us quantify a sediment budget for the event. We propose: 1) to develop a sediment budget for a headwater system of Clear Creek, IA during the June 19th flash flood event; 2) to quantify the proportions of recently eroded surface soils and channel-derived sediment in the suspended load using established sediment tracing techniques that involve naturally occurring radionuclides (7Be and excess Lead-210, 210Pbxs) and stable isotopes (Carbon-13, 13C, and Nitrogen-15, 15N); and 3) to compare the budget with simulations of coupled upland erosion and 1D/ 2D sediment transport models to test their predictive ability for simulating flash flood events. For this study, we are seeking support to conduct isotopic tracer analysis for the samples collected prior to and during the flash flood event to provide a more detailed budget and for modeling of the event. Although an inventory of geochemical tracers is available for Clear Creek through past research, these tracers cannot identify the origin of mobilized sediments solely associated to the flood event of June 19th, 2009. A powerful and established tracer for identifying recently mobilized sediment is the naturally occurring radionuclide, Beryllium-7 (7Be). 7Be is produced continuously in the atmosphere but delivered to the landscape surface in high concentrations mainly during precipitation events. Moreover, 7Be has a relatively short half-life of only 53 days, meaning that it will not remain long in the soil before decaying. Thus, there is a strong relationship between a single erosion event and high signatures of 7Be in the eroded surface soils. Past research has shown the unique ability of 7Be to differentiate uplands from stream banks and bed sediments. The samples to be analyzed have already been collected, but time is critical because of the short 53-day half-life of 7Be. Radionuclide analysis of the samples must begin immediately and be completed within 4 half-lives in order to capture detectable activities of 7Be for differentiating upland and stream bank/bed sources to the sediment load of the June 19th event. As with any catastrophic flood, frequently asked questions include how much material (e.g., water and sediment) was moved during the event. For this study, we will supersede this primary question by addressing not only how much material (in this case, sediment) was mobilized during the June 19th flash flood, but also from where the transported material was derived. This research is transformative because it will provide sediment flux vs. flow data for