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
Our goal in this study was to understand better how the delivery of sediment to streams in small intensively agricultural watersheds of the U.S. Midwest changes over time by determining the amount of sediment that comes from the fields and the stream banks during three consecutive runoff events, with the third event being a flash flood. This was accomplished by comparing the natural radioactivity of the suspended sediment in the stream during these events to that of the soil in the upland fields and the banks. We collected suspended sediment using different sampling techniques and the different methods produced the same amount of sediment despite their differences. The total sediment flux during each event was determined over a 24-hr period from the beginning of the rain event using the following methods: (1) we multiplied the measured suspended sediment concentrations by the stream flow for different blocks of time; (2) we developed individual stream flow – concentration regression relationships for each event; and (3) we developed a stream flow – concentration regression relationship using data collected over a 10 year period. We then measured the radioactivity of beryllium-7 and lead-210 that are naturally present in rainfall and attach to sediment moving across the landscape. We used a simple two end-member mixing model to determine how much eroded field soils and stream bank sediments were in the suspended loads of each event. The total sediment fluxes from the measurement-based values and individual event relationships were similar, within 10%, because they accounted for the changes in the relationship between stream flow and suspended sediment concentration during a rainfall event known as hysteresis. The long-term, stream flow-sediment concentration relationship did not account for these changes and under-estimated the loads of the first two events while over-estimating the load of the flood event. We quantified that 67% of the sediment carried in the stream during the first event were eroded upland field soils and was attributed to a "first flush" of readily available material from past events. For the second and third events, the amounts of upland field soils were 34% and 21%, respectively, because less material was readily available for mobilization. During the third event, the flash flood, stream bank collapse was observed and bank erosion estimates from multiple methods compared favorably with the load results. The results provided by the radioactivity measurements were confirmed with the changing hysteresis observed in the different events, as well. By quantifying the dominant sediment sources in watersheds, watershed managers can target more accurately the areas, which need BMPs to control sediment-related problems. Additionally, this research was transformative because it provided sediment flux and stream flow data for extreme events, like flash floods. These rare data can lead to better water quality models used to develop sound management strategies. In summary, we better understand how sediments and soils are mobilized by flash floods and identified the areas that significantly contributed to non-point source pollution in terms of sediment budgets. The broader impacts of this study are substantial, especially when considering climate change predictions in the U.S. Midwest call for increases in precipitation totals and rainfall intensity by the end of this century.
