Reports: DNI851950-DNI8: Fine-Sediment Storage and Bypass in Sandy Fluvial Systems

Elizabeth Hajek, Pennsylvania State University

Fine sediment (silt and clay) can occlude porosity in fluvial reservoirs; however, it is currently difficult to predict which formations may be predisposed to having abundant fine sediment intercalated into otherwise sandy riverbed deposits. To develop a better understanding of the conditions under which appreciable fine sediment is deposited and preserved within sandy river beds, work on this project has focused on sampling and measuring particle-size distributions in ancient river deposits and developing a series of experiments to directly test how river suspended-load concentration affects preserved bed-material particle-size distributions (and hence reservoir potential quality).

In order to evaluate how river and floodplain characteristics vary with basin-averaged and local paleo-sediment supply, samples were collected from the well-exposed Wasatch Formation (Paleocene/Eocene, Piceance Basin, Colorado, USA). The three members of the Wasatch Formation have different channel deposit sizes, architectural styles, and overall sandiness (net-to-gross). Channel and floodplain facies in each member were sampled, disaggregated, and evaluated for particle size distributions. Channel bed-material deposits in the Shire Member show a sizable fraction of silt- and clay-sized material. These channel sand bodies also exhibit prominent levees and deeply incised basal scour surfaces. These results suggest that abundant bed-material mud content in Shire rivers was associated with generally cohesive channel banks and floodplains. Ongoing work is aimed at better constraining the bulk, unit-wide sand-mud ratio in each member, and improved understanding of how paleo-bed-material load relates to channel architecture.

The centerpiece of this project is a series of physical experiments designed to determine whether, and the degree to which, suspended-sediment concentration influences the amount of fine-grained material trapped and preserved in active sandy riverbeds. Experiments are being conducted at the St. Anthony Falls Lab (University of Minnesota). Experimental conditions are designed to broadly emulate bed- and suspended-load transport conditions in the modern White River (South Dakota) and Niobrara River (Nebraska). These rivers have medium-sand bed material and very high (White River, ~10,000 ml/L) and moderate (Niobrara, ~1,000 ml/L) suspended-sediment concentrations. During each experiment active bed-material and suspended sediment will be sampled, and after the experiment is completed the bed will be excavated and sampled. Currently pilot runs are being conducted to prepare the flume; the main experimental runs will be completed during October and November, 2013. Particle-size analysis of samples collected during experimental runs will be completed in December 2013 and January 2014.