HR: 12:05h
AN: H52A-08    [Abstracts]
TI: A New Method for Identification of Tributary Sediment Sources using Hydroacoustics
AU: * Wright, S A
EM: sawright@usgs.gov
AF: U.S. Geological Survey, 2255 N. Gemini Dr., Flagstaff, AZ 86001 United States
AU: Topping, D J
EM: dtopping@usgs.gov
AF: U.S. Geological Survey, 2255 N. Gemini Dr., Flagstaff, AZ 86001 United States
AU: Melis, T S
EM: tmelis@usgs.gov
AF: U.S. Geological Survey, 2255 N. Gemini Dr., Flagstaff, AZ 86001 United States
AB: Identification of tributary sediment sources is important in geomorphologic studies because different tributaries within the same basin may deliver sediment with widely varying properties. In particular, tributaries may deliver sediment with very different grain-size distributions due to differences in lithology between tributary drainages. These differences have important implications for understanding the link between tributary sediment supply and main channel morphology. Hydroacoustic instrumentation has become popular in recent years for the study of sediment-transport processes in both marine and fluvial environments. Because suspended material scatters and attenuates acoustic energy, transducers designed to record the backscattered energy can be used to infer the suspended sediment concentration. The transducer records the energy received from direct backscattering from the particles, which is reduced by transmission losses that occur as the wave travels through the medium. These transmission losses are composed of: 1) geometrical spreading, 2) attenuation of energy by the fluid, and 3) attenuation of energy by the suspended particles. The backscatter and particle attenuation are functions primarily of the wave frequency, concentration of particles, and size of particles. Thus, for a known frequency and particle size, it is possible to invert the acoustic signal to determine particle concentration. Here, we present a new method that takes advantage of the relationship between particle attenuation and particle size in order to identify tributary sediment sources. The study site is on the Colorado River below Glen Canyon Dam, where a sideward-looking acoustic instrument has been bank-deployed since August 2002. Suspended-sediment samples were collected and a relationship was developed between suspended fine material (silt and clay) and particle attenuation (R²=0.98). However, substantial deviations occur from this relationship during flooding events from certain tributaries. These deviations can be directly traced to differences in the grain-size distribution of samples from these tributaries versus samples from the main supplier of fine sediment (the Paria River). They are relatively easy to recognize because the relationship between attenuation and grain size is highly non-linear (i.e. small changes in grain size lead to large changes in attenuation). Recognition of these deviations is critical in the context of continuous monitoring for a sediment budget, because the acoustic calibration predicts highly erroneous concentrations during these tributary flooding events which must be corrected using manual samples or other in-situ instrumentation (e.g. optical transmissometers/grain-size analyzers).
DE: 1625 Geomorphology and weathering (0790, 1824, 1825, 1826, 1886)
DE: 1848 Monitoring networks
DE: 1862 Sediment transport (4558)
DE: 1895 Instruments and techniques: monitoring
DE: 4863 Sedimentation (1861)
SC: Hydrology [H]
MN: Fall Meeting 2005