Suspended-Sediment Transport Rates and Recurrence Intervals at the Effective Discharge

Abstract

With renewed interest in rehabilitation and restoration of stream channels and the associated emphasis on bankfull geometry for channel design, characterization of sediment-transport conditions at the bankfull discharge becomes critical. Bankfull discharge has been ascribed various meanings and levels of importance over the past 50 years. However, in lieu of form-based estimates of the bankfull level, a flow of a given frequency and recurrence interval is more appropriate for the purpose of providing a consistent basis to compare transport rates in streams of varying stability and from different regions. Quantifying the effective discharge, (that flow that transports the greatest amount of sediment over the long term) serves this purpose. Historical flow and suspended-sediment concentration data from 475 sites in 17 ecoregions across the United States were analyzed in the context of determining the effective discharge, defined as the flow that represents the maximum product of flow frequency and suspended-sediment load. The average recurrence interval of the effective discharge for a given ecoregion based on the annual-maximum flow series ranged from 1.1 to 2.3 years. Data for a given ecoregion included measurements from both stable and unstable sites with drainage areas covering about four orders of magnitude. Based on these results, the use of the 1.5-year flow as an estimate of the effective discharge and a metric by which to compare suspended-sediment transport rates from streams draining watersheds of different size, geology, physiography and climate is appropriate. Data from more than 2,900 sites across the United States were analyzed in the context of estimating flow and suspended-sediment transport conditions at the 1.5-year recurrence interval (Q 1.5 ) discharge. Data were sorted into 84 Level III ecoregions to identify spatial trends in suspended-sediment concentrations and yields at the Q 1.5 . The highest median suspended-sediment concentrations occur in semi-arid environments (Southwest Tablelands, Arizona-New Mexico Plateau and the Mojave Basin and Range) owing to large quantities of available sediment, limited vegetative cover, and the flashy nature of runoff events. The highest suspended-sediment yields occur in humid regions with erodable soils and steep slopes or channel gradients (Mississippi Valley Loess Plains and the Coast Range).