Magnitude and Frequency Relations of Bankfull Flows and Bed-Material Entrainment in the Llano River Watershed, Central Texas, USA

Abstract

Bankfull channel geometry and bed-material entrainment are examined in the Llano River watershed in central Texas, which is characterized by mixed alluvial-bedrock boundaries and a highly variable flow regime. Field data, including cross-section surveys and sediment samples from 17 sites, are coupled with one-dimensional hydraulic models and flood-frequency analyses to compute recurrence intervals of bankfull and entrainment conditions. Recurrence intervals of flows that entrain median-sized bed material (d 50) are consistently less than 1 year, whereas flows that define bankfull morphology range from one to four years (median of 1.75 years). Recurrence intervals of flows responsible for incipient entrainment of relatively coarse bed material (d 84) are typically less than 1.5 years, but complete mobilization of this size fraction occurs less frequently than bankfull flows. At multiple sites, morphologic surfaces defining bankfull stage are discontinuous and set within a macro-channel, which is formed by high-magnitude flows with recurrence intervals greater than five years. Finally, the frequency of bankfull flow in the lowermost, confined, sandbar-dominated channels is similar to the frequency of the mobilization of relatively coarse low-flow channel gravels; but discontinuous, sandy bank deposits indicate that sediment exchange with channel bars is the most important process for development of bankfull indicators.