Sediment sorting and transport by flash floods

Abstract

Flash floods partition the sediment load of arid and semiarid watersheds into components that travel at different rates through the fluvial system and are deposited in characteristic settings. This paper examines sediment sorting and transport by flash floods within a small, sand‐dominated alluvial valley in a semiarid environment, upper Los Alamos Canyon, New Mexico. Floods in the study area partition the sediment load into two distinct facies: a coarse‐grained facies that travels near the channel bed and a fine‐grained facies that travels in suspension. The particle size distributions of channel and floodplain deposits resemble the measured textures of the bed load and suspended load, respectively. Calculations predict that typical flows sort the load into the same two fractions observed in the field. Whereas the transport rate of the coarse fraction depends on flow transport capacity, transport of the fine fraction is controlled by its supply. The long‐term discharge of both fractions is estimated by integrating instantaneous transport relationships over the probability distribution of flows. Over several decades the computed fluxes of the two fractions are approximately the same. Most of the load in both fractions is transported during small to moderate events that occur more than once per year. However, the two fractions are distinct because they are supplied by different sources, transported by different mechanisms, and stored in distinct locations in the valley floor. Sediment sorting by flash floods is an important mechanism in constructing floodplains and in determining the residence times of various particle size classes in alluvial valleys.