Scour and fill in steep, sand-bed ephemeral streams

Abstract

The traditional idea that entire long reaches of alluvial stream channels in semiarid regions are scoured at high flood discharges and subsequently filled in the waning flood phase (mean-bed scour and fill) can be challenged. The alternative concept that mean-bed elevation varies but little during a flood and that both scour and fill occur concurrently at different migrating loci within a reach (local scour and fill) is also consistent with published field data. Field and laboratory investigations reported herein suggest that mean-bed scour and fill in a straight uniform channel is minor compared to local scour and fill caused by bedform migration and, furthermore, that maximum local scour and fill may occur during the waning flood phase in some instances. The field experiment, utilizing a rectilinear array of maximum-scour indicators (scour-cords), produced data for contoured plots of maximum scour and fill in an ephemeral stream bed during two floods. In the first flood, 24 cm of scour and fill was measured for a bankfull flow depth of 23 cm. In the second, maximum scour and fill was at least 66 cm for a bankfull flow depth of 34 cm. Estimates of antidune amplitudes for the two floods, based on theoretical models and laboratory and field observations, are 27 to 61 cm and 44 to 92 cm, respectively. This indicates that all scour and fill measured by the scour-cord array could have been caused by antidune migration. Laboratory experiments were conducted in an 18-m long non-recirculating flume with automated controls for rates of sediment and water input. A series of experiments in a 26.7-cm-wide sand-bed channel with rigid walls, at grade for a simulated flood patterned after those typical of ephemeral streams, showed that mean-bed scour and fill was less than 3% of local scour and fill. For these experiments, mean sand size was 0.3 mm, channel slope was 0.009, maximum water depth was 40 mm, maximum local scour and fill was 22 mm, and maximum mean-bed scour and fill was 0.6 mm. Maximum mean-bed elevation variation was thus only two sand-grain diameters. Maximum local scour and fill took place near the end of the simulated floods, when bed-form amplitudes were the greatest. Antidune amplitudes calculated for flows in the Arroyo de los Frijoles, New Mexico, are larger than published values of scour and fill for unit discharges greater than 0.5 m 3 /m · s (5 cfs/ft). Below that threshold discharge, the antidunes that formed at maximum flood discharge are smaller than the dunes that probably form during the waning flood, and calculated antidune amplitudes are less than reported scour and fill.