Difficulties of Identifying Design Discharges in Steep, Coarse-Grained Channels in the Arid Southwestern US

Abstract

A study of three locations in a 16-km reach of the lower Verde River in Arizona demonstrates the problem of identifying generalized design flows in the arid and semiarid regions of the southwestern US, where flow regimes are highly variable and often non-continuous, where coarse bed material is supplied locally by steep ephemeral–flow tributaries, and where channel morphologies are forced by large, infrequent floods, canyon walls, tributary fans and alluvial terraces. At the three sites, channel slopes vary from 0.0027 to 0.0041, and the median size of the surface bed materials ranges from 81 to 145 mm. Hydraulic analysis (HEC-RAS) showed that, depending on the definition used, the bankfull discharge at the three sites ranges from 450 to 1,900 cms (1.7- to 8-yr RI), the critical discharge (� � = 1) for bed-material mobilization ranges from 398 to 811 cms (1.6- to 3.8-yr RI), and significant sediment transport (� � > 1.5) occurs at flows between 989 and 1,740 cms (4- to 8-yr RI). At any given flow above the critical discharge at one location, bed material can be immobile, in transport, or being deposited at any of the other two locations. Selection of flows for engineering design or restoration purposes in rivers with similar conditions is, therefore, highly constrained by local conditions, and site-specific geomorphic, hydraulic and sediment-transport analyses are required rather than generalized relations based on assumed relations between bankfull discharge, flood frequency, and effective discharge.