This paper describes methods of determining peak streamflows in arid areas, where unstable channels and ephemeral flows characterise many streams. Where these exist, special methods of obtaining peak flows are needed. Usually, flow is determined from a recorded stage and a relational curve of stage and discharge. In unstable channels, it may be difficult to obtain a peak stage because the flow may move horizontally away from the gage, or the sediment in the flow may bury the stage sensor. It the peak stage is measured, the flow may be difficult to determine because of the unstable rating. As a result, the peak flow determination for streams with unstable channels commonly has a high degree of uncertainty. A variety of techniques are used to reduce the uncertainty of peak-flow data. A variety of instruments that are used to measure a peak stage, streambed elevations, and velocities can improve data collection in some, but not all, streams. Attempts can be made to stabilise some channels in the area of measurement. Systems that alert field personnel when stream flow is high allow time for site measurement of discharge and verification of stage measurements. Ephemeral streams create special problems in maintaining the equipment during the dry season so that it will operate during periods of flow. Also, human activities such as bridge maintenance in the dry channels can add to the instability of the channel.
Abstract