Introduction
The collection of accurate and reliable flow records in open channel flow is dependent on several factors of which a stable stage-discharge relationship is crucial.
Flow monitoring site and hydraulic conditions that can impact a stable stage-discharge relationship comprises of unstable section control, sediment transport, debris, vegetation, off-channel storage, variable backwater effects and unsteady flow conditions.
Variable backwater, off-channel storage and unsteady flow conditions are all hydraulic conditions that can have a significant impact on stage-discharge relationship and accurate flow calculations. Flood-wave movement, operation of irrigation canals, tidal effects, stream junctions and flood control measures are some examples of both variable backwater and unsteady flow conditions.
There are number of established methods in defining a stage-discharge rating curve effected by variable backwater, off-channel storage, and unsteady flow conditions. The methods consist of direct measurements, analytical investigation using simplified approaches, modeling using physical-based approaches, index-velocity method and continuous slope are method. This technical note focuses on the application of the index velocity technique using a bank mounted acoustic doppler velocity meter, SonTek SL1500-3G instrument.