Factors influencing the calculation of periodic secondary circulation in a tidal river: numerical modelling of the lower Sacramento River,USA

Secondary circulation is the component of three‐dimensional (3D) flow in river channels perpendicular to the primary flow direction. Secondary circulation calculated from acoustic Doppler current profiler (ADCP) transects is sensitive to the calculation method and is affected by the transect angle relative to the mean flow direction and variations in the flow direction along a transect. To quantify bounds on transect alignment relative to river flow for field data collection and examine tidal time‐scale variability in secondary circulation, the 3D hydrodynamic model UnTRIM was applied to simulate the hydrodynamics in the lower reach of the Sacramento River (CA, USA). Secondary circulation was calculated using the Rozovskii and the zero net discharge methods on repeated transects extracted from the model results in regions of both relatively uniform and complex flows. When the depth‐averaged flow direction along a transect varied by more than about 5 °, occurring when the transect was as little as 10 to 20 ° out of normal to the mean flow direction, the Rozovskii method produced more realistic secondary circulation than the zero net discharge method. Analysis indicated that ADCP transects should be within 20 ° of perpendicular to the mean flow direction when calculating secondary circulation. Secondary circulation strength around two tidally influenced bends generally increased with increasing flow and broke down near slack water. However, the strength of the secondary circulation was not only a function of the flow magnitude, but also depended on the direction of the water flow and the transect location relative to the river curvature, which varied with the tidal flow direction. Copyright © 2015 John Wiley & Sons, Ltd.