CONVECTIVE ACCELERATIONS AND BOUNDARY SHEAR-STRESS OVER A CHANNEL BAR

Alternate bars are important features in alluvial channels as they determine flow and transport patterns. They appear fundamental to selection of meander wavelengths and the geometry of bends. Bend flow has been studied extensively: far less study has been made of flow over alternate bars. Field results from Solfatara Creek, a 5.2-m-wide, 0.2-0.7-m-deep gravel bed channel where flow exits an upstream bend and shoals over a bar in a straight reach, are used to examine patterns of flow and the fluid forces determining the flow field. Large cross-sectional area changes, tied primarily to variation in depth, force large stream-wise accelerations and substantial cross-stream flow off the central bar. The topographically driven downstream and cross-stream accelerations are sufficiently large that their influence upon the balance of forces is of the same order as the pressure gradient and the boundary shear stress. The importance of convective accelerations in the downstream flow equation in this straight reach concurs with bend flow results, but the similar importance of convective accelerations in the cross-stream equation contrasts with results from bend flow. While part of the difference may be attributed to the lower stage conditions herein, in the absence of significant curvature change the cross-stream force balance depends upon the flow going over and around the bar. Local boundary shear stress estimated from the law-of-the-wall and a roughness algorithm decreases out of the upstream bend, increases over the bar top to values approaching the threshold for motion, and then decreases in deeper flow. Strong bed surface coarsening maintains the topography in a stress field that would otherwise lead to planation of the bar top and filling of the deeper regions.