Influences of coarse bank roughness on flow within a sharply curved river bend

An experiment was performed to assess the influence of coarse bank roughness on flow within a sharply curved bend of the Ocklawaha Creek, a sand-bedded stream in northern Florida. This involved obtaining systematic measurements of flow velocity and water-surface topography when the outer bank was rough with natural vegetation, and obtaining an identical set of measurements after removing the vegetation and constructing a smooth wall along the outer bank. Results suggest that the roughness from bank vegetation systematically influences the flow field, particularly the secondary current strength and the position of the high-velocity core, because of its effect on the transverse boundary layer. The roughness essentially produces a backwater effect that inhibits outwardly directed surface flow from closely approaching the outer bank. This suppresses super-elevation on the outside bank and, therefore, weakens the inwardly directed transverse pressure gradient and secondary current. The flow is steered in a downstream direction, and the core of high velocity is nearly centered in the channel. In absence of roughness from vegetation, outwardly directed surface flows approach the outer bank more directly (and earlier in the bend), superelevation on the outside bank is enhanced, and the transverse pressure gradient and secondary current are strengthened. The core of high velocity is displaced toward the outer bank, and its magnitude is increased. Moreover, the streamwise position where the high-velocity core is closest to the outer bank shifts downstream from its position of closest approach in the presence of roughness. This, in principle, should contribute to asymmetrical bend migration, whereas migration in presence of roughness should be nearly in phase with bend curvature such that bends grow in amplitude, albeit slower, and with less asymmetry.