Experimental investigation of flow expansions over fans

We present experimental evidence on the occurrence of large angles of flow expansion with no flow separation over depositional fans. The evolution of a number of self-formed experimental fans was analyzed using overhead images and detailed topographic surveys taken from a run of the Experimental EarthScape basin at St Anthony Falls Laboratory. Instability phenomena produce narrow, upstream migrating channels coupled to lobes with large opening angles and sheet flow over them. Most of deposition appears to occur relatively rapidly during the initial perturbed expansion phase. Our analysis confirms that steady conditions can develop over large angles of expansion (half angle over 20 degrees). The steady condition is maintained with the help of a downstream-decreasing transverse bed curvature (with a normalized average value of 0.1). The bed curvature transfers flow momentum toward the banks, preventing flow separation and maintaining the high divergence angle. The bed curvature thus appears sufficient to explain the common occurrence of unchannelized, simple fans with opening angles, and hence rates of bedload divergence, much larger than would be predicted from jet theory. Furthermore, at equilibrium conditions, the deposit displays a conical shape, well described by a linear relation. We have also analyzed fan development. An initial channel perturbation causes an upstream migrating scour, which leads to an abrupt downstream increase in width and a consequent heart-shaped deposit. In this phase, flow shows the largest divergence angles values (half angle up to 50 degrees) in the upstream fan part. The fan then reaches a final cone-shape, developing an unchannelized low depth-width ratio flow.