Abstract:
We describe a flume study of the spatio-temporal evolution of particulate gravity currents. Time series of the vertical structure of flow in terms of the forward component of velocity, flow concentration and grain-size of suspended sediment were co-measured at a different position along the flume for each of a series of nominally identical flows. The data are combined to show the spatial evolution of a single idealised flow. Such a flow, fed into the flume from an overhead reservoir, first propagates as a quasi-steady jet. The subsequent evolution of coherent spatial and temporal trends in velocity, grain-size and concentration are developed by the internal action of the flow itself, rather than being inherited from the flow generation mechanism. Quasi-steady input currents evolved down flume into surge-type flows that wax very rapidly upon arrival then progressively wane. Thus the velocity history of currents at source may differ from that experienced downstream, indicating that flow steadiness measured (or interpreted) in downstream positions may not necessarily be indicative of the flow generation mechanism. The fore-most parts of the flow travelled more rapidly than the hind-most parts and thus gradually drew further away; as a consequence, the duration of the experimental currents systematically increased along the length of the flume (the flows 'stretched out'). Natural scale flows may therefore wane more rapidly in proximal regions than distal ones. This may impact upon sedimentation style, with higher sedimentation rates potentially more prevalent in proximal than in distal regions. Grain-size maxima are recorded in the head of the current. Convincing evidence of coarse tail lag behind the head is only patchily developed. Within the flow body a consistent pattern of upward fining then coarsening is observed. This can be related to an upward flux of coarse particles from the head, which subsequently settle downwards into the body. At the natural scale such a phenomenon may have implications for the development of coarse tail grading.
