UNCONFINED FLOWS CAUSE UNCONSTRAINED THINKING: 'SHEETFLOODS' SHOULD BE OUTLAWED

The term ‘sheetflood’ is common in sedimentological reports of ancient alluvial systems. The term appears, often unreferenced, as though it is well-defined and firmly rooted in fluid dynamics theory and observation of modern environments. But this term is unrecognized by geomorphologists and hydrologists, and the literature is riddled with contradictory use, sometimes even within a single article. Often, it is not clear if the term is being used to embrace a set of processes, as implied by the colloquial usage for laterally unconfined (i.e. sheet-like) flow of water down a slope, or to describe a particular deposit type. Articles frequently switch between both, ignoring established protocol to separate description from interpretation.

This might be dismissed as merely a matter of semantics were it not that ‘sheetflood’ has taken on a life in the sedimentology domain as a technical term that is intimately linked to a specific set of climatic and geomorphic parameters, most often a dryland alluvial-fan or ephemeral lake in a region dominated by convective rainstorms producing flash floods. Current usage implies the processes are restricted to drylands and there is something special about ‘sheetfloods’ that requires them to be distinguished from other instances of unconfined flow such as overbank flow onto a river floodplain. Circular reasoning is prevalent, with weak criteria for recognition of ‘sheetflood’ deposits being used to conclude the environment must have been arid.

Analysis of unconfined overland flow shows that the same processes claimed for ‘sheetfloods’ operate across a wide range of landscape and climatic settings. There is no subset that can meaningfully be split off into a ‘sheetflood’ category. Unconfined flows deposit a much wider range of lithofacies than typically claimed. The criteria quoted as diagnostic are not reliable indicators of the setting or the imagined processes. Previous reviews have separated ‘sheetflood’ from the ‘sheetflow’ of geomorphologists, but did so with unworkable criteria that ignore the continuum of process and product. Greater precision is urgently needed: we recommend this term should not be used in sedimentological accounts, thus forcing better lithofacies description and more careful interpretation.