Predicting Riparian Seedling Removal: A Mechanistic Scour Model Supported by Lab and Field Evidence

Riparian seedlings growing on river bars, islands, and floodplains create feedback processes with hydraulics, sediment transport, and channel morphodynamics. For example, intact seedling uprooting by the flow typically requires some degree of sediment scour before seedling removal. Estimates of the critical scour depth (Lc) that allows for individual seedling uprooting are often based on the resisting root force (FP). We develop a new mechanistic FP equation that includes root-sediment friction from different grain size distributions, root irregularities, and lateral root areas. Simple laboratory experiments demonstrate that root-sediment friction coefficients from previous studies may be orders of magnitude lower than those that occur in riparian areas with coarse grain size distributions. We test different forms of our equation using field measured FP to find that accurate FP predictions require: (a) a range of root-sediment frictional effects, (b) declining taproot diameter with below ground distance, and (c) root hair effects. Combining our FP equation with simple flow calculations, we calculate that Lc for an individual seedling is not constant as often assumed but depends on the bed grain size distribution, flow conditions, aboveground seedling patch density, and belowground and aboveground individual seedling biomass properties. We calculate similar Lc for invasive and native seedling species, suggesting that invasive seedling removal through targeted high-flow dam releases requires careful consideration of potential effects on native seedlings.