Flow-induced uprooting of young vegetation on river bedforms

Riparian vegetation stabilizes sediment by its roots and henceforth impacts riparian morphodynamics. After germination or vegetative reproduction on river bars or islands, juvenile plants are exposed to a high risk of mortality due to uprooting by floods. We distinguish two main types of root erosion by flow. As Type I root erosion, we defined a flow induced drag mechanism, which causes a nearly instantaneous uprooting of mainly very young vegetation with not fully developed root system by pullout drag exceeding root resistance. Type II root erosion arises as a combination of bedform erosion resulting in a decreased anchoring resistance of the roots and subsequent Type I uprooting. This second type applies to later stages of root development and is a delayed process induced by sediment erosion of morphodynamic origin. In laboratory experiments we tested the validity of both mechanisms. We investigated the first Type of root erosion mechanism with static uprooting experiments with 1550 seedlings of Avena sativa and Medicago sativa grown in low-cohesive sediment in order to quantify the distribution of their anchorage forces for different sediment size and moisture conditions as well as for varying root structure. Furthermore, we measured root strength of Avena sativa seedlings and compared pullout and breaking force of young vegetation with identical root structure. Type II root erosion mechanism, which is driven by the reduction of root anchorage due to sediment erosion, was investigated in laboratory flume experiments. The intensity of sediment erosion that was required before uprooting occurred increased with increasing root length. The higher the flow, the less time was necessary to erode a seedling of certain root length. Thus, the duration that a given flow requiresto erode a certain root structure, can be associated to a certain vegetation maturity stage. Following, Type II root erosion results from the balance of timescales of both vegetation growth and flood occurrence and duration.