Mediation of hydro-geomorphological process effects on flooding by vegetation in dryland channels

Vegetation effects are particularly significant in dryland region channels, where flooding is mainly as flash floods, because large vegetation commonly grows within the channels. Measurements at long-term study sites in ephemeral channels of southeast Spain show that such vegetation can be highly resilient to a range of flows.  However, extreme events, such as occurred in September 2012, can effectively zero the vegetation in the channels and valley floor.  The effects of the vegetation in mature pre-flood and in sparse post-flood state on the hydraulics of flow and on flood levels have been measured in the field and calculated using a range of assumptions on roughness effects.  The vegetation state is shown to have large effects on flow velocities and on flood stage and inundation extent. Since that extreme event the recovery of vegetation has been measured annually.  Differential rates of recovery are evident between sites and within sites, varying with spatial position in relation to the main channel. In some locations vegetation has regrown to pre-2012 states but elsewhere occurrence of a series of moderate to large flows has restricted growth.  Drought conditions have also occurred over the past decade and in earlier periods, affecting growth. The vegetation dynamics have complex interactions with the sediment and morphological changes in these channels and together they contribute to variations in flood capacity and hydraulics of flow. Feedback effects through erosion and deposition processes are identified. Results of modelling the effects of the different vegetation coverage, assemblages and heights of plants are discussed. Effects of timing of flows and of hydrological balances, countered by extreme temperatures, are analysed. It is shown that the dynamics of vegetation, through succession of different size events and varying conditions, have a significant effect on flood levels and spatial patterns in ephemeral channels and these need to be incorporated in flood modelling and predictions.  Consideration of flood management strategies needs to recognise that presence and dynamics of vegetation can pose both challenges and benefits, especially in highly erodible catchments with very high sediment fluxes and under conditions of climate change.