Reconstruction of a flash flood with large wood transport and its influence on hazard patterns in an ungauged mountain basin

The reconstruction of past flash floods in ungauged basins leads to a high level of uncertainty, which increases if other processes are involved such as the transport of large wood material. An important flash flood occurred in 1997 in Venero Claro (Central Spain), causing significant economic losses. The wood material clogged bridge sections, raising the water level upstream. The aim of this study was to reconstruct this event, analysing the influence of woody debris transport on the flood hazard pattern. Because the reach in question was affected by backwater effects due to bridge clogging, using only high water mark or palaeostage indicators may overestimate discharges, and so other methods are required to estimate peak flows. Therefore, the peak discharge was estimated (123±18 m3 s–1) using indirect methods, but one-dimensional hydraulic simulation was also used to validate these indirect estimates through an iterative process (127± 33 m3 s–1) and reconstruct the bridge obstruction to obtain the blockage ratio during the 1997 event (~48%) and the bridge clogging curves. Rainfall–Runoff modelling with stochastic simulation of different rainfall field configurations also helped to confirm that a peak discharge greater than 150 m3 s–1 is very unlikely to occur and that the estimated discharge range is consistent with the estimated rainfall amount (233± 27 mm). It was observed that the backwater effect due to the obstruction (water level ~7 m) made the 1997 flood (~35-year return period) equivalent to the 50-year flood. This allowed the equivalent return period to be defined as the recurrence interval of an event of specified magnitude, which, where large woody debris is present, is equivalent in water depth and extent of flooded area to a more extreme event of greater magnitude. These results highlight the need to include obstruction phenomena in flood hazard analysis. Copyright © 2012 John Wiley & Sons, LtdWe express our gratitude to the Spanish Ministry of Science and Innovation for financial support. This work was funded by the MAS Dendro-Avenidas project (CGL2010-19274) and the Geological Survey of Spain (IGME). We are grateful to the Tagus Water Authority, Environment Department of Castilla y Leon in avila, Caja avila, Asocio de avila and Navaluenga Council for their collaboration. Our special thanks to forester Jose Luis Galan for his assistance in the field. We would also like to mention Juan Ballesteros, Ignacio Gutierrez, Tasio Fernandez, Leticia Salas, Carolina Guardiola and angel Prieto for contributing suggestions and interesting discussions that significantly improved this paper. Thanks to the contribution of two anonymous reviewers, which improved the quality of the early version of this manuscript.Ruiz-Villanueva, V.; Bodoque, J.; Díez-Herrero, A.; Eguíbar Galán, MÁ.; Pardo-Igúzquiza, E. (2012). Reconstruction of a flash flood with large wood transport and its influence on hazard patterns in an ungauged mountain basin. Hydrological Processes. (9433):1-14. https://doi.org/10.1002/hyp.9433S114943