Understanding the long-term increase of flood risk in regulated rivers through combined use of CFD, paleo-hydrology and hydrological data

Session A1: Environmental Fluid Mechanics - Theoretical, Modelling and Experimental Approaches

This paper examines the increase of risk of flooding for a given size flood in urban areas upstream of dam as a consequence of in-channel sedimentation and silted-up reservoir over time. We selected one of the earliest gauged, dammed rivers in Spain: the Guadalquivir River. Our study combines high-resolution shallow water numerical simulations, characterization of paleoflood deposits as sedimentary sequences and botanical high-water marks, as well as hydrological instrumental data taken during large floods with return period of approximately 100 yrs. The co-existence of gauging records and in-situ imagery of water levels for more than one century allows us to calibrate the numerical model in two scenarios: (i) pre vegetation encroachment, and (ii) post vegetation encroachment due to upstream impoundment. Subsequently, the increase of fluvial risk was quantified with numerical simulations which accounted for in-channel sedimentation and spread of vegetation due to downstream impoundment. Numerical results were corroborated with inundation images from Terrasar-X satellite and paleoflood deposits. The main cause of reduction of channel capacity over time were identified, namely: in-channel sedimentation due to silted-up reservoirs, and the interaction between vegetation encroachment and sedimentation with in effect a positive feedback relationship. Similar to backwater effects that developed during floods at subcritical regime, channel aggradation started with silted-up reservoirs and extended further upstream up to urban areas faraway. Channel aggradation had a notorious impact on floods, as reported in the pioneer work by Fergus (1997) and recently across the continental USA by Slater et al. (2015). Contrary to common belief, our results indicate that dams do not always prevent floods. It is found out that flood risk mitigation strategies and flooding maps need to account for changes in the stage-discharge relationship over time due to sediment loads, readjustment of channel morphology and ecology, particularly for numerical projections of long-term changes in flood risk or frequency resulting from climate change.