Reduction of bend scour by an outer bank footing: Footing Design and Bed Topography

Protection of banks against erosion is an important but very expensive task in river management. The outer banks in river bends are most vulnerable to erosion and require an enhanced protection. This paper investigates, in an experimental flume, the efficiency of scour reduction and bank protection near the outer banks in open-channel bends by means of a horizontal foundation, called footing, protruding into the flow. First it is experimentally verified that bed mobility has a minor influence on the bed topography, which is mainly shaped by bend effects. Subsequently, the influence of the footing width and vertical elevation on the bed topography is investigated under clear water scour conditions. A maximum scour reduction of more than 40% was obtained with a footing placed at one-third of the maximum scour depth without bank protection and a footing width of about two-thirds of this maximum scour value. But a footing that is too narrow and/or not deep enough is vulnerable to underscour and subsequent bank failure. The experiments convincingly demonstrate the efficiency of this bank protection technique. The optimal footing parameters in the presented experiments should merely be seen as indicative, however, as they are expected to be case dependent

Modelling a torrential event in a river confluence

Torrential floods characterized by extremely high discharges and sediment transport rates are common in Mediterranean areas. Few field data is available for such floods due to sudden increase in water discharges and short flood duration. This paper presents the field data collected during a major flood event in a 90° confluence of two torrential rivers of the Mediterranean area. A two-dimensional model was used to describe the flow characteristics at the confluence. The influence on water levels of the torrential character of both rivers and some morphological processes as the tributary mouth gravel bar has been assessed. Although channel roughness coefficients and sediment deposits influence final water surface elevations they have lesser effect than the discharge ratio between main and tributary rivers

Geomorfología y restauración fluvial: seguimiento del derribo de presas en Gipuzkoa

Se presenta la metodología diseñada para el seguimiento geomorfológico del derribo de azudes, así como los resultados obtenidos hasta el momento en dos casos concretos: las presas de Mendaraz (río Urumea) e Inturia (río Leitzaran) en Gipuzkoa. Se han realizado secciones transversales, abundantes mediciones de procesos sobre testigos, análisis de los nuevos depósitos generados y muestreos granulométricos y morfométricos. En el caso de Mendaraz se han registrado rápidas movilizaciones de sedimentos e importantes cambios geomorfológicos aguas arriba y abajo del obstáculo. Este proceso de recuperación de la dinámica fluvial natural y de regularización del cauce fue favorecido y acelerado por la crecida extraordinaria de noviembre de 2011. Además de sus beneficios para el estado ecológico, se ha constatado que el derribo de presas es una medida eficaz para la restauración de la dinámica geomorfológica en cauces fluviales. El seguimiento geomorfológico es una labor fundamental para cuantificar y valorar la dinámica generada a raíz del derribo de la presa y toda su evolución posterior. A methodology for dam removal monitoring and the results of two case studies is presented: Mendaraz dam (Urumea River) and Inturia dam (Leitzaran River), both located in Gipuzkoa. This monitoring is conducted by river survey cross-sections, measurements of processes and granulometrical analysis. Fast sediment erosion and sedimentation together with geomorphological adjustments were detected after dam removal. In Mendaraz an extraordinary flood favored these processes. The recovery of fluvial dynamics shows benefits not only from the ecological point of view but also for the restoration of natural river dynamics. Geomorphological monitoring is a key tool to quantify and assess river evolution and dynamics after dam removal

Large wood debris that clogged bridges followed by a sudden release. The 2019 flash flood in Catalonia.

Study region: Francolí River (Catalonia, Iberian Peninsula). Study focus: The aim is the reconstruction of the October 2019 flash flood, that was documented through extensive field work: rainfall (300 mm in just a few hours), flood marks, times of flood passage and witnesses' snapshots and reports, channel changes, log drift (20,000 trees) and woody debris at bridges, as well as large damage and six fatalities. The methods are: hydrological model built for the rainfall-runoff in the basin and the flood routing in the river, use of hydraulic principles such as flow at waterfalls, flow against obstacles (trees), etc. and finally 1D/2D free surface numerical models. New hydrological insights: The uppermost 100 km2 produced discharges of 700 m3/s (up to 50 m3/s/km2, locally). Three bridges failed, but their cascading failure (when one failure triggers the next one downstream) was not proved. The main channel widened more than 10 times, dragging away soil and vegetation like a bulldozer. The resulting large wood debris that clogged two bridges worsened the inundation. An anomalous flow downstream, probably a surge of around 1090 m3/s, due to the failure of a woody jam at a narrow bridge, took two lives. Water Authority is now warning flood planners that vegetated, torrential basins may cause catastrophic floods in the valley towns, if their narrow bridges are sensitive to woody debris

Geomorfología y restauración fluvial: seguimiento del derribo de presas en Gipuzkoa

A methodology for dam removal monitoring and the results of two case studies is presented: Mendaraz dam (Urumea River) and Inturia dam (Leitzaran River), both located in Gipuzkoa. This monitoring is conducted by river survey cross-sections, measurements of processes and granulometrical analysis. Fast sediment erosion and sedimentation together with geomorphological adjustments were detected after dam removal. In Mendaraz an extraordinary flood favored these processes. The recovery of fluvial dynamics shows benefits not only from the ecological point of view but also for the restoration of natural river dynamics. Geomorphological monitoring is a key tool to quantify and assess river evolution and dynamics after dam removal.Se presenta la metodología diseñada para el seguimiento geomorfológico del derribo de azudes, así como los resultados obtenidos hasta el momento en dos casos concretos: las presas de Mendaraz (río Urumea) e Inturia (río Leitzaran) en Gipuzkoa. Se han realizado secciones transversales, abundantes mediciones de procesos sobre testigos, análisis de los nuevos depósitos generados y muestreos granulométricos y morfométricos. En el caso de Mendaraz se han registrado rápidas movilizaciones de sedimentos e importantes cambios geomorfológicos aguas arriba y abajo del obstáculo. Este proceso de recuperación de la dinámica fluvial natural y de regularización del cauce fue favorecido y acelerado por la crecida extraordinaria de noviembre de 2011. Además de sus beneficios para el estado ecológico, se ha constatado que el derribo de presas es una medida eficaz para la restauración de la dinámica geomorfológica en cauces fluviales. El seguimiento geomorfológico es una labor fundamental para cuantificar y valorar la dinámica generada a raíz del derribo de la presa y toda su evolución posterior