Alfredo Ollero

Channel Adjustments, Floodplain Changes and Riparian Ecosystems of the Middle Ebro River: Assessment and Management

In the middle Ebro River there is a 346 km reach, between Logroño and La Zaida, consisting of a free meandering channel on a wide floodplain. This reach contains a discontinuous riparian corridor, including valuable riparian forests and oxbow lakes. During the last 80 years this channel has witnessed substantial changes in channel morphology, area of bars, riparian vegetation and floodplain uses. The sinuosity growth, migrations and meander cut-offs have been frequent. There has been a progressive and significant decrease of both the area covered by water and the gravel bars without plant colonization. As a result of this, the riparian corridor width has been dramatically reduced for the benefit of human uses. The deceleration and nearly elimination of the Ebro channel free meander dynamics represents an important natural heritage loss. Dams, changes in land-use throughout the basin and river flood defences that restrict the channel have altered the system behaviour, which urgently needs a management plan combining both improvements and risk reduction.

Channel response to multiple damming in a meandering river, middle and lower Aragon River (Spain)

Small hydropower plants (SHP) affect river flow and sediment transport and thus impact river morphology. Eight hydropower schemes were studied along the meandering middle and lower reaches of Aragón River (Spain) to assess their effects on channel morphology and sediment dynamics from 1927 to 2010. GIS tools were used to measure changes in fluvial surfaces, channel planform and lateral and vertical dynamics. Three periods (early, middle and late twentieth century) were analysed to discern the effects of the main pressures, such as changes in land use, large reservoirs upstream and SHPs. Results were combined with field and topographical measurements and hydrological analysis. Active channel width and channel migration suffered a clear reduction in the whole period. They started as a consequence of land cover changes in the drainage basin, but their speed increased after a large reservoir was built upstream. More recent changes occurred since most of the SHPs were put into operation in the 1990s, especially in their short-circuited reaches and in the four more downstream ones. These changes are interpreted as a consequence of reduced discharge, transitory sediment trapping and reactivation of sediment transport after weirs became filled as well as by the impact of flood hydrology.

Channel Responses to Global Change and Local Impacts: Perspectives and Tools for Floodplain Management, Ebro River and Tributaries, NE Spain

In the last five decades, mountain and lowland rivers of the Iberian Peninsula have undergone noticeable hydrological and geomorphological change, in response to an overall reduction of discharge, floods and sediment supply. The causes are human-induced land use change, the building of reservoirs and gradual climate change. In lowland river floodplains, further significant impact comes from human interventions in the channels, such as the building embankments, in-channel gravel extractions and artificial meander cut-off. The case of the middle Ebro River and its tributaries (Aragon, Gallego and Cinca rivers) is very well suited to exemplify and analyse these processes and impacts, with morphological changes (incision, narrowing, simplification) and progressive reduction of channel migration and reduced presence of sediment bars. River migration was reduced from 15 m/year to values from 0 to 5 m/year since mid-twentieth century. Floodplain natural areas were reduced 40 % on average since 1927. In recent years, we have begun to seek solutions to mitigate these fluvial problems. Most of them are focused on floodplain management through the Fluvial Territory approach. Here we present this approach, the basic tools for its demarcation and the results of some actions of fluvial restoration already implemented. Four case studies of embankments removal are presented. Flood peak reduction was detected and morphological effects are being monitored. These initial actions could lead to new river management practices with improved river dynamics.

Geomorphology and river restoration: dam removal monitoring in Gipuzkoa

espanolSe presenta la metodologia disenada para el seguimiento geomorfologico del derribo de azudes, asi como los resultados obtenidos hasta el momento en dos casos concretos: las presas de Mendaraz (rio Urumea) e Inturia (rio Leitzaran) en Gipuzkoa. Se han realizado secciones transversales, abundantes mediciones de procesos sobre testigos, analisis de los nuevos depositos generados y muestreos granulometricos y morfometricos. En el caso de Mendaraz se han registrado rapidas movilizaciones de sedimentos e importantes cambios geomorfologicos aguas arriba y abajo del obstaculo. Este proceso de recuperacion de la dinamica fluvial natural y de regularizacion del cauce fue favorecido y acelerado por la crecida extraordinaria de noviembre de 2011. Ademas de sus beneficios para el estado ecologico, se ha constatado que el derribo de presas es una medida eficaz para la restauracion de la dinamica geomorfologica en cauces fluviales. El seguimiento geomorfologico es una labor fundamental para cuantificar y valorar la dinamica generada a raiz del derribo de la presa y toda su evolucion posterior. EnglishA 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.

Geomorphic classification of rivers: a new methodology applied in an Atlantic Region (Galicia, NW Iberian Peninsula)

The classification of rivers based on geomorphological criteria played, in the past, a secondary role for management decision making, although in the last years they have experienced a shift from the scientific field to that of the technical. Currently, managers require the most simplified form of classifications in order to use them in plans and planning projects, management and restoration. On the one hand, this means that classifications should be directed toward simplifying the diversity of fluvial environments in a number of manageable types, and on the other hand, to apply to each geomorphic type a management model. In this study, we have developed a method of “Geomorphic Classification of Rivers” according to specific stream power variables and median grain size. The new method is dynamic (ability to readjust and gain robustness with the incorporation of new data) and predictive. We obtained six types directly from the method, although we added a seventh type (bedrock rivers) for its special singularity. Each geomorphic type presents concrete values of specific stream power and median grain size, which additionally involves a hydraulic geometry adjusted to terms of balance. The change from a geomorphic type to another would reflect a geomorphologic imbalance in the form of greater power and sediment size. Our results have been compared with six commonly used classifications (e.g., Rosgen Classification System or River Styles Framework). “Geomorphic Classification of Rivers” stands as a flexible tool that allows the development of a “personalized” geomorphic classification for rivers of the same geomorphological province. Through the temporary revision of various sites that act as control points, we can learn, should the case arise, the intensity and geomorphic change of the site. “Geomorphic Classification of Rivers” acts as an alert system for any geomorphologic disturbance. Its simple application and interpretation facilitate the implementation in the administrative environment, or its attachment to other commonly used classifications.