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Effects of global change on hydro-geomorphological hazards in Mediterranean rivers

Final Report Summary - FLOODHAZARDS (Effects of global change on hydro-geomorphological hazards in Mediterranean rivers)

Rivers are dynamic and complex systems that transfer water and sediment from headwaters to deltas, estuaries and ultimately seas and oceans; they are organized through channel networks that facilitate the conveyance of water, sediments and associated components (e.g. pollutants). River channels are formed by sediments that have experienced episodes of entrainment, transport and deposition. These episodes are driven by natural disturbances such as floods. Floods cause the inundation of exposed areas (floodplains) that may be occupied by human activities. The severity of these inundations is not only directly related to the magnitude of the flood event, but also to the degree of modification of river channel geomorphology and the extent to which floodplains have been modified and occupied. Therefore, floods are seen as a major hydro-geomorphological hazard for society.

The present project aims to predict and assess the hydro-geomorphological hazards associated with global change in Mediterranean regions by coupling hydro-sedimentological and hydraulic-geomorphological models at different spatial scales, both basin and reach scale. This way, the project can be used to assess on the effect of channel metamorphosis on thresholds for flooding hazards associated with past and current situations as well as predicted global change scenarios. By following that coupled approach it can be also assessed whether an attenuation of the global change effects on hydro-geomorphology is observed or not by increasing the basin size. Key focal points for the project are i) understanding the evolution of the river features which determine the hydro-geomorphological hazards associated with flooding (i.e. water, sediment, channel deformation) in the study catchments, and ii) how these hazards will change as a result of both predicted climate (rainfall and temperature) and human-based (e.g. increasing urban areas or forest in headwaters) changes. Three interrelated objectives (O) will be addressed: i) O1. Analyse the historical conditions of selected catchments, their hydro-morphological patterns at the catchment scale, the transfer of water and sediments through the catchments and the effects at critical basin outlet points (reach-scale); ii) O2. Analyse the contemporary conditions of the catchments, their hydro-morphological patterns at the catchment scale, the transfer of water and sediments through the catchment and the effects at critical basin outlet points (reach-scale); iii) O3. Elucidate the causes of change between historical and contemporary patterns and use this understanding to forecast the impacts of global change on flooding hydro-geomorphological hazards. The project has studied 3 different nested basins in Spain affected by different anthropogenic impacts. Two of them were in the Iberian Peninsula, the Isábena (i.e. 445 km2) and the Ésera (i.e. 1,535 km2), and one more in the Balearic Islands, the Na Borges (i.e. 319 km2).
Floodhazards assesses, besides whether global change could alter flooding hazards in Mediterranean rivers (either increasing or decreasing it), i) the effect of nestedness (i.e. whether an attenuation of the global change effects on hydro-geomorphology is observed or not by increasing the basin size), ii) the effect of the different degree of ‘Mediterraneity’ of catchments characterized by different flow regimes, from snow-fed (i.e. the Rivers Isábena and Ésera) to purely-pluvial-fed with different hydrological regimes (i.e. the River Na Borges), iii) whether global change could also modify (either increasing or decreasing) the problems suffered by reservoirs threatened by different issues (i.e. Barasona), and iv) how climatic, environmental and socio-economical impacts can affect to water resources and flooding risks due to an increase of the exposure. Project is very important because of the urgent need to refine and implement the River Basin Management Plans (RBMPs) and water management policies required by the EU Water Framework Directive (WFD). RBMPs have yet to be implemented in Spain, so the project will greatly aid this process.

Description of the main results:
A comprehensive hydro-sedimentological dataset for the Isábena catchment, NE Spain, for the period 2010-2016 was compiled and uploaded to a public data repository, as well as published in a top-level SCI-listed journal. It was implemented for being available to anyone who wants to analyse water and sediment fluxes in a Mediterranean meso-scale catchment. The dataset contains an extensive data set of meteorological, hydrological and sediment data including time series, individual meterings and spectral reflectance data from multiple monitoring and sampling points throughout the Isábena river basin. The data will help for advanced understanding of water and sediment fluxes and budgets and related sediment connectivity processes and phenomena in mountainous Mediterranean catchments. In its high spatial and temporal resolution and comprehensiveness, the data set is expected to serve as a highly valuable benchmark for evaluating, calibrating and further developing hydro-sedimentological catchment models.
Suspended sediment loads and dynamics together with the effects of the Barasona reservoir on the Ésera–Isábena river fluxes (i.e. suspended sediment, C and N) were studied. The Barasona reservoir causes a considerable impact on the Ésera–Isábena river fluxes, reducing them dramatically as almost all the inputs are retained within the reservoir. Despite the very dry study year (2011–2012), it can be calculated that almost 300,000 t of suspended sediment were deposited into the Barasona Reservoir, from which more than 16,000 were C and 222 t were N. These values may not be seen as remarkable in a wider global context but, assuming that around 30 hm3 of sediment are currently stored in the reservoir, figures would increase up to ca. 2.6 × 106 t of C and 35,000 t of N. Floods are still responsible for more than 70% of the total suspended sediment load, indicating the importance of baseflows in the export of sediment from the basin. This fact is related to the high sediment availability, acting the Ésera-Isábena as non-supply-limited catchments due to the high productivity of the sources (i.e. badlands).
The effects of the Barasona dam on the flow regime, flood competence and bed disturbance (including topographic changes and bed mobility) in the Ésera-Isábena rivers were assessed by analysing flow and sediments dynamics. The flow regime of the Ésera-Isábena has been modified from nivo-pluvial regime, typical of humid mountainous environments, to that observed in dry-semiarid regions, in which high magnitude but low frequency floods are the dominant processes; this, together with the sediment deficit associated to the presence of the dam, have affected the channel morphodynamics downstream from the dam, showing notably physical changes, with channels clearly less dynamic below the dam. Moreover, the Ésera-Isábena rivers downstream from the dam experience no regular disturbance, but occasionally competent floods associated to dam operations, wash out the remaining gravels in the channel and scour the remaining relict forms. These processes leave a cobbled boulder bed, which has very little geomorphic similarities with its upstream counterpart. A hydraulic two dimensional numerical models was applied to study the morphological evolution of a specific river reach of the Ésera River. That reach was selected due that major morphological changes had occurred because of a massive extraction of sediments from the river bed, which extracted a complete massive gravel bar that was located in the centre of the reach. Results obtained for the evolution of the reach mimicked the natural behaviour of the river by forming an incipient new bar where the former was located.
When faced with unsatisfactory model performance, modellers may try to improve the model by addressing selected causes for the model errors. This leads to implementing certain “model enhancements” (MEs); however, deciding on which MEs to implement remains a matter of expert knowledge, guided by some sensitivity analyses at best. After implementing multiple MEs, a resulting improvement in model performance is not easily attributed, especially when considering different aspects of this improvement. To address that issue an approach for comparing the effect of multiple MEs in the face of several improvement aspects was proposed. The method was demonstrated by employing the mesoscale hydro-sedimentological model WASA-SED to the Isábena River. Built on extensive previous studies, different MEs were selected as most relevant enhancements to potentially improve model performance based on an existing base model configuration. The resulting effect of adding or withholding these ME on performance metrics was very diverse and depended on the metric and context. For the case study, although the employed model WASA-SED is a semi-process-based model for data-scarce situations, the additional information contained in the added MEs never produced the same increase in performance as the calibration. This suggests that for this model, the information content of the MEs is still quite low when compared to compensating this by mere calibration. Consequently, even for this catchment with relatively good input data situation, sufficient observations of the target variable are paramount for high model performance.
Suspended sediment transport and dynamics of two nested agricultural lowland Mediterranean catchments with a difference of two orders of magnitude in the surface area (i.e. FG1, 1 km2 and FG2, 264 km2), both included within the Na Borges catchment (Mallorca), were assessed. A detailed analysis of the rainfall–runoff–sediment transport relationships shows that the hydrological and sedimentological responses were extremely variable for both catchments. The hydrological response of the catchment was extremely variable, with the runoff coefficients ranging between 1% and 100% in FG1 and between 0.1% and 6.5% in FG2. The sedimentary response was also highly variable, with suspended sediment (SS) loads varying between 13 and 8,576 kg in FG1 and between 192 and 175,822 kg in FG2. After analysing the coincident floods at both catchments, it has been proven that the small catchment exerts a reduced (or even negligible) effect over the hydro-sedimentary response of the larger catchment. Unless some more tributaries are active, FG1 is not able by itself to generate a flood large enough to reach FG2. This is provoked by two issues: (a) the reduced sediment availability and (b) the disconnection of the sediment pathways because of the extensive work on soil conservation and sediment control since the Roman Age and the usual natural sediment re-deposition in different catchment compartments.
Flash floods and debris flows are iconic hazards in many mountainous regions prone to steep relief, high rainfall intensities, rapid snowmelt events, and readily available sediment; however, climate change can modify completely that; yet, an extremely heavy rainfall of up to 140 mm in four hours on 29 May 2016 (i.e. return period of between 200 and 1000 years) caused a number of destructive flash floods and debris flows in several municipalities in southern Germany, being the town of Braunsbach the most severely damaged, where a mix of water and some 42,000 m3 of sediment was responsible for destroying numerous buildings, cars, and town facilities (ca. €104 millions). A deep hydrological and geomorphic analysis of this event along its source catchment in the Orlacher Bach was carried out. The analysis showed that the high sediment concentration resulted from some 48 landslides, numerous locations of river-bank erosion, and remarkable (~2 m) channel-bed incision. It was estimated 40,950 m3 of non-organic natural sediments were carried in either suspended sediment or bedload (yielding more than 14,000 Mg/km2, among the highest documented in the world, although sourced from a humid temperate area of moderate relief), ~82% of which could be identified as bank erosion and channel incision while ~15% came directly from landslides. After comparison with similar-sized catchments around the region, the event happened in the Orlacher Bach was rather quite unique due to its high rank in landform characteristics within its drainage zone, such that in terms of the slope, planform and profile curvatures. Compared to similar past events and catchment conditions throughout the world, it was found that the landslide-triggering threshold is well above the 80th percentile; moreover when compared with other extreme flash floods in Europe, as well as when annual rates are considered, the event can be classified as extraordinary.
Over short and long timescales, wildfires can be an important cause of hydrological and geomorphological change; it is also considered one of the most important land use change as well as main driver of global change in Mediterranean environments. Such kind of rivers are part of a fire-prone and high-energy environment in which the timing of major storms in relation to fire influences the impact on fluvial systems; accordingly, the identification of major sources, stores and fluxes of sediments is essential for providing more effective post-fire management strategies. The study focused on two small and steep sub-catchments of the Na Borges river, where just one of the catchments was partially affected by a wildfire. The pre-fire dynamics indicated that surface soils were the main sediment source in these ephemeral creeks. Post-fire sediment dynamics were characterised by a single flood event with a short recurrence interval (i.e. return period ca. <1 year). Sediment generated from the burnt area contributed 12% on average to bed-stored sediments within the burnt catchment, which reduced downstream to 5% along the main channel of the Na Borges River.

Potential impact:
Floodhazards is a research project with a direct applicability and a high impact in society, by describing, defining, quantifying and providing a new approach to assess on the effects of global change on hydro-geomorphological hazards to the civil population and infrastructures in Mediterranean rivers. It is obvious that this issue should be taken into account in river basin management through the application of the WFD. Water has been identified as one of the most important environmental challenges of the early 21st Century, with a strong focus on society affectation. The WFD explicitly recognises the hydro-geomorphological hazards associated with global change and it is vital that management decisions in this regard are based on sound scientific principles that are underpinned by high quality research. Moreover, the WFD requires that all coastal and inland waters reach ‘a good ecological status’ and it was expected to be fully implemented by 2015, but it has not been implemented yet, at least in Spain. The integrated approach proposed will address this critical need, which is a topic of wide relevance in my home country with a large history of infrastructure and civil affectations by flooding (very important along the whole Mediterranean basin too). Thus, managers from both the industry (e.g. dam manages, Hydropower companies) and public sectors (e.g. Water Authorities) and governments have taken clear advantage of the results obtained by the project. Results have been already (or will be) delivered to the relevant and interested Agencies (i.e. Ebro Water Authorities, General Direction of Water Resources of the Balearic Islands). All that is supposed to be key for them as they include ideas, clues and advices that will help them to minimise flooding impacts to the society while preserving the river’s morphological and ecological functioning as well as to develop their RBMP.
Floodhazards, between all the other impacts already mentioned, has generated i) An effective co-operation between research disciplines; ii) A promotion further co-operation between research and environmental public agencies; iii) A reinforcement of the existing linkages between European researchers and research centres in the provision of ‘proximity support’ to researchers; iv) An enhancement of the knowledge transfer and information exchange within the EU, therefore promoting the European research excellence. The project has fostered the already existing collaboration between the University of Potsdam and the University of Lleida and has certainly extended and diversified the synergies between the groups. Other existing collaborations with the University of Liverpool and the University of the Balearic Islands have been reinforced and some new networks, as with the Polytechnic University of Valencia and the Helmholtz Centre for Environmental Research-UFZ, have been established. Therefore, the project has acted as a catalyst to strengthen and develop these links between research groups making a major contribution to the research efforts of this European scientific community.