Rare Floods Research Articles

The Sources of Belief in Personal Capability: Antecedents of Self-Efficacy in Private Adaptation to Flood Risk.

Self-efficacy is one of the strongest and most consistent drivers of private flood mitigation behavior; however, the factors influencing self-efficacy in the context of flooding remain unclear. The present study examines three potential antecedents of self-efficacy: personal and vicarious experiences of floods or building-related events, social norms for private flood preparedness, and personal competencies such as technical abilities and social skills. While controlling for other drivers in a protection motivation theory (PMT) framework, these antecedents are tested as precursors of self-efficacy and intentions to improve flood resilience. Structural equation modeling is applied to conduct mediation analyses with survey data of 381 flood-prone households in Austria. Contrary to theoretical expectations, personal and vicarious experiences do not predict self-efficacy, presumably because rare flood events and changing hazard characteristics do not facilitate generalizable performance accomplishments. Social norms strongly and consistently influence self-efficacy, especially for actions observable by others, and also directly influence protective responses. Personal competencies increase self-efficacy and support protective action, particularly with regard to preventive and structural measures. The strength and direction of the antecedents of self-efficacy as well as of other PMT determinants vary between general and specific protective responses. This study provides important insights for risk managers, suggesting that interventions involving social norms and personal competencies can be effective in stimulating self-efficacy and, in turn, private flood mitigation. Interventions and research should clearly differentiate between general intention and the implementation of specific measures, and should address cumulative, synergistic, or tradeoff interrelations between multiple measures.

Numerical hydrodynamic researches for justifying design of the Nizhny Novgorod low-head hydraulic system

Introduction. A problem area of the Volga river between the Nizhny Novgorod hydroelectric power station and the city of Nizhny Novgorod has been surveyed, where unfavourable conditions for navigation, power generation, and safe living in the downstream are formed as a result of the landing level. The only solution to the problem is construction of a low-head hydraulic system (NNGU) that will reduce intensity of relief re-formations in the downstream of the Nizhny Novgorod hydraulic system and stop lowering of the bottom and level marks in this area. Purpose of this research is to study processes that occur upstream and downstream from the site of the facility to identify hazardous trends and develop practical solutions to minimize negative impacts; as well as a review of mathematical models conducted in this area for improving navigation conditions. Materials and methods. Materials of previous researches on this subject, pre-design engineering surveys and layout drawings of the designed hydraulic system are used. The researches have been performed with numerical methods using Stream 2D software package that is based on the two-dimensional differential equation Saint-Venant system. Options for low-flow conditions are considered, taking into account passing of the Nizhny Novgorod hydroelectric power station, as well as rare floods. Results. Plans for distribution of velocity modules and vectors are created, which show that construction of the low-pressure hydraulic system results in decrease in slopes and velocities of water in the problem area of the Volga-Kama cascade, as a result of which intensity of bottom deformations decreases. Rare flow passage demonstrated that difference in pools is insignificant, while, at the same time, flow of water along the left-bank floodplain passes more than believed before. Calculations of low-flow conditions demonstrated a number of deficiencies in the design, which are associated with insufficient throughput and uneven distribution of flow rates in the discharge area of the waterfront. Conclusion The results demonstrated a practical importance of using mathematical simulation with numerical methods in a two-dimensional formulation, which allow us to consider processes in more detailed manner and change the hydraulic system design in a timely manner.

Modelling of rare flood meadow species distribution by a combined habitat surface water–groundwater model

AbstractFloodplains are highly complex and dynamic systems in terms of their hydrology. Thus, they comprise a wide habitat heterogeneity and therefore harbour highly specialized species. For future projections of habitat and species diversity, process‐based models simulating ecohydrological conditions and resulting habitat and species distributions are needed. We present a new modelling framework that includes a physically based, surface water–groundwater model coupled with a habitat model. Using the model framework, we simulate the occurrence of 23 flood meadow plant species in a Rhine River floodplain. To benchmark the data, results are compared with a conventional approach with simple spatial hydrological information. Our results show that models with predictors obtained from the surface water–groundwater model are significantly more accurate for rare and endangered species, as well as for typical flood meadow species. A total of 15 hydrological predictors were defined, which look relevant and promising for a good prediction, but at the same time reflect very different hydrological conditions. The standard deviation of the groundwater level, wet soil conditions, and inundation belong to the most relevant predictors for an accurate prediction. Therefore, we recommend including more specific hydrological information in habitat models of species in complex floodplain ecosystems. Such spatial explicit habitat models can also open up further possibilities, such as the projection of global change impact studies or nature conservation planning.

Envelope curves for the specific discharge of extreme floods in Malaysia

The relationship between peak specific discharge and watershed area is examined for large, rare and extreme floods in Malaysia using a distributed hydrological model. Envelope curves for specific discharge for these events and the uncertainty is quantified. The relationships between rainfall duration and intensity as a function of watershed size were also examined. As a result, three main regions were defined to estimate the peak discharge as a function of watershed size for large, extreme and rare floods. The average magnitudes for the PMP and the world’s extreme rainfall events were 5 and 12 times larger than the 100-year event, respectively. The envelope curves may assist engineers and other interested parties to estimate the peak discharge for watershed up to 100,000 km2, especially for ungauged watersheds.

Calculating the Risk of Rare Floods

The first spatially realistic catalog of synthetic flood event risk across the entire United States uncovers high-risk areas and estimates the probability of another Katrina–level flood loss.

High resilience of aquatic community to a 100-year flood in a gravel-bed river

Our understanding of ecosystem responses to exceedingly large rare flood events is currently limited. We report the resilience of aquatic community to a 100-year record-high flood, and how it varies depending on levels of water pollution, in a fourth-order gravel-bed river in northern Japan. We used data on riparian landscape structure, channel morphology, and community structure of aquatic fauna, which were collected in sites with and without effluent before (1 month–3 years) and after (10 months) the flood. Carbon and nitrogen stable isotope ratios of consumers and basal resources were measured only before (1 year) the flood. We observed aquatic food web with introduced rainbow trout (Oncorhynchus mykiss) as the top predator, with variable relative contributions of basal resources and their pathways to the rainbow trout, under the effects of water pollution. Biofilm-originating dietary carbon became the more dominant resource, with a slightly shorter food-chain length in the polluted sites. The flood led to a loss of riparian forest and a substantial increase in the proportion of exposed gravel bars (5–24%). While the average river-bed elevation changed a little, the localized scours of river bed down to > 2 m were observed with lateral shifts of channel thalweg. Despite the landscape-level physical and structural changes of ecosystem, aquatic community showed a remarkably high resilience exhibiting negligible changes in abundance, except in the polluted site where only fish abundance showed a slight decrease. This study suggests that the abundance of aquatic organisms in gravel-bed rivers is resilient to a flood of unprecedented magnitude in recent history.

Asynchronicity of fine sediment supply and its effects on transport and storage in a regulated river

PurposeA disconnected ephemeral tributary was reconnected to the regulated River Ehen (NW England) as part of a river restoration initiative, providing a renewed delivery of sediment to a highly stable and armoured channel. This paper (1) assesses spatial and temporal dynamics of suspended and stored sediments in the Ehen, (2) characterises the composition of stored sediment, (3) develops fine sediment budgets for downstream river reaches, and (4) assesses the controls on the storage of fine sediment in the riverbed.Materials and methodsA 3-km study section in the upper part of the River Ehen was divided into two reaches. Suspended sediments were monitored at the downstream limits of each reach over a 2-year period. In-channel storage was measured in three morphological units within the upper reach, on 13 occasions over the same period. Samples were used to assess changes in volumes of stored fine sediment, as well as the grain sizes and organic content of the material. A time-lapse camera facing the confluence of the tributary was used to conceptualise different flow scenarios. These scenarios reflect the degree of synchronicity between flows in the main-stem and those in the tributary. Fine sediment budgets were developed for each reach to assess the relative contribution of different sources of sediment.Results and discussionThe reconnection significantly affected suspended sediment loads in the Ehen. Bed storage increased twofold, with changes most evident in the slow-flowing morphological unit. Changes in the composition of stored sediment were less marked than changes in the quantity of material. Changes in bed storage were controlled by the degree of synchronicity between flows in the Ehen and those in the newly reconnected tributary. Results show that three generalised flow scenarios occur, with total asynchronicity between flows in the tributary and the Ehen being responsible for the main episodes of fine sediment deposition. Overall, the estimated sediment budgets provide insights into the importance of non-perennial sources of sediment in supply-limited systems such as the Ehen. Although bed storage values are within the range of those published for UK rivers, the increase observed since the reconnection, together with the persistence of a static pavement, highlights the ecologically critical conditions of the regulated main-stem River Ehen.ConclusionsIntermittent sources control fine sediment transport dynamics in the upper River Ehen. In this regulated river, ongoing deposition associated with increased low- and medium-sized flow events exerts more of a control on bed storage than large but rare floods. Management actions to limit delivery of material from lateral sources could help prevent further deterioration of habitat conditions for biota sensitive to fine sediment. Given the ongoing adjustment in the newly reconnected tributary, continued monitoring is needed to capture further morphosedimentary response in the main-stem.

Dry Dams in the Desert: Chasing Land Subsidence and Earth Fissures with Geophysics

Homogenous embankment dam flood retention structures (FRSs) were built throughout central Arizona desert basins in the mid-twentieth century to protect mostly agricultural lands and military installations from rare flood events. Many of these FRSs eventually suffered transverse cracking due to desiccation of the embankment soils in the hot, dry climate. While this dam safety hazard has been recognized and mitigated, another hazard was developing under several dams. Differential land subsidence was gradually tilting some FRSs by several feet; ground cracks known as earth fissures have developed near two FRSs. These earth fissures are large (typically) tension cracks in basin alluvium with lengths of hundreds of feet to several miles and depths up to perhaps hundreds of feet. They can become conduits for uncontrolled erosion and earthen structure failure. Compounding the impacts of these geohazards, continuous metropolitan growth was changing the land use that these FRSs protected so the consequences of dam failure were increasing dramatically.

Aridity and Risk Calculations in Saudi Arabian Wadis: Wadi Fatimah Case

BackgroundInfrequent rainfall and rare flash floods are the primary sources of water in semi-arid and arid regions and are the key aspects in the management of water supply. In these regions, rainfall varies temporally and spatially with infrequent high intensities and sporadic occurrences, making it impossible to systematically record measurements as a proper time series.PurposeThe main purpose of this paper is to present temporal (risk assessment) and spatial (isohyet maps) methods for the evaluation of monthly rainfall water volumes and infiltration rates. Additionally, an aridity index based on rainfall and temperature measurements is proposed for use in semi-arid and arid regions.MethodThe limited data available for semi-arid and arid regions are treated using effective probabilistic and statistical methods. Calculation of the volume of rainfall is achieved through a simple hydrological procedure which involves examining monthly isohyet maps based on a set of rainfall measurements from weather stations in scattered locations. Calculation of rainfall and infiltration risks are performed according to the logarithmic normal probability distribution function. Furthermore, the potential monthly rainfall and infiltration rates are calculated using a set of pre-selected risk percentages.Results Aridity index, rainfall volume, and risk values are presented for Wadi Fatimah, which is located in the western Saudi Arabia. Potential rainfall and infiltration rates are calculated on a monthly scale using a set of pre-selected risk percentages. These simple calculations provide a sound basis for future studies on groundwater resource storage strategy, operation, and management.ConclusionsWater management studies can be best advanced through simple but effective assessment methods, as presented in this paper with specific applications to the Wadi Fatimah drainage basin that joins the Red Sea.

Recent changes in extreme floods across multiple continents

Analyses of trends in observed floods often focus on relatively frequent events, whereas changes in rare floods are only studied for a small number of locations that have exceptionally long observational records. Understanding changes in rare floods is especially relevant as these events are often most damaging and influence the design of major structures. Here, we provide an assessment of changes in the largest flood events (~0.033 annual exceedance probability) observed during the period 1980−2009 for 1744 catchments located in Australia, Brazil, Europe and the United States. The occurrence of rare floods in spatial aggregate shows strong temporal variability and peaked around 1995. During the 30 year period, there are overall increases in both the frequency and magnitude of extreme floods. These increases are strongest in Europe and the United States, and weakest in Brazil and Australia. Physical causes of the reported short-term variability and longer-term changes in extreme floods currently remain elusive, because the key drivers vary between catchments. Nonetheless, this approach provides the basis for a more spatially representative assessment of changes in extreme flood occurrence.

Effect of Extraordinary Large Floods on at-site Flood Frequency

The peak flow of extraordinary large floods that occur during a period of systematic record is a controversial problem for flood frequency analysis (FFA) using traditional methods. The present study suggests that such floods be treated as historic flood data even though their historical period is unknown. In this paper, the extraordinary large flood peak was first identified using statistical outlier tests and normal probability plots. FFA was then applied with and without the extraordinary large floods. In this step, two goodness-of-fit tests including mean absolute relative deviation and mean squared relative deviation were used to identify the best-fit probability distributions. Next, the generalized extreme value (GEV), three-parameter lognormal (LN3), log-Pearson type III (LP3), and Wakeby (WAK) probability distributions were used to incorporate and adjust the extraordinary large floods with other systematic data. Finally, procedures with and without historical adjustment were compared for the extraordinary large floods in terms of goodness-of-fit and flood return-period quantiles. The results of this comparison indicate that historical adjustment from an operational perspective was more viable than without adjustment procedure. Furthermore, the results without adjustment were unreasonable (subject to over- and under-estimation) and produced physically unrealistic estimates that were not compatible with the study area. The proposed approach substantially improved the probability estimation of rare floods for efficient design of hydraulic structures, risk analysis, and floodplain management.

Metagenomic profiling of historic Colorado Front Range flood impact on distribution of riverine antibiotic resistance genes.

Record-breaking floods in September 2013 caused massive damage to homes and infrastructure across the Colorado Front Range and heavily impacted the Cache La Poudre River watershed. Given the unique nature of this watershed as a test-bed for tracking environmental pathways of antibiotic resistance gene (ARG) dissemination, we sought to determine the impact of extreme flooding on ARG reservoirs in river water and sediment. We utilized high-throughput DNA sequencing to obtain metagenomic profiles of ARGs before and after flooding, and investigated 23 antibiotics and 14 metals as putative selective agents during post-flood recovery. With 277 ARG subtypes identified across samples, total bulk water ARGs decreased following the flood but recovered to near pre-flood abundances by ten months post-flood at both a pristine site and at a site historically heavily influenced by wastewater treatment plants and animal feeding operations. Network analysis of de novo assembled sequencing reads into 52,556 scaffolds identified ARGs likely located on mobile genetic elements, with up to 11 ARGs per plasmid-associated scaffold. Bulk water bacterial phylogeny correlated with ARG profiles while sediment phylogeny varied along the river’s anthropogenic gradient. This rare flood afforded the opportunity to gain deeper insight into factors influencing the spread of ARGs in watersheds.

Reconstruction palaeoflood hydrology using slackwater flow depth method in the Yanhe River valley, middle Yellow River basin, China

Water depth above the flood deposits should be taken into account in calculations of the palaeoflood peak stages, which can provide more accurately estimate of palaeoflood stage. Here we present a new method, slackwater flow depth, to assess palaeoflood peak stage and to reduce the underestimation of palaeoflood stage. Palaeoflood slackwater deposits (SWDs) were identified by palaeohydrological criteria in cliff riverbank on the Yanhe River, middle Yellow River basin. Palaeoflood events recorded in four layers of SWD were dated by optical stimulated luminescence to 9.5–8.5ka. The estimation of palaeoflood maximum stage was 778.3m using the slackwater flow depth method and the palaeoflood peak discharge is 15,000m3/s using the step-backwater method. Palaeoflood results greatly extend the current flood data series in the Yanhe River basin. The regional flood history including gauged flood, historical and palaeoflood data was compiled and evaluated for the major tributaries of the middle Yellow River. The relationship between palaeoflood peak discharges and drainage areas in this region fit well with the global maximum curves. The results of site-specific and regional palaeoflood evaluations demonstrate that the approach estimates the true palaeoflood peak stage and discharges and improves the flood frequency analysis of extreme and rare floods for a particular basin. Meanwhile, the advantages and uncertainties of this method need ongoing discussion in palaeoflood investigations.

Cleanup and valuation of waters of the aquifer of M’zab Valley (Algeria)

Abstract The M’zab valley is a hyper arid region of average rainfall not exceeding 100 mm per year. However, the rare floods that occur in M’zab River drain large volumes of surface water. Thanks to the genius of the local population, traditional dams were made for artificial recharge of groundwater. Grace of traditional wells drilled in the valley, farmers irrigate their palm groves and gardens. However, since more than half a century, the contribution of deep drilling for the exploitation of the aquifer of the Continental Intercalary posed environmental problems. On the basis of investigations and surveys of the local population during the years 2010, 2011, 2012 and 2013, it appears that these modern techniques in water catchment caused harmful consequences to the region like the rising of water consumption, pollution of groundwater and soil salinity. Solutions and recommendations are outlined in this article.

Learning from and for rare floods in Dresden – how public officials interpret damage simulation results at the building type level

Public officials in Dresden are concerned about learning from and for rare flood events like the Elbe river flood in August 2002. This is interesting because research on individual as well as organizational learning from rare events indicates that this kind of learning faces significant difficulties (e.g., overestimation of rare events for decision-making based on “emotionalized event experience”). Up to now, only little is known what and how public officials in Dresden specifically learn from and for rare floods. Therefore, the paper follows an exploratory purpose in line with principles of qualitative social research. Firstly, the paper explores dealing with rare floods with reference to a conceptual framework that highlights relations between regulative, normative, and cognitive institutions on the one hand and learning of public officials on the other. Secondly, it adopts a single case study design in Dresden with embedded sub-cases that are defined with reference to organizations of FRM. The case study shows, among others, that regulations like the Floods Directive are important for justifying FRM with regard to rare flood events which is less obvious than it sounds. However, public officials display different interpretations of the term “rare flood event”, for instance, in the context of analysing the consequences of floods on the building stock. Furthermore, the case study findings indicate that public officials may follow alternative approaches to sustain commitment in the context of rare flood events (systematic versus pragmatic approach).

Precipitation and evapotranspiration controls on daily runoff variability in the contiguous United States and Puerto Rico

AbstractDaily runoff variability is an important driver of fluvial erosion but is difficult to incorporate into landscape evolution models due to limited observations and incomplete understanding of hydroclimatic controls on runoff distributions. Prior work in the contiguous U.S. showed how limitations can be overcome when mean runoff is correlated with the shape of the right tail of runoff distributions. However, which probability distribution functions best capture geomorphically important events and whether patterns in the contiguous U.S. transfer to other settings remain important open questions. Our analysis of large hydroclimatic data sets from the contiguous U.S. and Puerto Rico reveals that stretched exponential distributions provide a common probabilistic framework to evaluate daily rainfall and runoff variability. In both settings, daily runoff variability is correlated with the evapotranspiration ratio, aridity index, and the ratio of wet to dry days. Surprisingly, mean storm depth (estimated from average daily precipitation during wet days only) and storm depth variability are uncorrelated with daily runoff variability in either data set. These findings suggest that first‐order controls on runoff variability are processes that reduce runoff during intermediate frequency flows rather than processes that enhance the magnitude of rare floods. However, by normalizing local runoff variability by storm depth variability, some correlations collapse onto a single trend for the contiguous U.S. and Puerto Rico, suggesting a secondary role for rainfall variability on runoff variability. Taken together, this analysis provides a rationale for how hydroclimatic controls on runoff variability can be better incorporated into landscape evolution models from readily available data.

Collaborative governance and rare floods in urban regions – Dealing with uncertainty and surprise

Rare floods in urban regions like the Dresden region triggered significant changes in public policy and research. However, how actors are able to deal with uncertainty and surprise related to rare floods in the future is still open to many questions and debates in flood risk management research and practice. From an interpretative and agency-oriented perspective, the paper asks how dealing with uncertainty and surprise may be enhanced through processes of collaborative governance for rare floods in urban regions. The paper follows a conceptual purpose based on a series of completed projects and publications on flood risk management in the urban region of Dresden. Conceptual analysis highlights two strategic options for focusing collaboration of public and private actors: planning for flood risk reduction and searching for resilience. Both options are based on assumptions of collaborators about the predictability of the specific flood risk management problem at stake, especially with regard to analyzing surprise in retrospect. The paper elucidates on implications in the context of collaboration, participation, and governance.

Quantitative historical hydrology in Europe

Abstract. In recent decades, the quantification of flood hydrological characteristics (peak discharge, hydrograph shape, and runoff volume) from documentary evidence has gained scientific recognition as a method to lengthen flood records of rare and extreme events. This paper describes the methodological evolution of quantitative historical hydrology under the influence of developments in hydraulics and statistics. In the 19th century, discharge calculations based on flood marks were the only source of hydrological data for engineering design, but were later left aside in favour of systematic gauge records and conventional hydrological procedures. In the last two decades, there has been growing scientific and public interest in understanding long-term patterns of rare floods, in maintaining the flood heritage and memory of extremes, and developing methods for deterministic and statistical application to different scientific and engineering problems. A compilation of 46 case studies across Europe with reconstructed discharges demonstrates that (1) in most cases present flood magnitudes are not unusual within the context of the last millennium, although recent floods may exceed past floods in some temperate European rivers (e.g. the Vltava and Po rivers); (2) the frequency of extreme floods has decreased since the 1950s, although some rivers (e.g. the Gardon and Ouse rivers) show a reactivation of rare events over the last two decades. There is a great potential for gaining understanding of individual extreme events based on a combined multiproxy approach (palaeoflood and documentary records) providing high-resolution time flood series and their environmental and climatic changes; and for developing non-systematic and non-stationary statistical models based on relations of past floods with external and internal covariates under natural low-frequency climate variability.

Formation of gravel pavements during fluvial erosion as an explanation for persistence of ancient cratered terrain on Titan and Mars

In many terrestrial channels the gravel bed is only transported during rare floods (threshold channels), and rates of erosion are very slow. In this paper we explore how coarse debris delivered to channels on Mars and Titan from erosion may inhibit further erosion once a coarse gravel channel bed develops. Portions of the equatorial region of Titan are fluvially eroded into banded (crenulated) terrain, some of which contains numerous circular structures that are likely highly degraded large impact craters surviving from the late heavy bombardment. No mechanism that can chemically or physically break down ice (likely the most important component of Titans crust) has been unambiguously identified. This paper examines a scenario in which fluvial erosion on Titan has largely involved erosion into an impact-generated megaregolith that contains a modest component of gravel-sized debris. As the megaregolith is eroded, coarse gravel gradually accumulates as a lag pavement on channel beds, limiting further erosion and creating a dissected, but largely inactive, or senescent, landscape. Similar development of gravel pavements occur in ancient mountain belts on Earth, and partially explain the persistence of appreciable relief after hundreds of millions of years. Likewise, coarse gravel beds may have limited the degree to which erosion could modify the heavily cratered terrains on Mars, particularly if weathering were largely due to physical, rather than chemical weathering processes in a relatively cold and/or arid environment.

Flood frequency analysis of historical flood data under stationary and non-stationary modelling

Abstract. Historical records are an important source of information on extreme and rare floods and fundamental to establish a reliable flood return frequency. The use of long historical records for flood frequency analysis brings in the question of flood stationarity, since climatic and land-use conditions can affect the relevance of past flooding as a predictor of future flooding. In this paper, a detailed 400 yr flood record from the Tagus River in Aranjuez (central Spain) was analysed under stationary and non-stationary flood frequency approaches, to assess their contribution within hazard studies. Historical flood records in Aranjuez were obtained from documents (Proceedings of the City Council, diaries, chronicles, memoirs, etc.), epigraphic marks, and indirect historical sources and reports. The water levels associated with different floods (derived from descriptions or epigraphic marks) were computed into discharge values using a one-dimensional hydraulic model. Secular variations in flood magnitude and frequency, found to respond to climate and environmental drivers, showed a good correlation between high values of historical flood discharges and a negative mode of the North Atlantic Oscillation (NAO) index. Over the systematic gauge record (1913–2008), an abrupt change on flood magnitude was produced in 1957 due to constructions of three major reservoirs in the Tagus headwaters (Bolarque, Entrepeñas and Buendia) controlling 80% of the watershed surface draining to Aranjuez. Two different models were used for the flood frequency analysis: (a) a stationary model estimating statistical distributions incorporating imprecise and categorical data based on maximum likelihood estimators, and (b) a time-varying model based on "generalized additive models for location, scale and shape" (GAMLSS) modelling, which incorporates external covariates related to climate variability (NAO index) and catchment hydrology factors (in this paper a reservoir index; RI). Flood frequency analysis using documentary data (plus gauged records) improved the estimates of the probabilities of rare floods (return intervals of 100 yr and higher). Under non-stationary modelling flood occurrence associated with an exceedance probability of 0.01 (i.e. return period of 100 yr) has changed over the last 500 yr due to decadal and multi-decadal variability of the NAO. Yet, frequency analysis under stationary models was successful in providing an average discharge around which value flood quantiles estimated by non-stationary models fluctuate through time.