24 research outputs found
Regional parent flood frequency distributions in Europe – Part 1: Is the GEV model suitable as a pan-European parent?
Abstract. This study addresses the question of the existence of a parent flood frequency distribution on a European scale. A new database of L-moment ratios of flood annual maximum series (AMS) from 4105 catchments was compiled by joining 13 national data sets. Simple exploration of the database presents the generalized extreme value (GEV) distribution as a potential pan-European flood frequency distribution, being the three-parameter statistical model that with the closest resemblance to the estimated average of the sample L-moment ratios. Additional Monte Carlo simulations show that the variability in terms of sample skewness and kurtosis present in the data is larger than in a hypothetical scenario where all the samples were drawn from a GEV model. Overall, the generalized extreme value distribution fails to represent the kurtosis dispersion, especially for the longer sample lengths and medium to high skewness values, and therefore may be rejected in a statistical hypothesis testing framework as a single pan-European parent distribution for annual flood maxima. The results presented in this paper suggest that one single statistical model may not be able to fit the entire variety of flood processes present at a European scale, and presents an opportunity to further investigate the catchment and climatic factors controlling European flood regimes and their effects on the underlying flood frequency distributions
A process-based analysis of the suitability of copula types for peak-volume flood relationships
The work aims at analyzing the bivariate relationship between flood peaks
and flood volumes, with a particular focus on the type and seasonality of
flood generation processes. Instead of the usual approach that deals with an
analysis of the annual maxima of flood events, the current analysis includes
all independent flood events in a catchment. Flood events are considered
independent when they originate from distinguishably different
synoptic/meteorological situations. The target region is located in the
northern part of Austria, and consists of 72 small and mid-sized catchments.
On the basis of the discharge measurements with a time resolution of 1 h
from the period 1976–2007, independent flood events were identified and were
assigned to one of the three following flood generation type categories:
synoptic floods, flash floods and snowmelt floods. These were subsequently
divided into two seasons, thereby separating predominantly rainfall-fed and
snowmelt-fed floods. Nine frequently-used copula types were locally fitted
to the samples of the flood type and seasonal data. Their goodness-of-fit
was examined locally as well as analyzed in a regional scope. It was
concluded that (i) treating flood processes separately is beneficial for the
statistical analysis; (ii) suitability patterns of acceptable copula types
are distinguishably different for the seasons/flood types considered, (iii)
the Clayton and Joe copulas shows an unacceptable performance for all the
seasons/flood types in the region; (iv) the rejection rate of the other
copula types depends on the season/flood type and also on the sample size;
(v) given that usually more than one statistically suitable dependence model
exists, an uncertainty analysis of the design values in the engineering
studies resulting from the choice of model seems unavoidable; (vi) reducing
uncertainty in the choice of model could be attempted by a deeper
hydrological analysis of the dependence structure between flood peaks and
volumes in order to give hydrological support to the decision on model's
suitability in specific regions and for typical flood generation mechanisms
Land use change impacts on floods at the catchment scale: Challenges and opportunities for future research
Research gaps in understanding flood changes at the catchment scale caused by changes in forest management, agricultural practices, artificial drainage and terracing are identified. Potential strategies in addressing these gaps are proposed, such as complex systems approaches to link processes across time scales, long-term experiments on physical-chemical-biological process interactions, and a focus on connectivity and patterns across spatial scales. It is suggested that these strategies will stimulate new research that coherently addresses the issues across hydrology, soil and agricultural sciences, forest engineering, forest ecology and geomorphology
Changing climate both increases and decreases European river floods
Climate change has led to concerns about increasing river floods resulting from the greater water-holding capacity of a warmer atmosphere. These concerns are reinforced by evidence of increasing economic losses associated with flooding in many parts of the world, including Europe. Any changes in river floods would have lasting implications for the design of flood protection measures and flood risk zoning. However, existing studies have been unable to identify a consistent continental-scale climatic-change signal in flood discharge observations in Europe, because of the limited spatial coverage and number of hydrometric stations. Here we demonstrate clear regional patterns of both increases and decreases in observed river flood discharges in the past five decades in Europe, which are manifestations of a changing climate. Our results—arising from the most complete database of European flooding so far—suggest that: increasing autumn and winter rainfall has resulted in increasing floods in northwestern Europe; decreasing precipitation and increasing evaporation have led to decreasing floods in medium and large catchments in southern Europe; and decreasing snow cover and snowmelt, resulting from warmer temperatures, have led to decreasing floods in eastern Europe. Regional flood discharge trends in Europe range from an increase of about 11 per cent per decade to a decrease of 23 per cent. Notwithstanding the spatial and temporal heterogeneity of the observational record, the flood changes identified here are broadly consistent with climate model projections for the next century, suggesting that climate-driven changes are already happening and supporting calls for the consideration of climate change in flood risk management
On the possibilities of watershed parameterization for extreme flow estimation in ungauged basins
The estimation of design discharges and water levels of extreme floods is
one of the most important parts of the design process for a large number of
engineering projects and studies. Design flood estimates require a
consideration of the hydrological, meteorological and physiographical
situation, the legal requirements, and the available estimation techniques
and methods. In the last decades changes in floods have been observed (Hall
et al., 2014) which makes design flood estimation particularly challenging.
Methods of design flood estimation can be applied either locally or
regionally. A significant problem may arise in small catchments that are
poorly gauged or when no recorded data exist. To obtain the design values in
such cases, many countries have adopted procedures that fit the local
conditions and requirements. One of these methods is the Soil Conservation
Service – Curve number (SCS-CN) method which is often used in design flood
estimation for ungauged sites, including those in Slovakia. Since the method
was derived on the basis of the specific characteristics of selected river
basins in the United States, it may lead to significant uncertainties in
other countries with different hydrological conditions. The aim of this
study was to test the SCN-CN method and derive regional runoff curve numbers
based on rainfall and discharge measurements for selected region in
Slovakia. The results show that the classical CN method gives too high
estimates of event runoff depths and is not valid in the study area. To
avoid the overestimation of runoff caused by extreme rainfall events, the
use of the empirically derived regional runoff curves was tested and finally
proposed for practical application in engineering hydrology
Changes in the snow water equivalent in mountainous basins in Slovakia over recent decades
Changes in snowpack and duration of snow cover can cause changes in the
regime of snow and rain-snow induced floods. The recent IPCC report suggests
that, in snow-dominated regions such as the Alps, the Carpathian
Mountains and the northern parts of Europe, spring snowmelt floods may occur
earlier in a future climate because of warmer winters, and flood hazards may
increase during wetter and warmer winters, with more frequent rain and less
frequent snowfall. The monitoring and modelling of snow accumulation and
snow melting in mountainous catchments is rather complicated, especially due
to the high spatial variability of snow characteristics and the limited
availability of terrestrial hydrological data. An evaluation of changes in
the snow water equivalent (SWE) during the period of 1961–2010 in the Upper
Hron river basin, which is representative of the mountainous regions in
Central Slovakia, is provided in this paper. An analysis of the snow cover
was performed using simulated values of the snow water equivalent by a
conceptual semi-distributed hydrological rainfall-runoff model. Due to the
poor availability of the measured snow water equivalent data, the analysis
was performed using its simulated values. Modelling of the SWE was performed
in different altitude zones by a conceptual semi-distributed hydrological
rainfall-runoff model. The evaluation of the results over the past five
decades indicates a decrease in the simulated snow water equivalent and the
snow duration in each altitude zone and in all months of the winter season.
Significant decreasing trends were found for December, January and February,
especially in the highest altitude zone
Joint modelling of flood peaks and volumes: A copula application for the Danube River
Flood frequency analysis is usually performed as a univariate analysis of flood peaks using a suitable theoretical probability distribution of the annual maximum flood peaks or peak over threshold values. However, other flood attributes, such as flood volume and duration, are necessary for the design of hydrotechnical projects, too. In this study, the suitability of various copula families for a bivariate analysis of peak discharges and flood volumes has been tested. Streamflow data from selected gauging stations along the whole Danube River have been used. Kendall's rank correlation coefficient (tau) quantifies the dependence between flood peak discharge and flood volume settings. The methodology is applied to two different data samples: 1) annual maximum flood (AMF) peaks combined with annual maximum flow volumes of fixed durations at 5, 10, 15, 20, 25, 30 and 60 days, respectively (which can be regarded as a regime analysis of the dependence between the extremes of both variables in a given year), and 2) annual maximum flood (AMF) peaks with corresponding flood volumes (which is a typical choice for engineering studies). The bivariate modelling of the extracted peak discharge - flood volume couples is achieved with the use of the Ali-Mikhail-Haq (AMH), Clayton, Frank, Joe, Gumbel, Hüsler-Reiss, Galambos, Tawn, Normal, Plackett and FGM copula families. Scatterplots of the observed and simulated peak discharge - flood volume pairs and goodness-of-fit tests have been used to assess the overall applicability of the copulas as well as observing any changes in suitable models along the Danube River. The results indicate that for the second data sampling method, almost all of the considered Archimedean class copula families perform better than the other copula families selected for this study, and that for the first method, only the upper-tail-flat copulas excel (except for the AMH copula due to its inability to model stronger relationships). © 2016 George Papaioannou et al