Catchment similarity and spatial correlation: added value and impacts on hydrological predictions in ungauged basins

This Thesis presents a three-fold study aimed at deepening our understanding on the added value and impacts of catchment similarity and spatial correlation (or cross-correlation) on the regional prediction of flood quantiles and flow-duration curves (FDCs) in ungauged river cross-sections. First, we consider the reference procedure for design flood estimation in Triveneto, North-eastern Italy, which assumes the entire study area to be a single hydrologically homogeneous region. We highlight that Triveneto cannot be regarded as homogeneous in terms of flood frequency regime and show that a focused-pooling approach accounting for selected geomorphoclimatic descriptors leads to regional samples with significantly improved homogeneity. Nevertheless, focused pooling does not consider the effects associated with cross-correlation, which are instead considered by Generalized Least Squares (GLS) and Top-kriging (TK; geostatistical method), although in two different ways. Recent studies show that TK outperforms GLS for predicting empirical flood quantiles, but they also speculate that cross-correlation might affect their accuracy in predicting true flood quantiles. To investigate this aspect, we apply GLS and TK for predicting flood quantiles in a homogeneous pooling-group of sites in Triveneto under different cross-correlation scenarios through a Monte Carlo experiment. For both methods, we observe that an increasing degree of spatial correlation results in an increasing masking-effect on the true flooding potential. Morever, we confirm that TK significantly outperforms GLS when they both assume flood quantiles to scale with drainage area alone, yet, both methodologies significantly improve their accuracy when considering several catchment descriptors. Finally, concerning the prediction of FDCs in a large and heterogeneous region, the Danube river basin, we show that geostatistical models provide much more accurate predictions than multi-regression models. In summary, all the analyses confirm the added value for statistical regionalisation of properly handling hydrological heterogeneity, also highlighting the pivotal role played by cross-correlation in observed streamflow time-series

Pluvial flooding: high-resolution stochastic hazard mapping in urban areas by using fast-processing DEM-based algorithms

Climate change and rapid expansion of urban areas are expected to increase pluvial flood hazard and risk in the near future, and particularly so in large developed areas and cities. Therefore, large-scale and high-resolution pluvial flood hazard mapping is required to identify hotspots where mitigation measures may be applied to reduce flood risk. Depressions or low points in urban areas where runoff volumes can be stored are prone to pluvial flooding. The standard approach based on estimating synthetic design hyetographs assumes, in a given depression, that the T-year design storm generates the T-year pluvial flood. In addition, urban areas usually include several depressions even linked or nested that would require distinct design hyetographs instead of using a unique synthetic design storm. In this paper, a stochastic methodology is proposed to address the limitations of this standard approach, developing large-scale ~ 2 m-resolution pluvial flood hazard maps in urban areas with multiple depressions. The authors present an application of the proposed approach to the city of Pamplona in Spain (68.26 km2)

Geomorphic flood hazard mapping: from floodplain delineation to flood hazard characterization

Recent studies show how geomorphic descriptors, retrieved from digital elevation models (DEMs), can be used for flood hazard mapping. As they strictly depend on the accuracy of the input DEMs and reference flood hazard maps used for training, DEM-based flood hazard models may display severe inconsistencies. Our study shows the application of two advanced DEM-based models to a large study area, and presents two main innovative points. First, the delicate tasks of appropriately selecting the input DEM and flood hazard map are specifically addressed with newly defined methods. Second, the ability of DEM-based models to exploit their natural features to enhance flood hazard mapping over the study region is investigated. Our results show (a) the benefits of considering multiple geomorphic descriptors, (b) the potential of DEM-based models for completing the information of imperfect reference flood hazard maps, and (c) the advantages of continuous representation of hazard over binary flood maps

Climate, orography and scale controls on flood frequency in Triveneto (Italy)

The growing concern about the possible effects of climate change on flood frequency regime is leading Authorities to review previously proposed reference procedures for design-flood estimation, such as national flood frequency models. Our study focuses on Triveneto, a broad geographical region in North-eastern Italy. A reference procedure for design flood estimation in Triveneto is available from the Italian NCR research project "VA.PI.", which considered Triveneto as a single homogeneous region and developed a regional model using annual maximum series (AMS) of peak discharges that were collected up to the 1980s by the former Italian Hydrometeorological Service. We consider a very detailed AMS database that we recently compiled for 76 catchments located in Triveneto. All 76 study catchments are characterized in terms of several geomorphologic and climatic descriptors. The objective of our study is threefold: (1) to inspect climatic and scale controls on flood frequency regime; (2) to verify the possible presence of changes in flood frequency regime by looking at changes in time of regional L-moments of annual maximum floods; (3) to develop an updated reference procedure for design flood estimation in Triveneto by using a focused-pooling approach (i.e. Region of Influence, RoI). Our study leads to the following conclusions: (1) climatic and scale controls on flood frequency regime in Triveneto are similar to the controls that were recently found in Europe; (2) a single year characterized by extreme floods can have a remarkable influence on regional flood frequency models and analyses for detecting possible changes in flood frequency regime; (3) no significant change was detected in the flood frequency regime, yet an update of the existing reference procedure for design flood estimation is highly recommended and we propose the RoI approach for properly representing climate and scale controls on flood frequency in Triveneto, which cannot be regarded as a single homogeneous region

Influenza dei fattori climatici e topografici sul regime di frequenza delle piene nel Triveneto

Il presente studio prende in esame il Triveneto, vasta area geografica comprendente le Alpi Orientali italiane, ed ha tre obiettivi principali: (1) valutare l’influenza dei fattori climatici e di scala sul regime di frequenza delle piene; (2) verificare la possibile presenza di alterazioni del regime di frequenza dei massimi annuali di portata al colmo di piena; (3) sviluppare una procedura aggiornata per la stima delle piene di progetto in Triveneto impiegando tecniche di raggruppamento di bacini basate su criteri di similitudine idrologica col bacino di interesse (approccio della regione di influenza, Region of Influence, RoI). Lo studio è pervenuto alle seguenti conclusioni: (1) l’influenza dei fattori climatici e topografici sul regime di frequenza delle piene in Triveneto risulta simile a quella recentemente osservata a scala europea in analoghi contesti morfologici e climatici; (2) nonostante non si siano riscontrate evidenze di alterazioni significative e generalizzate del regime di frequenza delle piene, le indagini hanno messo in risalto come un singolo anno caratterizzato da piene eccezionali possa influenzare notevolmente i modelli regionali di frequenza delle piene; (3) infine, è confermata per l’area in esame una marcata eterogeneità spaziale del regime di frequenza delle piene; in quest’ottica si raccomanda per essa un aggiornamento della procedura di riferimento per la stima della portata di progetto sulla base di procedure analoghe al RoI, che consentano di rappresentare l’influenza di fattori climatici e topografici sul regime di frequenza degli estremi idrometrici.Our study focuses on Triveneto, a broad geographical region which includes the Italian Eastern Alps. The objective of our study is threefold: (1) to inspect climatic and scale controls on flood frequency regime; (2) to verify the possible presence of changes in flood frequency regime by applying non-parametric statistical tests to annual maximum series of flood peak discharges and by evaluating the possible presence of changes in time of regional statistics of hydrometric extremes; (3) to develop an updated reference procedure for design flood estimation in Triveneto by using a focused-pooling approach (i.e. Region of Influence, RoI). Our study leads to the following conclusions: (1) climatic and scale controls on flood frequency regime in Triveneto are similar to the controls that were recently found at European scale in an analogous morpho-climatic context; (2) although we found no significant uniform changes in the flood frequency regime in the study area, our analyses highlight how a single year characterized by extreme floods can have a remarkable influence on regional flood frequency models; (3) finally, our analyses indicate for the study region a high heterogeneity degree in the flood frequency regime; therefore we recommend an update of the existing reference procedure for design flood estimation, referring to procedures similar to the RoI approach, which properly represents climate and scale controls on flood frequency regime

Prediction of streamflow regimes over large geographic areas: interpolated flow-duration curves for the Danube region

Flow–duration curves (FDCs) are essential to support decisions on water resources management, and their regionalization is fundamental for the assessment of ungauged basins. In comparison with calibrated rainfall–runoff models, statistical methods provide data-driven estimates representing a useful benchmark. The objective of this work is the interpolation of FDCs from ~500 discharge gauging stations in the Danube. To this aim we use total negative deviation top-kriging (TNDTK), as multiregression models are shown to be unsuitable for representing FDCs across all durations and sites. TNDTK shows a high accuracy for the entire Danube region, with overall Nash-Sutcliffe efficiency values computed in a leave-p-out cross-validation scheme (p equal to one site, one-third and half of the sites), all above 0.88. A reliability measure based on kriging variance is attached to each interpolated FDC at ~4000 prediction nodes. The GIS layer of regionalized FDCs is made available for broader use in the region.JRC.D.2-Water and Marine Resource

Streamflow data availability in Europe: a detailed dataset of interpolated flow-duration curves

The dataset consists of a GIS vector layer of the contours of 24,148 elementary catchments in Europe and the associated representation of the streamflow regime in terms of empirical flow–duration curves (FDCs). FDCs are estimated by means of the geostatistical procedure termed total negative deviation top-kriging (TNDTK), starting from the empirical FDCs available for 2484 discharge measurement stations across Europe. Together with the estimated FDCs' percentiles, for each catchment, indicators of the accuracy and reliability of the performed large-scale geostatistical prediction are provided. The file is stored using the ESRI Shapefile format in the ETRS89 (European Terrestrial Reference System 1989) – LAEA (Lambert Azimuthal Equal Area) datum and geographic coordinate system