Most of the hydrological and hydraulic studies refer to the notion of a return period to quantify design variables. When dealing with multiple design variables, the well-known univariate statistical analysis is no longer satisfactory, and several issues challenge the practitioner. How should one incorporate the dependence between variables? How should a multivariate return period be defined and applied in order to yield a proper design event? In this study an overview of the state of the art for estimating multivariate design events is given and the different approaches are compared. The construction of multivariate distribution functions is done through the use of copulas, given their practicality in multivariate frequency analyses and their ability to model numerous types of dependence structures in a flexible way. A synthetic case study is used to generate a large data set of simulated discharges that is used for illustrating the effect of different modelling choices on the design events. Based on different uni- and multivariate approaches, the design hydrograph characteristics of a 3-D phenomenon composed of annual maximum peak discharge, its volume, and duration are derived. These approaches are based on regression analysis, bivariate conditional distributions, bivariate joint distributions and Kendall distribution functions, highlighting theoretical and practical issues of multivariate frequency analysis. Also an ensemble-based approach is presented. For a given design return period, the approach chosen clearly affects the calculated design event, and much attention should be given to the choice of the approach used as this depends on the real-world problem at hand.

The time of concentration is a primary parameter for a variety of modern hydrological models adopted in professional and scientific communities. Nevertheless, a universally accepted working definition of this parameter is currently lacking and several definitions can be found in the technical literature along with related estimation procedures. This study brings to light the inherent variability of these definitions through the empirical analysis of four small basins. These case studies demonstrate that available approaches for the estimation of the time of concentration may yield numerical predictions that differ from each other by up to 500%. Editor D. Koutsoyiannis Citation Grimaldi, S., Petroselli, A., Tauro, F. and Porfiri, M., 2012. Time of concentration: a paradox in modern hydrology. Hydrological Sciences Journal, 57 (2), 217–228.

A design hyetograph is a synthetic rainfall temporal pattern associated with a return period, usually determined by means of statistical analysis of observed mean rainfall intensity through intensity—duration—frequency (IDF) curves. Since the univariate approach is simple to apply and data availability is scarce, only the mean intensity of a rainfall storm is usually analysed statistically. The other characteristics of a rainfall storm, such as peak (maximum intensity), total depth and duration, are found indirectly throughout the several phases of hydrological analysis by suitable work assumptions. The aim of this paper is to apply a multivariate approach in order to analyse jointly observed data of critical depth, peak and total depth. In particular, bivariate analysis of peak—total depth conditioned on critical depth is developed using a 3-copula function to define the trivariate joint distribution. Following the proposed procedure, once design return period and related critical depth are sele... Résumé Un hyétogramme de projet est un schéma temporel de pluie synthétique associé à une période de retour, généralement déterminé par l'analyse statistique d'intensités pluviométriques moyennes observées par l'intermédiaire de courbes intensité–durée—fréquence (IDF). L'approche univariée étant simple d'application et la disponibilité des données étant faible, seule l'intensité moyenne de l'événement pluvieux est généralement analysée statistiquement. Les autres caractéristiques de l'événement pluvieux, comme le pic (intensité maximale), la hauteur totale et la durée sont estimées indirectement au fil des étapes de l'analyse hydrologique grâce à des hypothèses de travail adéquates. L'objectif de cet article est d'appliquer une approche multivariée afin d'analyser conjointement les données observées de hauteur critique, pic et hauteur totale. En particulier, l'analyse bivariée pic-hauteur totale conditionnée par la hauteur critique est développée en utilisant une fonction 3-copules pour définir la distrib...

In this study, a parsimonious hydrological modelling algorithm is proposed based on the automated DEM-based geomorphic characterization of runoff dynamics in scarcely monitored river basins. The proposed approach implements the instantaneous unit hydrograph (IUH) concept, estimated using the width function (WF), for characterizing the travel time distribution using just one parameter, the river network flow velocity. Hillslope velocities are defined using spatially-distributed empirical formulas based on slope and soil-use information extrapolated from digital topographic data. Case studies are presented for testing model performance and comparing simulated and observed hydrographs of 25 selected flood events, as well as investigating the differences with the geomorphological instantaneous unit hydrograph (GIUH) model results. The calibration of the WFIUH channel flow velocity parameter using the concentration time is investigated providing interesting insights for the use of such a method for hydrological prediction in ungauged basins. Editor D. Koutsoyiannis Citation Grimaldi, S., Petroselli, A. and Nardi, F., 2012 Grimaldi, S. , Petroselli, A. , Porfiri, M. and Tauro, F. 2012. Time of concentration: a paradox of modern hydrology. Hydrological Sciences Journal, 57(1) in press [Google Scholar] . A parsimonious geomorphological unit hydrograph for rainfall–runoff modelling in small ungauged basins. Hydrological Sciences Journal, 57 (1), 73–83.

Devastating floods are observed every year globally from upstream mountainous to coastal regions. Increasing flood frequency and impacts affect both major rivers and their tributaries. Nonetheless, at the small-scale, the lack of distributed topographic and hydrologic data determines tributaries to be often missing in inundation modeling and mapping studies. Advances in Unmanned Aerial Vehicle (UAV) technologies and Digital Elevation Models (DEM)-based hydrologic modeling can address this crucial knowledge gap. UAVs provide very high resolution and accurate DEMs with low surveying cost and time, as compared to DEMs obtained by Light Detection and Ranging (LiDAR), satellite, or GPS field campaigns. In this work, we selected a LiDAR DEM as a benchmark for comparing the performances of a UAV and a nation-scale high-resolution DEM (TINITALY) in representing floodplain topography for flood simulations. The different DEMs were processed to provide inputs to a hydrologic-hydraulic modeling chain, including the DEM-based EBA4SUB (Event-Based Approach for Small and Ungauged Basins) hydrologic modeling framework for design hydrograph estimation in ungauged basins; the 2D hydraulic model FLO-2D for flood wave routing and hazard mapping. The results of this research provided quantitative analyses, demonstrating the consistent performances of the UAV-derived DEM in supporting affordable distributed flood extension and depth simulations.