The need for high resolution data to improve urban flood 1 risk assessment 2

Abstract 12 Cities are particularly vulnerable to rainfall-generated floods that are typically characterised 13 by their rapid onset and localised nature. This implies that precipitation and catchment 14 information need to be available at high resolution to reliably predict hydrological response 15 and potential flooding. On the contrary, urban areas constitute a major knowledge gap as most 16 flood risk studies have concentrated on natural basins and records of rain gauges and water 17 level gauges in cities are scarce. While increase in intense precipitation as a result of climate 18 change is expected in many areas around the world, it is at present not possible to assess how 19 this will affect urban pluvial flood risk. Collection of reliable, high resolution data in cities 20 needs to start urgently to build up datasets in support of urban flood risk assessment and to 21 enable detection of changes in flood risk whether these are induced by climate change, 22 urbanisation or other future developments. This study shows how implementation of 23 polarimetric X-band radar can contribute to filling the knowledge gap of flood risk 24 quantification in cities. 2

Fractal analysis of urban catchments and their representation in semi-distributed models: imperviousness and sewer system

Fractal analysis relies on scale invariance and the concept of fractal dimension enables one to characterize and quantify the space filled by a geometrical set exhibiting complex and tortuous patterns. Fractal tools have been widely used in hydrology but seldom in the specific context of urban hydrology. In this paper, fractal tools are used to analyse surface and sewer data from 10 urban or peri-urban catchments located in five European countries. The aim was to characterize urban catchment properties accounting for the complexity and inhomogeneity typical of urban water systems. Sewer system density and imperviousness (roads or buildings), represented in rasterized maps of 2m  ×  2m pixels, were analysed to quantify their fractal dimension, characteristic of scaling invariance. The results showed that both sewer density and imperviousness exhibit scale-invariant features and can be characterized with the help of fractal dimensions ranging from 1.6 to 2, depending on the catchment. In a given area consistent results were found for the two geometrical features, yielding a robust and innovative way of quantifying the level of urbanization. The representation of imperviousness in operational semi-distributed hydrological models for these catchments was also investigated by computing fractal dimensions of the geometrical sets made up of the sub-catchments with coefficients of imperviousness greater than a range of thresholds. It enables one to quantify how well spatial structures of imperviousness were represented in the urban hydrological models

Two months of disdrometer data in the Paris area

The Hydrology, Meteorology, and Complexity laboratory of École des Ponts ParisTech (hmco.enpc.fr) has made a data set of optical disdrometer measurements available that come from a campaign involving three collocated devices from two different manufacturers, relying on different underlying technologies (one Campbell Scientific PWS100 and two OTT Parsivel2 instruments). The campaign took place in January–February 2016 in the Paris area (France). Disdrometers provide access to information on the size and velocity of drops falling through the sampling area of the devices of roughly a few tens of cm2. It enables the drop size distribution to be estimated and rainfall microphysics, kinetic energy, or radar quantities, for example, to be studied further. Raw data, i.e. basically a matrix containing a number of drops according to classes of size and velocity, along with more aggregated ones, such as the rain rate or drop size distribution with filtering, are available. Link to the data set: https://zenodo.org/record/1240168 (DOI: https://doi.org/10.5281/zenodo.1240168)

Inputs of X-band radars to high resolution urban water management

International audienc

Improving Universal Multifractal parameter estimation for large data sets, a case study in the Greater Paris

International audienc

Development and analysis of a simple model to represent the zero rainfall in a universal multifractal framework

High-resolution rainfall fields contain numerous zeros (i.e. pixels or time steps with no rain) which are either real or artificial – that is to say associated with the limit of detection of the rainfall measurement device. In this paper we revisit the enduring discussion on the source of this intermittency, e.g. whether it requires specific modelling. We first review the framework of universal multifractals (UM), which are commonly used to analyse and simulate geophysical fields exhibiting extreme variability over a wide range of scales with the help of a reduced number of parameters. However, this framework does not enable properly taking into account these numerous zeros. For example, it has been shown that performing a standard UM analysis directly on the field can lead to low observed quality of scaling and severe bias in the estimates of UM parameters. In this paper we propose a new simple model to deal with this issue. It is a UM discrete cascade process, where at each step if the simulated intensity is below a given level (defined in a scale invariant manner), it only has a predetermined probability to survive and is otherwise set to zero. A threshold can then be implemented at the maximum resolution to mimic the limit of detection of the rainfall measurement device. While also imperfect, this simple model enables explanation of most of the observed behaviour, e.g. the presence of scaling breaks, or the difference between statistics computed for single events or longer periods

Analyse multifractale en hydrologie. Application aux séries temporelles

This book provides a simplified description of the procedures to be used to perform an analysis of hydrological data within a multifractal framework. After a review of multifractal theory and the presentation of one model for identifying scale invariance properties, examples of applications to rainfall and discharge time series are given.Cet ouvrage propose une procédure simplifiée pour réaliser une analyse de séries hydrologiques dans un cadre multifractal. Après un rappel de la théorie des multifractales, il décrit un modèle permettant d'identifier les propriétés d'invariance d'échelle, et présente des exemples d'application à des séries temporelles de pluie et de débit. Il intéressera les enseignants et chercheurs de la discipline tant au niveau national qu'international