Auguste Gires

Impacts of small scale rainfall variability in urban areas: a case study with 1D and 1D/2D hydrological models in a multifractal framework

In this paper the sensitivity to small scale unmeasured rainfall variability (i.e. at scales smaller than 1 km by 1 km by 5 min in time, which are usually available with C-band radars) of a 1D/2D model with a 10 m resolution and a semi-distributed 1D model of the same 1.47 km2 urban area is analyzed. The 1D/2D model is the open source numerical platform Multi-Hydro, which couples (open source) distributed models of involved hydrological/hydraulic processes. The methodology implemented to evaluate the uncertainties consists of generating an ensemble of realistic rainfall fields downscaled to a resolution of 12.3 m in space and 18.75 s in time with the help of a stochastic universal multifractal model. The corresponding ensemble of hydrographs is then simulated. It appears that the uncertainty is significant and that Multi-Hydro unveils much more uncertainty than the simpler 1D model. This points out a need to develop high resolution distributed modelling in urban areas.

Multifractal analysis of a semi-distributed urban hydrological model

The aim of this paper is to quantify the sensitivity of an operational semi-distributed urban hydrological/hydraulic model to rainfall variability. A 3400 ha urban area located in Seine-Saint-Denis (North-East of Paris, France), is used as a case study. First, spatio-temporal multifractal downscaling techniques are implemented on the C-band radar data (whose resolution is 1 km in space and 5 min in time) of the 9 February 2009 rainfall event to quantify the uncertainty associated with small scale unmeasured rainfall variability. It appears that the variability should not be neglected. This finding highlights the need to implement X-band radars (whose resolution is hectometric) in urban areas. Then, multifractal tools are used on both rainfall and simulated discharges that also exhibit a scaling behaviour. It appears that the rainfall drainage system basically transmits the rainfall variability without damping it, at least in terms of multifractal statistics.

Multifractal comparison of the outputs of two optical disdrometers

ABSTRACT In this paper a universal multifractals comparison of the outputs of two types of collocated optical disdrometers installed on the roof of the Ecole des Ponts ParisTech is performed. A Campbell Scientific PWS100 which analyses the light scattered by the hydrometeors and an OTT Parsivel2 which analyses the portion of occluded light are deployed. Both devices provide a binned distribution of drops according to their size and velocity. Various fields are studied across a range of scales: rain rate (R), liquid water content (ρ), polarimetric weather radar quantities such the horizontal reflectivity (Zh) and the specific differential phase (Kdp), and drop size distribution (DSD) parameters such as the total drop concentration (Nt) and the mass-weighted diameter (Dm). For both devices, good scaling is retrieved on the whole range of available scales (2 h–30 s), except for the DSD parameters for which the scaling only holds down to few minutes. For R, the universal multifractal parameters are found to equal 1.5 and 0.2 for α and C1, respectively. Results are interpreted with the help of the classical Zh–R and R–Kdp radar relations. Editor D. Koutsoyiannis; Associate editor E. Volpi

Consequences to water suppliers of collecting rainwater on housing estates

The collection, storage and use of rainwater from roofs reduce the need for potable water. However, if water suppliers are to decrease their infrastructure costs as well as their operational costs (due to water savings), the rainwater system has to provide most of the time a significant percentage of the water demand. This paper adopts the viewpoint of the water suppliers and investigates how reliable this source of water is in the case of a housing estate, considering WC flushing as the only water demand. A housing estate was modelled and different realistic input scenarios (water demand for WC flushing, storage capacity, roof area, and rainfall) were defined. Three indicators were exhibited. The variability generated by each input on the indicators was evaluated. The indicators were estimated for 63 homogeneously distributed French cities. Among the indicators exhibited in this paper, the most relevant one is the percentage of water supplied from the tank that is secured during 95% of the days of the simulation. The main conclusion is that the optimum way of determining the storage capacity of the rainwater collection system is not the same from the viewpoint of the users than from the viewpoint of the water suppliers. Indeed, water suppliers tend to require bigger tanks in order to take into account the rainwater collection systems in their management plan.

2DVD Data Revisited: Multifractal Insights into Cuts of the Spatiotemporal Rainfall Process

Data collected during four heavy rainfall events that occurred in Ardeche (France) with the help of a 2D video disdrometer (2DVD) are used to investigate the structure of the raindrop distribution in both space and time. A first type of analysis is based on the reconstruction of 36-m-height vertical rainfall columns above the measuring device. This reconstruction is obtained with the help of a ballistic hypothesis applied to 1-ms time step series. The corresponding snapshots are analyzed with the help of universal multifractals. For comparison, a similar analysis is performed on the time series with 1-ms time steps, as well as on time series of accumulation maps of N consecutive recorded drops (therefore with variable time steps). It turns out that the drop distribution exhibits a good scaling behavior in the range 0.5-36 m during the heaviest portion of the events, confirming the lack of empirical evidence of the widely used homogenous assumption for drop distribution. For smaller scales, drop positions seem to be homogeneously distributed. The notion of multifractal singularity is well illustrated by the very high-resolution time series.

Multi-hydro hydrological modelling of a complex peri-urban catchment with storage basins comparing C-band and X-band radar rainfall data

ABSTRACT The spread of impervious surfaces in urban areas combined with the rise in the intensity of rainfall events as a result of climate change has led to dangerous increases in storm water flows. This paper discusses a new implementation of the fully distributed hydrological model Multi-Hydro (developed at École des Ponts ParisTech), when operating storage basins, and its ability to deal with high-resolution radar rainfall data. The peri-urban area of Massy (south of Paris, France) was selected as a case study for having six of these drainage facilities, used extensively in flood control. Two radar rainfall datasets with different spatiotemporal resolutions were used: Météo-France’s PANTHER rainfall product (C-band) and ENPC’s X-band DPSRI. The rainfall spatiotemporal variability was analysed statistically using Universal Multifractals (UM). Finally, to validate the application, the water level simulations were compared with local measurements in the Cora storage basin located next to the catchment’s single outlet.

Multifractal Analysis of Snowfall Recorded Using a 2D Video Disdrometer

AbstractUniversal multifractal (UM) analysis was used to investigate the scaling properties of snowfall at high temporal and spatial resolutions. Snowfall data were recorded using a 2D video disdrometer (2DVD) in the Swiss Alps. Six 1-h-long periods of snowfall, half in calm and half in light wind conditions, were selected for analysis. UM analysis was performed on reconstructed 35-m vertical columns of snowfall structure, snowfall time series at 100-ms resolution, and two-dimensional snowflake accumulation maps over a 5.12 5.12 cm2 area. Multifractal scaling was observed for the vertical structure of snow particle number concentration, for scales between about 35 and 4.4 m, and sometimes down to about 0.5 m. At smaller scales, no scaling was observed. In high-resolution time series of snowfall, evidence of scaling was found for scales between about 7 min and ~26 s in most of the analyzed hours. Snowflake accumulations within a subset of the small sampling area of the 2DVD showed no scaling properties, su...