"Le travail de thèse de Clément Fagour a porté sur un sujet innovant et d'une grande importance sociétale, axé sur une problématique de mécanique des fluides environnementaux. Sa recherche se distingue par des résultats originaux et novateurs qui dépassent le cadre des sciences de l'ingénieur, mettant en avant une approche interdisciplinaire.
La présentation s'est déroulée de manière claire et pédagogique, avec une structure facilitant la compréhension des enjeux et des résultats. Clément Fagour a su établir un équilibre entre les aspects théoriques de son travail et des mises en situation concrètes, enrichies par des références à des évènements actuels.
Au cours de la soutenance, le candidat a démontré une maîtrise approfondie de son sujet, tant sur le plan expérimental que dans les différentes dimensions scientifiques abordées. Les résultats obtenus ont été présentés avec clarté, reflétant ses capacités de synthèse et d'analyse.
Les échanges qui ont suivi la présentation ont été constructifs et enrichissiants. Clément Fagour a répondu aux question avec honnêteté et rigueur scientifique, tout en faisant preuve d'une qualité d'écoute et d'un esprit de dialogue qui ont favorisé des discussions approfondies."




THESIS

Pollution caused by urban flooding : Experimental modelling of pollutant transport due to punctual releases.

Abstract

Pollution caused by urban flooding remains overlooked by both the scientific community and management stakeholders, compared to other well-studied risks (such as the destabilisation and sweeping away of vehicles, urban furniture, buildings, and pedestrians). The effect of complex hydrodynamic processes in a flooded street network on pollutant transport has been scarcely studied. This PhD investigates the pollutant transport coming from punctual releases of matter such as chemical or biological substances found in street runoff that could infiltrate homes, close to inhabitants. The PhD is based on laboratory experiments carried out in a simplified and reduced-scale urban street network model. We examine a steady-state flood flow, already studied in previous works. The novelty of this PhD lies in examining pollutant transport within this flow. To do so, we measure mass discharges and concentration fields within streets, within an urban block with openings, and at the outlets of the street network, using conductimetry and colorimetry techniques. The first objective of the PhD is to identify the predominant hydrodynamic effects on pollutant transport and the influence of pollutant release locations. The second objective is to identify parameters governing the dynamics of polluted water introduced through openings into an urban block by varying the number of openings and the duration of the pollutant release. The main results of the PhD are as follows. (i) The distribution of pollutant mass discharges in the several streets differs from that of water discharges, both within the streets and at the outlets of the street network. This is explained by a mode of the pollutant transport that is spatially heterogeneous within the street network. The most intense mixing occurs downstream of intersections. Turbulent diffusion and dispersion due to secondary currents are the dominant mixing modes, contributing to the homogenisation of pollutant concentration. At intersections, pollutant transport is primarily driven by advection in both longitudinal and transverse directions. The pollutant plumes merge or separate following the time-averaged flow streamlines. Consequently, (ii) the pollutant distribution in the street network is highly sensitive to the exact release location relative to the bifurcation and crossroad dividing streamlines. (iii) A simplified model is proposed to describe and predict pollutant distribution at intersections, and its accuracy depends on the ability to ix precisely locate the dividing streamlines in intersections. (iv) In a porous urban block, the number of openings significantly affects maximum concentrations within the block, as well as filling times, residence times, and spatio-temporal concentration variations. (v) The initial filling duration strongly influences the flushing time of polluted water after the pollution source stops. However, the renewal rate of pollution or clear water remains constant, regardless of the filling duration, and is solely governed by the hydrodynamics of the porous urban block. (vi) An adaptation of the aggregated dead zone model is finally proposed to fit the spatially averaged concentration within the block. A link between these results and health risk implications is also explored at the end of the PhD.

Keywords

Resilience, Discard studies, Urban metabolism, Urban political ecology, Social issue.

Thesis directors

Emmanuel Mignot (LMFA), Sébastien Proust (INRAE RiverLy) 

Doctoral School

ED 162 MEGA

Laboratory

UMR 5509 LMFA, INRAE RiverLy

Defence Date

7 January 2025

Defence Language

French

Thesis Jury Members

  • Pietro Salizzoni, President
  • Emmanuel Mignot, thesis director
  • Sébastien Proust, thesis director
  • Philippe Gourbesville, rapporteur
  • Koen Blanckaert, rapporteur
  • Lylia Kateb, examiner