Reperméabiliser les sols urbains : une démarche d’amélioration de la qualité de eaux pluviales qui tarde à se mettre en place à Montpellier ?

International audienceUrban pavements, one of the objectives of which is to evacuate rainwater as quickly as possible, have become, in a short time, the source of many problems. They generate large amounts of runoff that aggravate flooding and degrade the quality of surface water. They also contribute to rising temperatures in cities - as well as hindering biodiversity. Our article, which is based on interdisciplinarity, evokes the situation of the Metropolis of Montpellier (Southern France), which is experiencing one of the strongest demographic and urban growth in France. It raises the question of the impact of increasing construction on the quality of water in a small and fragile coastal catchment area by the Mediterranean. It also questions local public policies in an attempt to identify the brakes and levers on the depaving actions encouraged by certain government agencies - which promote alternative green techniques (and the return to pervious urban soils) for managing runoff more efficiently. To do so, it mobilises empirical material collected during a sociological survey - in addition to hydrological and chemical measurements. We conclude our reflection by suggesting ways to democratically think about the dismantling of modern coating infrastructures, to slow down urban water flows, and thus to improve the habitability of our cities.L’imperméabilisation des sols urbains, dont l'un des objectifs est d'évacuer au plus vite le ruissellement pluvial, est devenu, en peu de temps, la source de nombreux problèmes. Cela contribue à augmenter le volume ruisselé et par là même aggrave le risque d’inondation et dégrade la qualité des eaux de surface. Cette imperméabilisation, le plus souvent réalisé en bitume noir ou en béton gris, contribue également à la hausse des températures dans les villes - ainsi qu'à la baisse de la biodiversité. Notre article, qui s'appuie sur l'interdisciplinarité, évoque la situation de la Métropole de Montpellier (Sud de la France), qui connaît l'une des plus fortes croissances démographiques et urbaines de France. Elle pose la question de l'impact de l'augmentation des constructions sur la qualité de l'eau dans un bassin versant côtier petit et fragile du bord de la Méditerranée. Elle interroge également les politiques publiques locales pour tenter d'identifier les freins et les leviers aux actions de désimperméabilisation encouragées par les agences gouvernementales - qui promeuvent des techniques vertes alternatives (et le retour aux sols urbains perméables) pour mieux gérer les ruissellements. Pour ce faire, il mobilise du matériel empirique collecté lors d'une enquête sociologique - en plus de mesures hydrologiques et chimiques. Notre réflexion permet de suggérer des pistes pour penser démocratiquement le démantèlement des sols urbains imperméables, pour diminuer et freiner les ruissellements des zones urbaines, et ainsi améliorer à la fois l'habitabilité de nos villes et la qualité des eaux côtières

Antidotes aux urgences

Antidotes aux urgence

Utilisation et qualification d'un débitmètre à effet doppler (ADC) sur une rivière intermittente

Intermittent rivers pose different challenges to stream rating due to high spatial and temporal gradients. Long dry periods, cut by short duration flush flood events explain the difficulty to obtain reliable discharge data, for low flows as well as for floods: problems occur with standard gauging, zero flow period, etc. Our study aims to test the use of an acoustic Doppler current meter (ADC) for improving stream rating curves in small catchments subject to large variations of discharge, solid transport and high eutrophisation levels. The study is conducted at the outlet of the river Vette, a small coastal river (67 km2) located close to the city of Montpellier (France). The low flow period lasts for more than 6 month; during this period the river flow is sustained by effluents from urban sewage systems, which allows development of algae and macrophytes in the riverbed. The ADC device (Sontek ODArgonaut SW) is a pulsed Doppler current profiling system designed for measuring water velocity profiles and levels that are used to compute volumetric flow rates. It is designed for shallow waters (less than 4 meter depth). Its main advantages are its low cost and high accuracy (1% of the measured velocity or 0.05 m/sec, as reported by the manufacturer). The study evaluate the improvement in rating curves in an intermittent flow context and the effect of differences in sensitivity between low and high water level, by comparing mean flow velocity obtained by ADC to direct discharges measurements. The study also report long-term use of ADC device, by considering effects of biofilms, algae and macrophytes, as well as solid transport on the accuracy of the measurements. In conclusion, we show that the equipment is reliable and is able to provide additional information. However we show also that it is always difficult to measure low flow

Tache d’Observation BV-Thau, Système d’Observation Recherche en Environnement Côtier de Thau (REC-Thau) poster

International audienc

Antidotes

25-030-A-30info:eu-repo/semantics/publishe

Spatial and temporal trends in water quality in a Mediterranean temporary river impacted by sewage effluents

This paper analyzes how changes in hydrological conditions can affect the water quality of a temporary river that receives direct inputs of sewage effluents. Data from 12 spatial surveys of the VSne river were examined. Physico-chemical parameters, major ion, and nutrient concentrations were measured. Analyses of variance (ANOVA) and multivariate analyses were performed. ANOVA revealed significant spatial differences for conductivity and major ion but no significant spatial differences for nutrient concentrations even if higher average concentrations were observed at stations located downstream from sewage effluent discharge points. Significant temporal differences were observed among all the parameters. Karstic springs had a marked dilution effect on the direct disposal of sewage effluents. During high-flow periods, nutrient concentrations were high to moderate whereas nutrient concentrations ranged from moderate to bad at stations located downstream from the direct inputs of sewage effluents during low-flow periods. Principal component analysis showed that water quality parameters that explained the water quality of the VSne river were highly dependent on hydrological conditions. Cluster analysis showed that when the karstic springs were flowing, water quality was homogeneous all along the river, whereas when karstic springs were dry, water quality at the monitoring stations was more fragmented. These results underline the importance of considering hydrological conditions when monitoring the water quality of temporary rivers. In view of the pollution observed in the VSne river, "good water chemical status" can probably only be achieved by improving the management of sewage effluents during low-flow periods