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

Remodelage d'un seuil de station hydrométrique et continuité hydrologique Compromis entre sensibilité hydrométrique et franchissabilité.

International audienc

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

Implication of two in-stream processes in the fate of nutrients discharged by sewage system into a temporary river

The aim of this study was to better understand the fate of nutrients discharged by sewage treatment plants into an intermittent Mediterranean river, during a low-flow period. Many pollutants stored in the riverbed during the low-flow period can be transferred to the downstream environments during flood events. The study focused on two processes that affect the fate and the transport of nutrients, a physical process (retention in the riverbed sediments) and a biological process (denitrification). A spatial campaign was carried out during a low-flow period to characterize the nutrient contents of both water and sediments in the VSne River. The results showed high nutrient concentrations in the water column downstream of the treated wastewater disposal (up to 13,315 mu g N/L for ammonium and 2,901 mu g P/L for total phosphorus). Nutrient concentrations decreased rapidly downstream of the disposal whereas nutrient contents in the sediments increased (up to 1,898 and 784 mu g/g for total phosphorus and Kjeldahl nitrogen, respectively). According to an in situ experiment using sediment boxes placed in the riverbed for 85 days, we estimated that the proportion of nutrients trapped in the sediments represents 25% (respectively 10%) of phosphorus (respectively nitrogen) loads lost from the water column. In parallel, laboratory tests indicated that denitrification occurred in the VSne River, and we estimated that denitrification likely coupled to nitrification processes during the 85 days of the experiment was significantly involved in the removal of nitrogen loads (up to 38%) from the water column and was greater than accumulation processes

Antidotes

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