Carolina Gonzalez-Merchan

Computational fluid dynamics modelling of flow and particulate contaminants sedimentation in an urban stormwater detention and settling basin

Sedimentation is a common but complex phenomenon in the urban drainage system. The settling mechanisms involved in detention basins are still not well understood. The lack of knowledge on sediment transport and settling processes in actual detention basins is still an obstacle to the optimization of the design and the management of the stormwater detention basins. In order to well understand the sedimentation processes, in this paper, a new boundary condition as an attempt to represent the sedimentation processes based on particle tracking approach is presented. The proposed boundary condition is based on the assumption that the flow turbulent kinetic energy near the bottom plays an important role on the sedimentation processes. The simulated results show that the proposed boundary condition appears as a potential capability to identify the preferential sediment zones and to predict the trapping efficiency of the basin during storm events.

Monitoring of clogging evolution in the stormwater infiltration system and determinant factors

Infiltration is widely used to manage stormwater in cities and their suburbs. Despite its advantages, questions remain about its long-term performance, in particular the potential for clogging. To address this problem, a field study was undertaken to assess the evolution of clogging over time by means of hydraulic resistance (R) measurements. The experiment was carried out on an infiltration basin continuously monitored for 7 years from 2004 to 2010. The work shows that clogging takes place progressively and can be slowed down by the growth of vegetation. It also showed that the variation from one event to another presented high variability. The variation of successive R-values per time unit has been statistically explained by the variation per time unit of two groups of factors: one linked to physical aspects (water volume and particle load), and more interestingly one linked to biological clogging (solar energy and air temperature), which is generally not considered for these systems. L’infiltration est aujourd’hui largement utilisée dans la gestion des eaux pluviales en milieu urbain. Malgré ses avantages, son efficacité sur le long terme pose encore problème en particulier leur aptitude au colmatage. Pour traiter ce problème, des observations de terrain ont été menées de manière à évaluer l’évolution du colmatage au cours du temps à l’aide de mesures de résistance hydraulique (R). L’observation a été menée sur un bassin d’infiltration suivi en continu sur 7 ans de 2004 à 2010. Le travail montre que le colmatage est très progressif est peut être ralenti par la croissance de végétation. Il montre également une grande variabilité d’un évènement à l’autre. La variation entre deux valeurs de R par unité de temps a pu être expliquée statistiquement par les variations par unité de temps de deux groupes de facteurs : l’un lié à des aspects physiques (volume d’eau et charge particulaire) et de manière plus intéressante à des facteurs biologiques (énergie solaire et température d’air) non considérés habituellement.

Review on physical and chemical characterizations of contaminated sediments from urban stormwater infiltration basins within the framework of the French observatory for urban hydrology (SOERE URBIS)

Urban stormwater infiltration basins are designed to hold runoff from impervious surfaces and allow the settling of sediments and associated pollutants. However concerns have been expressed about the environmental impacts that may be exerted by the trapped pollutants on groundwater, soils and ecosystems. In this context, sediment characterization represents a key issue for local authorities in terms of management strategies. During the last two decades, several studies were launched including either physical or chemical characterization of stormwater sediments but without real synthesis of data and methods used. Consequently, there is an important need for reviewing the current experimental techniques devoted to the physico-chemical characterization of sediment. The review is based on the outcomes of two experimental sites for which long term monitoring and data collection have been done: the Cheviré basin (near Nantes) and the Django Reinhardt basin (near Lyon). The authors summarize the studies dealing with bulk properties, pollutant contents, their potential mobility and speciation. This paper aims at promoting the significant progresses that were made through a multidisciplinary approach involving multi-scaled and combined experimental techniques.

Stormwater retention basin efficiency regarding micropollutant loads and ecotoxicity.

Retention basin efficiency in micropollutant removal has not been very well studied, in particular for pollutants highlighted by the European Water Framework Directive of 2000 such as pesticides, polybrominated diphenyl ethers (PBDEs) and alkylphenols. This study is based on in situ experiments carried out on a stormwater retention basin with the aim of estimating the basin efficiency in trapping and removing micropollutants from stormwater run-off from an industrial catchment drained by a separate sewer system. Along with stormwater, the basin receives some dry weather effluent flows, which are supposedly non-polluted. Ninety-four substances from five families (metals, polycyclic aromatic hydrocarbons (PAHs), PBDEs, alkylphenols and pesticides) were analyzed during 10 event campaigns in urban wet weather discharges at the inlet and outlet of the basin. The ecotoxicity of the samples was also tested. The results show high inter-event variability in both chemical and ecotoxic characteristics. They indicate good event efficiency concerning heavy metals and most PAHs. The studied pesticides, mainly found in the dissolved fraction, were not trapped. Particulate fraction study highlighted that settling is not the main process explaining micropollutant removal in a retention basin, as was noted for alkylphenols and PBDEs.

Influence of spontaneous vegetation in stormwater infiltration system clogging

The paper presents the role of spontaneous vegetation on the hydraulic performance of an infiltration basin. The objective of the research was more particularly to study this role of different types of spontaneous vegetation found in situ in an infiltration basin near Lyon. The saturated hydraulic conductivity of three areas covered by Phalaris arundinacea, Polygonum mite, Rumex crispus and similar non-vegetated zones was compared. Eight field campaigns were carried out from July 2010 to May 2011 in order to compare the performance of each type of vegetation and its evolution over time. The results suggest a positive impact of vegetation on hydraulic performance in particular in summer during the growth of the plants. The hydraulic conductivity in this period was twice to four times higher than in bare areas or in vegetated zones during the plant rest periods. Some species were also found more appropriate to limit clogging (Phalaris arundinacea) likely due to its specific structure and growth process.

Ecotoxicological characterisation of sediments from stormwater retention basins

Retention-detention basins are important structures for managing stormwater. However, their long-term operation raises the problem of managing the sediments they accumulate. Potential uses for such sediments have been envisaged, but each sediment must be characterised beforehand to verify its harmlessness. In this paper we address this issue through the development of a battery of bioassays specifically adapted to such sediments. We tested the method on samples taken from four retention basins in the region of Lyon (France). This battery focuses on the toxic effects linked to both the solid phase (ostracod and Microtox(®) solid-phase tests) and the liquid-phase (interstitial water) of sediments (rotifer and Microtox(®) liquid-phase tests). The results obtained permit the sorting of sediments presenting little toxicity, and which could therefore be potentially exploitable, from those from more polluted areas presenting higher toxicity that limits their use.

A method for evaluating the evolution of clogging: application to the Pampulha Campus infiltration system (Brazil)

In order to evaluate the hydraulic performance of stormwater infiltration trenches, a study was undertaken to assess clogging and its distribution between the bottom and the sides. The method used was based on the calibration of the hydraulic resistance event by event according to Bouwers model and applied to a demonstration trench in Belo-Horizonte monitored in the framework of the European Project Switch. The calibration was performed by minimizing the distance between measured and modeled infiltration flow rates and by using continuous measurements of rainfall, inflow, water temperature and depth in the trench. The study showed that the methodology and particularly Bouwers model was able to produce satisfactory results. It revealed a significant clogging evolution within a year, with global resistance increasing by a factor of 9. A significant difference between the bottom and the sides was observed; the bottom being more rapidly prone to clogging. Sudden fluctuations of the hydraulic resistance of the bottom were found that could be explained by very high concentrations of total suspended solids from inflows (about 2,000 mg/L). Clogging of the sides evolves over the time but with a very low rate.