Gislain Lipeme Kouyi

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.

Spatial variability of sediment ecotoxicity in a large storm water detention basin

Detention basins are valuable facilities for urban storm water management, from both the standpoint of flood control and the trapping of pollutants. Studies performed on storm water have shown that suspended solids often constitute the main vector of pollutants (heavy metals, polycyclic aromatic hydrocarbon (PAH), etc.). In order to characterise the ecotoxicity of urban sediments from storm water detention basins, the sediments accumulated over a 6-year period were sampled at five different points through the surface of a large detention basin localised in the east of Lyon, France. A specific ecotoxicological test battery was implemented on the solid phase (raw sediment) and the liquid phase (interstitial water of sediments). The results of the study validated the method formulated for the ecotoxicological characterization of urban sediments. They show that the ecotoxicological effect of the sediments over the basin is heterogeneous and greater in areas often flooded. They also show the relationship between, on one hand, the physical and chemical characteristics of the sediments and, on the other hand, their ecotoxicity. Lastly, they contribute to a better understanding of the dynamics of the pollution close to the bottom of detention basins, which can be useful for improving their design. The results of this research raise particularly the issue of using oil separators on the surface of detention basins.

Typology of the flow structures in dividing open channel flows

ABSTRACT This paper aims at describing two different recirculation structures observed both numerically and experimentally in the lateral branch of a dividing open channel flow with identical downstream weirs. The flow conditions leading to each identified recirculation structure are also investigated. The first recirculation pattern is a classic two-dimensional closed recirculation, typically described in the literature. The second one is a three-dimensional helix-shaped recirculation that differs from the first one notably regarding the streamline behaviour. We show that the occurrence of each structure of recirculation can be predicted by means of the Froude number and the aspect ratio in the upstream channel of the bifurcation.

Chemical, microbiological, and spatial characteristics and impacts of contaminants from urban catchments: CABRRES project

Chemical and microbial contaminations of urban waters including stormwater runoff, wastewaters, and combined sewer overflow waters have been shown for a number of years. It is clearly demonstrated, for example, that metallic ions, polycyclic aromatic hydrocarbons (PAH), pesticides, and microbial pathogens can be conveyed by such waters, and they have been found strongly associated with suspended matters. The “Water Framework Directive” (WFD) led to the launching of several research initiatives toward the “Best Management Practices – BMP,” through novel or improved technologies aiming at reducing the ecotoxicological impacts and health risks associated with these waters. Some of these techniques consist at keeping these waters for a certain amount of time in a confined system in order to favor settling of their suspended particles and lead, in part, to the natural biological degradation of their contaminants. These systems are typically impervious basins that can receive stormwaters, e.g. detention basins, biofilters, artificial wetlands, or combined sewer overflows, e.g. stabilization ponds of wastewater treatment lagoons. Several key mechanisms occur in these systems (Fig. 1). The deposits formed constitute areas of high levels of contamination. These structures (basins) are ecosystems with an important biological diversity. They can represent a high risk of contamination of the connected surfaces, streams, or groundwater environments into which they are discharged. Investigations regarding their efficiency at retaining and transforming pollutants and killing undesirable microbes are thus required. When landscaped, these structures may lead to the development of urban public spaces. In this context, they are subjected to social practices generally associated with public gardens, practices which accidentally or regularly expose the public to their contaminants. In addition, these structures require maintenance and specific management, exposing workers to their contaminants. It is therefore essential that the chemical and microbiological contaminants present in these systems are accurately characterized and measured (i.e. which chemical forms and microbial species and genotypes, and how much?), and their health hazards and risks be inferred or estimated. In this context, several biofilters, detention basins, and stabilization ponds have been monitored and investigated around the world. The CABRRES project (Chemical, microbiological, spatial characteristics and impacts of contaminants from urban stormwater detention basins: Assessment and Management of Environmental and Human Health Risks, http://www.graie. org/cabrres), funded by the French National Research Agency (ANR), is one of these research initiatives. It is an interdisciplinary research project (Fig. 2) aiming at better defining the interactions between chemical pollutants (including toxic Responsible editor: Philippe Garrigues

DSM-flux: A new technology for reliable Combined Sewer Overflow discharge monitoring with low uncertainties

In the past ten years, governments from the European Union have been encouraged to collect volume and quality data for all the effluent overflows from separated stormwater and combined sewer systems that result in a significant environmental impact on receiving water bodies. Methods to monitor and control these flows require improvements, particularly for complex Combined Sewer Overflow (CSO) structures. The DSM-flux (Device for Stormwater and combined sewer flows Monitoring and the control of pollutant fluxes) is a new pre-designed and pre-calibrated channel that provides appropriate hydraulic conditions suitable for measurement of overflow rates and volumes by means of one water level gauge. In this paper, a stage-discharge relation for the DSM-flux is obtained experimentally and validated for multiple inflow hydraulic configurations. Uncertainties in CSO discharges and volumes are estimated within the Guide to the expression of Uncertainty in Measurement (GUM) framework. Whatever the upstream hydraulic conditions are, relative uncertainties are lower than 15% and 2% for the investigated discharges and volumes, respectively.

Priority substances in accumulated sediments in a stormwater detention basin from an industrial area

One of the most adopted solutions in developed countries to manage stormwater is detention/retention basins which generate large quantities of sediments that have to be removed regularly. In order to manage them properly, accurate data are needed about their physical and chemical characteristics, particularly on micropollutant concentrations and their associated risk. This work consisted in a two-year sampling of dry sediments from a detention-settling basin. Priority substances, including pesticides, polybrominated diphenyl ethers (PBDE), alkylphenols and bisphenol A (BPA), were monitored. Different sites in the basin bottom were sampled in order to investigate spatial distribution of the contamination. Results show that the increase of the sediment thickness in the basin was heterogeneous with a maximum of 15 cm after two years. Pesticides and PBDE were, if detected, mainly found in low concentrations from 2 ng/g to 286 ng/g. Conversely, alkylphenols and bisphenol A were always quantified at concentrations varying from 6 ng/g to 3400 ng/g. These high levels suggest that these sediments should be managed with precautions. Spatial heterogeneity of alkylphenol ethoxylates and BPA concentrations was observed, with higher contamination of alkylphenol ethoxylates in anaerobic zones and BPA levels correlated with total organic carbon and in a lesser extent to fine particles.