Long term monitoring of sewer sediment accumulation and flushing experiments in a man-entry sewer.

This paper presents continuous field experiments carried out during 4 years in a man-entry egg-shaped combined sewer in Lyon, France in order to contribute to the knowledge and the modelling of sediment accumulation and sediment removal by means of a Hydrass flushing gate. The 250 microm sediments are mainly mineral, and their physical and chemical characteristics appear as rather stable in time and space. Long-term sediment monitoring reveals: (i) a regular asymptotic increase of both the sediment mass and the slope of its longitudinal profile, (ii) a clear correlation between local sediment profile irregularities and sewer ancillaries but without significant influence on the global and long term accumulation. Simple sediment accumulation modelling shows: (i) a good suitability of a three parameters conceptual model to reproduce asymptotic sediment volume accumulation, and (ii) a good suitability of the Velikanov model to reproduce sediment profiles. Both models reproduce observations with an acceptable margin of uncertainty for operational management purposes but are very sensitive to input data and parameter values. The Hydrass flushing gate is efficient and it appeared that the mass of sediments moves downstream linearly with the number of flushes.

Statistical analysis of pollution in stormwater infiltration basins.

In order to characterize soil pollution in stormwater infiltration basins, four basins of the Lyon area were selected and sampled. Altogether, 66 soil samples were collected. Each sample was analyzed for pH, cationic exchange capacity, particle size distribution, total organic carbon, total nitrogen, total phosphorus, metals, total hydrocarbons and polycyclic aromatic hydrocarbons (PAHs). Data was explored in a classic way (soil profiles) and using multivariate analysis techniques. Two methods were applied: Principal Component Analysis (PCA) and Cluster Analysis (CA). Main results are presented in this paper. Pollutant concentrations decrease rapidly with depth while pH and grain size increase. Sustainable concentrations are reached at a 50-cm depth, even after 21 years of operation. Multivariate analysis shows how pollution affects each sampling depth.

Hydraulics of a sewer flushing gate.

This paper presents field experiments carried out in a 1.8 m height egg-shaped sewer in Lyon, France in order to contribute to the knowledge and the simulation of Hydrass flushing gates. The main results are: i) definition of an empirical relationship giving the flow discharged by the gate as a function of the upstream water level and its use in modelling the gate behaviour, and ii) observation of the flush propagation along the sewer and evaluation of the potential cleansing efficiency.

Accounting for sensor calibration, data validation, measurement and sampling uncertainties in monitoring urban drainage systems.

Assessing the functioning and the performance of urban drainage systems on both rainfall event and yearly time scales is usually based on online measurements of flow rates and on samples of influent effluent for some rainfall events per year. In order to draw pertinent scientific and operational conclusions from the measurement results, it is absolutely necessary to use appropriate methods and techniques in order to i) calibrate sensors and analytical methods, ii) validate raw data, iii) evaluate measurement uncertainties, iv) evaluate the number of rainfall events to sample per year in order to determine performance indicator with a given uncertainty. Based an previous work, the paper gives a synthetic review of required and techniques, and illustrates their application to storage and settling tanks. Experiments show that, controlled and careful experimental conditions, relative uncertainties are about 20% for flow rates in sewer pipes, 6-10% for volumes, 25-35% for TSS concentrations and loads, and 18-276% for TSS removal rates. In order to evaluate the annual pollutant interception efficiency of storage and settling tanks with a given uncertainty, efforts should first be devoted to decrease the sampling uncertainty by increasing the number of sampled events.

Evaluation of uncertainties in urban hydrology: application to volumes and pollutant loads in a storage and settling tank.

It is necessary to assess adequately measurement uncertainties of experimental data in urban hydrology in order to draw pertinent and valuable conclusions from measurement results. This paper first gives a brief presentation of the concepts relative to standard uncertainties and to the law of propagation of uncertainties. Then it presents an example of calculation of the uncertainties in some quantities describing the behaviour of the Vénissieux (France) storage and settling tank during one rainfall event. These quantities are the flow rates and the volumes entering into the tank, the inflow and outflow mean concentrations and masses of TSS (Total Suspended Solids), and the TSS removal rate. Variographs are used to account for the autocorrelation of time series data. The results lead to the following relative uncertainties 8% for the total volume, 30% for the inflow concentration, 38% for the outflow concentration, 31% for the inflow mass, 39% for the outflow mass and 138% for the removal rate.

The purification performance of infiltration basins fitted with pretreatment facilities: a case study.

In the south-east of France, the evacuation of stormwater by infiltration into the ground is being developed for large aquifer systems such as the ground water in the Rhône valley and in the eastern part of Lyons. A study proposal has been presented to the water management department of the conurbation of Lyons, aimed at quantifying, within a reasonably short space of time, the effects, in terms of transport of pollutants, of the stormwater infiltration system in the underground water in eastern Lyons. To this end, a one year duration experiment was carried out on the Vénissieux infiltration basin which drains stormwater from a 380 hectares industrial catchment area. Its peculiar configuration also made it possible to acquire new knowledge on the qualitative operation of a few pretreatment facilities. After describing the operation of the basin and the experimental protocol, we shall present a body of data that we monitored and our conclusions about the behaviour of the pollution throughout the facilities. Then, we present methods used to assess the pollution removal performance of the infiltration basin and its pretreatment devices, the results obtained, and our conclusions about the impact of the infiltration basin on groundwater and soil.