Bernard De Gouvello

Rainwater harvesting to control stormwater runoff in suburban areas. An experimental case-study

On a 23 ha urban watershed, 10 km East of Paris, rainwater tanks have been installed on 1/3 of the private parcels to prevent stormwater sewer overflows. This paper investigates the macroscopic effect of rainwater harvesting on runoff, and thus the potential of this technique for stormwater source control. The analysis is performed using the SWMM 5 model, calibrated on rainfall- runoff measures from two measurement campaigns, before and after the equipment. The availability of two data-sets allows the authors to point out changes in the catchments behaviour. The main findings are that: (1) catchments evolution, mainly caused by individual land-cover modifications, produces non-stationarity of the hydrologic behaviour; (2) the rainwater tanks installed, although they affect the catchment hydrology for usual rain events, are too small and too few to prevent sewer overflows in the case of heavy rain events.

Effect of substrate depth and rain-event history on the pollutant abatement of green roofs

This study compares the effectiveness of two different thickness of green roof substrate with respect to nutrient and heavy metal retention and release. To understand and evaluate the long term behaviour of green roofs, substrate columns with the same structure and composition as the green roofs, were exposed in laboratory to artificial rain. The roofs act as a sink for C, N, P, zinc and copper for small rain events if the previous period was principally dry. Otherwise the roofs may behave as a source of pollutants, principally for carbon and phosphorus. Both field and column studies showed an important retention for Zn and Cu. The column showed, however, lower SS, DOC and metal concentrations in the percolate than could be observed in the field even if corrected for run-off. This is most probably due to the difference in exposition history and weathering processes.

Has water privatization peaked? The future of public water governance

The privatization of water supply and institutional restructuring of water management – through decentralization and the penetration of global firms in local and regional markets – have been promoted as solutions to increase economic efficiency and achieve universal water supply and sanitation coverage. Yet a significant share of service provision and water resources development remains the responsibility of public authorities. The papers in this issue – with case evidence from Argentina, Chile, France, the USA, and other countries – address critical questions that dominate the international agenda on public versus private utilities, service provision, regulations, and resource development.

Use of green roofs to solve storm water issues at the basin scale – Study in the Hauts-de-Seine County (France)

At the building scale, green roof has demonstrated a positive impact on urban runoff (decrease in the peak discharge and runoff volume). This work aims to study if similar impacts can be observed at a basin scale. It is particularly focused on the possibility to solve some operational issues caused by storm water. For this purpose, a methodology has been proposed. It combines: a method to estimate the maximum roof area that can be covered by green roof, called green roofing potential, and an urban rainfall-runoff model able to simulate the hydrological behaviour of green roof. This methodology was applied to two urban catchments, one affected by flooding and the other one by combined sewage overflow. The results show that green roof can reduce the frequency and the magnitude of such problems depending on the covered roof surface. Combined with other infrastructures, they represent an interesting solution for urban water management.

Consequences to water suppliers of collecting rainwater on housing estates

The collection, storage and use of rainwater from roofs reduce the need for potable water. However, if water suppliers are to decrease their infrastructure costs as well as their operational costs (due to water savings), the rainwater system has to provide most of the time a significant percentage of the water demand. This paper adopts the viewpoint of the water suppliers and investigates how reliable this source of water is in the case of a housing estate, considering WC flushing as the only water demand. A housing estate was modelled and different realistic input scenarios (water demand for WC flushing, storage capacity, roof area, and rainfall) were defined. Three indicators were exhibited. The variability generated by each input on the indicators was evaluated. The indicators were estimated for 63 homogeneously distributed French cities. Among the indicators exhibited in this paper, the most relevant one is the percentage of water supplied from the tank that is secured during 95% of the days of the simulation. The main conclusion is that the optimum way of determining the storage capacity of the rainwater collection system is not the same from the viewpoint of the users than from the viewpoint of the water suppliers. Indeed, water suppliers tend to require bigger tanks in order to take into account the rainwater collection systems in their management plan.

Changing paradigms in water and sanitation services in Argentina: towards a sustainable model?

Within a very short period of time, the Buenos Aires metropolitan region has implemented a number of different water and sanitation service models: a federal welfare model (Obras Sanitarias de la Nación, OSN, created in 1912), a regional decentralized model (1981), concessions to the private sector (1993), and a new public organization (2006). Analysis of various facets of the sustainability of this new organization in Argentine cities demonstrates that it seems to approach the OSN model, but with territorial limitations and some features inherited from the “private parenthesis”, such as institutionalized regulation and social control of services.

A new method for modelling roofing materials emissions on the city scale: Application for zinc in the City of Créteil (France)

Today, urban runoff is considered as an important source of environmental pollution. Roofing materials, in particular, the metallic ones, are considered as a major source of urban runoff metal contaminations. In the context of the European Water Directive (2000/60 CE), an accurate evaluation of contaminant flows from roofs is thus required on the city scale, and therefore the development of assessment tools is needed. However, on this scale, there is an important diversity of roofing materials. In addition, given the size of a city, a complete census of the materials of the different roofing elements represents a difficult task. Information relating roofing materials and their surfaces on an urban district do not currently exist in urban databases. The objective of this paper is to develop a new method of evaluating annual contaminant flow emissions from the different roofing material elements (e.g., gutter, rooftop) on the city scale. This method is based on using and adapting existing urban databases combined with a statistical approach. Different rules for identifying the materials of the different roofing elements on the city scale have been defined. The methodology is explained through its application to the evaluation of zinc emissions on the scale of the city of Créteil.

Privatization: Lessons from Argentina

In the 1990s Argentina was the testing ground for the privatization of water and sanitation services by international consortia. Touted as the solution to economic woes and poor service, three major concession contracts, Aguas Argentinas S.A., in the Buenos Aires Metropolitan Region—the largest concession in the world—and Aguas del Aconquija in Tucuman and Azurix in the province of Buenos Aires together became a global reference model both for management and financing. Yet all three were terminated prematurely, with companies and the government each blaming the other. A detailed analysis of external and internal factors delves into the complex dynamics at work in each of the privatizations to explain what led to the failure of the model.