Influence of the topography on odour dispersion from a landfill located in complex terrain

Abstract : Odour emissions from municipal solid wastes create secondary atmospheric pollution leading to complaints within communities. Due to waste accumulation orography within a landfill is continuously modified. Terrain-induced variability of the airflow exerts an important influence on odour dispersion. This study aims at investigating the impacts of the orography variation within a landfill located in complex terrain. High-resolution large-eddy simulation approach has been used to study odour dispersion around the site. Numerical simulations have been performed with the Advanced Regional Prediction System (ARPS) model using several nesting levels down to a 100-m horizontal resolution. Small-scale modelling is required to take account of the whole frequency range of the fluid and odour motions. Terrain elevation was accurately measured within the landfill area and was processed using a geographical information system (GIS). Terrain elevation as well as physiographic data were carefully integrated in the ARPS model. An eulerian model of dispersion has been developed as an add-on to the ARPS code to follow odours. The model uses a realistic hourly emission inventory realized during a measurement campaign within the landfill site. Numerical simulations have been conducted to investigate local atmospheric dynamics using different topographies within the landfill area: the actual topography, the topography representing the site at the beginning of the exploitation 30 years ago and an estimation of the topography at the end of the exploitation in about 25 years (a 90- m thickness of wastes filling the small valley). This study focuses on a 3-day episode favourable to odour pollution as high-pressure systems located over the area produced clear skies and weak thermally driven winds. Results stress that topography changes modify odour dispersion over a large distance around the site. Local-scale circulations are affected in terms of wind direction and the atmospheric stability may be reinforced or weakened. Therefore the terrain elevation variability throughout the landfill site is likely to modify location and intensity of odour pollution for given atmospheric conditions and must be taken into account for prospective studies and landfill management.