The settling processes of solid particles in stormwater detention basins are not well known and modelled. There are difficulties to transpose the results obtained on small scale physical models in laboratory to real sewer system basins because of: complex geometry, variability of fluxes, hydrodynamics, solids characteristics. Consequently, within the framework of OTHU (Field Observatory for Urban Water Management www.othu.org), we carry out specific experimental and modelling study of the Django Reinhardt stormwater settling-detention basin located in Chassieu (Lyon, France) in order to improve design and management of big size stormwater settling-detention basins. The calibrated Rubar 20 2D CFD software based on convection-diffusion terms coupled to Barre de Saint Venant equations enables to reproduce hydrodynamics with errors less than 15 % and shows good capacity to estimate sedimentation distribution at the bottom of the basin. Some 3D simulations with Fluent CFD software highlight good correlations between skin friction coefficient, Reynolds stress according to Ox axis, Rubar20 results on sedimentation distribution and the observed zones of settling.