Overland flow under rainfall : some aspects related to modelling and conditioning factors

This study concerns the theory and some practical aspects of overland flow under rainfall. Of the conditioning factors and processes which govern the generation of overland flow, the following were studied: depression storage, infiltration, morphology and wind. Special attention was paid to wind-driven rain and its effect on raindrops, raindrop splash and wind induced shear stress on the water-air boundary in relation to overland flow.

The study was carried out by means of laboratory experiments and mathematical modelling, both numerical and analytical. The laboratory experiments included the use of two rainfall simulators and a wind tunnel. Impermeable surfaces (concrete, plastic, plywood and Perspex) and pervious surfaces (loess from Limburg, the Netherlands, and a clay loam from Alentejo, Portugal) were used. A photographic set-up was also used for the assessment of the factors affecting raindrop splash anisotropy.

In the modelling of the overland flow process, the kinematic wave approach (along with linear and nonlinear assumptions) was used, both for the numerical and analytical solutions. Two numerical models were developed: model KININF for overland flow on an infiltrating surface, and model WROF for overland flow under wind-driven rain. An analytical solution was derived for the rising limb of overland flow over infiltrating surfaces of parabolic shape. The upper boundary conditions used on overland flow studies were also discussed, both with and without wind effects.