Field evidences and theoretical analysis of the gravity-driven wetting front instability of water runoffs on concrete structures

Abstract

A series of field observations of the evolution of water runoffs over several vertical concrete walls directly exposed to rain falls is reported in this note. In all the cases, the main water flow originated from the top horizontal surface of the walls. The observations show that the gravity-driven wetting front may propagate in a very unstable way by developing well defined and quite regularly spaced vertical finger-like features. The mean width and the mean growth's velocity of the fingers appear locally constant, but may vary from a wall surface to another. A simple relationship between and is deduced from the field data and the narrower the fingers the higher the growth's velocity. The fingering process is tentatively interpreted by using the theoretical analysis developped by Glass et al (1989b) for the wetting front instability of infitration in unsaturated homogeneous layered soils. It is shown the model accounts qualitatively well for our observations. The variation in the geometry and kinematics of the instabilities from a wall surface to another may therefore be related to variations of the concrete structure at the microscopic scale. The relationship between and reflects the effects of the microstructure. The gravity driven-wetting front instability provides a powerful echanism for a fast and over large distance moisture transfer along concrete constructions. It also leads to an heterogeneous distribution of the moisture content along the wall surface, which may eventually result in large spatial variations of the moisture-induced damages of the building structures.Civil Engineering and Geoscience