Etude hydrogéologique et dimensionnement par modélisation du système-traçage du réseau karstique de Han-sur-Lesse (massif de Boine-Belgique)

Abstract

The cave of Han-sur-Lesse is a shallow-hole (gouffre de Belvaux) – resurgence (trou de Han) system excavated by the Lesse river in the Givetian limestones (380 to 360 Ma) of the Boine Massif. Since 2004, a large experimental spread (17 recording stations) was developed on the site, aiming to better understand the hydro-geological behaviour of this karstic system. Hydrographs, piezometric data and physico-chemical parameters were recorded. Tracer tests, during different hydrological conditions, were carried out. This information allowed us to suggest a conceptuel model of the hydrogeology of the Boine massif. During low water conditions, the underground Lesse course can be compared to a one-dimensional drain with a succession of underground river stretches and flooded areas. Underground river stretchs are accessible and can be easily proportioned. It is not the case for flooded areas. Based on comments of divers, these areas are considered as free surface high flow sections developed in intensely fractured and karstified limestones. One of the aims of this work is to proportion these areas. A tracer test with sampling sites located upstream and downstream of flooded areas was carried out in September ‘09 in low water conditions. The measured breakthrough curves showed that the crossing of flooded areas led to a slowing down of the tracer cloud and a delay in the restitution of a part of the tracer. The tracer results were modelled with the river flow and transport model OTIS (One-dimensional Transport with Inflow and Storage) (Runkel, 1998). This model is based on the temporary storage processes described by Bencala and Walter (1983). In our model, the flooded areas of the Han-sur-Lesse karstic system are considered as high flow sections composed of a sum of small flow sections in interaction with storage zones due to the heterogeneousness of the flow media. The fitting of the modelled curves on the experimental data are accurate showing that the model could be used not only to quantify the value of transport parameters but also to estimate the dimension of the saturated karst system. This model was also applied to other active karstic systems of the Boine Massif. The results are also conclusive.

Date of Award	23 Sept 2011
Original language	French
Awarding Institution	University of Namur
Supervisor	Vincent Hallet (Supervisor), Pauline Collon-Drouaillet (Jury), Yves Quinif (Jury), E. Deleersnijder (Jury), Ph Meus (Jury) & Johan Yans (President)