SATURATED CONTRIBUTIVE AREAS IN MEDITERRA NEAN CATCHMENTS - CONDITIONFOR APPEARANCE AND CONSEQUENT FLOODS

Observations with tensiometers and piezometers near the Maurets River in the Real Collobrier basin supply interesting information about hydr ological mechanisms during floods. Series of weekly time-step measurem ents show a good correlation between river flow and piezometric levels near the river. These levels can rise only when the soil capillary pr essure before the storm event is approximately zero. More detailed obs ervations of the suction values were recorded with a half-hour time st ep. They show that only high intensity rainfall can provoke a quick in crease of positive pressures in the soil profiles (saturation), as lon g as these pressures are very close to zero before the rain. Associate d with these positive pressures a quasi-null vertical hydraulic head g radient and a strong flow in the river are observed. In such a case, t he soil saturation can be of a very short duration. Therefore, in Medi terranean conditions, saturated contributing areas can appear and deve lop causing big floods, but they seem to be very limited in time.