Effect of fast drainage in karst sinkholes on surface runoff in Larzac Plateau, France

Abstract

Study region: A 1 km² mountainous karst watershed on the Larzac Plateau in Southern France. Study focus: The village of La Vacquerie, located at the outlet of the watershed, is punctually affected by flash floods during storm events. On the contrary, no runoff is observed in the village during other rainfalls. A sinkhole, draining surface flows at the entrance of the village, plays a role on these differences. Descriptions of short and intense rainfalls in 2015 (avg. 40 mm/h over 5 h) and long and light events in 2014/2016 (avg. 8 mm/h over 25 h) provide valuable data to calibrate inputs of a distributed physic-based flow model, in particular the sinkhole drainage capacity. This study aims to quantify drainage capacity changes with rainfall characteristics and to find the rain intensity threshold at which runoff exceeds sinkhole capacity. New hydrological insights for the region: The hydrological water balance of the hydrosystem is affected by the intensity variations for each of the different rain events. It shows the limited part of drainage in sinkhole (10%) during long light rains but its significant increase (40%) during short and intense rains. Drainage in sinkhole is affected by intensity variations rather than by the total amount of precipitation. When intensity exceeds 110 mm/h the sinkhole is submerged. However, this result is obtained in a case of Plateau configuration, where the saturated level, located deep below the surface (here 500 m), does not retro-affect drainage in the sinkhole.

Keywords

• Karst
• Interface
• Flood
• Hydrology
• Modeling
• Mediterranean