Karst aquifers are often considered natural reservoirs that store rainfall and attenuate floods. However, under certain conditions, they can also become major sources of flooding. Their highly heterogeneous structure, combining rapid flow through conduits and fractures with slower storage in the surrounding rock, creates complex interactions between rainfall, groundwater and surface runoff.

My research on the Fontaine de Nîmes and Lez karst systems in southern France has investigated how aquifer structure, groundwater storage and hydraulic conditions control flood generation in Mediterranean catchments.

Fontaine de Nîmes: Groundwater overflow and urban flash floods

The Fontaine de Nîmes karst system, located beneath the city of Nîmes, is exposed to intense Mediterranean rainfall events. Research conducted during major storms, particularly the September 2005 event, revealed the significant contribution of groundwater to urban flash flooding.

Two complementary overflow mechanisms were identified:

• Temporary groundwater springs (boulidous): rapid aquifer saturation causes groundwater to emerge through temporary outlets, particularly in the Eaux-Bouillies area, contributing directly to flows in the temporary streams (cadereaux).
• Overflow from the main karst drainage system: during particularly intense events, the main conduit network can also overflow through existing openings, notably when spring discharge exceeds approximately 13 m³/s.

These observations demonstrated that groundwater flooding cannot be explained by a single, uniformly connected conduit network. Different parts of the aquifer can develop distinct hydraulic responses during storms.

The research also highlighted the importance of antecedent groundwater conditions. A cumulative rainfall of approximately 145–150 mm over the preceding 25 days was identified as an indicator of increased risk of aquifer saturation and overflow.

These findings contributed to the development of rainfall–runoff modelling and the integration of real-time groundwater monitoring into the ESPADA flood forecasting system in Nîmes.

Lez karst system: Groundwater storage and flood dynamics

The Lez karst aquifer, near Montpellier, provides another example of the strong influence of groundwater dynamics on Mediterranean river floods. Research investigated how the initial hydraulic state of the aquifer controls the partitioning of rainfall between groundwater recharge, storage and surface runoff.

Two contrasting situations were identified:

• Unsaturated aquifer: available storage capacity allows rainfall infiltration and delays the transfer of water towards the river, contributing to flood attenuation.
• Saturated aquifer: reduced storage capacity favours surface runoff and rapid groundwater transfers, potentially increasing flood discharge.

The Lez studies also explored the possibility of using groundwater abstraction to modify aquifer storage conditions and influence flood responses. Conceptual hydrological models and artificial neural networks were developed to reproduce groundwater-level variations and spring discharge under different rainfall and pumping scenarios. This work highlighted the potential contribution of active groundwater management to flood-risk assessment and mitigation.

A common conceptual framework

The Fontaine de Nîmes and Lez studies demonstrate that karst aquifers can act both as natural flood buffers and as sources of additional floodwater.

Their response depends on three main processes:

1. Infiltration and storage in the epikarst, fractures and surrounding rock.
2. Rapid groundwater transfers through connected fractures and karst conduits.
3. Groundwater overflow through temporary springs and surface outlets when hydraulic storage capacity is exceeded.

These findings emphasize that groundwater levels and aquifer saturation must be considered alongside rainfall and surface runoff to understand and forecast karst flash floods. They also demonstrate the importance of long-term groundwater monitoring and hydrogeological modelling for operational flood warning and integrated water-resource management.

Related publications

Version française - crues karstiques