Identification of leakage and potential areas for internal erosion combining ERT and IP techniques at the Negratín Dam left abutment (Granada, southern Spain)

Abstract At the Negratin Dam (southern Spain) two main springs derived from the water reservoir were detected on the west side of the mountain ridge, matching the location of the left abutment of the dam. The seepages appear when the level of the stored water exceeds 622 m above sea level (m a.s.l.), where some paleochannels are embedded in the silts. The water level was at 631 m a.s.l. on the day of the measurements (13/12/23), close to the historical maximum (633.78 m a.s.l.). Although the water crossing the leaks does not drag particles nowadays, this sector may potentially cause internal erosion or piping phenomena implying a loss of water resources and slope instability problems. The infiltration areas were evaluated by electrical resistivity and induced polarization methods, measured along one profile on the mountain ridge between the water reservoir and the springs. We determine that the infiltrations zones are characterized by low resistivity and high chargeability distributed in small areas. We demonstrate that these methods are a reliable tool to detect and characterize infiltration zones, and they are the first step to planning mitigation actions to avoid the piping phenomena.