Reconstructing the geochronological evolution of large landslides by means of the trenching technique in the Yesa Reservoir (Spanish Pyrenees)

Detailed geomorphological mapping has revealed that the slopes upstream of the Yesa Dam are affected by several flysch landslides. The largest landslides correspond to La Refaya rotational slide (11 million m3), situated just upstream of the spillway and El Vertedero translational slide (6 million m3), located 1.5 km upstream of the dam. A portion of El Vertedero landslide (3 million m3) was reactivated in 2006 due to overloading caused by an earth heap. A higher earth dam will be built on the downstream side of the existing gravity concrete dam to raise the maximum water level of the reservoir by 23 m. The retrodeformation analysis of the deposits exposed in two trenches excavated in the upper part of these landslides together with the obtained absolute ages (C14 and OSL) have provided information on the kinematics and evolution of these slope movements. The main findings derived from the 45 m long trench dug in the closed depression developed at the foot of the head scarp of La Refaya landslide include: (1) The landslide is older than 12 ka and has undergone a cumulative rotation higher than 17°, yielding a mean rotation rate of about 1.4°/millennium. (2) Most of the rotation (14°) occurred before 5.8 ka and according to the trench stratigraphy the kinematics of the landslide has been dominated by progressive displacement; no evidence of catastrophic reactivation has been found. The 45 m long trench excavated across the scarp and bench situated at the head of the non reactivated portion of El Vertedero translational slide reveals that: (1) The landslide is older than 38 ka. (2) The trenched portion has undergone at least one reactivation subsequent to 20 ka and has remained inactive for a long time as suggested by the highly degraded bedrock scarp, with a slope of 18° compared to the 60° dip of the underlying failure plane. Based on this study, corrective measures with an estimated cost of 11.5 million euros have been designed for this slope. The investigation illustrates the usefulness and cost-effectiveness of the trenching technique allied with geochronological methods to obtain information on the precise limits, age, failure mechanism, kinematics (progressive vs. episodic displacement, movement rate) and chronological evolution of landslides.