Analysis of changes in the structural behaviour of the Lar Dam and its potential risks using radar interferometry and field experiments

Introduction
Dams are defined as installations used to provide water for various use [1]. Dams also contribute to socio-economic developments by not only providing shelter for the downstream regions by suppressing floods, but also forming reservoirs used for various purposes including irrigation, human consumption, and hydropower. These notwithstanding, current trends in the climate change together with improper management of water resources have increased the risks of flood events and drought as global crises. The collateral damage from these events have also been imposed on dam structures [2].
Case Study
The case study includes the Lar Dam, located at 85km distance from the North East of Tehran. The area receives an annual precipitation of 600 mm, more than 60% of which is often accumulated as snow. The reservoir is however dry throughout the summer, contributing to only 6% of annual precipitation. Nearly 70% of the total precipitation in the region occurs in winter and spring. The right abutment is located on calcareous formations, while the left abutment is situated on the alluvial sediments and layers of lava originating in the Mount Damavand. The main leakages from the dam have been reported in the karst regions developed on the calcareous formations.
Material and methods
Data requirements for this study were supplied by acquiring SLC images from the Sentinel-1A sensor the Soyuz satellite of the European Space Agency. The images were taken in 2014 in single polarisation mode (VV) from 28 orbits. Images with similar orbits were initially identified and the 30-meter SRTM digital elevation model was used to process the images. The required DEM files were generated using the GMTSAR software available at (https://topex.ucsd.edu/gmtsar/demgen). The data processing was conducted using interferometric synthetic aperture radar (InSAR) technique. This technique calculates the differences in the phase of waves returning to the sensor to generate an image called the interferogram; which is the differential of phase of two geometrically aligned images taken at two different time StaMPS.
Results and discussion
Noisy interferograms obtained for a pair of images were eliminated and a time series of interferograms with the least amount of noise and highest pixel count were identified for later processing. The results were indicative of increasing trends in subsidence at certain points in the dam, from 2015 to 2020. The highest amount of vertical change was identified in the form of subsidence in the left abutment area, progressing towards the Delichay River. From 2018, protrusions started to form in the abutment as it started to swell, requiring further investigations in terms of geology of the region and dam behaviour.
Based on the results of interferometry, the total subsidence and swelling points were observed in three main areas; the middle of the right abutment where vertical deformations were observed in the form of subsidence at downstream and upstream. The highest amount of subsidence in the area were measured at 20 mm throughout the study period, with a sudden increase in vertical deformation from 2018 onwards. The sudden escalation in deformation in the left abutment and dam body is a major cause for concern about the stability and safety of the dam. The main cause of these deformations was identified as breaches in the calcareous and karstic ducts within the reservoir, causing water to seep under the dam structure. Leakage from the karsts and calcareous ducts cause an increase in water flow, which then causes porewater in the alluvial layers to flow to the calcareous ducts. The alluvial particles then fill up the calcareous pores, reducing the shear strength of soil. As a result, the soil particles are carried away from the embankment by water seeping through the dam, causing internal erosion. The prolongation of erosion together with exploitation of the dam throughout the years have caused further subsidence of the dam structure, increasing the odds of a sudden dam failure and the formation of a sink hole. Further cause for concern is the difference in height of the dam riprap overlay. These conditions of impending failure are similar to those of the Mosul Dam, highlighting the need for constant monitoring of the Lar Dam and use of geological data to develop an alarm system for mitigating the potential impacts of a dam failure and increasing safety.