Dam Leak Detection
Earthen embankments such as levees, tailings dams, and earthen dams are often very large, many are old, and all may use different materials in their structural components due to their geographic location, size, and operational needs. These variables present challenges in understanding and analyzing these structures as well as challenging problems with operation, maintenance, inspection, verification of structural integrity, and safety. Many of these structures are in the vicinity of downstream communities and protected lands, so great importance is being placed on monitoring due to their vulnerability and the potential catastrophic damage that could result. This has lead to an increasingly important and complex regulatory environment requiring dam leak detection for verification of structural integrity and monitoring to ensure long term viability.
HGI has developed cost effective geoelectrical dam leak detection methods for locating leaks and assessing integrity of earthen dams and levee structures…
Historically, these types of earthen structures have relied on visual inspection and while this simple tool cannot be replaced it does have obvious limitations. These structures can be so massive that visual scrutiny cannot possibly survey the entire structure effectively and efficiently. Failures can occur in the substructure with no manifestations at the surface. Structural compositional weakness can be extremely hard to detect visually, and warning signs can easily be missed.
To help understand and mitigate challenges with earthen structures, HGI has developed cost effective geoelectrical methods for detecting leaks and assessing the integrity of earthen dams and levees. The method employs a network of electrodes placed on the exterior and, where feasible, in the interior of the structure as well. A resistivity data acquisition system is used to acquire time lapse 2D and 3D electrical resistivity images of the materials used in the structure and the geologic formation that supports it. This requires a baseline set of resistivity measurements, and then at a pre-determined point in the future, repeat measurements are collected to help delineate zones of maximum change. Since it is reasonable to assume that the intrinsic resistivity of the construction materials and geologic formation will not vary appreciably over time, areas of maximum percent change from background (lower resistivity in a relative sense) will most likely be due to an increase in pore water content or the ionic strength of the pore water. This methodology is an adoption of the electrical resistivity method and has been used successfully to map subsurface geology, such as fine grain versus coarse grained lithological units, relative spatial distribution of variations in pore water content above and below the water table, monitoring fluid infiltration through geologic materials, and delineating preferential groundwater flow pathways.
Case Study – Patagonia Lake State Park
The Patagonia Lake Dam has been rated as a high hazard facility by the Arizona Department of Water Resources (ADWR). ADWR and Arizona State Parks (ASP) have monitored this dam periodically using piezometers and land surveys. The dam’s design is based on climate and flood models and engineering practices in the late 1960’s. Failure of this dam could cause 4.6 billion gallons of water and sediments to be transported down Sonoita Creek towards the Santa Cruz River, devastating the surrounding riparian environment and infrastructure, and drowning the community of Rio Rico. In 2009, with cooperation from ASP, HGI instrumented the face of the dam with 533 resistivity electrodes installed along four transects. The dam as characterized (graphic above) for the baseline survey, from which all subsequent surveys will be compared. Decreases in resistivity indicate an increase in saturation and it is anticipated that problems deep in the dam’s core can be detected long before they manifest at the surface.
The 2D resistivity images illustrate the benefit of time lapse resistivity monitoring applied to these structures. Though subtle, the difference in resistivity between the two images is significant enough to identify an anomalous low resistivity zone at the toe of the structure due to an increase in pore water content, perhaps the result of a preferential pathway for fluid movement.