Patagonia Lake Dam Assessment Study

The Patagonia Lake Dam has been rated as a high-hazard facility by the Arizona Department of Water Resources (ADWR), and Arizona State Parks has monitored this dam periodically using piezometers and land surveys. The dam's design is based on climate and flood models and engineering practices of the late 1960s.??Failure of this dam could transport 4.6 billion gallons of water and sediments down Sonoita Creek towards the Santa Cruz River, devastating the surrounding riparian environment, infrastructure, and the growing community of Rio Rico.

HGI instrumented the face of Patagonia Dam with 533 resistivity electrodes placed along four electrical resistivity transects.

With cooperation from Arizona State Parks, HGI instrumented the face of Patagonia Dam with 533 resistivity electrodes placed along four electrical resistivity transects. Decreases in resistivity indicate an increase in moisture content/saturation, and it is anticipated that problems deep in the dam's core can be detected long before they manifest at the surface. In addition, the lake bottom was surveyed upstream of the dam for sediment accumulation using bathymetry and marine electrical resistivity.

The electrical resistivity section below shows two survey lines collected across the dam's crest and on the downstream dam face. The survey lines were long enough to image the entire dam structure and the native rock. The crest line (top) shows a conductive core comprising finer-grained soils and clay to minimize seepage. There is a concrete spillway at the dam's base to control flow, and conductive material is shown extending to the bottom of the profile, around position 600, representing this flow. The native material on the left side is slightly more resistive than the right side, but we think we can see some influence of lake water into the native rock for a couple hundred feet. The resistivity line on the face shows the dam material becoming less conductive as it becomes drier.??This is expected for a well-functioning dam.??The host material does show some conductive material as well, perhaps due to increased moisture or a response to one of the faults in this region.

Marine electrical resistivity surveys are conducted using equipment similar to land-based surveys, the main difference being that the cable is floated behind a vessel, and data is collected continuously as the vessel performs the survey.??The resistivity data was tied to coincident measurements of GPS position and bathymetry of the lake to produce the cross-sections of the underlying lake sediments and bedrock. An example of one section of the marine electrical resistivity survey is shown below, where the more conductive (blue shades) sediments can be seen overlying the more resistive (olive to red shades) bedrock material.

These results can be used to determine sediment volume behind the dam, identify structural features, such as the potential fault at 3100 meters along the cross-section represented by the sharp lateral change in resistivity, and locate zones of preferential flow upstream of dam structures.? Combined with the bathymetry survey, shown below for the Patagonia Lake, this provides an unparalleled understanding of sediment build-up behind the dam and how this varies spatially.

As a leader in geophysical imaging, HGI is at the forefront of ensuring the safety and stability of critical infrastructure like the Patagonia Lake Dam. Our innovative use of electrical resistivity and bathymetry surveys provides insights into structural integrity and sediment accumulation, allowing for early detection of potential issues before they escalate.??Partner with us to ensure the longevity and resilience of your essential assets—because when it comes to protecting communities and the environment, precision and foresight are paramount.??