Real-time GPS Monitoring of Structures in the
Pacific Northwest
Howard Hansen Dam
Howard Hansen dam is a leaking earthen embankment structure that threatens Tacoma and nearby cities in the southern Puget Sound region. The dam was completed in 1961 with the primary purpose of flood control and providing water to Tacoma and the Green River Valley. In January of 2009 the US Army Corp of Engineers (USACE) discovered seepage and two surface depressions in the right abutment of the dam, suggestive of internal erosion of the embankment by the seepage. Although the USACE does not believe the dam is at immediate risk of failing in any style similar to the Teton dam, concerns about its potential for failure have led to the injection of a grout curtain within the embankment, the drawdown of the resevoir levels behind the dam, and the installation of a host of new monitoring technologies, include high-precision GPS monitoring of 14 points around the dam operated by the Pacific Northwest Geodetic Array at Central Washington University.
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Quantifiable position measurements of dam deformation are made possible by installing solid monuments with real-time GPS receivers across the dam. This study is intended to provide real-time position estimates across multiple transects of the dam and local geology to aid dam operators and engineers in hazard mitigation. The overarching goal is to determine typical deformation limits of the Howard Hansen dam caused by seasonal run-off/temperature changes, landslides and/or earthquakes. As a consequence, the applicability of real-time GPS data and experiment design will also be assessed.
Central Washington University (CWU) has partnered with the United States Army Corps. Of Engineers (USACE), Seattle Public Utilities (SPU) to instrument the Howard Hansen dam. Experiment design and GPS instrumentation was completed by SPU, the USACE provided the test site and constructed all the solid GPS monumentation located directly on the dam, CWU PANGA installed three solid bedrock reference sites and designed/constructed communications between all the receivers. This project was designed by employing the methods used in SPU's successful Tolt Dam monitoring prototype. Data processing and archiving will be completed at the PANGA laboratory at Central Washington University. The research staff of PANGA will provide professional expertise in GPS data interpretation and evaluation of monitoring techniques and methodology to the USACE, US Bureau of Reclamation and all others responsible for dam safety.
The short-baselines afforded by this dense GPS network geometry allows sub-centimeter resolution at data rates of a sample of second. These GPS data, therefore, will provide instantaneous strain measurements of the Howard Hansen Dam during earthquakes, landslides, floods and regular seasonal temperature/rainwater run-off events -important to ascertain the nature of structural response and to discriminate between normal recoverable dam motion and irreparable damage.
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SPU's Tolt pilot study used the existing Washington State Reference Network as an active reference framework
for hierarchical monitoring program involving a suite of motion engines to detect motion and relative positional
changes over short and long baselines. This tiered approach enables detection of motion resultant from regional
influences (i.e. tectonic plate movement) and more localized influences (e.g. pool level, landslides).
At Howard Hansen, an array of 13-16 GNSS stations, known as nodes, have been placed on various key
features identified by the
USACE geology and dam safety representatives. The antennae, receivers, mounts, cables, and communications are
provided on loan from CWU and WSRN partners for the duration of the study.
GPS antennae are mounted on key features in groups, with antenna cables connected to four enclosures that house the receivers, backup power, and communications equipment. Each enclosure sends data via radio links to a central processor. External communications links to CWU from the site will be handled by radio link from a nearby peak and will be independant of public communications grid. Installed auxillary power will also avoid data interuption during power outages expected to exist during an earthquake or other natural disaster.
As part of this study, a Continuously Operating Reference Station (CORS) is also established onsite
and will become part of the National Geodetic Survey's National CORS program to provide a permanent
tie to the National Spatial Reference System (NSRS) for use by USACE and as a public amenity for the region.
All materials for the CORS have been provided by CWU and WSRN partners. These CORS data are streamed out
to CWU for use in the scientific studies of PANGA with additional raw streams to SPU for purposes of providing
an offsite mirror of the processing for QA/QC purposes. Two onsite servers run in the data-house, one will be for
USACE use only and will not be connected to the offsite processing of SPU or CWU.
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The second server will be a mirror for CWU and SPU use as a QA/QC tool and as a backup for storing raw observations should external comms be lost. The raw data streams will not be made available to any other parties without express consent of USACE.
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Results or reports from the CWU or SPU processing are solely based on interpretations of the raw data as probable positional changes and do not represent any conclusions as to actual dam safety conditions. CWU is simply studying a site and conditions for academic purposes only, and SPU is simply seeking to study the nature of structural movement as one of many studies that may possibly benefit future initiatives.
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