120-5 Subsidence Rates in SE Texas from Repeated GNSS Measurements on NGS Benchmarks

Session: Land Surface Subsidence: Processes, Impacts, and Ongoing Challenges

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Abstract:

Land subsidence is a known problem in the Houston, Texas region as well as in the coastal areas of Louisiana.  However few studies have been conducted in the area of southeast Texas between Houston and the Texas-Louisiana border.  This study resurveys National Geodetic Survey (NGS) survey markers, that were monumented between 1918 and 2006, using a survey-grade GNSS system.

The first resurvey campaign took place between October 2010 and October 2012.  This resurvey was designed to compare the remeasured benchmark orthometric heights (elevations) to the oldest orthometric heights published on the NGS datasheets which were assumed to have been measured when the benchmarks were monumented.  Subtracting the new elevations from the old elevations and dividing that elevation difference by the time difference between the oldest and recent measurements yielded a land subsidence rate (-) or uplift rate.  With this first resurvey campaign, 359 benchmarks were remeasured with vertical rates calculated to be -15.5 mm/yr (subsidence) to +5.5 mm/yr (uplift) over a 5-to-92-year period depending on the benchmark.

The second resurvey campaign took place in two phases with a total of 312 benchmarks found and measured between June 2023 and May 2025.  The same instrumentation and methods were used for the second campaign, as well as the same geoid and datums.  This more recent data set was compared to the previous resurvey campaign to again determine the subsidence and uplift rates, but with more accuracy due to the known times of the measurements with no assumptions.  A maximum subsidence rate of -16.6 mm/yr and a maximum uplift rate of +10.0 mm/yr were determined between the two surveys which were between 11 and 15 years apart.

These more recent rates are like the maximum and minimum observed between the first resurvey campaign and the original benchmark measurements during monumentation, however spatially the rates do not necessarily correlate between the two time periods.  This indicates that the original rates measured assuming the oldest elevations were measured during monumentation are in error or that subsidence and uplift rates change over time, depending on the driving force for the subsidence and uplift.  Comparison of vertical rate maps from both surveys indicate the differences and if the rates are persistent or transitory.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

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