201-11 Changes in Recent Subsidence and Uplift Rates in SE Texas Based on Resurveying of National Geodetic Survey Benchmarks

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 southeast Texas between Houston and the Texas-Louisiana border.  This study resurveys National Geodetic Survey (NGS) survey benchmarks, that were monumented between 1918 and 2006, using a survey-grade GNSS system.

The first resurveying 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 by the NGS, 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 resurveying campaign, 359 benchmarks were remeasured with vertical rates calculated between -15.5 mm/yr (subsidence) to +5.5 mm/yr (uplift) over a 5 to 92-year period.

The second resurveying 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 was compared to the previous resurveying campaign to again determine the subsidence and uplift rates, but with more accuracy because the dates and times of the measurements during both surveys is known, 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 first and second resurveying campaigns which were 11 to 15 years apart.

These more recent rates are similar to the maximum and minimum rates observed between the first resurvey campaign and the original benchmark measurements during monumentation, however the rates do not necessarily correlate spatially between the two time periods.  This indicates that the original rates, which were determined by assuming the oldest elevations were measured during monumentation may be in error or that subsidence and uplift rates changed over time.  Comparison of vertical rate maps from both surveys indicate where the differences occur, if the rates are relatively consistent or if they change dramatically over time.

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