Figure 1.9. Relative sea level measured at Pier 21 in Galveston, 1910 to 1980. 
principally because of groundwater removal (19). Subsidence along the bay at Clear Lake Bayou near 
the NASA Space Center measures 5.5 feet. A housing subdivision near Baytown is now submerged 
beneath several feet of bay water, thus contributing the complex chemicals of developed properties 
and roads to the bay system. More than 30 percent of the park land (130 acres) subsided into the bay 
at the San Jacinto Battleground. Recently, the subsidence rate in these areas has decreased, in part 
related to better management of groundwater pumping regulated by the Harris County Subsidence 
District. 
Not all man-induced subsidence relates to ground-water pumpage; some subsidence clearly 
relates to oil and gas production, especially as production includes reservoir water as well as oil and 
gas. In the Galveston Bay area a larger net subsidence represents the integration of pumping ground 
water and petroleum. The two localities of maximum subsidence, Pasadena and Baytown, probably 
experienced exploitation of both fluids. 
Faults 
Faults related to the original deposition of sediments and to subsequent formation of salt domes 
persist as planes of weakness and remain active today on the land surface (19) and on the seafloor of 
the bays and Gulf (7). Depositional and compactional faults generally form arcuate trends, more than 
20 miles long, subparallel to the Gulf shoreline. Faults associated with salt diapirs typically form a 
peripheral complex of horsts and grabens constructed of short straight faults with a radial pattern. 
Natural escarpments at the surface, which reflect the vertical offsets of the faults, are generally less 
than 3 feet high. The natural fault scarps may be frequently very subtle features. 
Because natural faults are commonly planes of weakness susceptible to further displacement 
from subsidence, larger surficial offsets and high fault scarps may occur. Elevation differences on 
each side of the Hockley escarpment measure as much as 45 feet in 1 mile. Detrimental effects of active 
faults underneath transportation routes and buildings on land and under or along pipelines in the 
bay can be significant. 
Erosion and Accretion 
Erosion is a predominant, nearly ubiquitous, process around Galveston area bays and on Gulf 
beaches (Table 4) (17,18), except where deltation or spits naturally develop. This erosion and conse¬ 
quent land loss represents the summation of (1) sea-level rise, (2) a wave-dominated shallow bay, (3) 
episodic tropical storms and northers, and (4) minor subsidence. Land losses along the Gulf shoreline 
reflect a deficit in sediment supply and relative sea-level rise or compactional subsidence (18). High¬ 
est rates of natural accretion occur at the bayhead delta of the Trinity River, where the shoreline 
advanced as much as 42.6 feet per year between 1851 and 1982. 
The largest rates of accretion or erosion are invariably related to human activities. Inordinately 
18 
