Coastal Construction Manual - National Ready Mixed Concrete ...

3 IDENTIFYING HAZARDS
and a step-by-step process to follow. Although the publication was written with USACE projects in mind,
the guidance will be helpful to those planning and designing coastal residential buildings.
3.3.4.2 Subsidence and Uplift
Subsidence is a hazard that typically affects areas where (1) withdrawal of groundwater or petroleum has
occurred on a large scale, (2) organic soils are drained and settlement results, (3) younger sediments deposit
over older sediments and cause those older sediments to compact (e.g., river delta areas), or (4) surface
sediments collapse into underground voids. The last of these four is most commonly associated with mining
and rarely affects coastal areas (coastal limestone substrates would be an exception because these areas
could be affected by collapse). The remaining three causes (groundwater or petroleum withdrawal, organic
soil drainage, and sediment compaction) have all affected coastal areas in the past (FEMA 1997). One
consequence of coastal subsidence, even when small in magnitude, is an increase in coastal flood hazards due
to an increase in flood depth. For example, Figure 3-16 shows land subsidence in the Houston-Galveston
area. In portions of Texas, subsidence has been measured for over 100 years, and subsidence of several feet
has been recorded over a wide area; some land areas in Texas have dropped 10 feet in elevation since 1906.
Subsidence also complicates flood hazard mapping and can render some flood hazard maps obsolete before
they would otherwise need to be updated.
Figure 3‐16.
Land subsidence in the Houston-Galveston area, 1906–2000
SOURCE: HARRIS-GALVESTON SUBSIDENCE DISTRICT 2010
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