1.0 Introduction  

The San Francisco Bay Rocks Removal Project is part of a proposal by the United States Army Corps of Engineers (USACOE) to deepen and selectively widen the San Francisco Bay Bar Channel.  The deepening of the channel is linked to the removal of five underwater geologic features located near the designated ship navigation lanes (Figure 1).  Four of the five underwater geologic features are known to be bedrock outcrops identified as Blossom Rock, Harding Rock, Shag Rocks, and Arch Rock.  The fifth underwater geologic feature is a unique underwater geological formation that is referred to as Golden Gate Mound.  

Historically, underwater rocks in central San Francisco Bay, California have been problematic to navigation and several efforts have been made in the past to eliminate the hazards created by these underwater geologic features.  In the past, Blossom Rock, Harding Rock, Arch Rock, and Shag Rocks have had their tops removed by underwater blasting[1].  In modern times, these underwater rock masses have been reported to extend above -40' elevation, as referenced to the mean lower low water (MLLW) vertical datum, and pose a hazard to deep draft shipping.   

Recently, the Harbor Safety Committee declared these submerged rocks to be a hazard to navigation, and has recommended that the rocks be lowered or removed in order to decrease the possible threat of a major oil spill[2].  In support of the Harbor Safety Committee's recommendation to lower or remove these submerged geologic features, the U.S. Army Corps of Engineers (USACOE) and the California State Lands Commission funded a marine geophysical investigation to map the rock formations for future rock-core borings, and to characterize the material properties of the rock masses for engineering analyses (Sea Surveyor 2001).
 

[1] In 1870 Blossom Rock was reduced from approximate elevation of –5 feet MLLW to below –24 feet MLLW.  At the beginning of the 1900's, Blossom Rock, Arch Rock, and Shag Rocks had their tops removed to a depth below -30' MLLW.  Again in the 1930's, Arch Rock and Shag Rocks had their tops removed to a depth below -35' MLLW and Blossom Rock was lowered to -42' MLLW.  In 1932, Harding Rock had its top lowered by blasting to -35' MLLW. 

[2] As mandated by the State Oil Spill Prevention and Response Act, representatives from government agencies, industry, recreation, economic, and environmental groups comprise the Harbor Safety Committee.