When a coastal structure is photographed at low tide, an accurate permanent 

 record of all visible armor units is obtained. Through the use of stereoscopic, 

 photogrammetric instruments in conjunction with photographs, details of structure 

 geometry can be defined at a point in time. By direct comparison of photographs 

 taken at different times, as well as the photogrammetric data resolved from each set 

 of photographs, geometric changes (i.e. armor unit movement and/or breakage) of 

 the structure can be defined as a function of time. Thus, periodic inspections of the 

 structures will capture permanent data that can be compared and analyzed to deter- 

 mine if structure changes are occurring that indicate possible failure modes and the 

 need to monitor the structure(s) more closely. The Humboldt Bay Jetties, 

 Eureka, CA, were nominated for periodic monitoring by the U.S. Army Engineer 

 District, San Francisco (CESPN). 



An additional CESPN project has been monitored previously under the Periodic 

 Inspections work unit. Conditions have been defined for the Crescent City Harbor 

 Breakwater, Crescent City, California (Markle, Melby, and Kendall 1995). 



Project Location and History 



With the discovery of Humboldt Bay in 1 849 by the Josiah Gregg party, came a 

 fever of land speculation in San Francisco (O'Hara and Graves 1 99 1 ). The Bay is 

 located on the northern California coast (Figure 1). It is approximately 22.5 km 

 (14 miles) 1 long (north to south) and varies from 0.8 to 5.6 km (0.5 to 3.5 miles) in 

 width. The discovery of the Bay improved access to trade with the Trinity gold 

 mines. Passengers aboard the schooner Laura Virginia, bound for Humboldt Bay, 

 hoped that they would be able to establish businesses which would supply the 

 mines. Hans Henery Buhne piloted the schooner into Humboldt Bay in 1 850, after 

 spending days in treacherous seas attempting to find a navigable channel through 

 the crescent-shaped sandbar blocking the Bay entrance. This piloting experience 

 was the beginning of a long battle with the sea to create a stable navigable channel 

 into Humboldt Bay. 



During the 30 years following the successful piloting of the Laura Virginia into 

 Humboldt Bay, vessels relied on experienced pilots, many of whom were part of 

 Buhne's piloting business, to aid in navigating through the ever-shifting crescent- 

 shaped sandbar at the entrance to the Bay. However, the severe wave climate of the 

 northern California coast, effects of the bar on wave breaking, and the frequent 

 shifting of channels through the bar often prevented deep-draft vessels from servic- 

 ing the growing timber and agricultural businesses in the area. As commerce 

 increasingly felt the constraints placed on it by the unpredictability of navigation 

 into the Bay, concerns were voiced and drew the attention of the Federal Govern- 

 ment and ultimately the U.S. Army Corps of Engineers. 



'Units of measurement in the text of this report are shown in SI (metric) units, followed by non-SI 

 (British) units in parentheses. In addition, a table of factors for converting non-SI units of measure- 

 ment used in figures in this report to SI units is presented on page vi. 



Chapter 1 Introduction 



