SURVEYS OF COASTAL STRUCTURES USING GEOPHYSICAL TECHNIQUES

Abstract

Coastal engineers have long relied upon bathymetric surveys to determine the extent of underwater damage to jetties and breakwaters. Though such surveys supply important information about variations in water depth around structures, they alone do not show in sufficient detail the nature of the material on the structure, the extent of subbottom features, or the nature of the subbottom upon which the structure sits. However, by conducting bathymetric surveys in conjunction with other remote-sensing techniques and diving observations, it is possible to obtain more complete knowledge of the subaqueous condition of coastal structures. During the summer of 1983 and the spring and summer of 1984, we conducted side scan sonar and shallow subbottom surveys in conjunction with bathymetric and diving surveys along three northern California coastal structures to determine the condition of the structures before extensive damage occurred. Then, we evaluated the applicability of the data collection techniques for condition surveys in general. Two of the structures surveyed are the parallel jetties that protect the entrance to Humboldt Bay, California, and the third structure is the outer breakwater at Crescent City, California. Bathymetric records and sonographs from Humboldt Bay show deep holes along much of the inside of the south jetty and off the heads of both jetties. The subbottom record from inside Humboldt Bay shows a subsurface fault, the extension of which would run under the south jetty. Sonographs from Crescent City show significant bedrock outcrops throughout the area outside the breakwater, making it difficult in places to identify the toe of the structure. The subbottom record shows that pockets of sand exist amidst the bedrock, but they are generally less than 2 m (6 ft) in thickness. We found that the side scan sonar is an excellent tool for defining the toe of the structure; also, when waves are low, it can be useful in determining armor types and the slope of structures Our subbottom system provided information on fault and bedrock locations; however, other systems need to be tested to see if any of them can locate buried armor near the structures.