Seabed obstructions

Should a desk study indicate the potential presence of wrecks, ordnance, sub-marine pipelines or other obstacles in the borrow area or the fill area, more detailed investigations are required. 

Apart from damage to the dredging equipment, a wreck itself can sometimes be of archaeological importance and may not be disturbed. Therefore, all wrecks and obstacles should be localized with a wreck survey and if necessary removed from the seabed before dredging operations begin. Large wrecks or 

Obstructions can be detected either by side scan sonar or with a magnetometer. The side scan sonar cannot identify buried objects. A magnetometer can only detect metal objects which cause a magnetic anomaly. Therefore, one can never be sure whether the subsurface is free of obstacles. Shallow geophysical methods can be helpful for this. 

The following types of hazards and obstacles may seriously hinder dredging operations  

Desk study Research into available archives (Wreck charts. Wreck database) 

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Localization is no easy task, especially when bearing in mind that many of the wrecks and other objects that have to be removed before dredging takes place are not visible to the naked eye or to standard hydrographical survey techniques (Dredging News Online, 2000). An overview of some common wreck survey techniques together with their advantages and drawbacks is given in appendix B/section 4 detection of seabed obstructions. 

Over the past ten years the high-resolution marine geophysical survey field has witnessed significant advances in survey investigation equipment. New equipment is based on optical, acoustic, electrical, and magnetic sensors. Some of these techniques can be used to detect and map pipelines, cables, wrecks and other seabed obstructions. Common used techniques include ... 

Acoustic methods are ideal for deep or turbid waters (unclear because of stirred-up sediment). Unlike optical methods, such as a laser or underwater camera, water characteristics such as turbidity and light penetration do not affect the acoustic sonar sensor. When a seabed obstruction is not completely covered by sand, it can easily be detected by sonar. However when a seabed obstruction is completely covered, standard sonar-based surveying techniques are not sufficient. In these cases, sub-bottom profiling and magneto-gradiometry are used to detect and localize the obstructions. Another problem arises since although these techniques can see beneath the seabed, they are not as accurate as the side scan or multibeam sonar. 

The following paragraphs give a more detailed description of techniques that can be used for the detection of seabed obstruction. Table B.8 gives a summary of some important characteristics. 

Table B-8 presents an overview of frequency ranges, coverage and resolution of the discussed survey techniques used for the detection of seabed obstructions.

Wide Scan Echosounder . The Wide Scan Echosounder allows the scanning of the area in front of the vessel and the equipment carrier in order to circumnavigate obstructions like wrecks or similar. This is necessary due to the low height of the sensor carrier above seabed (approximately 2 m) and the aimed for high survey speed (approximately 2.5 kn). 

Under Section 10 of the Rivers and Harbors Act, USAGE has authority for permitting the construction of any obstructions, such as artificial islands, installations, or structures, in U.S. navigable waters and on the seabed of the OCS but does not have authority for the operations of these obstructions. In addition, USAGE does not have authority over offshore wind farm worker safety. ... 

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