64 



Onshore portions of profiles are surveyed using standard land survey tech- 

 niques and equipment. Equipment commonly used in surveys includes 

 transits, levels, or theodolites, which are used for siting survey rods. Detailed 

 information concerning the techniques and equipment available for surveying 

 can be found in several surveying textbooks (i.e. Brinker and Wolf 1984). 



Surveys are preferably conducted during low tide, when the profile line can 

 be extended as far seaward as possible. Extending profile lines offshore 

 beyond wading depths requires boats or amphibious vehicles. Amphibious 

 vehicles are better suited to this task because they can traverse the sea-land 

 boundary and maintain the continuity of profile lines. Although acoustic echo 

 sounders can be used for continuous profiling seaward of the breaker zone, 

 the signals are usually disrupted by breaking waves, and boats suitable for 

 offshore use cannot approach the shore close enough to connect directly with a 

 land profile. High-precision electronic navigation is recommended if the sur- 

 veys extend offshore more than a few hundred meters. 



During calm weather conditions, sea sleds have been successfully used to 

 obtain shoreface profiles close to shore. A sea sled consists of a long, upright 

 stadia rod mounted vertically on a base frame with sled-like runners 

 (Clausner, Birkemeier, and Clark 1986) or a sled-mounted mast with a prism 

 for use by a total station survey system (Fredette et al. 1990). The sled is 

 towed, winched, or otherwise propelled along the profile lines (self-propelled, 

 remote-controlled sea sleds are currently being developed) while frequent 

 depth and position data are determined using onshore instruments. Because 

 the sea sled does not float, elevations are not subject to wave or tide 

 variations, thus providing a more accurate comparison between repeated 

 surveys. At present, it is not possible to obtain bottom samples with a sea 

 sled; these must be obtained from a boat or amphibious vehicle working in 

 conjunction with the sled. Sleds are currently limited to use within 4 km of 

 the coast and water depths of 12 m, less than the height of the sled masts. 



A helicopter bathymetric surveying system has been in use at the USAE 

 District, Portland, since the 1960's. The big advantage of this procedure is 

 that land-accuracy surveys can be conducted offshore in high waves and near- 

 structure conditions under which a boat could not perform. A helicopter is 

 fitted with a weighted, calibrated cable and prisms. A total-station survey 

 system is set up onshore to measure the location of the cable. Soundings are 

 commonly taken at 8-m intervals along profile lines up to 2,500 m offshore. 

 Operations are limited by poor visibility or winds over 15-20 m/sec 

 (30-40 knots). 



The Coastal Research Amphibious Buggy (CRAB), a unique self-propelled 

 vehicle, was developed by the U.S. Army Corps of Engineers for making 

 continuous onshore-offshore profiles and obtaining concurrent bottom samples. 

 The CRAB consists of a structural tower mounted on wheels and is self- 

 propelled by hydraulic motors. It can traverse under its own power across 

 both the beach and shoreface to a depth of about 9 m. It has been widely 

 used at the CERC Field Research Facility at Duck, NC. Both the CRAB and 



Chapter 3 Field Data Collection and Observation 



