2. Background and Objectives . 



Research in the harsh physical environment of the nearshore zone has 

 always been difficult to implement. The problem area is the zone of 

 breaking waves where navigation and the placement and operation of instru- 

 ments have had many failures. This zone is of interest to those involved 

 in the study of nearshore sediment transport mechanics, the distribution 

 of longshore currents, and the dispersal and mixing of water masses, the 

 physical description of which has suffered for the lack of continuity in 

 surveys between the beach and the offshore area. Since the wave climate 

 is seldom constant, these surveys must be rapidly conducted, the sensors 

 used must be rugged enough to withstand the wave impact forces,, and the 

 environmental parameters measured should be analyzed in real time to 

 collect representative data. 



The first requirement is for a device which will negotiate the surf 

 and carry oceanographic instruments at the same time. Several precedents 

 for such an implement exist, and most result from the interest in military 

 amphibious operations during World War II, although the type developed by 

 Isaacs (1945) was intended to carry demolition charges. Isaacs' sled was 

 perhaps the first of a generation of similar devices designed explicitly 

 to negotiate the surf. Another who experimented with self-propelled 

 (or wave- action propelled) sleds was Johnson (1949) in measuring water 

 depth . 



The sled described in this report (Fig. 1) was originally designed by 

 Robert Sears of the U.S. Army Engineer District, Baltimore. Kolessar and 

 Reynolds (1966) named it the Sears sled. It was engineered to be a towed 

 stadia rod (a mast riding on a frame) for sounding the nearshore zone. 

 The sled could be winched to shore from 9.14 meters (30 feet) of water 

 or less but had to be deployed again at the beginning of the next survey 

 line; Kolessar and Reynolds engaged a helicopter for this phase of the 

 work. 



Certain aspects of the Coastal Engineering Research Center (CERC) sea 

 sled (using Johnson's (1949) terminology), such as the sled's framework 

 and onshore incremental movement in one mode of operation, reflect its 

 heritage. However, the use of the sled in the measurement of nearshore 

 currents and waves is a departure from previous experience. The sled is 

 a component in TODAS (Towed Oceanographic Data Acquisition System) , which 

 includes the amphibious craft towing the sled, the sensors and the asso- 

 ciated telemetry, data conversion and storage units, future improvements 

 (cathode ray tube (CRT) minicomputer), auxiliary dye studies, and aerial 

 photography. At present the capabilities of TODAS extend to measuring 

 currents and waves in shallow water (in 0.91 meter (3 feet) < depth < 

 9.14 meters (30 feet)) in "quasi-real time," with provisions made for 

 increased capabilities (more and varied-purpose instrumentation and on- 

 site data analysis) . 



The development of TODAS was a 3-year effort, during which the system 

 was continually upgraded and tested under both laboratory and field con- 



