66 



an onshore receiver and recorder. Bottom-mounted pressure gages measure 

 wave parameters by sensing the pressure changes with the passage of each 

 wave. They can be either self-recording or can be connected to onshore 

 computers and recorders with cables. Divers must retrieve data periodically 

 from self-recording gages. Both types of systems require regular 

 maintenance. Structure-mounted wave gages are the most accessible of the 

 non-directional gages, allowing convenient maintenance. Unfortunately, 

 offshore structures are not always located near project sites. 



Directional wave gages are used mainly in buoys or bottom mounts of 

 single units (Figure 24) or multiple arrays in a fixed configuration. 

 Directional buoy-type wave gages are often designed to measure other 

 parameters, especially meteorologic ones. Pressure-type gages can measure 

 wave direction using an accompanying electromagnetic current meter or by 

 combining multiple synoptic pressure measurements from individual gages in a 

 known geometric array. 



Wave hindcasting is widely used for obtaining wave statistics by analysis of 

 weather maps using techniques developed from theoretical considerations and 

 empirical data (Coastal Engineering Research Center 1984) (Figure 25). Over 

 the last several decades, since wave hindcasting came into common use, 

 numerous improvements have been made in the technique and reasonably 

 reliable information on wave climate in given areas can be computed (Abel 

 et al. 1989; Hubertz and Brooks 1989; Jensen, Hubertz, and Payne 1989; 

 Corson et al. 1987; Corson and Tracy 1987). Advantages of hindcasting 

 include the long-term database associated with weather maps and the useful 

 statistical information. 



A large amount of wave data is available in the form of visual wave 

 observations from ships at sea and from shore stations along the coasts of the 

 United States. Although observations are less accurate than measured data, 

 experienced persons can achieve reasonably accurate results and the large 

 database of available observations makes it a valuable resource. Shipboard 

 wave observations have been compiled by the US Navy Oceanographic 

 Research and Development Activity in the form of sea and swell charts and 

 data summaries such as the Summary of Shipboard Meteorological 

 Observations. Areal coverage by these sources is extensive, but the greatest 

 number of observations come from shipping lanes. 



The second important source of observations has been collected by CERC 

 under the Littoral Environmental Observation (LEO) program (Schneider 

 1981; Sherlock and Szuwalski 1987). The program, which was initiated in 

 1966, makes use of volunteer observers who make daily reports on conditions 

 at specific sites along the coasts of the United States (Figure 26). A variety of 

 data from over 200 observation sites are available from CERC. As shown, 

 LEO data include more than wave parameters, and encompass information on 

 winds, currents, and some morphologic features. LEO data are best applied 

 to a specific site, and do not provide direct information regarding deepwater 

 statistics. 



Chapter 4 Investigation of Environmental Factors 



