Table 3 



Self-Contained and Cable-Telemetry Wave Gages; Advantages and 



Disadvantages 



I. Self-contained gages 



A. Advantages 



1 . Deployment is often simple because compact instrument can be handled by a small dive 

 team. 



2. Gage can be easily attached to piles, structural members, or tripods. 



3. Field equipment can be carried by airplane to remote sites. 



4. Gage will continue to function in severe storms as long as the mount survives. 



B. Disadvantages 



1 . Gage must be periodically recovered to retrieve data or replace data tapes. 



2. Data collection time is limited by the capacity of the internal memory or data tapes. 

 Researcher must compromise between sampling density and length of time the gage can 

 be gathering data between scheduled maintenance visits. 



3. Battery capacity may limit the deployment time. 



4. If bad weather forces delay of scheduled maintenance, gage may reach the limit of its 

 storage capacity. This will result in unsampled intervals. 



5. While under water, gage's performance cannot be monitored. If it fails electronically or 

 leaks, data are usually lost forever. 



6. Gage may be struck by anchors or fishing vessels. The resulting damage or total loss may 

 not be detected until the next maintenance visit. 



C. Notes 



1 . Data compression techniques and advances in low-energy memory have dramatically 

 increased the storage capacity of underwater instruments. Some can remain onsite as 

 long as 1 2 months. 



2. If a gage floods, data from electronic memory systems are usually irretrievably lost. On 

 the other hand, a wet data tape can sometimes be saved by flushing with fresh water and 

 carefully drying. 



3. Onboard data processing can extend deployment times by reducing the need to store raw 

 data. 



II. Data transmission by cable. 



A. Advantages 



1 . Data can be continuously monitored. If a failure is detected (by human analysts or error- 

 checking computer programs), a repair team can be sent to the site immediately. 



2. Because of the ability to monitor the gage's performance, infrequent inspection visits may 

 be adequate to maintain systems. 



3. Frequency and density of sampling are only limited by the storage capacity of the shore- 

 based computers. 



4. Gage can be reprogrammed in situ to change sampling program. 



5. Electrical energy is supplied from shore. 



B. Disadvantages 



1 . Cable to shore is vulnerable to damage from anchors or fishing vessels. 



2. Shore station may be damaged in severe storms, resulting in loss of valuable storm data. 



3. Shore station and data cable are vulnerable to vandalism. 



4. Backup power supply necessary in case of blackouts. 



5. Installation of cable can be difficult, especially in harbors and across rough surf zones. 



6. Installation often requires a major field effort, with vehicles on beach and one or two 

 boats. Heavy cable must be carried to the site. 



7. Cable eventually deteriorates in the field and must be replaced. 



8. Cable may have to be removed after experiment has ended. 



C. Notes 



1 . Some cable-based gages have internal memory and batteries so that they can continue to 

 collect data even if cable is severed. 



2. Ability to constantly monitor gage's performance is a major advantage in conducting field 

 experiments. 



Chapter 3 Field Data Collection and Observation 



33 



