150 



can be installed easily. Unfortunately, piers are seldom built in locations ex- 

 posed to the sea; rather, they are located in protected regions such as coves, 

 bays, or where sheltered by offshore islands. At the same time piers can be 

 found that provride suitable locations for gages to study local wave action. When 

 piers are available a surface-type gage such as the Beach Erosion Board Step 

 Resistance Gage can be installed which records the actual variation of water sur- 

 face elevation (Beach Erosion Board, 1948). Surface fluctuations caused by 

 tides and local wind chop are recorded. Pressure- type wave recorders also 

 can be installed easily off piers. By suspending the pressure recorder close to 

 the water surface, pressure records corresponding closely to the surface pro- 

 file can be obtained. The extent to which the pressure recorder will reproduce 

 the locally-generated short period waves can be controlled by adjusting the depth 

 at which the gage is installed below the surface. 



In some locations offshore oil well structures are available to provide 

 mountings for wave gages. One installation has been completed at Cape Henry., 

 Virginia (U.S. Naval Ordnance Laboratory, 1951) in which a step resistance 

 gage was attached to a pile specially driven for the purpose of installing the gage. 

 The pile, 60 feet in length, was driven 25 feet into the sand bottom at a site 2500 

 feet from shore, where the water depth was 20 feet. The gage was connected 

 by armored cable to recorders located at a shore station. 



Surface-type gages, although requiring cleaning at 3 to 4 month intervals, 

 can be operated for several years without major repair. Several pressure-type 

 gages are also available that provide continuous service for at least one year 

 without repair or maintenance. Either type gage should be installed at least 3 

 pile diameters from the nearest pile. 



Tripods For Sub-surface Pressure Gages - Pressure-type gages, supported by 

 small tripods resting on the ocean bottom, provide a practical method of record- 

 ing the wave action where there are no piers. The tripods vary in design, 

 depending upon the nnaterial available, the equipnient available for handling the 

 tripod during installation, the size and type of marker buoy to be used, and the 

 size and shape of the pressure head. The two primary requirements of the tri- 

 pod are as follows: 



a. The tripod should have sufficient weight to keep it in place on the bot- 



tom. Marker buoys, when acted upon by heavy seas, may exert a 

 considerable overturning force on the tripod, if the marker buoy 

 cable is attached to the top of the tripod. Also, the electrical cable 

 usually attached to the base of the tripod may tend to drag the tripod 

 when the cable is acted upon by longshore currents. Normally, tri- 

 pods weigh between 250 pounds and 20000 pounds, depending prima- 

 rily on the marker buoy size and the equipment available for handling 

 the tripod. 



b. The tripod should have sufficient height to prevent the "sanding down" 

 of the pressure head; sand nnovements are known to occur consider- 

 able distances offshore, often changing the bottom elevations by sev- 

 eral feet. This is especially true of the seasonal movement of sand 

 onshore during the summer months and offshore during the winter 

 months. 



The Woods Hole tripod (Woods Hole Oceanographic Institution, 1947) has 

 a special design feature which allows the instrument to be detached from its con- 

 crete base in case the unit becomes covered with sand and cannot be lifted with- 

 out possible breaking of the lifting line. The tripod is made in two parts: a 

 concrete base weighing approximately 300 pounds and a pipe framework which 

 supports the pressure head. A shear pin, whose strength is less than that of 



