already so steep that there is little if any increase 

 in height just before breaking. Thus, swell 

 often defines the period of the breakers even 

 though the wind waves appear to predominate 

 in deep water. A long, low swell in deep water 

 may be obscured by choppy wind waves and 

 be detectable only on the beaches. 



10-10 Surf. — The zone of breakers, termed 

 surf, includes the region of white water between 

 the outermost breaker and the waterline on the 

 beach. During a storm, it may be difficult to 

 differentiate between surf inshore and white- 

 caps in deep water just beyond. 



10-11 Wave Refraction and Longshore 

 Currents. — Wlien waves approach a coastline 

 at an angle, they tend to swing around and 

 break parallel to the beach. The waves are 

 slowed down as they come into shallow water 

 by the inhibiting effect of the bottom, and 

 owing to the change in velocity, are deflected, 

 or refracted from their original parts. Conse- 

 quently, refraction is only noticeable on beaches 

 with gradual profiles, since the bottom must 

 influence the waves over several wavelengths. 

 When waves do not swing all the way around 

 before breaking and break at an angle with the 

 shoreline, a current in the direction of the open 

 angle is generated. The strength of this current 

 depends chiefly on the height, period, angle of 

 approach of the waves, and beach configuration. 

 If a longshore current develops during unload- 

 ing operations, it may swing vessels sideways 

 and broach them. 



10-12 Effect of Tidal Currents.— If tidal 

 currents acquire velocities of 2 or 3 knots or 

 greater, they affect the waves which travel 

 into the area of their influence. Waves opposing 

 currents tend to steepen and increase in height; 

 those moving with currents flatten and decrease 

 in height. Unless the current is effective over a 

 considerable area with a moderate to high 

 velocity, wave changes are not usually notice- 

 able. Near headlands and in tidal races, how- 

 ever, they ma.y be appreciable, and zones of 

 whitecaps where the waves are breaking 

 because of this effect occur at many places. 



10-13 Effect of Shoals.— Waves (either sea 

 or swell) in passing over shoals or bars tend to 

 steepen and increase slightly in height. Long 

 swells may feel the effect of a deeper shoal 

 more than wind waves and may steepen more 

 noticeably, but will not break as quickly because 

 their height to length ratio is initially very low. 

 Wind waves, on the other hand, may suddenly 

 steepen and break. 



10-14 Wave Reflection. — At steep coasts, 

 where there is no beach and deep water is close 

 inshore, wind waves and swell will travel to 



the coast without undergoing shallow water 

 transformation. Under these conditions, the 

 waves will be reflected from the shoreline and 

 proceed seaward, causing an interference pat- 

 tern. If this phenomenon is pronounced, one 

 can see pyramidal waves shooting upward 

 where the crest of a reflected wave meets the 

 crest of an oncoming wave. Beach gradients 

 generally must be steeper than 1 in 10 before 

 reflection occurs. 



10-15 INSTRUCTIONS FOR OPERAT- 

 ING THE ELECTRIC WAVE STAFF.— The 

 Electric Wave Staff is an instrmnent designed 

 to record wave heights and periods at sea. 

 It consists of three 12-foot lengths of water- 

 tight 3-inch aluminum tubing, a 3-foot circular 

 steel damping disk, a Brush recorder, a trans- 

 former-rectifier circuit, electrical cables, floats, 

 retrieving lines, and balancing weights. The 

 upper section of tubing has 36 contact points 

 set 4 inches apart, and is covered with black 

 waterproofing material. It is called the step- 

 resistance gage and has a connection for the 

 electrical cable at the top. The remaining 

 two sections of tubing provide proper buoyancy 

 to the step-resistance gage so that it will float 

 vertically in the water witli one-half of its length 

 exposed. Below the end of the lower tube is 

 suspended the damping disk. Its function 

 also is to provide weight to keep the staff 

 vertical and to damp the tendency of the 

 staff to rise and fall with the passing waves. 

 As these waves pass, the water rises and falls 

 along the step-resistance gage, thereby increas- 

 ing and decreasing the resistance in the gage as 

 it passes successive contact points. This 

 variation in resistance is transmitted to the 

 Brush recorder aboard ship by the electrical 

 cable and transformer-rectifier circuit. A 

 record of the wave height and period is thus 

 recorded on a strip chart. The usual length of 

 recording time is from 7 to 20 minutes. These 

 observations are made when the ship is lying 

 to and the wave-staff assembly put overboard 

 to windward so that the normal drift of the ship 

 will be away from the staff. The overboard as- 

 sembly of the wave staff" is shown in figure 10-1. 



10-16 The Recorder and Transformer- 

 Rectifier Assembly. — Tlie Brush recorder and 

 transformer-rectifier are mounted in a single 

 case. A cover on the top of the case gives access 

 to the Brush recorder. The transformer-recti- 

 fier, electrical cable connection to the wave 

 staff, and the power supply cable are reached 

 by a door in the back of the case. The dimen- 

 sions of the case are 22 x 15 x 20 inches and the 

 assembly weighs about 97 pounds. The 

 electrical power requirements are 120 volts, 



104 



H. O. 607 



