be determined satisfactorily in some instances by the electronic wave 

 gage apparatus and the oscillograph. In other studies, if determination 

 of waveforms are desired when breaking on a structure, high-speed photog- 

 raphy may be used. The direct measurement of particle velocity is seldom 

 necessary in model studies of coastal structures. However, in research 

 studies of wave-induced sediment motion (which may affect the stability 

 of coastal structures by erosion at the toe of the structures) or in 

 basic studies of wave forces on the structures, the kinematics of the 

 water particles are of fundamental importance. Orbital velocities in 

 progressive waves have been measured by introducing neutrally buoyant 

 particles in the waves and photographing their motion by a high-speed 

 movie camera (Marlow, 1957). This method was determined fairly accurate, 

 but it was not considered completely satisfactory because the points of 

 measurement could not be controlled; the effort in obtaining photos and 

 the reduction and analysis of data were difficult and required consider- 

 able time and effort. As a result, other methods of measuring orbital 

 velocities have been investigated, including the hot-wire anemometer, 

 the thermistor, and the hot-film sensor (Eagleson and Van de Watering, 

 1964; Kolpak and Eagleson, 1969). 



Wave height measuring devices for model use consist of a wave gage, 

 a power supply, an amplifier and control system, and an oscillograph 

 (Fig. 6-15). This apparatus measures changes in water surface elevation 

 with respect to time by measuring and recording the corresponding varia- 

 tions in current flow with respect to time by resistance or capacitance 

 wave gages. The first wave gages used at WES were the step-resistor type, 

 but most laboratories now use the two-wire or printed-circuit resistor 

 gages or the one-wire capacitance gages. The capacitance wave gage has 

 been described by Killen (1952), Campbell (1953), Tucker and Charnook 

 (1954), Ippen and Eagleson (1955), and Moore (1964). The development of 

 the printed-circuit gage was described by Hanes (1957), who listed the 

 following advantages and disadvantages of the four types of wave gages. 



(a) Step-Resistor Gage . Advantages are good linearity, 

 rugged construction, and simplicity of operation. Disadvantages 

 are high cost of construction and of auxiliary equipment, dis- 

 tortion of wave shape, obstruction of wave fronts, difficulties 

 in maintaining waterproofness, and relatively poor accuracy. 



(b) Two-Wire Electrode Gage . Advantages are low cost of 

 wave gage construction, minor obstruction of wave fronts, good 

 linearity and accuracy over small ranges of wave height, and 

 long service life. Disadvantages are high cost of auxiliary 

 equipment, distortion of wave shape due to finite distance 

 between elements, and small ranges of linearity. 



(c) Capacitance Wave Gage . Advantages are good linearity 

 and dynamic response, minor obstruction to wave fronts, and low 

 cost of wave gage construction. Disadvantages are high cost of 

 auxiliary equipment and the requirement of a bridge network of 

 capacitors and resistors (which must be accurately matched), 

 that is sensitive to temperature variations. 



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