A SIMPLIFIED METHOD OF DETERMINING DURATIONS AND FREQUENCIES OF 

 WAVES GREATER OR LESS THAN A SPECIFIED HEIGHT 



by 



Thorndike Saville, Jr. 



Beach Erosion Board, Corps of Engineers 



In planning for many types of marine operations it becomes important 

 to know the length of time over which, and frequency with which, waves 

 greater or less than a specified height may be expected to occur. For 

 example, a condition of 3-foot waves is about the maximum under which 

 accurate hydro graphic survey data may be obtained by small boats or 

 DUKWS in the nearshore area, higher waves serving to mask out small 

 depth changes indicated on an echo sounder record due to the resultant 

 (up-and-down) motion of the vessel. A 3-foot wave height is also 

 regarded as being roughly the upper limit for some types of geophysical 

 survey operations, and it is the critical height for operation of one 

 type of barge used for transporting crude oil ashore from offshore wells. 

 Accurate knowledge of the frequency of occurrence of waves above or 

 below these limiting heights is then of considerable importance in 

 determining the economic feasibility of operating in an area, in choosing 

 between operation in one area or another, in selecting the most suitable 

 time for carrying out such operations, and in overall planning of the 

 operation (particularly as regards probable amount of lost time). 



Frequency diagrams of occurrence of various wave heights are needed 

 for design of structures subject to wave action, and are frequently 

 obtained by hindcasting from weather maps. However, it is generally 

 only the higher waves that are of interest, those above, say, 8 or 10 

 feet. The lower waves, however, account for a much greater percentage 

 of the time involved, and methods of forecasting which would automatically 

 eliminate thorough consideration of these waves would result in much 

 more rapid and economical determination of design criteria. 



In such cases as these the wave height is of interest only as it 

 is of greater or less magnitude than a particular value; i.e. the actual 

 wave height is not of importance. There would seem to be, therefore, 

 nothing particular to be gained in forecasting the actual height rather 

 than just whether or not the wave is over the limiting value. 



This can be done much more simply than by making a complete forecast 

 of the actual height, as curves showing the values of wind velocity and 

 duration, and fetch length and decay necessary to produce waves of a 

 particular specified height may be drawn rather easily from the revised 

 Sverdrup-Munk curves presented by Bretschneider*. if the particular 



*Bretschneider, C. L. - Revised Wave Forecasting Relationships, Proc. of 

 the Second Conf. on Coastal Eng'g., Council on Wave Research, Eng'g. 

 Foundation, 1952. 



