IV. EXAMPLE PROBLEM 



This example illustrates the method used to estimate the maximum and mini- 

 mum wave height factors, R H L and R H g, whether the current can be 

 neglected completely in computing wave height, H, from the fluctuating bot- 

 tom pressure magnitude, P, and if not what range (if any) the current can be 

 neglected. 



GIVEN : A bottom-mounted wave pressure gage is located near an inlet at an 

 elevation of -19.68 feet (-6 meters) referenced to the 1929 National Geo- 

 detic Vertical Datum (NGVD). The tide elevation ranges from -3.28 to 3.6 

 feet (-1 to 1.1 meters) NGVD. Current speeds from to 4.92 feet (1.5 

 meters) per second are known to occur at the location of the gage. The wave 

 climatology indicates that the dominant waves have periods between 6 and 15 

 seconds and can travel at any angle with respect to the current. 



FIND: 



(a) The largest and smallest values of R„ (Ru t and R^ g) for the 

 conditions expected and the range of wave periods of interest (periods 

 between 6 and 15 seconds). 



(b) Whether the currents can be neglected for the entire range of wave 

 periods of interest when bottom pressure observations are used to determine 

 wave heights. 



(c) The range of wave periods over which currents can and cannot be 

 neglected if they cannot be neglected over the entire range of interest. 



SOLUTION: 



Step 1 . Set 6S and 9L. Since the angle between the direction of wave 

 travel and the current can take on any value, 



9S = 0; 9L = 180° = it rad 



Step 2. Set VL, VS, and TS. From the given values of the problem, 



VL = 4.92 ft (1.5 ia)/s 



VS = 

 TS = 6 s 



Step 3. Compute d T g and d™ , . From the tide elevation and bottom 

 elevation information given, 



d T s = 19.68 -3.28 = 16.4 ft (5 m) 



d T L = 19.68 + 3.60 = 23.28 ft (7.1 m) 



Step 4 . Compute QL using equation (14). 



2(3.14159) / 23.2 ft \ 1/2 



fiL = = 0.89 (dimensionless) 



6 s \32.2 ft/s 2 / 



