*************** EXAMPLE PROBLEM H*************** 



GIVEN ; Lake Erie has a mean depth d = 18.6 meters (61 feet) and its length 

 £g is 354 kilometers (220 miles) 



FIND : The fundamental period of oscillation T . , if j = 1 . 



SOLUTION ; From equation (2-84) for an enclosed basin, 



. ''b 1 



^' =T"(idTi/2 



^ 2(354,000) 1 



1 1 [9.8(18.6)]l/2 



Tj = 52 440 s or 14.57 h 



Considering the variability of the actual lake cross section, this result is 

 surprisingly close to the actual observed period of 14.38 hours (Platzman 

 and Rao, 1963). Such close agreement may not always result. 



*************************************** 



Note: Additional discussion of seiching is presented in Chapter 3, Section 

 VIII, 4. 



4. Wave Reflection from Plane Slopes, Beaches, Revetments, and Breakwaters . 



The amount of wave energy reflected from a beach or a manmade structure 

 depends on the slope, roughness, and permeability of the beach or structure, 

 and also on the wave steepness and angle of wave approach. Battjes (1974) 

 found that the surf similarity parameter given by 



C= ^ ;" (2-86) 



coteyH^/L^ 



is an important parameter for determining the amount of reflection of waves 

 approaching a beach or structure at a right angle. In equation (2-86), 6 

 is the angle the beach or structure slope makes with a horizontal, H^ the 

 Incident wave height, and L^ the deepwater wavelength (see Fig. 2-64). The 

 amount of reflection is given by the reflection coefficient 



X ^ (2-87) 



Hi 



in which Hj. is the height of the reflected wave, and H^ the height of the 

 incident wave. 



2-116 



