where Q is the overtopping rate (volume/unit time) per unit structure 

 length, g is the gravitational acceleration, H' is the equivalent 

 deepwater wave height , h is the height of the structure crest above the 

 bottom, d is the depth at the structure toe, R is the runup on the 

 structure that would occur if the structure were high enough to prevent 

 overtopping corrected for scale effects (see Sec. II, WAVE RUNUP), and a and 

 * 

 Q are empirically determined coefficients that depend on incident wave 



characteristics and structure geometry. Approximate values of a and Q as 



functions of wave steepness H'/gT and relative height d /H' for various 



o so 



slopes and structure types are given in Figures 7-24 through 7-32. The 



* * 

 numbers beside the indicated points are values of a and Q (Q in 



o o 



parentheses on the figures) that, when used with equation (7-10) or (7-11), 

 predict measured overtopping rates. Equations (7-10) and (7-11) are valid 

 only for < (h-dg) < R . When (h-dg) ^ R the overtopping rate is taken as 

 zero. Weggel (1976) suggests a procedure for obtaining approximate values of 

 * 

 a and Q where more exact values are not available. His procedure uses 



theoretical results for wave overtopping on smooth slopes and gives conserva- 

 tive results; i.e., values of overtopping greater than the overtopping which 

 would be expected to actually occur. 



It is known that onshore winds increase the overtopping rate at a 

 barrier. The increase depends on wind velocity and direction with respect to 

 the axis of the structure and structure slope and height. As a guide, 

 calculated overtopping rates may be multiplied by a wind correction factor 

 given by 



(7-12) 



where W/- is a coefficient depending on windspeed, and is the structure 

 slope (0 = 90° for Galveston walls) . For onshore windspeeds of 60 mi/hr or 

 greater, Vf = 2.0 should be used. For a windspeed of 30 mi/hr, V^ = 0.5 ; 

 when no onshore winds exist, W^ = . Interpolation between values of Wj? 

 given for 60, 30, and mi/hr will give values of W^ for intermediate wind 

 speeds. Equation (7-12) is unverified, but is believed to give a reasonable 

 estimate of the effects of onshore winds of significant magnitude. For a 

 windspeed of 30 mi/hr, the correction factor k' varies between 1.0 and 

 1.55, depending on the values of (h-d )/R and sin . 



* 

 Values of a and Q larger than those in Figures 7-24 through 7-32 



should be used if a more conservative (higher) estimate of overtopping rates 



is required. 



Further analysis by Weggel (1975) of data for smooth slopes has shown that 

 for a given slope, the variability of a with incident conditions was 



relatively small, suggesting that an average a could be used to establish 



* 

 the Q valjue that best fit the data. Figure 7-33 shows values of the 



average a (a) for four smooth, structure slopes with data obtained at three 



different scales. An expression for relating a with structure slope (smooth 



7-44 



