From Figure 13, 



Therefore, 



Q* = 3 x 10" 3 (22) 



Q 0wen = (3 - 5 x 10_3) T S H S = (3 ' 5 * 10" 3 )(8)(32.2)(5) (23) 



=4.5 ft 3 /sec/ft of structure 



42. Owen recommends an unverified correction to account for seawall 

 roughness. He assumes the effect of roughness on overtopping can be accounted 

 for by assuming the structure has a higher effective freeboard. This higher 

 freeboard is estimated using the SPM (1984) roughness and porosity correction 

 factor : 



F .. . . = - (24) 



effective r 



where 



^effective = Owen's effective freeboard for rough slopes, L 

 F = freeboard (see Figure 1), L 

 r = SPM roughness and porosity correction factor 



The effective freeboard is used to determine F* for Equation 20. However, 



because Owen's laboratory tests were done only for smooth slopes, this 



roughness correction is unverified. 



Example 6 



43. Using Owen's method, an estimation can be made of the volume rate 

 of water which will overtop the seawall in Example 5 when made of typical 

 rubble-mound construction. The SPM recommends r = 0.5 - 0.55 for two layers 

 of rough, angular quarrystone. To be conservative, r = 0.55 is used: 



F . E . 5_lt _ 

 effective r 0.55 y 



F» = F effective = F 9 ' 1 _ = 0.089 (25) 



T VgH7 [8 V32.2 (5)] 



s 

 Using Equation 20 and Table 1 as an equivalent alternative to Figure 13, 



24 



