more specific scour regimes such as scour at vertical seawalls, additional 

 assumptions can be made that result in an additional reduction of important 



parameters . 



Scour Prediction at Rubble -Mound Structures 



27. Depth of scour at the toe of rubble-mound structures is extremely 

 difficult to isolate and measure. This is due to the subsidence or lowering 

 of the stone (or other materials from which the structure is constructed) 

 which accompanies scouring of the toe foundation. This subsidence typically 

 fills the void caused by scoured sediment and makes direct measurement of the 

 actual scoured depth virtually impossible. In light of this, very few 

 researchers have attempted to develop prediction equations for depth of scour 

 at toes of rubble-mound structures. According to the SPM (1984), "No 

 definitive method for designing toe protection is known," however, Markle 

 (1989) does provide design guidance for toe berm armor for stability, even 

 though scour-related problems are not addressed. Usually, general or 

 approximate guidelines based on laboratory and field studies are used to 

 design jetties, breakwaters, and revetments that have varying degrees of toe 

 scour protection. Most often, the type and amount of toe scour protection is 

 given as a rule of thumb or in terms of the mean weight of the individual 

 stable armor unit. This mean weight of the stable individual armor unit W a , 

 lb f , is determined by various empirically derived equations, the most common 

 of which is that developed by Hudson (1961) : 



w = Uli < 39 > 



K D (S r - l) 3 cot e 



For the above, -y r is the specific weight of armor unit, H D is the design wave 

 height at the structure site, S r is the specific gravity of the armor unit 

 relative to the water at the structure given by S r = 7 r /7, where 7 is the 

 specific weight of the water, 9 is the angle (in degrees) of the structure 

 slope measured relative to the horizontal plane, and experimentally determined 

 K D is the stability coefficient which varies with the type of armor unit as 

 well as other parameters. The reader is advised to consult the Shore 

 Protection Manual for additional information regarding the use of Equation 36. 



28. The following section briefly describes various methods for 

 approximating amounts of scour and, where appropriate, recommended guidelines 

 for planning toe protection methods. For additional information on scour at 

 rubble-mound structures, consult Sawaragi (1966), Hales (1980), Eckert (1983), 



28 



