PART VI: SUMMARY 



54. It may well be unreasonable to expect that a single scour predic- 

 tion method will yield consistent and reliable results for all cases. This 

 report has briefly discussed the merits and shortcomings of the several scour 

 prediction techniques for various coastal scour situations. In general, 

 prediction techniques for scour at structure toes are either rule-of-thumb 

 methods or semi-empirical equations based on limited laboratory and field 

 studies. Table 2 contains recommended methods for estimating maximum scour 

 depths for inclusion in coastal structure designs. Probably sufficient 

 guidance exists for vertical walls, piles, and pipelines. Additional research 

 is needed in the area of rubble-mound structure scour prediction methods. 



Rubble -Mound Structures 



55. Very little guidance is available for prediction of scour depths at 

 the base of rubble-mound structures. This is not due to a lack of laboratory 

 or field study efforts - basically, the magnitude of the scour is difficult to 

 measure directly since the structure typically collapses onto itself and fills 

 the holes. In addition to research efforts associated with scour at rubble- 

 mound structures being conducted as a part of the " Laboratory Studies on 

 Scour" work unit, a three-dimensional scaled laboratory model study entitled 

 "Scour Holes at the Ends of Structures" under the U.S. Army Corps of Engi- 

 neers, Coastal R&D Program, is being conducted to gain understanding of 

 processes that occur during formation of scour holes at structures. The 

 majority of efforts in this area have focused on predicting the size and 

 amount of toe protection which should be used to avoid significant damage to 

 the structures. Usually, general guidelines based on laboratory and field 

 studies are used to design jetties, breakwaters, and revetments which have 

 varying degrees of toe scour protection. Hales (1980) conducted a survey of 

 scour protection practices in the United States and found that a rule of thumb 

 for minimum toe scour protection is a toe apron measuring 2.0 to 3.28 ft thick 

 and 4.9 ft wide. Eckert (1983) subsequently found that toe scour protection 

 should be designed to accomodate the maximum scouring force that exists where 

 wave downrush on the structure face extends to the toe. According to Eckert, 

 the rule of thumb for minimum toe scour protection suggested by Hales will be 

 inadequate under certain conditions (see section IV) involving water depth and 

 wave reflection. 



56. Laboratory model studies by Markle (1989) produced the most recent 

 guidance for sizing toe berm armor stone and toe buttressing stone. Guidance 

 is given in terms of the stability number N s as defined in paragraph 33 



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