WAVE LOADING ON VERTICAL SHEET-PILE GROINS AND JETTIES 



by 



J. Richard Weggel 



I. INTRODUCTION 



The Shore Protection Manual (SPM) (U.S. Army, Corps of Engineers, Coastal 

 Engineering Research Center, 1977) provides guidance for the design of verti- 

 cal wall structures subjected to either breaking, nonbreaking, or broken 

 waves; however, the design methods presented are for waves approaching perpen- 

 dicular to a structure. The SPM also presents a rational, though untested, 

 procedure for reducing forces when waves approach a structure at an angle. The 

 SPM force reduction appears to be valid if the direction of wave approach is 

 not too different from a perpendicular, i.e, a > 70°, where a is the angle 

 between a wave crest and a perpendicular to the structure. For structures 

 essentially normal to shore such as groins and jetties, waves usually prop- 

 agate along the axis of the structure rather than normal to it. For this sit- 

 uation, the SPM force reduction factor, which varies as sin a, underpredicts 

 the maximum wave force since a ->■ 0° and sin ex ^ 0. Furthermore, the instan- 

 taneous distribution of force along the structure is not uniform when a is 

 small since wave crests act along some parts of the groin or jetty while wave 

 troughs act along other parts. Consequently, the maximum wave force acts over 

 only a small part of the structure at any one time and the point on the struc- 

 ture where the maximum force acts moves landward along the structure as the 

 wave propagates to shore. This report outlines a design procedure for esti- 

 mating the force acting on a groin or jetty when waves propagate along the 

 axis of the structure, i.e., when a, the angle between the incoming wave 

 crests and a perpendicular to the structure, is less than about 45°. 



II. SPM METHODOLOGY FOR COMPUTING FORCES AInID MOMENTS 



The SPM presents three procedures for calculating wave forces on vertical 

 or near-vertical walls. For nonbreaking waves, the Miche-Rundgren method is 

 given. 'The SPM design curves for this method were developed from Rundgren's 

 (1958) equations and, for small values of the incident wave steepness H./gT , 

 from Sainflou's (1928) equations where H- is the incident wave height, g 

 the acceleration of gravity, and T the incident wave period. For breaking 

 waves, the Minikin (1963) method is given in the SPM. When waves break 

 against a structure they exert extremely high impac.t pressures of very short 

 duration. The impact with the structure of the translatory water mass associ- 

 ated with the moving wave crest causes the high pressures. The Minikin method 

 attempts to describe these pressures but in applying the method the force is 

 assumed to act statically against the structure. The problem is in reality a 

 dynamic one. Wave pressures exceeding those predicted by the Minikin method 

 have been measured; however, their duration is so short that the assumption 

 that the force is a static one makes the method extremely conservative even 

 though the force itself may be underpredicted. Since it is the impact of a 

 translatory mass of water that results in the high pressure, it seems doubtful 

 that waves approaching a vertical wall at an angle could cause them. The 

 Minikin method is thus inappropriate for calculating forces by waves approach- 

 ing a structure at an angle. For broken waves, the SPM presents a rational 

 method for calculating forces based on computing the stagnation pressure. 

 Again, this method is for normal wave approach. 



