28 



not an exact procedure, the MWL is not a critical parameter, as h is only used to 

 locate the origin of coordinates in the potential function. Note that the result is a 

 local MWL, which could be quite different than the global still water level (SWL). 

 The mean water level is the more appropriate reference frame, as the method is local, 

 and only considers a small window in time at once. The elevations of the water 

 surface nodes are found independently, without any requirement that the resulting 

 mean water level be at the origin. Thus it is not necessary that the origin is at the 

 exact MWL, only that the it is near the surface. In shallow water, the bed could be 

 used as the origin (Fenton 1986), but this would not work well in deeper water, as 

 the area of primary interest, the water surface, would be far from the origin. 



The Eulerian Current 



The depth uniform Eulerian current, t/, appears as a known parameter in the 

 potential function, Eq. 3.1. Unfortunatel}^, a measured pressure record gives no in- 

 dication of the local current. Unlike the mean water level, the Eulerian current is a 

 critical parameter that defines the propagation medium, and varying the value used 

 for the current will have a considerable effect on the results of the computation. In 

 fact, as pointed out by Fenton (1985a), any attempt to apply a wave theory without 

 knowledge of the local current will be inaccurate, even at only first order. 



The situation is not hopeless, however. The presence of the Eulerian current 

 term in the potential function draws attention to the fact that it is an important 

 parameter, even if it is taken to be zero. This should encourage investigators planning 

 field experiments to establish a method of estimating the local current. This may be 

 as straightforward as placing a current meter nearby, or as simple as using tidal 

 data to estimate the local current. Caution must be taken with the later method, 

 as the accuracy of the estimate may not be consistent with the use of high order 

 interpretation methods. 



It is also assumed in the LFI method that local current is depth uniform. Although 

 this is unlikely to be strictly true, Kishida and Sobey (1988) demonstrated that for 

 steady waves, a current profile with a realistic level of vorticity was likely to yield very 



