the indicated scour is attributable to an anchorage dredged in this area 

 in the prototype. South of the abandoned channel, the model indicates 

 two areas of rather substantial fill that are not borne out by prototype 

 experience. The inner area is in extremely shallow water and is typical 

 of the unnatural model-bed formations developed in shallow water when 

 using crushed coals as bed material and with exposure to significant 

 wave action. 



The objectives of the original model study were to determine a means 

 for protecting the north jetty from being undermined by tidal currents 

 and to determine the shoaling characteristics of the inner and outer sec- 

 tions of a deepened entrance channel. One plan tested in the model was 

 constructed in the prototype. The feasibility of the plan was partly 

 judged on model results indicating that although a deepened channel along 

 the proposed alinement would require increased maintenance dredging, the 

 additional cost would be partially offset by a seaward shift in the shoal- 

 ing distribution. 



An analysis of pertinent data 6 years after construction of the plan 

 by Letter and McAnally (1977) indicates that the model: 



(a) Correctly predicted that the channel realinement and 

 dredged-material disposal would halt undermining of the north 

 jetty next to the outer bar channel; 



(b) correctly predicted that total maintenance dredging 

 voliimes would increase for the proposed channel but under- 

 predicted the magnitude of the increase; 



(c) erroneously predicted that the inner bar channel would 

 experience net erosion and require no maintenance dredging be- 

 yond the second year; 



(d) correctly predicted the approximate absolute and rela- 

 tive increases in maintenance dredging volumes for the outer bar 

 channel ; 



(e) correctly predicted that maintenance dredging volumes 

 for the approach channel would increase, but underpredicted the 

 magnitude of that increase due to a marked absence of shoaling 

 in the seaward extension of the approach channel; and 



(f) correctly predicted a seaward shift in the channel 

 shoaling volume distribution but overpredicted the magnitude 

 of the shift; this overprediction was due to the erroneous 

 scour prediction for the inner bar channel. 



Therefore, it is concluded that the model satisfied most qualitative 

 objectives but failed to accurately meet some quantitative objectives. 



515 



