energy transmission characteristics of the proposed structure in a two-dimensional 

 model using a scale large enough to ensure negligible scale effects. A cross section 

 then was developed for the small-scale, three-dimensional model that would provide 

 essentially the same relative transmission and reflection of wave energy. Therefore, 

 from previous findings for structures and wave conditions similar to those at Nome 

 Harbor, it was determined that the correct wave-energy transmission and reflection 

 characteristics could be closely approximated by increasing the size of the rock used 

 in the 1 :90-scale model to approximately two times that required for geometric 

 similarity. Accordingly, in constructing the rubble-mound structures in the Nome 

 Harbor model, rock sizes were computed linearly by scale, then multiplied by 2 to 

 determine the actual sizes to be used in the model. 



Ideally, a quantitative, three-dimensional, movable-bed model investigation 

 would best determine the impacts of harbor modifications with regard to sediment 

 deposition in the vicinity of the harbor. However, this type of model investigation is 

 difficult and expensive to conduct, and each area in which such an investigation is 

 contemplated must be carefully analyzed. In view of the complexities involved in 

 conducting movable-bed model studies and due to limited funds and time for the 

 Nome Harbor project, the model was molded in cement mortar (fixed-bed), and a 

 tracer material was obtained to qualitatively determine sediment patterns in the 

 vicinity of the harbor. 



Model and Appurtenances 



The model reproduced approximately 3,350 m (1 1,000 ft) of the Alaskan shore- 

 line, the existing harbor and lower reaches of the Snake River, and underwater 

 topography in the Norton Sound to an offshore depth of 12.2 m (40 ft) with a 

 sloping transition to the wave generation pit elevation of -27.4 m (-90 ft). The total 

 area reproduced in the model was approximately 1,225 sq m (13,200 sq ft), 

 representing about 9.8 sq km (3.8 sq miles) in the prototype. Vertical control for 

 model construction was based on mean lower low water (mllw), and horizontal 

 control was referenced to a local prototype grid system. Figure 5 is a general view 

 of the model. 



Model waves were reproduced by a 24.4-m-long (80-ft-long), electrohydraulic, 

 unidirectional spectral wave generator with a trapezoidal-shaped, vertical motion 

 plunger. The wave generator utilized a hydraulic power supply. The vertical motion 

 of the plunger was controlled by a computer-generated command signal, and 

 movement of the plunger caused a displacement of water, which generated the 

 required experimental waves. The wave generator also was mounted on retractable 

 casters, which enabled it to be positioned to generate waves from the required 

 directions. 



An Automated Data Acquisition and Control System, designed and constructed 

 at WES (Figure 6), was used to generate and transmit wave generator control 

 signals, monitor wave generator feedback, and secure and analyze wave data at 

 selected locations in the model. Through the use of a Microvax computer, the 

 electrical output of parallel-wire, capacitance-type wave gauges, which varied with 



Chapter 2 The Model 



