Design of Tracer Material 



As discussed previously, a fixed-bed model was constructed and a tracer material 

 selected to qualitatively determine movement and deposition of sediment in the 

 vicinity of the harbor. Tracer was chosen in accordance with the scaling relations of 

 Noda (1972), which indicate a relation, or model law, among the four basic scale 

 ratios, i.e., the horizontal scale X ; the vertical scale u ; the sediment size ratio T| D ; 

 and the relative specific weight ratio T] y . These relations were determined experi- 

 mentally using a wide range of wave conditions and bottom materials and are valid 

 mainly for the breaker zone. 



Noda's scaling relations indicate that movable-bed models with scales in the 

 vicinity of 1 :90 (model to prototype) should be distorted (i.e., they should have 

 different horizontal and vertical scales). Since the fixed-bed model of Nome Harbor 

 was undistorted to allow accurate reproduction of short-period wave and current 

 patterns, the following procedure (which has been successfully used and validated 

 for undistorted models) was used to select a tracer material. Using the prototype 

 sand characteristics (median diameter, D 50 = 0. 15 mm, specific gravity = 2.7) and 

 assuming the horizontal scale to be in similitude (i.e., 1:90), the median diameter for 

 a given vertical scale was then assumed to be in similitude and the tracer median 

 diameter and horizontal scale were computed. This resulted in a range of tracer 

 sizes for given specific gravities that could be used. Although several types of 

 movable-bed tracer materials were available at WES, previous investigations (Giles 

 and Chatham 1974, Bottin and Chatham 1975) indicated that crushed coal tracer 

 more nearly represented the movement of prototype sand. Therefore, quantities of 

 crushed coal (specific gravity = 1.30; median diameter, D 50 = 0.29 - 0.54 mm) were 

 selected for use as a tracer material throughout the model investigation. 



Chapter 2 The Model 11 



