The model grid system is located on the model site to delineate areas 

 which may require rough grading with heavy equipment. The grid system is 

 then accurately established on the site for use in positioning templates. 

 The templates are set by nailing to wooden stakes and are roughly graded 

 (Fig. 3-10). The ground between the templates is then smoothed to an 

 elevation about 4 to 6 inches below the final model grade. Sand is placed 

 between the templates and smoothed to within 2 or 3 inches of the final 

 grade, and the templates are final graded. The templates are then painted 

 with a sealer to provide a waterproof joint between the concrete and the 

 template, and twenty-penny nails are driven through the templates to pre- 

 vent differential settlement of the concrete slabs. At this point (Fig. 

 3-11), the model is ready for paving. 



Concrete grout (cement, sand, and water) is poured between the tem- 

 plates and rough graded by screeds (only for areas of very low relief) or 

 critical contours are sketched in the wet grout to define details between 

 the templates (Fig. 3-12). The concrete is then molded to conform to the 

 sketching. During the molding process, both the position and elevation 

 of physical features between the templates a.re accurately checked. The 

 concrete surface is finished, and roughness is added to the model while 

 the concrete is still wet (Fig. 3-13). 



Different types of model roughness are used, often in a single model. 

 In relatively deep water, boimdary roughness will probably not generate 

 sufficient mixing for satisfactory reproduction of vertical salinity gra- 

 dients. In this case, vertical roughness elements (strips, rods, or bars) 

 are used. At WES, copper or stainless-steel strips of widths varying be- 

 tween about 0.25 and 1.0 inch are preferred. The tops of the strips are 

 cut off just below the elevation of low water to avoid interference with 

 surface currents. The strips are initially placed about 1 per 1 to 5 

 square feet (normally considerably more than required), and the excess 

 strips are bent down during the verification process. In shallow water 

 and areas of low velocity, the turbulence generated by vertical roughness 

 elements will not yield sufficient model friction; therefore, boundary 

 roughness is used. This can be achieved by scratching the concrete sur- 

 face before it sets, adding a thin layer of stucco and roughening it with 

 a mason's float, or by applying gravel, small cubes, etc., to the model 

 bed. It may be necessary to smooth or further roughen the boundary rough- 

 ness during the verification process. 



If an investigation of a new or enlarged navigation channel is planned, 

 a trough is constructed in the model on the proper alinement of sufficient 

 width and depth to permit installation of the enlarged channel at a later 

 date. The trough is then filled with concrete and molded to the existing 

 conditions (Fig. 3-14). 



Concurrent with construction of the model, construction and installa- 

 tion of model appurtenances is accomplished. Installation of drainpipes 

 (and sometimes water supply pipes) must be done before pouring the concrete. 



b. Tide Generation . Several different types of tide generation mecha- 

 nisms are employed at the various hydraulic laboratories throughout the 



77 



