III. WATER LEVEL CONTROL 



1. Necessity . 



The SPTB plumbing system is shown in Figure 6. A constant water depth 

 was maintained throughout a test to eliminate, or minimize, the effect of 

 changing water depth on the (a) instrument carriage-to-water level dis- 

 tance, (b) position of the shoreline, and (c) generated and reflected wave 

 conditions. 



2. Procedure for Establishing Control . 



In order that data be comparable in a given test area from 1 year to 

 the next, water level criteria were established in 1970 according to cri- 

 teria which had been used from the beginning of service of the particular 

 test area within the SPTB. 



The north basin was filled to the approximate desired depth and then 

 adjusted until the average of the depths along ranges 1, 3, 5, 7, and 9 

 at the toe of the concrete slope was 2.340 feet (the reference depth in 

 earlier experiments in the 10-foot tanks). A 4-inch-long (10 centimeters) 

 notch was made with a hammer and chisel at the east end of the concrete 

 slope at the SWL intercept. The reference depth in the 6-foot tank was 

 established in a similar manner at 2.330 feet and a black line was drawn 

 at the SWL intercept on the concrete slope. 



3. Procedure for Maintaining Control . 



To monitor the water level while the wave generators were running, 

 a point gage was rigidly mounted about 1 foot inside the tank wall adja- 

 cent to the hydrant (Fig. 7). The rubble absorber in this area provided 

 good to excellent damping depending on the wave period used. With the 

 SWL intersecting the concrete slope at the previously marked reference, 

 the point gage was carefully adjusted to read some easily remembered value 

 which was used as a constant for the test season. A 2-inch feeder line 

 continuously added water to the basin during testing to offset the losses 

 from leakage and evaporation (Fig. 7). 



The water level was checked and recorded three times during each run: 

 the start, midway, and near the end. However, readings were made more 

 frequently when conditions warranted. 



4. Problems Encountered . 



Three conditions which commonly caused difficulty in maintaining the 

 desired water level during test intervals were (a) improperly adjusted 

 feeder line valve, (b) change in water-main pressure when filling the 

 large wave tank, and (c) rain. 



The practical tolerance in water level was ±0.002 foot (±0.6 milli- 

 meter). Factors affecting the establishment of this tolerance were (a) basin 



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