(Fig. 3-6). Examples of velocity station layouts are shown in Figures 

 3-4 to 3-8. Velocity observations should be made at various depths on 

 each station. In depths of 6 feet or less, only the middepth observa- 

 tions are required; in depths of about 6 to 15 feet, only surface and 

 bottom observations are normally required. In greater depths, the ver- 

 tical observation interval depends largely on the expected degree of 

 salinity stratification. In well-mixed estuaries, surface, middepth, 

 and bottom observations are sufficient; in estuaries with a higher 

 degree of stratification, observations should be made at the surface 

 and bottom and at the 1/4-depth points or at depth intervals of 6 to 

 10 feet. Surface measurements should be made at 1 to 3 feet below the 

 water surface, and bottom measurements at 2 to 4 feet above the bottom. 



Current velocities (magnitude and direction) should be observed at 

 each designated depth at each station at 1/2-hour intervals over a com- 

 plete tidal cycle. Where the tides are mixed or diurnal (Pacific and 

 gulf coasts), a complete tidal cycle refers to a 24.84-hour period; where 

 the tides are semidiurnal (Atlantic coast), a complete tidal cycle is a 

 12.42-hour period. If the number of stations is relatively small, it is 

 recommended that sufficient personnel and equipment be assembled to moni- 

 tor all stations during a single tidal cycle. If this is not possible, 

 the survey should be conducted during as few consecutive days as possible 

 and during a period when successive tides are predicted to have a reason- 

 ably uniform amplitude. In such cases, one station should be established 

 as the control station, and it should be monitored on each day of the 

 survey period to determine the effects of varying tidal conditions on the 

 magnitude and phasing of velocities. To minimize the time required to 

 con5)lete the survey, a single boat may concurrently monitor more than one 

 station, if the stations are located close enough for the boat to monitor 

 each station every 30 to 45 minutes. 



(5) Salinity . Salinities should be measured concurrently with 

 velocity measurements at all ranges, stations, and depths specified for 

 velocity observations; however, it may be necessary to extend the salin- 

 ity survey upstream to the extent of saltwater intrusion. These data are 

 sufficient to define the lateral and vertical salinity distributions 

 throughout the system for particular freshwater inflow conditions and 

 to evaluate the change of the salinity with tidal phase in all critical 

 areas. However, if the velocity measuring program does not cover a sig- 

 nificant part of the year to adequately evaluate the response of the 

 salinity regimen to major changes in freshwater inflow, a supplemental 

 long-term program of salinity measurements may be required. This can be 

 accomplished by establishing a network of key salinity stations through- 

 out the system, and salinity observations can be made at intervals (e.g., 

 at the time of high water slack every few days) over a period of time in 

 which freshwater inflow varies from minimum to maximum. The sampling 

 network for the long-term" salinity survey can be random, in which case 

 the variation of salinity with time is determined at each point; or the 

 stations can be located along the length of the main channel, in which 

 case the longitudinal salinity profile for each sampling period is also 

 determined. Both types of measurements have previously been successful. 



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