higher velocities laminae become less distinct); and (5) occurrence of regres- 

 sive ripples (regressive ripples indicate relatively higher velocities). 



Measurements of bed forms can be accomplished on exposed sand banks at 

 low water using surveying techniques or large-scale aerial photographs. 

 Dimensionless parameters of ripples and other bed forms can indicate deposi- 

 tional environment (Tanner 1967). The flow directions can be assessed in 

 terms of the trace of the crestline (Allen 1968). Wave-formed structures 

 reflect the velocity and direction of the oscillatory currents as well as the 

 length of the horizontal component of orbital motion and the presence of 

 velocity asymmetry within the flow (Clifton and Dingier 1984). The flow 

 strength for inter-tidal estuarine bed forms can also be estimated for a given 

 flow depth by the velocity-depth sequence of bed forms (Boothroyd 1985). 



Navigation and Positioning Equipment 



Accurate positioning is essential for most geological monitoring studies. 

 Several types of positioning and navigation systems are available for coastal 

 studies, with the most common being Loran-C and Global Positioning Systems 

 (GPS). Other technologies, such as short-range microwave and optical 

 systems, are also in common use (Fredette et al. 1990). 



Loran-C computes microsecond time differences using pulsed low- 

 frequency radio waves between networks and receivers. The differences are 

 then computed as lines of position. The receivers can be used up to about 

 2,000 km from the networks with reasonable accuracy. The absolute accuracy 

 of Loran-C varies from 180 to 450 m, while the repeatable accuracy varies 

 from 15 to 90 m. 



GPS is a satellite navigation system developed by the U.S. Department of 

 Defense. An array of satellites collectively provides precise, continuous, 

 worldwide, all-weather, three-dimensional navigation and position for land, 

 sea, and air applications. Use of GPS for hydrographic surveying is 

 expanding, and procedures and equipment are improving rapidly. 



Navigation (positioning) error standards have been established for US ACE 

 hydrographic surveys. Three general classes of surveys have been defined 

 (HQUSACE 1991): 



Class 1 - Contract payment surveys 

 Class 2 - Project condition surveys 

 Class 3 - Reconnaissance surveys 



Although the requirements of geologic site surveys may not be the same as 

 those of USACE hydrographic surveys, the accuracy standards are useful 

 criteria when specifying quality control requirements in contractual 



Chapter 3 Field Data Collection and Observation 



55 



