To convert the raw sensor output R (A/D units) to concentration C (gll), 

 the following algorithm was used 



C = G ( R ~ 0) * B (6-1) 



204.8 



where G is the sensor gain (g/l/V), O is the clear-water offset (A/D units), 

 204.8 is the conversion factor from A/D units to volts, and B is the 

 background turbidity present in the channel (g/l). Absolute measurements of 

 background turbidity were not made during the project, and, consequently, B 

 is assumed to be zero. 



Data file format 



Data were recorded on a PC in binary format. A sample record consists of 

 8 bytes of time in one of two formats, a ground channel (2 bytes) and N 

 channels of OBS output (2 bytes each), where N is the number of sensors 

 deployed for that run. For Runs A0509A - A0915A, time was stored as four 

 2-byte integers (HH MM SS HH); for all subsequent runs, time was stored as 

 an 8-byte real number in seconds since midnight. These two formats reflect 

 differing clock outputs; the former being taken from the A/D clock (which 

 was set to WWV), and the latter directly from the WWV clock. 



A common number of channels recorded was 34; thus, the corresponding 

 record would be 8 + 2 x 34 = 76 bytes. Subsequent records (at 10 Hz) are 

 simply appended to the file. Typically, a 70-min run had 42,000 records and 

 was 3.2 MB in size. All data files have been archived in two formats, on 

 QIC-40 tapes in the original PC format and on optical disk in UNIX format. 



Sample data 



A 20-min segment of suspended sediment concentration data (gll) collected 

 during Run A1417A (70-min duration) is shown in Figure 6-5. During this 

 random wave forcing run, the peak wave period T p was 8 sec, the zero- 

 moment wave height H^ was 0.5 m, and 7, the TMA spectral width pa- 

 rameter, was 3.3. Figure 6-5 shows the time-dependence of nearbed 

 concentration (z = 3.5 cm) at five cross -shore positions progressing from off- 

 shore (panel A) to onshore (panel E). General observations of sediment dy- 

 namics evident in Figure 6-5 are: 



a. Seaward of the surf zone, suspended sediment concentrations are con- 

 siderably lower (panels A, B) than shoreward of the breakpoint (pan- 

 els C - E). 



b. The highest peak concentrations are found at the innermost positions, 

 panels D and E (note scale change). 



Chapter 6 Suspended Sediment Concentration 



115 



