Acoustic Doppler Current Profilers (ADCP) 



These profilers operate on the principle of Doppler shift in the backscat- 

 tered acoustic energy caused by moving particles suspended in the water. 

 Assuming that the particles have the same velocity as the ambient water, the 

 Doppler shift is proportional to the velocity components of the water within 

 the path of the instrument's acoustic pulse (Bos 1990). The backscattered 

 acoustic signal is divided into parts corresponding to specific depth cells, often 

 termed "bins." The bins can be various sizes, depending upon the depth of 

 water in which the instrument has been deployed, the frequency of the signal 

 pulse, the time that each bin is sampled, and the acceptable accuracy of the 

 estimated current velocity. Much excitement has been generated by ADCP's, 

 both among scientists working in shallow water and in the deep ocean (a 

 comprehensive bibliography is listed in Gordon et al. 1990). A great 

 advantage of using ADCP's in shallow water is that they provide profiles of 

 the velocities in the entire water column, providing more comprehensive 

 views of water motions than that from strings of multiple point source meters. 

 ADCP data are inherently noisy, and signal processing and averaging are 

 critical to the successful performance of the gages (Trump 1990). 



Indirect estimates of currents 



Indirect estimates of current speed and direction can be made from the 

 orientation, size, and shape of bed forms, particularly in shallow water. Sedi- 

 mentary structures on the seafloor are caused by the hydrodynamic drag of 

 moving water acting on sediment particles. The form and shape of bottom 

 structures reflect the effects of and interaction among tidal currents, waves, 

 riverine flow, and longshore currents. These complex interactions especially 

 affect bed forms in tidal channels and other restricted waterways. Bed forms 

 reflect flow velocity, but are generally independent of depth (Clifton and 

 Dingier 1984; Boothroyd 1985). Their shape varies in response to increasing 

 flow strength (Hayes and Kana 1976). Bed form orientation and associated 

 slipfaces also provide clues to flow direction (Morang and McMaster 1980; 

 Wright, Sonu, and Kielhorn 1972). Widespread use of side-scan sonar has 

 made this type of research possible in bays, inlets, and offshore. 



Grab Sampling and Samplers 



Seafloor sediments in coastal areas can show great spatial and temporal 

 variation. The surface sediments may provide information about the energy of 

 the environment as well as the long-term processes and movement of 

 materials, such as sediment transport pathways, sources, and sinks. Bed 

 surface sediments are typically collected with grab samplers and then analyzed 

 using standard laboratory procedures. These tests are described briefly in this 



Chapter 3 Field Data Collection and Observation 



41 



