38 



changes. Cell circulation, the combined effect of both types of currents, may 

 explain or assist in identifying regularly spaced features along many coasts. 

 The longshore migration of such cells may also cause accompanying landform 

 migration. 



An example of the interaction of topography and hydrodynamic forces is 

 provided by rip currents. Rip currents extend perpendicular from the shore- 

 line through the surf zone and serve as a conduit for water to escape from a 

 zone of elevated water. The spacing of rip currents may be controlled by 

 edge waves or other wave height variations along the surf zone (Bowen 1969; 

 Bowen and Inman 1969; Tang and Dalrymple 1989). However, such wave 

 height variations may not necessarily be the only cause of rip currents. 

 Irregular nearshore topography, manifested by shoreline protuberances, may 

 produce nearshore circulation cells (Sonu 1972). Rip currents are important 

 geologically because they have been shown to carry significant amounts of 

 sand offshore (Davidson-Arnott and Greenwood 1976; Sonu 1972). Rip 

 currents can be located by observers at the shore and from aerial photographs. 

 Their positions can also be identified indirectly by side-scan sonar when their 

 characteristic rip-scoured channels are imaged on the seafloor (Morang and 

 McMaster 1980). 



The configuration of the shoreline can provide information regarding 

 littoral currents. Shoreline protuberances, especially in the vicinity of 

 structures, headlands and barriers, and tidal inlets are useful indicators of the 

 prevailing longshore littoral drift (Komar 1976). Such indicators cannot 

 generally be used for quantitative estimates of the sediment transport rate. 

 Usually, transport rate must be calculated from: (a) direct evidence, such as 

 sand impoundment in front of structures; (b) physical measurements of 

 currents and sediment size and type; and (c) the use of longshore transport 

 formulas, provided that local waves can be measured or hindcast. 



General techniques of current measurement 



The observation of hydraulic phenomena can be accomplished by two 

 general approaches. One of these, Lagrangian, follows the motion of an 

 element of matter in its spatial and temporal evolution. The other, Eulerian, 

 defines the motion of the water at a fixed point and determines its temporal 

 evolution. Lagrangian current-measuring devices are often used in sediment 

 transport studies, in pollution monitoring, and for tracking ice drift. Eulerian, 

 or fixed, current measurements are important for determining the variations in 

 flow over time at a fixed location. Recently developed instruments combine 

 aspects of both approaches. 



Four general classes of current-measuring technology are presently in use 

 (Appell and Curtin 1990): 



• Radar and Lagrangian methods. 



• Spatially integrating methods. 



Chapter 3 Field Data Collection and Observation 



