In some places, the hydraulic head of water carried onshore by waves may 

 return across the surf zone as a concentrated jet, known as a rip current (Bow- 

 en 1969; Bowen and Inman 1969). Rip currents are relatively narrow and 

 strong, capable of moving sediment seaward through the breakers and the in- 

 shore bar field. Although near-bottom flow usually dissipates rapidly, surface 

 flow may persist for greater distances. The geologic signature of rip currents 

 can be seen on the seafloor well seaward of the surf zone (Morang and 

 McMaster 1980). Rip currents tend to be found at positions of lowest breaker 

 height. 



The existence of rip currents, and the manner in which they are fed, 

 depends upon whether the nearshore current system is dominated by a cell cir- 

 culation system or a longshore current. Typically, both situations are present 

 simultaneously (Shepard and Inman 1950). In the cell circulation system, the 

 rip currents are fed by a series of longshore currents that increase in velocity 

 from a minimum midway between two adjacent rips to a maximum just before 

 turning seaward into the rip (Figure 6). If the wave approach angle is large, 

 at least 5 deg to 10 deg, a strong longshore current will be generated that is 

 continuous along the shoreline. The intermediate condition with a smaller 

 breaker angle is known as general circulation. General circulation may result 

 in an asymmetrical current pattern, with velocities at their minimum level 

 updrift from the rip current and increasing to a maximum before turning 

 seaward to the next rip current (Komar and Inman 1970). 



Tidal currents are horizontal water movements caused by the rhythmic 

 rise and fall of the sea surface. Their magnitude generally increases with 

 increasing tidal range. Tidal currents force water to move both at the surface 

 and at depth, and generally have the greatest velocities during the flood and 

 ebb, at mid-stage between high tide and low tide. In the open ocean, tidal 

 current velocities rarely exceed 1 m/sec. In restricted passages and shallow 

 coastal shelves, however, velocities are greater and can exceed 4 m/sec in 

 some channels. 



Storm Surges 



Sustained and strong wind stresses on a water body, often developed under 

 tropical and extratropical storm systems, not only create waves but also pro- 

 duce a horizontal flow of water in the general direction of the wind. When 

 this flow approaches a coast and is affected by shoaling, there is a sustained 

 increase in the water levels, known as a storm surge. Storm surges may be as 

 much as 3 to 6 m during major hurricanes or extreme seiches on the Great 

 Lakes, and may be augmented by decreases in atmospheric pressure. This 

 anomalously high water level can cause flooding of the beach, dunes, and 



Chapter 2 Relevant Processes and Factors 



