494 BOUNDARIES OF THE SEA 



clay in the water is much higher in the tidal flat area than in the 

 open sea. This should cause diffusion of clay toward the outlets, 

 in the same way as freshwater from the land soon disappears. 



Less obvious mechanisms have now been discovered, which 

 evidently play an important part in the storage of the clay. These 

 have been termed "lag effects." As the water slackens towards 

 the time of high tide its turbulence decreases and the clay pellets 

 and floccules start to settle out. But before they have reached 

 the bottom they have been carried inward a certain distance. 

 As the average maximum current velocities decrease from the 

 inlets to the mainland shore, the particle reaches the bottom at a 

 point with lower current speeds than occur at the locality where 

 it started to settle. Hence, the accelerating ebb current will not 

 be able to pick it up when the same stage in the ebb tide has been 

 reached at which the foregoing flood current started to drop the 

 particle. The retreating tide will have to approach closer to its 

 maximum velocity before the particle can be dislodged. Less of 

 the ebb cycle can therefore be used for the outward journey, and 

 the obvious result is that the particle cannot be carried back to the 

 point from which it started its inward journey. 



To this "settling lag effect" is added a "scour lag effect." This 

 results from the fact that a stronger current is needed to dislodge 

 a sediment particle from the bottom than is required to keep it 

 in motion, once it has been set going. Then, the muddy suspension 

 is spread out in a thin sheet at high tide, but is concentrated in 

 deep gullies at low tide. This means that slowly settling particles 

 can be deposited at high tide but not at low tide. This in turn 

 renders the lag effects more efficient. Finally, the period of stag- 

 nant water lasts much longer at high tide than at low tide (Postma, 

 personal communication). Hence, more lutum reaches the bottom 

 and suffers scour lag. 



Soon the combined results of these lag effects cause a particle 

 to be carried so far inward that the ebb current cannot pick it up 

 again. What happens is that with each tidal cycle the particle is 

 transferred to a new mass of surrounding water that lies relatively 

 farther from the inlet. There is a kind of diffusion away from the 

 open water. This reasoning explains why there can be a much 



