154 



migrating and "disappearing" due to lack of sediment supply (Otvos, 1979). 

 See also Fig. 6.1. Over the past -100 years, this factor seems to have 

 been far more important than (absolute) sea level rise. 



The East Frisian Islands in Germany illustrate trends of an opposite 

 nature . These islands and the corresponding inlets are believed to have 

 been in existence for the past few millennia (Luck, 1976) . Unlike the 

 Mississippi barriers, however the islands have accumulated additional 

 sediment from the West Frisian Islands. Fig. 10.1 shows three of these 

 islands (and four inlets) [Norderney (N) , Baltrum (B) , and Langeoog (L) ] , 

 plus ends of two [Juist (J) and Spiekeroog (S)]. It is observed that 

 between 1750 and 1960, the barrier as a whole gained sediment, resulting in 

 greater land area above mean high water (MHW) . To some extent, these 

 developments have been influenced by coastal protection works at the 

 eastern ends of Norderney and Baltrum (Kunz , 1987). Luck (1976) has 

 accumulated map-based evidence on the changes at East Frisian Islands and 

 has provided a qualitative explanation for the observed historic behavior. 

 Unfortunately, present day technology precludes the possibility of 

 predicting changes in island chain configuration over even the next 

 10 years . 



10.3 ESTUARINE SEDIMENTATION 



It is common for inlets to have sandy beds and upland estuarine waters 

 to consist of fine-grained material. A relatively rapid sea level rise 

 could cause a correspondingly rapid landward migration of the sandy barrier 

 island, thus exposing back barrier fine-grained deposits to open coastal 

 wave action (Everts, 1987). In this way, a marine source of fine-grained 

 sediment is created. Some low coasts lack barriers, for example west 

 Florida north of Tarpon Springs. Barriers have not developed here because 

 of the lack of sandy material, which in turn is due to the limestone 

 formations through which the coastal rivers drain (Shepard and Wanless, 

 1971) . Here again there is an offshore source of fine-grained sediment 

 which generates coastal turbidity mainly during storms. 



The significance of a marine source of fine-grained sediment is that 

 this material tends to enter the estuary with the upstream salinity- driven 

 residual current, which typically occurs in the mixing zone of the estuary 



