224 • Marine Minerals: Exploring Our New Ocean Frontier 



Photo credit: A. Crosby Longwell. National Marine Fisheries Service 



Atlantic mackerel eggs sorted out of plankton from 

 surface waters of the New York Bigfit. 



Alteration of Wave Patterns 



Mining in shallow water may change the form 

 and physiography of the seafloor. Wave patterns 

 may be altered as a result of removing offshore bars 

 or shoals or digging deep pits. When changes in 

 wave patterns and wave forces affect the shoreline, 

 coastal beaches can erode and structures can be 

 damaged. The best example of these dangers oc- 

 curred in the United Kingdom in the early 1900s 

 when the town of Hallsands in Devon was severely 

 damaged by wave action following large scale 

 removal of offshore sandbars to build the Plymouth 

 breakwater (see photograph). Coastal erosion is 

 now the first consideration in the United Kingdom 

 before mining takes place; dredging is limited to 

 areas deeper than 60 feet. This criterion is based 

 on studies that imply sediment transport is unlikely 

 at depths greater than 45 feet; the additional 15 feet 

 were added as an extra precaution.^" Current work 



by the U.S. Army Corps of Engineers suggests that 

 concern with water depth alone may not be suffi- 

 cient to avoid beach erosion^' ^^ and that detailed 

 on-site modeling should be considered in pre-plan- 

 ning anadysis. For example, the U.S. Army Corps 

 of Engineers of the New Orleans District built a 

 beach and dune on Grand Isle, Louisiana for ero- 

 sion control in 1983. The project required 2.8 mil- 

 lion cubic yards of sand obtained by digging two 

 large borrow holes one-half mile offshore (about 

 twice this amount was actually dredged to achieve 

 the design section). Shortly after completion, cus- 

 pate sand bars began to form on the leeward side 

 of the dredged holes and the beach began to erode 

 adjacent to the newly formed bars. During the win- 

 ter and spring of 1985, heavy storms exacerbated 

 the areas of beach loss adjacent to the cuspate bars 

 (e.g., see opposite page).^^ This unexpected re- 

 sponse of beach formation and erosion as a result 

 of altered wave patterns around the borrow areas 

 illustrates the importance of site- specific assessment 

 before mining large volumes of sediment from the 

 seafloor. 



Seasonal 



During certain times of the year, e.g., when eggs 

 and larvae are abundant, the effects of offshore min- 

 ing may have a more negative impact on the ocean 

 community than at other times. Juvenile stages of 

 fish and shellfish are transported by water currents 

 and therefore are less able to actively avoid adverse 

 conditions. They are generally more susceptible to 

 high concentrations of suspended sediments than 

 swimming organisms that can avoid such conditions. 

 For example, striped bass larvae in the Chesapeake 

 Bay develop more slowly when particulate levels 

 are high.^* Therefore, restricting offshore mining 



^»R.W. Drinnan and D.G. Bliss, The U.K. Experience on the Ef- 

 fects of Offshore Sand and Gravel Extraction on Coastal Erosion and 

 the Fishing Industry, Nova Scotia Department of Mines and Energy, 

 Open File Report 86-054. 



^'Joan Pope, U.S. Army Corps of Engineers, OTA Workshop on 

 Environmental Concerns, Washington, D.C., Oct. 29, 1986. 



"R.J. Hallermeier, A Profile Zonation for Seasonal Sand Beaches 

 from Wave Climate, U.S. Army Corps of Engineers, Reprint 81-3 

 (Fon Belvoir, VA: Coastal Engineering Research Center, April 1981). 



"A.J. Combe and C. W. Soileau, "Behavior of Man-made Beach 

 and Dune, Grand Isle, Louisiana," Coastal Sediment '87, 1987, p. 

 1232. 



"A.H. Aula and J.R. Schubel, "Effects of Suspended Sediment 

 on Fish Eggs and Larvae; A Laboratory Assessment," Estuarine and 

 Coastal Marine Science, No. 6 (1978), pp. 153-164. 



