OCEANOGRAPHIC INFORMATION FOR SUBMARINE CABLES 1089 



5.2 Turbidity Currents — • Slumps 



The turbidity current is a flow of sediment-laden water which occurs 

 when an unstable mass of sediment at the top of a relatively steep slope 

 is jarred loose and slides down the slope. As the slide or slump travels 

 { down the slope, more and more water becomes mixed in the mass, giving 

 it high fluidity combined with its high density. The currents reach very 

 high velocities, enabling them to spread for vast distances across the 

 abyssal plains. Such slides occur at the edge of continental slopes, in 

 particular, in the vicinity of river mouths, off prominent capes, or near 

 banks. They are triggered by earthquakes, hurricanes, or floods. 



Three excellent pieces of evidence support the turbidity-current the- 

 ory and all have important implications for submarine cables. The 

 turbidity-current hypothesis was first introduced to explain the erosion 

 of submarine canyons in the continental slopes. Supporting evidence 

 was found in submarine cable breakage following the Grand Banks 

 earthquake of 1929^ and the Orleansville (Algeria) earthquake of 1954.^ 

 In both cases, submarine cables were broken consecutively in the order 

 of increasing distance downslope from the epicenter of the earthquake. 

 All of the cables were broken in at least two widely separated places 

 (100 miles or more apart). The sections between breaks were swept 

 away and/or buried beneath sediment deposited by the current so that 

 repair ships were never able to locate a large proportion of these sections. 

 Fig. 26 shows the area of the Grand Banks turbidity current. 



After study of this evidence, Heezen and Ewing concluded that the 

 area covered by the current would show graded sediments as a result of 

 deposition by the turbidity current. This was substantiated by the 

 evidence of cores obtained from the locations shown on Fig. 26. 



Cable damage resulting from turbidity currents due to slumps at a 

 river mouth is well illustrated by that off the mouth of the Magdalena 

 River (Columbia, S. A.). On August 30, 1935, the disappearance of 480 

 meters of the western breakwater and most of the ri\'er bar resulted 

 from a slide which produced a 30-foot channel across the bar. The same 

 night, tension breaks occurred in the submarine cable from Barranquilla 

 to Maracaibo, which crosses the submarine canyon 15 miles off the 

 mouth of the Magdalena in a depth of about 700 fathoms. The cable, 

 ^vhen brought up for repair, was tightly wrapped with green grass of a 

 type which grows in the marshes near the jetties. 



The same pattern has been repeated 15 times since the cable was laid 

 in 1930. The breakage of the cables has occiu-red most fi-equently in 

 August and late November to early December; the two periods of the 



