Slumping 



The slumping or sliding of bottom sediments is a potential hazard to 

 underwater cables and structures. The kind and rate of sedimentation 

 required to produce an unstable slope occurs only in areas of relatively rapid 

 sediment accumulation. These areas include deltas and canyon heads in the 

 inner shelf slopes. The stability of a sedimentary deposit on a given slope 

 depends basically on the shear strength of the deposit and the rate of increase 

 of this strength with depth of burial. 



The slumping of bottom sediments can jeopardize structures or cables 

 from either the resultant cascading sediments or turbidity currents. A classic 

 example of turbidity currents caused by slumping was the submarine landslide 

 which occurred in the Grand Banks region in 1929 following an earthquake.* 

 The speed of these turbidity currents has been estimated to be about 50 knots. 

 It becomes apparent, then, that a thorough bottom survey, including corings, 

 of the site and above-site slopes should be made to ensure against slumping. 



Temperature and Salinity 



The range of temperature and salinity in the oceans is anywhere from 

 about -2°C and 20% to about 30°C and 37%, with some areas having higher 

 or lower readings. Many of the extreme conditions are found in the Red Sea 

 and the Persian Gulf. The layers of the ocean are generally classified as: 



Troposphere: down to about 300 feet 



Thermocline: from a depth of 600 to 984 feet 



Subtroposphere: from a depth of 984 feet to about 4,000 feet 



Stratosphere: below 4,000 feet 



Most of the temperature and salinity variations exist, along with the 

 strongest currents, in the troposphere. In the stratosphere the temperatures 

 and salinities are more uniform. 



The maximum vertical temperature and salinity gradients are found 

 between 300 and 600 feet, in the layer between the troposphere and the 

 thermocline. An exception to the above classification occurs in the subtropics, 

 where the thermocline is found at a depth of 500 feet at 20 degrees south and 

 600 feet at 20 degrees north. This layer rises steadily to a depth of 160 feet 

 at the equator and 10 degrees north. Intensity of the thermocline is greatest 

 in the equatorial areas. 



Beneath the thermocline, the vertical temperature and salinity gradients 

 decrease with depth down to the subtroposphere. 



B. C. Heezen and M. Ewing. "Turbidity currents and submarine slumps and the Grand 

 Banks earthquake," American Journal of Science, 1952, pp. 849-873. 



