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| Geomorphological units | Terrain and landform, substrate, and shallow stratigraphic features | Disaster factors | Countermeasure |
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| Continental shelf | Yangtze River subaqueous delta | The terrain is flat, with microlandforms such as scouring and sandwaves, and there are many obstacles. The sediments are mainly silt and clay, and the buried layer is soft, which is suitable for submarine cable excavation and shallow gas development | (1) Obstacles
(2) Scouring
(3) Sandwave
(4) Shallow gas | (1) Avoid obstacles
(2) Scouring- and sandwave-developed area should be buried as deep as possible and increasing the length of the submarine cable appropriately
(3) The construction of the submarine cable should take shallow gas preemission measures and increase the length of the cable appropriately |
| Large erosion depressions | The terrain is flat with many obstacles, and four obvious sand ridges are developed. The sediment at the top of the sand ridge is mainly fine sand, and the sandwave landform is developed. The sediment at the bottom of sand ridge is mainly silt or clay, and shallow gas is developed | (1) Obstacles
(2) Developed shallow gas at the bottom of the sand ridge
(3) Hard seabed at the top of the sand ridge, developed sandwave, and hard for deep burial | (1) Avoid obstacles
(2) The construction of the submarine cable at the bottom of the sand ridge should take shallow gas preemission measures and increase the length of the cable appropriately
(3) The length of the sea cable can be increased appropriately when it is difficult to bury deeply at the top of the sand ridge |
| Stacked terrace | Flat terrain, development of small sandwave or sandwave, obstacles, and fishing activities
The substrate is composed of sandy silt and silty sand, and the local sea area is developing ancient river channel/ancient lagoon | (1) Occasional obstacles
(2) Hard seabed, hard for deep burial | (1) Avoid obstacles
(2) Buried as deep as possible otherwise increase the length of the sea cable appropriately |
| Ancient Delta plain |
| Ancient trendy sandy ridge group |
| Residual accumulation plain |
| Land slope | Large terrain changes, hard for burial construction, developed submarine valley, developed stepped terrain, abundant pockmarks on the submarine surface, turbidity, and submarine bedrock are developed in submarine trench | (1) Large slope of the terrain, hard for burial construction
(2) Submarine trench, rock, stepped terrain, and high-density turbidity
(3) Pockmarks | (1) Slow construction; burial operation and properly strengthen the structure and strength of the protective layer of the optical fiber cable
(2) Avoid submarine trench, rocky, stepped terrain, and high-density turbidity
(3) Avoid pockmarked area |
| Okinawa trough | Undulating seafloor terrain, with gullies, abyssal hills/seamounts, rocky and hard strata exposed
The substrate is composed of clayey silty sand, with shallow stratigraphic layers and locally developed shallow gas, faults and hard strata, or rock outcrops | (1) Gullies, abyssal hills/seamounts
(2) Shallow gas (pockmarks)
(3) Fault
(4) Hard strata or rock outcrops
(5) High-density turbidity | (1) Complex terrain, buried construction is prone to plowing accidents, and it is recommended to lay construction, while avoiding trench valleys, abyssal hills/seamount areas, choose relatively flat terrain area, and properly strengthen the structure and strength of the optical cable protective layer
(2) Avoid shallow gas developed area
(3) Avoid fault zones as far as possible, and when unavoidable, shall try to choose shallow stratigraphic structure stable area to cross
(4) Avoid hard strata or rocky outcrop areas |
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