Why does climb gradient at Changi be at minimum of 6 and 7 percent?
Geographical location of Changi airport

Singapore is on the Straits of... Singapore. This straits is the southern continuation of the Straits of Malacca, both form an international maritime road between Indian Ocean and Pacific Ocean, on a par with Suez and Panama.


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To add to the bustle, the Changi Naval Base, opened in 2004, is a mere 2 km East of the runways:


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If that was not enough, there is a shipping channel at the North of the airport, with two terminals near the runways: A ferry terminal, and a cargo terminal.

View of the waters South of Changi airport:


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Obstructions and climb gradient

Very large ships, including military ones can be close to the runway ends. When this happens, and to take a long takeoff into account, a large climb gradient is in use.

AIP for Changi includes instruction for obstacle clearance calculation.

On the North the climb gradient is at least 3.3%, but is increased based on ships actual height when ships exceed a specified height, and depending on the runway e.g. for RWY 02C:


  When there are no reports of vessel movement along the northern
  shipping channel, or where the reported vessel height is less than 70m
  (230 ft) AMSL, the aircraft minimum net climb gradient shall be at
  3.3%.
  
  Where the reported vessel height is 70m (230 ft) AMSL or higher, ATC
  shall advise pilots of the vessel height. Pilots, on receipt of this
  information, shall apply the minimum net climb gradient in
  accordance with the following table.
  
  


For RWY 02L, the height threshold is as small as 35 m and the gradient for a 110 m ship will be 10.7%, i.e. the gradient is twice.

On the South, because the traffic is larger and permanent, the guidance is different:


  All departures on Runway 20C/20R shall be on a minimum net climb
  gradient of 5% until reaching or passing 400 ft. Thereafter, the
  minimum net climb gradient shall be 3.3%.


So what happened for this NOTAM to be published, requiring a higher gradient of 6 and 7% for RWY 20C/20R? Note this is only 1 or 2% steeper than it is usually. Different possibilities, no certainty:


The South protected area border is temporarily closer to the coast.
There is a very tall ship in the vicinity.
There is an obstacle on the land.


The general formula used for MOC calculation, for North and South departures:


  For the above calculations, operators shall use the following
  information:  
  
  a. The most penalizing obstacle is a tall vessel which is
  on the extended center line of the runway. (ATC shall advise pilots of
  the height of the tall vessel.)  
  
  b. The required MOC is 0.8% of the
  distance (d) from the departure end of runway (DER) to the obstacle,
  in accordance with Volume II of ICAO Doc 8168 -  Procedures for Air
  Navigation Services Operations (PANS-OPS)  where, in the case of
  Singapore Changi Airport, the DER is defined as the end of the
  clearway.  
  
  c. The distance (d) for departure Runways 02L/02C is
  measured from the DER to the shipping channel north of Changi. The
  distance (d) for departure Runways 20C/20R is measured from the DER to
  the boundary of the restricted waters south of Changi wherein tall
  vessels of height above 49m (161 ft) AMSL are not permitted. The
  distance (d) for the various departure runways is as follows
  
    


The procedure above can be used by crews to calculate an optimized climb gradient, in case the default 5% is not retained. This procedure is used to provide pre-calculated MOC for North departures.

Now I want to see the actual departure from RWY 02C! Fly me!