Rainbowing is the method of choice to discharge huge quantities of sand in shallow locations or ashore.
Rainbowing does not require a floating pipeline, and a landline, with all the associated extra costs.
This technique was used extensively in the construction of the Palm Islands and "The World", Dubai.
When rainbowing, following parameters are important for production output of the dredger:
A. Vertical angle of the rainbownozzle
Some ten years ago, most dredgers were equipped with a rainbownozzle in a 45° angle to the vertical.
This was indeed the best angle to reach long distance, from a ballistic point of view.
Since then, it was observed that a 45° nozzle creates large craters on the sandfill area, and that a large part of the sand flows back towards the dredger.
A 30° nozzle is now standard. The sand is projected in a more flat trajectory, and bounces futher on the sandfill. Backflow is minimal. The final distance reached is comparable to a 45° rainbow nozzle.
Another factor to be kept in mind is the height of the rainbownozzle (and the height of the discharge area) above the waterline. Rainbow distance can be calculated using this link: http://hyperphysics.phy-astr.
B. Nozzle design
Left photo: an "old-style" rainbownozzle, with a sharp decline in diameter. This causes turbulence during rainbowing, with higher pressure and less flow.
Nowadays, nozzles are much more sleek (right and below); resulting in a higher exit velocity of the sand-water mixture, thus higher production for the same power output.
C. Nozzle opening diameter
The most important parameter of all is nozzle diameter.
A small diameter nozzle means less flow, hence less hourly production output, but the sand will be projected over a longer distance, because of the higher exit velocity. With the power available on most jumbo´s today, reaching distances in excess of 150 meter is no problem, but at the cost of some 30% extra discharge time.
Jumbodredgers can reach peak productions > 25.000 m3/hour in the first ten minutes of rainbowing, which is massive by any standard.
(It is said that, at the trials of "Vasco Da Gama", there was a secret project to rainbow sand at a speed exceeding Mach 1, simply by reducing the nozzle diameter.)
To be more flexible in the choice of rainbow distance versus production, large jumbo´s are equipped with two nozzles of different diameter; which can be used together if maximum output is required.
D. Loaded draught of the dredger (and especially: forward draught)
The only prerequisite for rainbowing is close access of the loaded ship to the rainbow location. This may often require to beach the ship.
Draught can be a limitation, as the ship may not be able to reach the sandfill.
However, in most cases, the dredger can change forward draught fast during the first minutes rainbowing, spraying massive amounts of sand, then pushing forward on the beach. Again, being able to play with nozzle diameters during rainbowing is an advantage.
Beaching the ship -pushing the bow through the sand- wears down the steelplates in contact with the sand slope. It´s wise to check this during the next drydock, if the ship saw extensive rainbow action. I know of at least one jumbo who had steelplates replaced in the lower bow section for this reason.
Some dredgers are limited by design and dare not touch the underwater sand slope. Design limitations are mostly: position of cooling water intake and strengthening of the bow.
"Nielsen" dredgers filling a caisson by rainbowing.
"Vasco Da Gama" on a special mission: the rainbownozzle points to starboard and allows for trenching, with the dredged material sidecasted.
Thus, the trenching operation is continuous; no time lost on dumping, and the ship remains light and manoeuvrable during dredging.
"Gerardus Mercator" during rainbowing action offshore the Dubai coast. The material is sprayed underwater. Sand -especially the finer fraction- can travel long distances underwater and forms large "plumes".