Deep wading or deep fording is a technique used by some heavy amphibious vehicles to traverse water that is no more than a few metres deep - the vehicle drives on the riverbed/lakebed/seabed and uses screens or a pipe (a snorkel) that reaches above the water surface for an air supply. The technique has been used by armoured military vehicles such as tanks.
In contrast, lighter amphibious vehicles that float on the water surface are not limited by the depth of the water.
World War II
Although Duplex Drive allowed landing craft to release tanks farther from shore, the alternative deep wading gear permitted a tank to drive partially or completely underwater on the sea floor rather than swim. Deep wading Churchills took part in the 1942 Dieppe raid, and also operated during the D-Day assault. These tanks were given waterproofed hulls and air intake and exhaust trunking to allow them to come ashore from shallow water. Tall ducts extended from the engine deck to above the turret top and they needed to stay above water. The front duct was the air intake for the engine, the rear duct vented the exhaust. This device saw use in many amphibious operations, it was also used on light tanks and tank destroyers. The US had similar devices for trucks and jeeps.
The Germans gave their Tiger tank a long snorkel, essentially a long tube on the commander's hatch that allowed it to wade through four metres of water. The German Tauchpanzer, a modification of the Panzer III and Panzer IV, drove on the sea-bed. A rubber hose supplied the engine and crew with air and gave the waterproofed tank a maximum diving depth of 15 metres (49 ft) making it an extreme example of a wading tank. The Germans converted 168 Panzer IIIs and 42 Panzer IVs for use in Operation Sea Lion.
The Leopard 2 tank carries a snorkel that is in fact a series of rings which can be stacked to create a long tube. This tube is then fitted to the crew commander's hatch and provides air and an escape route for the crew. The height of the tube is limited to around three meters.
All modern Soviet/Russian tanks like the T-90 are also able to perform deep fording operations, however unlike the Leopard, the Russian snorkel is only a few inches round and does not provide a crew escape path, although it is more practical and can be stored on the tank.
Deep fording capability is important for military vehicles because bridges may be destroyed during conflict to prevent crossing, and smaller bridges may not be able to support heavier tanks to begin with. Making a vehicle capable of navigating shallow water counters this problem without the expense and technical difficulties of floating the entire vehicle.