User:Pbsouthwood/Wall diving



Wall diving is underwater diving along the face of a near vertical cliff wall, particularly if the bottom is below the range of the diver's equipment and certification. The context is usually recreational diving, technical diving, or scientific diving on scuba. This diving environment requires good buoyancy control.

Environment
Wall diving is diving along vertical or near vertical sided of coral reefs, or rocky cliffs, or other near vertical underwater terrain. Walls can vary from a few metres high on part of a reef to faces that drop down for hundreds of meters. The wall face may be fairly short, or may continue for kilometres, and may be interrupted by ledges, less steep areas, or overhangs, or may be almost continuously vertical. The bottom may or may not be visible from diveable depths. Visibility may be anything from very good to very bad, and there may be currents along the wall in almost any direction, sometimes with strong turbulence, vortices and overfalls. This can be a very challenging diving environment.

Walls may be the continuation of a steep rocky cliff shoreline, or may be a considerable distance offshore, both of which may be difficult to access from the shore. In other cases, an easy beach entry with a short swim may provide access to the top of a reef edge drop-off starting near the surface. In some places the terrestrial terrain may suggest the presence of underwater cliffs, and in other areas the land may be flat and low-lying, with fringing coral reefs with steep and deep dropoffs. Most wall dives are in the sea, but others can be found in flooded quarries, in fresh water, and in sinkholes.

Depending on the geology, the wall face may be relatively smooth, blocky, or riddled with overhangs, caves, crevices, and tunnels. Lava faces and coral reefs tend to be more varied in texture. Karst shorelines tend to have larger caves.

By their nature, there is a reliable fixed visible reference for vertical position and movement monitoring along a wall if there is any visibility.

Habitat
Walls generally have relatively high biodiversity compared with other sites in the same region. This is partly due to the large range of depths, but is also influenced by the rugosity.

Hazards
In many cases there is no effective bottom, so there is a significant risk of exceeding the planned depth limit for the dive. Vertical currents or surge are possible, which can make depth control difficult, and lateral currents are fairly common. As the dive is generally conducted close to the wall, and there may be overhangs or projecting organisms which can foul a surface marker buoy or decompression buoy line, it may be necessary to do drift dives without a marker to identify the position of divers to the boat, as a snagged line in a current can cause great difficulty to control depth, and can damage fragile organisms living on the wall.

Skill requirements
Some shallower walls and walls with a less vertical slope or many ledges may be suitable for less skilled divers, if the currents are mild and there are no vertical currents or strong turbulence at the wall edge, but generally, good buoyancy and depth control skills and management are necessary for safe diving. At the end of a dive it may be prudent to move away from the wall before deploying a decompression buoy, to make sure it clears the reef, and this requires deployment in midwater, and depth control is more complex while inflating the float. Many wall dive sites are relatively deep, and nitrox or trimix certification may be required.

Some walls allow groups with mixed skill levels to dive at the same site, with novices remaining in the shallows and more competent and adventurous divers with suitable equipment and skills exploring the deeper parts, while still having an interesting ascent and decompression environment. The common recreational dive plan is to start deep and ascend slowly and relatively continuously, so that there is no major decompression obligation at the end of the dive. Scientific divers may use a similar general strategy, but will plan around the dive task, which may require a different profile.

Equipment
Good buoyancy control requires both suitable skill and suitable equipment. When decompression is planned, it is critical to diver safety to be able to ascend at a controlled rate and remain at suitable depths for decompression, so a decompression buoy serves both as a marker identifying the location of the diver, and a means of easily maintaining a stable depth for decompression when above the top of the wall, and can be deployed at the end of the dive when there is less risk of fouling on the wall above the diver. A decompression buoy also gives the diver more options if they have a problem with buoyancy of a dry suit or buoyancy compensator. Other equipment often recommended includes a light for looking into crevices, overhangs and caves,