Broadly speaking, there are three types of Spillway available for wall mounted Waterfalls.
Wall Descent spillways need a flow rate of 50 to 60 litres per minute (lpm) per metre spillway width.
Falling Sheet features require a faster flow rate to achieve the desired "Water Fall" effect. 100 to 120 lpm per metre spillway width is usually required.
Projecting Sheet units are pressurised and require at least 150 lpm per metre spillway width. More may be required depending on the distance the water is to project forward. A Rain Effect Projecting Sheet spillway is also available. This breaks the sheet of water up into many small streams, permitting the water to form into individual drops as it falls.
Projecting Sheet features can be ordered with short spillway lips (20 to 50 mm), or longer ones - usually 125, 150 mm or 230 mm. The longer lips allow the manifold to be mounted behind a brick or stone wall. Inbuilt fibre optic lighting or attachable LED lighting is also available with some models.Depending upon Manufacturer, Waterfall features can be made of:
Now for a bit about Hydraulics.
With Falling Sheet and Projecting Sheet features, the curtain of water "pulls in" as it falls. This is due to the surface tension of water. The extent to which it pulls in varies, depending on flow rate. With a very slow rate of flow (say only 30 to 50 lpm per metre), the pull in is quite dramatic. At higher rates it is less so.
However, even at the recommended levels, expect the width of the sheet of water to reduce by around 15 cm per edge (30 cm overall) over a 1 metre drop.
After a one meter drop (or even less) the sheet of water will start to break up. From this point, instead of a single sheet, water will form into large "drops" and start to behave like water falling from several separate spouts. This is also due to the surface tension of the water and changes with flow rate.
Finally, it is most important to make sure the plumbing feed to the feature is properly thought through. Turbulence and friction loss in pipes can reduce the flow rate by a staggering amount if the plumbing layout is not designed properly. At the relatively large flow rates required by spillways, all pipes should be at least 25 mm diameter for smaller spillways, and up to 40 mm (or more) for large spillways. They should also be kept as short as possible and there should be no sudden changes in direction. To achieve this latter point, avoid the use of " Elbows" wherever possible. A smooth 90 degree curve is much better.