a sheer decent is used for waterfalls that produce a sheet of water.
The edge needs to be very smooth, over hang so the water falls off the edge and doesn't "roll" off, needs to be fed from a smooth reservoir to prevent turbulence and needs to be as level as possible. THe flow needs to be controlled too little and you'll just get a dribble too much and you'll get a flood. Here's a good site to look at.
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What an interesting link. Thanks for posting it ! Steve