The sand trap is often the last part of the inlet tunnel before the water flows down the penstock and on to the turbine. It is a chamber that has a larger volume than the rest of the tunnel, which makes that the water slow down when it enters the sand trap.
The figure illustrates the structure of a typical Norwegian hydropower plant, with the location of the sand trap between the surge chamber (left) and the penstock (right).
The reduced speed of the water causes particles in the water (pebbles and gravel) to fall to the bottom of the sand trap and sediment there. The sediment-reduced water will flow on to the penstock and the turbine.
In Norway, two types of sand trap are mainly used:
Open sand traps
Just an open empty space, where the increased volume in the sand trap causes the water to lose speed and the particles fall to the bottom of the room (as sediments). The sediments are exposed to turbulence in the water, resulting to the distribution shown in the figure below (a & c).
(+) cheap to create/empty the sand traps, (-) less effective at separating and protecting the sediments.
Ribbed sand trap
The sand trap has ribs that lie above a deeper chamber. When the water moves with reduced speed over the ribs, the particles will fall between the ribs and settle on the bottom of the chamber. The ribs lead to the sediments being better protected from the turbulence in the water, see the figure below (b & d).
(+) effective at separating and protecting the sediments, (-) more expensive to create/empty the sand traps.
The figure shows an open sand trap seen from the side (a) and from above (c), and a ribbed sand trap seen from the side (b) and from above (d).