Side, drop and frontal intakes

1. Side intake

The opening is made in the river bank, at an angle (normally 90-110°) to the flow, beside the weir. It has a sill, a trash rack and a gate.

 river ==========================>
        weir crest  ||||||||||||||
 bank   +----------------------+
   rack |  intake    gate  -----+--> canal

• Used where the river carries bed load and boulders: the sill is raised and the undersluice keeps the intake clear. It is the most common type in Nepal's RoR plants (steep gravel-boulder rivers).
• The flow must turn through about 90°, so some sediment still enters; a gravel trap is therefore needed.

2. Drop (bottom / Tyrolean) intake

A trench with inclined bars (a rack) is built in the river bed across the flow; the water drops through the bars into a chamber and flows on into a pipe or canal.

 flow ====>   inclined rack
        ______\\\\\\\\\\______
              |  chamber  |---> conduit

• Used in steep mountain streams with small flow and coarse bed load, where the boulders roll over the rack.
• The collection chamber lies below the river bed, so some head is lost; the rack lets fine sediment through, so a settling basin is needed downstream. It is liable to clogging by leaves and gravel and needs regular cleaning.
• Not suited to a large discharge.

3. Frontal intake

The opening is placed in line with the flow (in the weir or the dam face).

 river ===> |intake| ----> conduit

• Entry losses are small and the structure is simple, but it takes in much of the bed load, so it is used only where the water is clean (e.g. a reservoir or a clean channel) or with a good sediment excluder in front.