I'd use a tapered approach (1:20 or so) between your pipe and a "trash rack" to reduce the velocity of the water through the rack. I say a "trash rack" for two reasons. I trash rack is the correct terminology and it the right application for what you want to do. Screens tend to plug naturally because of their elements perpendicular to stream flow. Track racks are pretty much self- cleaning if designed properly. For your application the rack needs to be in a fast section of the stream and be perpendicular (not about 90 deg to stream flow but absolutely 90 deg to the stream flow. If less then 90 deg water flowing into the rack tends to try and force debris into the grating while more then 90 deg eddy's are produced which will tend to keep debris in circulation on the face of the grating. If the grate is at 90 deg to stream flow then the streams own water tends to wash debris off the grate. The grating should be flat steel stock (3/16 x 1 inch stripe) spaced approx half of your nozzle openings (impulse wheel) of half your blade spacing (reaction turbine). Frank said, "I'm thinking of parallel rods like refrigerator shelf" That's a pretty good description of what it should look like. The grating (strips) must be parallel with stream flow and maybe have only one vertical support (for mechanical stability) on the backside of the grate. For your application I'm thinking about a grate surface of 8 x 14 inches or so.
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