Pre-filters are used when pumping water from a pond, or a slow moving stream or river. These instructions are based on a 20 GPM water supply, if you need more water you will need to use a larger pipe and hole, or manifold multiple pipes together. This is a very basic outline of what is required, you will obviously need to adapt it to your situation.
Dig a 3 foot deep 10 foot long hole in the stream or pond bed . Next make a intake manifold, it is just a piece of 2″ SCH 40 PVC pipe with a cap on one end and your intake pipe connected to the other end. You then drill one hundred each, 3/8″ diameter holes in the manifold pipe to allow the water to flow into the manifold. Put a plastic liner on the bottom of your hole. Now put 6″ of 3/4″ diameter gravel in the hole. Then lay the intake manifold in the hole on the gravel and pack another 12″ deep layer of 3/4″ gravel around and over it. On top of that you place another 18″ deep layer of 3/8″ gravel.
The gravel acts as a “pre-filter”. As the water moves slowly down through the gravel layers to the intake manifold the sand and much of the algae gets trapped in the gravel. This is generally enough to protect the pump. You still need a filter after the pump and it still might need to have a auto-flush mechanism.
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To slow down a fast moving small creek to allow pumping water from it the classic method is to create a small, low, dam using sand bags. Not only is the use of sand bags easy, it is also temporary and the sand bags can be removed seasonally if desired.
Remember that in most countries there are lots of rules and regulations that apply to disturbing stream beds. In the USA, at a minimum, you should contact the US Fish and Game Department and EPA for advise. There are probably also State and Local regulations as well. The fines for violating rules or regulations can be very extreme! Make sure you get a ll required permits.