Question
Hi Matt:

I've finally been given a permit to pump out of an irrigation canal adjacent to my property.  I have no problem installing things myself, but I've never tackled a project like this before and don't want to do it wrong.

The spot where I'm allowed to pull water out is in a curve of the canal, so the water moves a little faster there. Additionally the canal is only about 1 foot deep. The engineer who approved the permit said he would allow me to put a sump in the canal and he recommened I use a 12" diameter pipe for the sump.  He said to put it deep enough "to get adequate submergence over your suction".

Do you have any clue how deep I would need to go below the bottom of the canal to make this work? Also, how far above the bottom of the canal should the top of the pipe be?

I'm only allowed to pump out 45GPM and the pump I've been approved to use is the Goulds Irrigator GT-20. I plan to use a 1" pipe in submerged in the sump and the pump will be within 75 feet of the canal. The land is very level and the level of the pump will pretty much be the same as the canal level.

I'm trying to irrigate about 1.5 acres.  If you have any ideas on these questions I'd really appreciate hearing them.

Thanks!


Answer
Thanks for the detailed question. You've provided enough information to give a good answer.
When the pump is running it can draw a vortex of air down from the surface. It will look like the water going down the drain from your bathtub. A sump give the perfect environment for this to happen. The top of the filter should have 18" of water over it. So it will depend on what type and size filter or suction fitting you are using as to the depth of the sump. I would suggest taking your 12" sump pipe and drilling a series of 1-1/2" to 2" holes around the pipe about 4" down from the top. Then fashion a screen to go around the holes and also over the top of the sump, sort of like a hat. Place the top of the sump just below the water level. This will give you flow from the top and also the sides which tends to disturb the vortex effect.
The problem you are most likely going to encounter is soil and sediment filling the bottom of the sump. You will need to make you suction system removable so you can periodically clean out the bottom of the sump. A plastic union just before the filter would be adequate.
I would suggest using a 2" suction pipe. A 1" suction will only allow 12-18 GPM to flow and will put an unnecessary strain on you pump. Also don't forget to use a check valve at the sump end of the suction.
Finally, the most important item with the suction of a pump are air leaks. Absolutely, no suction leaks are allowed. NONE!  A tiny pin hole will give you a multitude of problems.   
Good luck, and if you have further question, please ask.