I am a drainage contractor looking for info on backfilling seepage drainage on downhill slopes in a heavy rainfall area in the mountains. Sand backfill is required to meet the particle size requirements of the native soil. Trenches will be 2 to 3 feet deep. Our plan is to place the sand in sand bags to cover the seepage drainage, and backfill the top portion of the trench with compacted native soils. Trenches would be sodded and anchored with sod staples. I am concerned however on the 20 and 30 degree slopes that the recompacted native soils would still tend to erode. Are there other things I can do in the backfill process to help stabilize the trenches?
  If a trench was filled with just sand, would the erosion start in the bottom of the trench as the sand column saturates, or at the top of the trench from surface water velocity?

Golfer2 - is this seepage out of a "limited" area - such as a spring?  In many highway projects, they typically will place a coarse stone on the slope to stabilize although, in my view, there should be a filter fabric suitable tied down and then, say a 3/4 inch to 3 inch stone placed on top - then larger stone - say 6 to 10 inch above that.  This should stop local seepage distress.  You could also use gabion baskets with geotextile behind.  The sand will just wash out.  

Now if this is a water collection system (french drain) that would be placed at the base of a slope and running parallel to the road so as to draw down the water below the road level, I would suggest that you line the trench with geotextile and then fill with 20-40mm clear stone.  Some like to put in pipes.  I don't think a french drain filled with sand will be suitable - not enough capacity.  There are plenty of references showing french drains - we use them in Laos in some heavy seepage areas.

If major stability problems exist, you might actually have to use lateral drilled-in drains extending back into the hillside to draw the water level well down and behind the slope.
 Hope this helps -