Do you have a picture showing the slope?

Is this a northern situation or southern?

When you say the seed and plants are flushed from the ground, is that caused by rain or your irrigation?

The house is about 300' elevation on the edge of Piedmont Plateau east of Raleigh NC and faces nearly directly south. The slope Left to right drops about 20' in 120' parallel to the road and about 6' horizontally to the road. The road was cut through a natural ridge when the addition was laid out.

The problem is around the bed in the foreground and below the Tree in
the middle of the picture. That tree is the one I mentioned in my
original request. Last winter the area in the right of the picture lost most of the vegetation that was there.

I do not have irrigation but with this last batch of seed I water about a 40 minutes each evening, unless it rained. We have lost plants in the bed, because of too much water.