Berm

Shape rules

• Top width ≈ height. A 1-ft-high berm needs roughly a 1-ft-wide top to stand. A knife-edge ridge washes out in one storm.
• Side slopes 3:1 or flatter. Steeper sides slump and won't hold grass. This is the same number the swale page uses, on purpose.
• Compaction is the material. A berm is not a pile; it is soil placed in lifts and walked in. Uncompacted berms are the single most common self-inflicted failure in yard drainage.
• Set the top level. A berm that tips downhill becomes a spillway; a berm that tips uphill sends water back at the house. One pass with a line level — the 30-minute job — costs nothing and prevents both.

The soil arithmetic

You will move more soil than the visible ridge. A 40-ft berm, 1 ft high, 1 ft top, 3:1 sides carries a cross-section of about 4 square feet. That is 160 cubic feet of compacted soil, roughly 6 cubic yards before settling — and every yard of it must come from somewhere. Dig the channel first and stack the spoil downhill; importing soil because the berm came first is the expensive order.

When a berm makes it worse

• Placed uphill of a house it dams water toward the foundation. The 10-ft setback on the swale page applies to the berm too, and then some.
• Across a neighbor's natural flow it becomes a liability, not a feature. Water crossing a property line is a legal fact before it is an engineering one — read your code before you redirect it.
• Without a low point or overflow, every pool overflows somewhere. Design the spillway; do not let the storm pick it.