Controlling Hillside Runoff

When runoff comes from above, from a slope on your own lot or from uphill properties, the fix is almost never at the low spot where water ends up. It’s about catching the water earlier and giving it a controlled path around your house.

Why runoff is worse here

• Clay and decomposed granite don’t absorb fast. In heavy rain, much of the water runs off instead of soaking in, so slopes shed a lot of it.
• Hardscape adds up. Roofs, driveways, patios, and pool decks uphill all send water downhill faster.
• Winter storms are intense. NorCal rain often comes in concentrated bursts during atmospheric rivers, not gentle drizzles.

The main tools for runoff control

Swales

A swale is a shallow, gently sloped channel shaped into the landscape that redirects surface water. It’s simple, low maintenance, and can be planted or lined with rock so it blends into the yard. On many sloped lots, a well-placed swale uphill of the house does most of the work.

Interceptor French drains

A French drain installed across the slope, perpendicular to the flow, catches both surface water and shallow groundwater moving downhill and carries it around the house to a safe outlet. It’s especially useful when water is seeping through the soil, not just running over the top.

Catch basins

Where runoff concentrates at a low point, like the bottom of a driveway or a corner where two slopes meet, a catch basin collects it into a solid pipe before it spreads.

Dry creek beds

A rock-lined channel that carries water during storms and looks like landscaping the rest of the year. Great where you need to move a lot of water and want it to look natural.

Erosion control

On bare or steep slopes, ground cover, erosion blankets, mulch, and terracing keep soil from washing down with the water. That silt otherwise ends up clogging drains and burying beds.

When water comes from a neighbor’s property

Natural runoff flowing downhill is common and usually your job to manage on your side. If a neighbor has recently redirected a drain or downspout straight at your property, that’s worth a friendly conversation first. Either way, an interceptor drain or swale along your uphill edge protects your yard regardless of what’s happening next door.

What not to do

• Don’t just build up a berm at the bottom. Water pools behind it and finds another way.
• Don’t send runoff into a retaining wall backfill.
• Don’t trench across a slope without a planned outlet. Collected water has to go somewhere.