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Prepping for a landslide (Part 2): Controlling the water

By The SHTF App Team

The geologist father had a one-sentence theory of landslides that he repeated at every dinner table argument about retaining walls: “You don’t fight the hill, you negotiate with the water.” A neighbor once spent a fortune on a beautiful concrete wall, and the father pointed at the downspout dumping 600 gallons per rain inch onto the soil behind it. “You built a dam,” he said, “and then you aimed a hose at it.” The wall bowed within two winters. The negotiation had been lost before it started, and the failure was plumbing, not geology.

My dad was a geologist, so while other kids heard bedtime stories about princesses, I heard stories about plate tectonics and “pore water pressure.” He taught me early on that the earth looks solid, but it acts like a sponge. A dry hillside is held together by friction, but when water saturates that soil, it pushes the particles apart, friction drops, stuff gets slippery, and gravity takes the wheel.

If you don’t manage the water falling on your property, you are essentially lubricating the slide that will take your house down the hill, or the hill down on your house. By controlling where the water flows, capturing it from the roof and diverting it away from the slope, you maintain the friction that keeps your homestead standing. This is Part 2 of the landslide series.

Disclaimer: The information provided in this article is for general informational and educational purposes only. It is not intended as, and should not be considered, geological or engineering advice. Check local laws before diverting water.

The Readiness Audit

Where does your roof water actually go?

  • Green: Every downspout pipes water 10 to 20 feet away from the foundation, and your drains are flushed annually.
  • Yellow: You have gutters but no extensions, or drains you have never checked.
  • Red: Your downspouts dump at the foundation and your slope gets irrigated.

If you are Yellow or Red, execute Phase 1 immediately.

Phase 1: The Roof Catchment (Concentrated Fire)

Goal: Turn the roof from a cannon into a hose aimed safely away.

Your roof is a massive water collector, a 1,000-square-foot roof catches about 600 gallons of water for every inch of rain. If you dump 600 gallons onto a single spot of dirt next to your foundation, you are drilling a hole in your own stability.

The gutter imperative: concentrated water cuts canyons, and a gutter without a downspout extension is just a canyon-maker aimed at your footing. Install hard-piping (PVC) or flexible extensions on every downspout, releasing the water at least 10 feet (ideally 20) away from the foundation onto a flat, stable area or into a storm drain, never onto the top of a steep slope.

The clean-out: if gutters clog, water spills over the edge and saturates the soil right against the house walls, causing “hydrostatic pressure” that can buckle basement walls. Keep them clear.

The Takeaway: Six hundred gallons per inch per thousand square feet is a fire hose aimed at your footing, and the extension pipe is the nozzle redirect that costs ten dollars.

Phase 2: Surface Defense (Swales and Berms)

Goal: Intercept the sheet before it gains speed.

When rain hits a hill, it flows down in a “sheet.” If that sheet hits loose soil, it takes the soil with it.

The swale (the ditch): a shallow, gravel-lined ditch dug across the slope on contour. It catches the water rushing down the hill before it gains speed. Unlike a gardening swale (meant to sink water), a drainage swale needs a slight (1 to 2%) downhill grade to carry the water away to a safe discharge point like a rocky creek bed or street gutter.

The berm (the speed bump): a small mound of compacted earth on the downhill side of the swale. It acts as a backup dam, if the swale overflows, the berm holds the water back and forces it to move sideways rather than down the sensitive face of the slope.

The Takeaway: The swale catches and steers, the berm is the backup plan, and together they turn a falling sheet into a controlled walk to the gutter.

Phase 3: Sub-Surface Defense (The French Drain)

Goal: Relieve the pressure you cannot see.

Sometimes the water isn’t on top of the ground, it’s moving through it. When groundwater builds up behind a retaining wall or under a slope, it exerts massive pressure, the “hydraulic hammer” that can blow out concrete walls and liquefy soil.

The French drain solution: a trench lined with landscape fabric, filled with gravel, containing a perforated pipe at the bottom. Water always seeks the path of least resistance, so instead of pushing against the dirt, it flows into the gravel, drops into the pipe, and flows away safely. Install these behind every retaining wall and along the uphill side of your house foundation.

The Takeaway: The French drain gives groundwater a polite exit, and it is the difference between water flowing and water pushing.

Phase 4: The “Don’t” List (Common Mistakes)

Goal: Stop doing the things that sabotage everything else.

  • No dumping on the slope: never pipe your gutters or pool backwash onto the steep part of a hill, you are essentially power-washing the earth away.
  • No irrigation on the edge: do not put lawn sprinklers on the top edge of a steep bank. The constant artificial soak weakens the “brow” of the hill, causing it to slump. Use native plants that don’t need water there.

The Takeaway: Every gallon you add to the slope is a gallon of lubricant, so the don’t list is really the don’t-water-the-slide list.

The Essential Kit Checklist

  • The extensions: flexible downspout extenders or 4-inch PVC pipe for every corner of the house.
  • The shovel: a sharp trenching shovel for digging swales.
  • The fabric: non-woven geotextile landscape fabric to line drains.
  • The gravel: bulk drainage rock (jagged rock that locks together, not pea gravel).
  • The boots: rubber muck boots for the rain walk inspection.

Scenario Planner (Contingencies)

“The neighbor’s runoff is flooding my yard.” The trap: ignoring uphill water. The fix: the curtain drain, dig a French drain along the property line between you and the neighbor to capture their runoff before it enters your soil matrix and divert it to the street. Check local laws on diverting water, but protecting your foundation is priority one.

“My French drain is clogged.” The trap: “set it and forget it”, roots love drain pipes. The fix: the clean-out, when you install the drain, leave a “Y” access point sticking out of the ground, and once a year run a hose or plumber’s snake through it to ensure it’s clear.

Next Steps

  1. Walk the downspouts today and measure. Anything releasing closer than 10 feet from the foundation gets an extension this week.
  2. Mark the swale line with flags before you dig, following the contour, and confirm the 1 to 2% grade with a level before moving dirt.
  3. Read Part 3: Rooted resistance next, because water management stops the trigger, and roots stop the soil from letting go when the trigger comes anyway.

Originally adapted from public USDA and USGS drainage guidance (public domain).