Why Basements Flood in Thornton, CO

Where does the water in a Thornton basement usually come from?

Six routes, in rough order of how often they turn up: lower-level plumbing and appliances, surface water at the foundation, window wells, a failed sump pump, a backed-up drain line, and creek or storm drainage overflow. The first three account for most calls on an ordinary week.

Homeowners tend to picture a creek when they picture a flooded basement, and in this city the creeks are real and mapped. But the water on the floor of a finished basement in a 1990s subdivision has usually traveled a few feet, not a few miles: out of a water heater, down a window well, or through the bottom of a foundation wall after a thunderstorm dropped an inch in twenty minutes on ground that could not take it.

That is good news, because short routes are fixable. Grading, downspouts, window well covers, a backup pump and a hose you replace before it splits are all cheap next to what a lower level costs to rebuild. The point of this page is to help you work out which route your water took, since that is the decision the repair depends on.

Which creeks and drainages run through Thornton?

Three are named in the city's flood insurance study: Big Dry Creek, Brantner Gulch and Grange Hall Creek. The Urban Drainage and Flood Control District mapped Big Dry Creek as floodprone from I-25 to 168th Avenue, and FEMA revised its flood elevations in 1999 between East 136th Avenue and just below I-25.

Those are the stretches where rising water is a real design consideration rather than a worry. The South Platte River runs down the east side of the metro and takes the whole basin's runoff, which is why a storm over the foothills can be a Thornton problem a day later even if nothing fell here.

September 2013 is the event that showed the scale: rain of up to twelve to twenty inches across the foothills and adjacent plains put the South Platte basin into flood, and the United States Geological Survey has been cited putting Colorado's damage around three billion dollars. No Thornton-specific damage figure from that event was found in a source we would publish, so we are not going to invent one.

The practical step is to find out where your own address sits relative to the mapped floodplain, because a homeowners policy generally excludes rising surface water. Flood damage restoration describes the different cleanup that outside water demands, since it arrives with soil and whatever the yard and street gave it.

How does surface water get through a foundation wall?

Through the joint where the wall meets the floor, through cracks in the wall, and around penetrations. Water does not need a hole: saturated soil against a foundation puts pressure on it, and the path of least resistance in a Front Range basement is usually the cold joint at the bottom of the wall.

Follow the water uphill and the cause is nearly always outside the house. A downspout that ends at the wall, a patio or a flower bed graded toward the foundation, a swale somebody filled in, a tree root that cracked the apron, an irrigation zone running against the south wall for twenty minutes every evening. Each of those puts water exactly where it can be driven inward.

Clay makes it worse in a specific way. The Colorado Geological Survey describes expansive soils as prevalent the length of the Front Range, where clay swells when it absorbs water and shrinks when it dries. That cycle moves foundations differentially, opens cracks that were hairline last year, and leaves last summer's shrinkage gaps along the wall ready to channel the first heavy rain straight down.

The fix is a drainage fix, not a drying fix. Extend downspouts well clear of the wall, re-establish fall away from the house across the first several feet, keep irrigation off the foundation, and seal only after the drainage is right. Sealing a wall that is still being soaked moves the leak rather than stopping it.

Why do window wells put water on the floor?

A window well is a hole next to your foundation with a window at the bottom. Its gravel drains until it silts up, and once the well holds water, the only route out is through the window frame. It is the most common single-point entry in a suburban basement.

Look for the signature: a clean puddle directly under one basement window, dry floor elsewhere, and often a fan of soil or mulch washed across the carpet. Egress wells in finished basements are the worst offenders because they are wide, deep and often set below the surrounding grade in exactly the place a lawn drains to.

Three things help and all are cheap. Keep the well clear of leaves and mulch so its gravel can drain, fit a clear cover so rain and snowmelt never enter it, and make sure the ground around it falls away rather than toward it. If the well has a drain pipe to a footing drain, a plumber can tell you whether it still works; silted wells are routine on houses twenty or thirty years old.

What happens when a sump pump fails?

The pit fills, then the floor floods from the pit outward. A sump pump is one motor, one float switch, one check valve and one discharge line, with no redundancy unless you added it, and it typically fails at the moment it is working hardest or during the power cut that came with the storm.

Test it on a quiet day instead of finding out in the dark. Pour a bucket of water into the pit and watch: the float should rise, the pump should start, the water should go, and the check valve should keep it from running back in. Listen for a motor that hums without moving water, and look for a discharge line that freezes shut in winter or dumps right beside the foundation, where the water simply returns to the pit.

Redundancy is the real answer. A second pump, a battery backup or a water-powered backup, and a loud alarm on the pit all exist because the primary pump is a consumable part. Note too that a basement filling because a pump quit is a backup-type loss, which on a Colorado policy generally needs its own endorsement, so how water damage insurance claims work in Colorado is worth reading before you need it.

• Pour a bucket into the pit every few months and watch the full cycle.
• Check that the discharge line runs well away from the foundation and has not frozen or crushed.
• Clear the pit of gravel and silt that can jam the float.
• Add a backup pump or battery and an alarm; the primary pump is a wear part.
• Replace on age rather than on failure, especially before the storm season.

What about the plumbing already down there?

The lower level holds the water heater, the laundry, often a bathroom, the main shut-off and sometimes a wet bar. Every one of those is a water source sitting on the floor you are trying to keep dry, and a failure there floods the room from the inside with no weather involved.

Water heaters are the classic. A tank that lets go empties its contents and then keeps feeding from the supply until somebody closes a valve, which is why knowing where your shut-off is matters more than any other preparation in this article. Washing machine supply hoses are next: they are rubber, they age, and they fail under constant pressure.

Then there are the slow ones. A weeping fitting inside a finished basement wall, a condensate line that backed up, a toilet supply that dripped behind the base for a season. Those do their damage invisibly, and the first symptom is a soft baseboard, a musty smell or a stain that appears after months. Water heater leak damage and appliance leak water damage cover how each is traced and dried.

How does a drain line send water back into the house?

When the line leaving the house cannot take what is coming, the water finds the lowest opening indoors, which is a basement floor drain, a shower pan or a laundry standpipe. That water is contaminated, so the cleanup differs from a clean-water loss from the first minute.

Colorado districts generally put the service lateral on the homeowner and the main on the utility, but Thornton's own rule is not something we found on an official page, so ask the City of Thornton or your sewer provider directly where responsibility ends at your address. Get the answer before there is water on the floor, and write it down.

Treat a backup as contaminated regardless of how it looks. Keep people and pets out of the room, do not run more water into the system, and expect porous materials that soaked it to leave rather than be dried. Sewage backup cleanup sets out the containment and disposal sequence. Call (833) 490-5225 as soon as the room is clear; the hazard is biological and it grows.

What should you do in the first hour of a flooded basement?

Make it safe before you make it dry. Water and electricity share that floor: the panel, the furnace and the water heater are usually down there. Shut off power to the level if you can do it from a dry position, close the water supply if the source is plumbing, then start documenting.

Do not wade into standing water in a basement that still has power to its circuits, and do not reach for a cord or a breaker while standing in water. If the panel is in the wet area, call your utility or an electrician rather than improvising. The furnace and water heater should be left alone by anyone who is not sure, and gas appliances that have been submerged are not safe to relight.

Once it is safe, get photographs of everything before you move it, lift what you can off the floor, and get the water out. Standing water in a cool below-grade room is a clock: the longer it sits the more material it writes off, which is why extraction leads every job. Crews go out 24/7 as soon as one is available, and emergency water removal is what that first visit is for.

• Keep out of standing water until power to the lower level is off.
• Close the main water shut-off if the source is inside the plumbing.
• Photograph and video the room before anything is moved.
• Lift contents and furniture out of the water where it is safe to.
• Leave submerged gas appliances off and have them checked before relighting.