Heavy Rain and Runoff in the Ozarks: Designing Outdoor Spaces for Storms

About 47 inches a year, with spring the wettest season.

NOAA's 1991 to 2020 normals for Fayetteville Drake Field put average annual precipitation at 46.96 inches. May is the wettest month at 5.89 inches, followed by April at 5.01; January and February are the driest at 2.75 and 2.56. The same normals count an average of 31.3 days a year with half an inch or more, 14.0 days with an inch or more, and 3.0 days with two inches or more.

Annual totals are not what damage a yard, though. What matters is how much falls in a short time, and that is what NOAA Atlas 14 measures.

A storm with a 10 percent or a 1 percent chance of happening in any given year, not one that arrives on a schedule.

Engineers design drainage to a return period. A 10-year, one-hour storm is the amount of rain in one hour that has a one in ten chance of being matched or exceeded each year. Over the life of a patio you should expect to see it more than once. A 100-year storm has a one in a hundred chance each year; it is the event you design an overflow route for, so that when it comes the water goes somewhere harmless.

The two towns are almost identical, and the short-duration numbers are the ones to notice.

Point estimates from NOAA Atlas 14, Volume 9, for central Bentonville and central Fayetteville, partial duration series. A 10-year, 15-minute burst of 1.13 inches is falling at about 4.5 inches an hour while it lasts. For comparison, the City of Rogers lists a design rainfall rate of 3.6 inches per hour in its climatic design criteria.

More than people expect: one inch of rain on 1,000 square feet is about 623 gallons.

The arithmetic is simple. An inch of rain is one twelfth of a foot, so 1,000 square feet collects about 83 cubic feet, and a cubic foot is about 7.48 gallons. Put the Atlas 14 numbers through it and a 600 square foot patio in Bentonville sheds about 830 gallons in the 10-year, one-hour storm. Add a covered structure's roof and the house roof draining onto the same area and the volume doubles or triples.

The same 600 square feet in Fayetteville's 100-year, one-hour storm, 3.46 inches, sheds about 1,290 gallons in an hour. A 400 square foot pavilion roof in Bentonville's 10-year, one-hour storm delivers about 550 gallons to its downspouts. None of those are exotic events; they are the numbers a drainage design is checked against.

A lawn would have soaked some of that in. A hard surface sends all of it sideways, concentrated at the edges and low corners, which is where the damage starts.

Because the soils here take water in slowly, so a heavy burst runs off rather than soaking in.

The USDA's descriptions of common Benton County soils are consistent on this. The Nixa series has very slow permeability and a surface runoff index of high to very high. The Tonti series, typed two miles east of Lowell, has a fragipan 15 to 31 inches down and a perched water table within 18 to 30 inches of the surface from December through April. In spring, when Atlas 14 storms are most likely and the ground is already wet, there is very little room left in the soil.

On the Springfield Plateau there is a second route: water finds joints in the limestone. That is fine for rain falling on a lawn and a problem when a pipe dumps a roof's worth of water into one spot. Our karst guide explains why.

Spring and autumn, mostly, but no month is safe.

NOAA's monthly normals for Drake Field count the average number of days with an inch of rain or more. May leads with 1.8, then September and October with 1.5 each and April with 1.4. Even December and January average 0.7 each. Days with two inches or more are rarer, 3.0 a year in total, and they cluster in April, September and October at 0.4 each.

Winter rain has its own problem. December through February each bring 2.5 to 3.1 inches of rain on average, into a season with around 20 freezing nights a month. Water that lands on a patio in January and does not drain away is water that freezes in the base. Storm design and freeze design turn out to be the same design: get the water off and out.

An open excavation fills like a pond, and saturated clay at the bottom of it is no longer fit to build on until it dries or is dug out.

That is why the order of work matters on this ground. Drainage outlets go in early so the excavation can empty. Bare soil gets silt fencing or straw where water leaves the site, so a storm does not carry the job into the street or the neighbour's garden. Base stone is not placed onto mud to keep a schedule. A crew that has built through Ozark springs plans for the downpour rather than hoping to avoid it.

Every hard surface needs a planned route for its water, sized for the burst, with an overflow for the big one.

That starts with fall: the finished surface slopes away from the house and toward a place the water is allowed to go. Where a patio meets a door, a channel drain at the threshold stops a sheet of water running inside. Downspouts get piped past the patio, not onto it. On a slope, a shallow swale above the patio intercepts water coming down the hill before it reaches the paving. And somewhere on the lot there is a route, usually across lawn, that carries the 100-year storm safely past the house when the pipes are overwhelmed.

Our drainage and grading guide covers the mechanics of fall and outlets. This guide is about the size of what they have to carry.

They help, but on slow-draining ground they still need an outlet.

Permeable pavers let water through the joints into a deep open-graded stone base that stores it while it soaks away. That works well over soil that takes water in. Over a fragipan or tight clay, the stored water has nowhere to go quickly, so the base fills, and in winter a full base is a frozen base. On most Benton County lots a permeable system is designed with an underdrain or overflow to a planned outlet, which makes it a slower, better-behaved form of surface drainage rather than a way to make the water disappear.

Each one concentrates water in its own way, and each needs its own plan.

A retaining wall on a slope is catching the runoff of everything above it, and saturated soil behind a wall is far heavier than dry soil. Drain aggregate and a perforated pipe with a real outlet are not optional in this rainfall. A pool deck has to shed water away from the pool rather than into it, and away from the house at the same time. A covered patio or pavilion roof collects every drop that falls on it and delivers it to one or two corners; the gutters and downspouts need sizing for the burst, and the water needs a destination before the roof goes up.

Not onto a neighbour, and on some lots not into the easement you were planning to use.

Pushing concentrated runoff onto a neighbour's land is how drainage disputes start, and subdivision plats commonly reserve drainage easements that may not be built over. Before choosing an outlet, check the plat for easements and look at where the water currently goes. The best outlet is usually the route nature already uses, improved rather than redirected.

Walk the lot in a real downpour before the design is final if you can. Where water sheets across the lawn, where it pools against the house and where it leaves the property tell you more about the drainage a patio needs than any map. A photo or a short video of the garden mid-storm is one of the most useful things you can bring to a site visit.

Projects with us start around seven and a half thousand dollars. Francisco Gallardo, the owner, walks the ground himself before any number is given, because on this geology the number depends on what is under the turf. The visit costs nothing. Financing is available through Acorn Finance. Call (479) 379-2226 or send the project form.