How To

Water Wins: Lessons for Dirt Roads and Heavy Rains

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A road turned into a river at the height of a recent flash flood in the American Southwest.

A road turned into a river at the height of a recent flash flood in the American Southwest.

If you’ve been following the news about the rains in the Southwest, you’ll know that Colorado got hit with some once-in-500-years rainfall. It’s once-in-100-years for New Mexico, which also got its share of the late “monsoon rains” that flow northward from Baja from July through September. The “monsoons” are what keeps the high desert reasonably cool and brings us as much as 75 percent of the annual rainfall for these parts, which is about 10 inches.

For the last two years, the Southwest had been experiencing a serious drought. That dry spell continued through July 1. At that point a total of three inches of rain had fallen from January through June, which put the high desert of New Mexico on schedule to have a third year of record drought. Then, on July 1, the skies darkened and a procession of storms began to stream from the south into the high desert area of northern New Mexico.

In July and August almost 10 inches of rain came down in these parts. Each part of the country has its own type of rain. The Midwest has thunderstorms, but April showers bring May flowers, and so on. Along the West Coast, rains come from October through April. In the Willamette Valley in Oregon, during that time it will cloud over and rain steadily for days at a time, resulting in large redwoods and cedars.

A road with a gully created by heavy rainfall.

A road with a gully created by heavy rainfall.

In the high desert, the sky is entertainment, as you can never tell what it’s going to do and you may see clouds coming from several different directions at the same time. Rain comes in thunderstorms. It may rain cats and dogs in one area, and five miles away they may not get a drop. But, when it does rain, it can pour with passion and that can cause flooding.

In the middle of September, in one 10-day period, 10 inches of rain fell in the Santa Fe area, concluding with four inches in six hours. This rainfall set the all-time record for Albuquerque. At Cochiti Lake, a popular fishing spot, the heavy rains have carried so much debris into the lake that the boat-launching ramp has been closed. Over at the Las Vegas National Wildlife Refuge in New Mexico that was bone dry last year, they now have lakes and ducks and geese streaming in, according to refuge staff.

During the last heavy-rain period, a neighbor lost his driveway, which traversed a normally-dry arroyo. This was bad for him, but in these parts many of the roads are dirt, and when flash floods come, those roads are in jeopardy because the normal dryness makes the soil very light and easily swept away by a torrent of water. At the height of the four-inches-in-six-hours rain, a torrent of water was flowing down the road. The force of the water was strong enough that granite rocks the size of footballs were swept along like dry leaves. And where previously there was a graded road, there suddenly were ravines being carved out as a foot of water came pouring down the roadbed.

You can see a video of an arroyo flooding from the recent rains embedded below.

Experiences like this carry lessons. After the clouds part and the sun comes out, you suddenly are faced with what once was a road that had become a river for a little while. Holes and ruts are the norm. The obvious response is to get some dirt to fill the ruts. A friend who lives on a road that is bisected by an arroyo 40 yards wide said that at the height of the rain, there was a torrent of water two feet deep rushing through the arroyo, leaving ruts, rocks, trees, and so on. The size of the arroyo warranted trucking in 12 truckloads of fill to restore the washed-out road. The fill amounted to 350 tons.

The ruts in roads ran as deep as 3-1/2 feet. Fortunately again, the neighbor who lost his driveway has a Kubota tractor with a scoop and a grader. That has been a very important asset as the three quarters of a mile of dirt road that comes into the area where we live is all downhill, and ruts in the road were everywhere.

A section of washed-out road prior to repair.

A section of washed-out road prior to repair.

After speaking with local experts and researching on the Internet, the neighborhood consensus was that the best way to fix the road was with rocks to create a foundation as well as dirt. Begin with the largest rocks you can find, metamorphic if at all possible. Sedimentary rock like shale or limestone will break apart under the weight of a vehicle.

The first layer of rocks should be the biggest. Then you put in a layer of soil a couple of inches deep. The soil helps to keep the rocks in place. Then the next layer should be medium-sized rocks. Again, cover them with some soil. And finally, put on another layer of soil, and cover it with gravel. Vehicles passing over this road will gradually compress the layers, and if there is any clay in the soil, it will become like concrete. For normal rains, this will hold.

You can also create diversion ditches to channel water away from the road. Here again, layers of rocks to build the initial diversion that steers water into the diversion ditch are crucial. And, in existing ditches, you can use rocks to create small dams that will slow the rate of flow of the water. This will cause silt and sand deposits behind the dams that can be later shoveled out. It is the rate of flow that causes the damage so these mini-dams also help avert problems.

If possible, direct the diversion ditches toward low-lying areas where ponds can form. The ducks will really appreciate this.

The normal rainfall will generally be handled by taking some simple precautions, following common sense, and maintaining roads. But, as folks learn when you get the rare flash flood, the water almost always wins. An ounce of prevention for the worst is worth a truckload of repair.

Images by James Swan, video by Matt Eastwood

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