July 15, 2015 12:26

Going with the flow

Water always follows the path of least resistance. Usually, this isn’t a problem. Irrigating our crops, filling our reservoirs, even filtering our groundwater is all made possible by gravity working on water. Sometimes, though, gravity is not your friend. Sometimes water pools where it shouldn’t and causes drainage problems, great and small.

Let’s take a look at some major and minor drainage problems and solutions, and finally, some constructive measures that can help you avoid the problem altogether.

In 2010, Schultz Peak, just north of Flagstaff, Arizona, was devastated by a massive fire. For two days, firefighters from as far away as Idaho fought the blaze. Thanks to their efforts, no one died and no buildings were lost, but the fire consumed 15,000 acres, an area roughly the size of Manhattan. It removed vegetation and baked the soil to the point where it became hydrophobic, repelling water rather than absorbing it.

“The fire was on a very steep mountainside with pretty loose, volcanic soils, as well as ponderosa pine and spruce fir trees, all up high,” explained Allen Haden, an aquatic ecologist and principal at the Flagstaff-based Natural Channel Design, Inc. “Within two weeks of the fire being out, we had a very big rainstorm,” Haden said. “Obviously, the watershed was in the worst condition it could ever be in, and it flooded a neighborhood that had been built at the base of the mountain.”

The flooding was bad, but the sediment released from the mountainside was even worse. That single storm released so much sediment that the flooding left entire basements filled with material. The whole neighborhood was without flood insurance, since no one had ever predicted that there’d be a problem. Now, though, they lived at the base of a flood plain, and only emergency measures would keep every hard rainfall from flooding them out.

After the fire had been put out, the Army Corps of Engineers had predicted that flooding as a result of the well-baked ground and reduced vegetation would be a problem. However, they failed to predict the massive amount of sediment that the flooding would release. Once they understood the extent of the problem, local officials took action. Natural Channel Design was given the job of determining where the sediment was coming from, how it was getting there, and drawing up plans on how to stop it.

Haden and his team determined that the floodplains located at the base of the mountain were the culprit. “What we found was that the flooding had gullied through several alluvial fans on about eight different watersheds,” said Haden.

“So the alluvial fans, which had stored sediment for ages, were now releasing sediment. It was also being released into the neighborhoods which were built on the lower portions of the alluvial fans.” Natural Channel Design estimated that sediment from the alluvial fans made up 85 percent of the released deposits, with the rest coming from the mountainside or the road.

Jody Greene, who works in environmental compliance for Wetland Studies and Solutions in Gainesville, Virginia, explained how this erosion works. “The first way a channel erodes is down; the energy is then contained in the channel, which compounds the erosion problem, and then it erodes out.”

“Ultimately, over many thousands of years, you would establish a new flood plain, which is much lower, that could accommodate these flows.” For the case in Arizona, such a flood plain would establish itself in people’s houses, an unacceptable outcome.

A basin could be made to hold the matter back, but the basin itself can erode, and should the basin fail, the town would be knocked back to square one. Instead, Haden and his coworkers advised returning the flood plain to its original state. The area could be redesigned to return it to the alluvial fan it was before the fire.

This was most impressive, because the fan had to be considerably shortened, so that the residents who suddenly found themselves living at the bottom of a flood plain would be out of the danger zone. “We had to shorten the fan, putting all the water that would be spreading out over a quarter-mile width into a channel that’s only a few dozen feet across,” said Haden. “Then we had to fit that channel down through the neighborhoods, between the houses.”

Haden applied his company’s namesake, natural channel design. The brainchild of David Rosgen, natural channel design is a system hydrologists can use to restore rivers to a sustainable level of flow after they have been altered by erosion.

Jody Greene explained how natural channel design can solve the problem without taking eons to do it. “It uses stone or log structures that help to steer the water around bends. You can use the same method to build pools that dissipate the energy in the pool, rather than in the stream banks or beds.”

Haden and his team applied these principles on a number of projects for the county. By surveying local channels, and looking at the fire history in those channels, they were able to come up with reference reach dimensions. They used these to rebuild the alluvial fans, now with attached channels that would allow them to resist flooding. The neighborhood would once again be high and dry.

Over the last five years, the county has made significant progress in implementing the plans. Select areas have been re-graded, and vegetation has been replaced to return the plant life that once kept the soil in check. They’ve built a series of log rundowns, log ramps which allow the water to drop off the flood plain into a channel without the erosion that would occur if it dropped straight down. Built at a 2:1 slope, these rundowns are built using rocks and wood exclusively from the forest itself. All the projects have been thoroughly rained on, and nobody has wet feet yet.

FOR SMALLER SCALE drainage issues, we turn to Sahuarita, Arizona, where it is hot, dry and averages under a foot of rainfall per year. It’s probably just about the last place you’d expect to have drainage issues, but the bounceback from the recession brought a construction boom to Arizona, especially around Phoenix.

Margaret Cooper, owner of Sahuarita-based Hot Desert Landscaping, LLC, says that rushed construction work is often the cause of drainage issues she fixes. “They originate when the building is done very quickly; they go through and grade the land, but they don’t do and adequate inspection.”

Cooper finds that the construction inspectors can miss major issues because their focus is internal, not external. “When you’re going through inspections for the house, you’re not looking at the grade for the land. Then, five years down the road, you’re having mold underneath your foundations, cracks in the walls, or runoff from rainfall.”

When a person buys one of these properties, and then discovers that water is pooling next to their house and seeping in through basement walls, they call Cooper. “That’s where we come in and can take care of the drainage on the outside.

We facilitate it, to get it away from the foundation.”

The water they move away from the house must go in one of two directions: out front to the street or out back to the yard. Ideally, Cooper prefers to put in a swale, which corrects the grade of the land, and lets the water flow out to the street. Unfortunately, swales require that the house be far enough above the street to allow sufficient grading.

When a house is lower than the street, Cooper has another option, the French drain. She digs a trench from the house down to the backyard and lines it with erosion control fabric, a barrier impervious to dirt which will still allow water through. She fills the trench with dirt, gravel and an embedded PVC pipe, then covers it to match the lawn. Begin the trench at the problem area, with a hose running from the downspout to the trench, and the French drain can then direct all the water out to the backyard.

The depth and water retention of the trench means that even a slight gradient is enough to shunt water away from the foundation. Cooper stands by this solution as a surefire wet-basement buster. “We’ve been installing French drains for about ten years, and we haven’t yet had any issues with them.”

Sometimes, however, the house is below the street and the backyard isn’t available. Cooper has the answer for that, too. “That’s where a pump has to be used. What we do is take a sump pump and configure it so we can get our PVC into the pump. Then, we go ahead and put the pump below-ground, and we bury a line going either to the back or front.” The average sump pump they use runs at about one-half horsepower, which pumps very quickly, and is enough to handle any level of rain, short of biblical.

Once they’ve installed the system, they check back after the next major rain to make sure everybody is high and dry. In all her years of correcting drainage, only once has Cooper had to revisit a job. One customer was worried that his pump had been installed incorrectly, so Cooper’s team came out and reinstalled it, just to be safe.

So far, we’ve discussed how to fix an existing drainage problem, but how do you prevent a problem in the first place? There will always be rainstorms big enough to overrun our protections and cause flooding, but there’s still a lot we can do to soften the blow and ensure that only the largest rainstorms cause problems. Steve Rodgers of Eco Express, LLC, is very conversant with the techniques that can be incorporated into a design to stop drainage problems before they start.

His Wilmington, North Carolina company offers the standard menu of retention ponds, underground stormwater cisterns and bioswales, but they also offer some more advanced options to head drainage off at the pass by encouraging infiltration.

Allowing permeable areas to accept rainwater and leach down into the soil is a natural and expedient way to overcome drainage problems. Recently, Eco Express did some work at the Camp Lejeune Marine Base in Jacksonville, North Carolina, where they installed a number of features designed to retain stormwater. They installed a bio-retention pond, which is lined with materials to encourage infiltration, rather than just slowing and storing stormwater.

They also installed permeable grass pavers, a web of plastic cells which is laid atop a compacted mixture of aggregate base course and concrete sand. “Then we’ll backfill it with sand, and lay sod down on top of it.” Rodgers said the Marine base uses them for emergency access lanes around buildings. “Once the sod’s established and is rooted into it, you can drive on this stuff. It’s rated for a fully-loaded fire truck. At the same time, you’re getting a permeable surface.”

The very act of construction can reduce infiltration simply by compacting the earth around the construction site with the weight of heavy machinery. Rodgers even has an answer for this. “We have some products that we incorporate in the soils at the end of a construction project which alleviate some of the compaction issues, increase infiltration, and are extremely beneficial for the vegetation establishment.”

These soil enhancers work through a biochar which improves soil microbiology and reduces soil compaction. Rodgers can make use of these enhancers on any surface that isn’t permeable, even the bioretention ponds.

Drainage, like every environmental issue, is communal. Good fences may make for good neighbors, but they aren’t drainage solutions, and they won’t stop a flood. When your basement is starting to fill up, it’s already too late. It’s better by far to keep gravity on your side and let your problems flow away.

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In many ways, we are fortunate that, in our chosen profession, we are able to help people when certain disasters occur: the tornadoes in Missouri, Oklahoma, Louisiana and Georgia, the flooding in Louisiana, the snows in the northeastern part of the country, the rain in California, and the snow in Colorado....

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