Neal Bolton

By Neal Bolton

At most landfills there are soil-covered slopes. And at most landfills, rain or snowmelt flows down those slopes. If those two characteristics apply to your landfill, there will be soil erosion - period. Whether the erosion becomes an annoyance or a disaster often depends on the origin of water.

If the erosion is a result of rain or snow that falls directly onto the slope, it is usually not a major problem except on especially long slopes. In those cases, you might need to install drainage benches to control the runoff.

The shallow rills (channels eroded into the soil by flowing water) caused by localized runoff can often be self-correcting if a good stand of vegetation can be established before the next wet season.

Will that work at your landfill? I don't know, but it's worth a shot. The key is to stabilize the slopes as quickly as possible, before the rills become too deep and permanent.

And when you place seed, it's also a good idea to stabilize the slopes with hand-placed or blown straw. Straw is relatively inexpensive and can work well to hold the soil in place until the vegetation gets established.

But before you rush out and deal with the symptoms (the erosion), make sure that you don't have a more serious underlying problem. At many landfills, the underlying problem is runoff water that's coming from atop the landfill and flowing uncontrolled down the slope. This kind of runoff can cause significant damage to the slopes. Under these conditions, the slopes never self-correct - they self-destruct.

What's the solution? Contain the runoff. Direct it to a few specific points using soil berms and then install downdrains to control it as it flows down the slope(s). Downdrains are culverts or lined ditches that are placed on steep slopes. In many instances, downdrains are used to carry runoff water down a steep slope, such as from one horizontal bench to another.

What type of downdrain is best? It depends on your situation. Culverts are often the most secure way to contain runoff water. Once the water is in the pipe, it (usually) can't get away. You will need to inspect your downdrains regularly, however, especially prior to each wet season. Most landfills settle. Most pipes can't compensate without separating or buckling.

If you decide to use pipe, consider using corrugated high-density polyethylene instead of the traditional corrugated metal pipe. Plastic pipe is easier to handle and easier to install and offers some flexibility, so it can move as the slope settles.

To avoid the problems sometimes associated with pipe downdrains, many landfills construct lined ditches as downdrains. Keep in mind that, similar to pipes, concrete or asphalt-lined ditches don't hold up well when the slopes settle. As an alternative, downdrain ditches lined with riprap or interlocking concrete blocks are often used. Because these materials can move with the landfill, they are much more durable than concrete or asphalt.

Some creative landfills even use leftover HDPE liner material to line their downdrains. A ditch is graded down the slope. A sheet of HDPE is placed in the ditch. If more than one sheet is needed, be sure to start at the bottom and overlap (like shingles) on the way to the top. As long as the edges are tucked in, plastic-lined downdrains function very well, and they're inexpensive.

As a final note, if you use pipe downdrains, make sure you design the pipe large enough to easily carry the maximum flow of runoff - and the random pieces of litter that are sure to find their way to the inlet. And don't install a grill or a grate to keep litter out. Chances are it will plug up with litter and overflow.

If it rains or snows at your landfill, you will have to deal with runoff. By preplanning the type and location of your downdrains, you can control the water and eliminate potential problems.

Neal Bolton is a consultant specializing in landfill operations and management. He is principal of Blue Ridge Services in Atascadero, CA.

Neal Bolton is a consultant specializing in landfill operations and management. He is principal of Blue Ridge Services in Atascadero, CA, and author of The Handbook of Landfill Operations

MSW - May/June 2003