Whether your landfill is in the path of a hurricane and got 20 inches of rain last week or in a desert environment that receives 5 inches of rain every year, one fact remains: when it does rain, you must control stormwater. And in most cases, that boils down to understanding and implementing straightforward best management practices (BMPs).
Let’s start at the top and work our way down. Most landfills are designed and constructed based on a few basic geometric shapes. The top deck of most landfills, where filling is occurring lift-by-lift, is sloped to drain. Typically, that drainage is flowing along a slope that varies from 2–5%, with 3% being most common. Now, most engineers, surveyors, and other folks involved in heavy construction and earth moving know that any slope measured as a percentage can be expressed as a certain amount of elevation change per 100 feet.
As a point of reference, a typical paved parking lot—for example, at a Walmart—is probably sloped somewhere around 2%, which means that that grade elevation changes 2 feet every 100 feet. To an unpracticed eye, 2% looks flat. On the other hand, a 5% grade is typically about the steepest you’ll find on an interstate highway, and anything steeper than 5%, say 6% or even 7%, will be marked as very steep with plenty of warning signs to go slow.
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The top deck of your landfill may slope in one direction or there may be a cross slope or a radial slope as well where it might drain outward from some high point in the center of the top deck.
Regardless of how much of a slope you have, the stormwater is going to flow in the direction of that slope across the top portion of your landfill. This flow is normally referred to as sheet flow. If that top is a well-graded plane surface, that is just how the water will move. It will sheet across the surface rather than concentrating in a specific flowline. However, we normally find that over some distance, the natural unevenness of that top deck will cause water to concentrate and be focused into one or more swales or flow lines. This may not create serious problems on the top deck unless the soil is extremely erosive. But if that top deck water just flows over the edge of the landfill onto a steeper side slope, that concentrated water will cause significant erosion.
To minimize that kind of erosion, you must control the top water—the water sheet-flowing across the top deck. Perhaps the best way to do that is to simply establish some soil berms along the perimeter of that top deck to direct the flow toward one or more downdrains that serve the specific purpose of carrying stormwater safely down the slope. There are many types of downdrains used at landfills. Some of the most elaborate have cast-in-place concrete inlets with an apron that gathers the water and directs it into a pipe to carry the top water down to the bottom. These types of downdrains also have the capacity to collect runoff from one or more benches.
Managing municipal solid waste is more than landfilling: publicity, education, engineering, long-term planning, and landfill gas waste-to-energy are specialties needed in today’s complex environment. We’ve created a handy infographic featuring 6 tips to improve landfill management and achieve excellence in operations.6 Tips for Excellence in Landfill Operations.Download it now!
Moving down the scale of sophistication, many landfills will use flumes that are constructed with corrugated metal. In some cases, these are built from scratch from pieces of sheet-metal like you would use on the roof of a shop, or they may use manufactured inlets and flume sections of corrugated metal. These flumes can also carry the stormwater down the slope, collecting any bench water on the way.
Perhaps the most common type of downdrain consists simply of sandbags at the top to form a funnel or inlet that directs water to a pipe downdrain.
Those downdrains may be constructed with corrugated metal pipe (CMP) or they may be an even simpler system using high-density polyethylene (HDPE) pipe. One benefit of using the HDPE pipe is that it is relatively lightweight and can often be installed or adjusted by hand. Some downdrain systems use no flume or pipe but simply a grade swale that works as a flowline for water to flow down the slope. Of course, this graded swale must be lined with something other than just soil, or it will quickly erode.
Finally, one of the most innovative systems for lining that swale is simply using a leftover strip of landfill liner material. A long run of HDPE can be used to line that swale and can be anchored along the edges with a series of sandbags. These systems are relatively inexpensive, and they will continue to work even as the landfill settles (and remember, all landfills settle). This is an important consideration when deciding what type of downdrain system you want to construct.
Generally, it is best to go with something that is economical, simple, and flexible, meaning it can adjust and move as the landfill settles without being damaged or requiring replacement.
Next time, we will move downhill and discuss stormwater on the side slopes of your landfill.