The earliest retention ponds were all unlined because there was little understanding of why these ponds need lining so much. Yet, many of the issues with stagnation and flooding experienced with early installations were solved specifically by the addition of flexible liners. Even if state and local regulations allow for the construction of a new, unlined retention pond, it’s best to line it for reduced maintenance and greater security against water loss. An exception are detention ponds, which are temporary holding basins designed to allow the water to infiltrate the ground. These ponds are still sometimes lined so that only certain drainage areas release water into the soil. No matter the type of retention or detention pond that needs an impermeable liner, there’s a product from BTL Liners to match.
Reasons to Line a Retention Pond
Lining a retention pond is a complex decision. Durable geomembranes, like reinforced polyethylene (RPE), do add to the cost of the project. However, the benefits of lining this kind of water feature vastly outweigh the costs.
Rapid Filling During a Storm
First, retention ponds tend to fill rapidly during even the shortest rainstorm. Water rushing in to fill a partially empty retention area can result in erosion or destabilization of the banks. Lined ponds are much better at handling the routine rush of water suddenly flowing in, whether that’s over the banks or through the correct inlets. Waves generated by the filling and emptying of the pond, or just the wind moving over the surface of the water, won’t erode and damage the banks. It’s important to reinforce these kinds of ponds and basins that see a lot more use than standard fish or decorative ponds that stay stable all year round.
Unstable or Loose Soil
Almost all retention ponds require some amount of excavation to create enough space for adequate water storage and flood control. Yet digging into a loose, sandy, or unstable soil will just result in failure of the banks and dams that allows for flooding in the area around it. Using low sloped banks, reinforcement grids, and an impermeable liner to prevent seepage can help even the loosest soils hold their stability. Concrete alone isn’t a good solution for keeping these kinds of soils from moving. If water manages to seep through tiny cracks or openings in the concrete, it could destabilize the soil underneath to create voids that collapse concrete. Impermeable liners like reinforced polyethylene (RPE) control soil conditions under concrete by preventing seepage and leaks alike.
Bank and Edge Erosion
Lining a retention pond protects the banks from more than just erosion caused by filling and draining cycles. Whether the liner is exposed or covered with large amounts of soil, having an impermeable barrier in the construction will help stabilize it. For best results, the material needs to extend past the edges of the banks and well into the area around the pond. This is known as an apron and it helps anchor the material so it can’t slide or crinkle as weight compresses the soil above it.
Seepage Through the Soil
Seepage is a problem for many types of storage ponds and holding basins, including retention ponds. This is the technical term for water that travels through the soil under and around the pond. Even the thickest clay soils will lose water this way over time, because the ground has plenty of invisible gaps between the individual particles of dirt. Adding bentonite clay and similar natural products won’t necessarily solve the seepage problem either. Even installing a thicker layer of concrete doesn’t prevent seepage and leaks due to the regular development of tiny cracks. Only a flexible, impermeable liner from a source like BTL Liners can truly guarantee the control of water. Retention pond liquids are often unsafe to release into the surrounding soil or water, so liners play an essential role in keeping the contaminants in the storage basin or pond.
Water Quality Issues
Allowing the water in a retention pond to come in contact with the soil can lead to quality issues. If the water’s clean enough to release, there’s no need to add extra silt or potential contaminants from the ground where the pond is built. Water that is high is contaminants itself must stay separated from the soil to prevent negative effects on the ground water supply.
How to Line Retention Ponds
Now that you know why retention ponds need impermeable liners, it’s important to learn how to choose the right material. Installing the wrong pond liner could be worse than putting in no liner at all. Make the right choice with the following guide.
Material Options
There are a number of materials available for pond liners today, but only a few are a good fit for retention ponds. This is due to factors like the use of fish and amphibians to control pests, the need for healthy water for plants, and the possibility of damage during routine maintenance. Ranked from best to worst, the options are:
- Reinforced polyethylene (RPE), a tough and flexible material that offers strength while being thinner than other materials. This reduces shipping weight and makes installation easier. RPE resists breakdown from potential runoff contaminants like oil and fuel, is available in proven fish and plant safe formulations and can be highly UV resistant for exposed installation.
- High density polyethylene (HDPE), which is tough and can be fish-safe, but which suffers from stiffness that makes it heavy and hard to install. Getting natural and attractive curves around the irregular edges of a pond is hard with the thickness and reduced flexibility of the HDPE. HDPE also doesn’t have great puncture, tear, or abrasion resistance,;three features essential in retention ponds that are cleaned and maintained on a regular basis. It’s a better choice for landfill lining and strictly decorative ponds that are rarely dredged or accessed.
- Low density polyethylene (LDPE), a thinner material that is highly flexible for conformation to curves and corners. Unfortunately, it trades in strength and durability for the extra flexibility. RPE offers similar amounts of flexibility while still providing plenty of strength due to its reinforced scrim design. LDPE also varies in its fish and plant safety rating. On top of that, it tends to lack the UV resistance offered by RPE liners. In general, LDPE is best limited to being one of the materials used in RPE rather than used on its own as a pond liner.
- Polyvinyl chloride (PVC), once the most common pond liner material but now far from the best choice. PVC might be acceptable for small backyard ponds, but it’s rarely fish-safe and plant-safe. It’s also weak against UV exposure and easy to tear during installation. Choosing a reinforced PVC liner won’t compensate for the material’s weakness to many chemicals found in storm water runoff, such as oil and gas residues.
- Chlorosulfonated polyethylene (CSPE), a material that is primarily used for potable water storage. It’s durable but not resistant to most chemicals, making it a poor choice for retention purposes. It’s also far more expensive than RPE and hard to produce in large sheets without expensive specialty fees.
Minimizing Seams with Custom Fabrication
Even after settling on RPE as the material for your retention pond project, it’s a good idea to look for a manufacturer that offers custom fabrication. Sticking with standard manufacturing sizes limits you to somewhat narrow rolls that must be sealed on site. The seam sealing process for RPE is relatively easy compared to other materials, but it’s still best to minimize it whenever possible. Every inch of seam between panels of material represents a potential source of leaks in the future. Custom fabrication results in large factory welded sheets that are leak resistant while remaining easy to install with a relatively small crew.
Choose BTL Liners to find the RPE and RPP pond liners you need for retention and storm water projects. We can supply fish-safe, plant-safe, and chemical-resistant materials that will last for decades. Contact our team of experts today to tap our 40 years of industry experience and get recommendations on choosing the right pond liners.
