Every region is unique—but California’s Central Valley is among the most unique. Despite being one of the largest agricultural regions in the world by the sheer volume of product created there, it’s also one of the more drought-prone regions in the world. Some parts of the state experience extreme heat, some parts of it feature desert terrain—and all of these factors plus others can drastically affect the performance of even the most durable liners used for ponds, irrigation, and rain harvesting.
In Northern California, conditions are quite a bit different. Here, freezing temperatures are common in the winter, the soil is rocky, and organic material is abundant. These are also factors that can have a major impact on geomembranes.
If you’re looking at rainwater harvesting and drought mitigation measures in this region, then flexible geomembranes are likely going to be a key part of your system. Therefore, it’s important to learn how climate and soil conditions throughout the region can affect liners so that you can plan to make the most of your system.
Soil Composition
Elsewhere, we’ve discussed the advantages and disadvantages of clay liners used for drought mitigation and rainwater harvesting. While it isn’t necessarily recommended to rely on clay liners alone in these circumstances, clay can be an integral part of a successful reservoir and canal system, even when these facilities are lined with a geomembrane.
The biggest reason for this is because clay helps to stabilize soil. In California, loose, sandy soils are quite common—and these types of soils are prone to shifting, which can place pressure on a geomembrane. Soils that are high in clay are less likely to shift, which provides a stable base for the geomembrane. Adding clay to the soil is one way to help ensure the longevity of drought mitigation facilities.
However, because the addition of clay may prove cost-prohibitive or infeasible for various reasons, there are other techniques one can adopt in order to help protect geomembranes from shifting soils. One of the chief ways to do this is making ponds or irrigation ditches as shallow as possible. When soil shifts beneath a geomembrane, it is more likely to tear because the water above presses down on the liner and causes it to stretch. Shallower depths result in less pressure placed on the geomembrane, thus reducing the chances that it will be damaged.
Organic Material
This is a concern that will present itself more often in Northern California—and much like sandy, loose soil, it is an issue that can cause geomembranes to stretch and tear unnecessarily. What happens is when soil features heavy organic material such as leaves, vegetation, stumps, or extensive root systems? These things will all break down and decompose with time. As that happens, voids open beneath the geomembrane. Pressure from the water above presses the geomembrane into these voids, thus stretching it out and increasing the likelihood that it will eventually tear.
There are a couple of solutions to this. One is to reduce the depth of water collection reservoirs and canals—but this doesn’t stop the voids from happening. It only reduces the stress placed on the geomembrane from above.
The better solution is to thoroughly clear the soil of organic materials as much as possible prior to installation of the geomembrane. Of course, using a properly reinforced liner material also helps, but good site preparation will always be your best bet.
Rocky Soil
Some areas of California deal with exceptionally rocky soil, and this poses problems to even the sturdiest of geomembranes. Water pressure from above can cause sharp rocks below the membrane to punch through the liner, which causes leaks and potentially thousands or tens of thousands of gallons of water lost.
Here again, the solution is to thoroughly prepare the soil before installing the geomembrane. This could simply mean flattening soil with a bulldozer, but in particularly rocky areas, you may want to till the soil and then use a tractor and pull-behind rock picker to remove as many rocks as possible. Repeating the tilling and rock-picking process at least a couple of times will ensure that you unearth and remove as many rocks as possible, thus lessening the potential for damage to your geomembrane.
Dry Liners
One problem that is more prevalent in California than elsewhere is the yearly seasonal cycle. These days, California’s rainy season (when not in an extreme drought period) happens in November and December. Throughout most of the state, the rest of the year is relatively dry. Hence, rainwater collection reservoirs and storm detention facilities fill up during these rainy months, and slowly drain throughout the rest of the year as those water supplies are used up for various purposes.
In facilities where geomembranes are used, this leads to exposed liners—and that is highly problematic because exposed liners break down much faster than those that are covered.
What’s the solution for this issue? First and foremost, select a high quality, UV resistant liner. Ideally, it’s best to not let water storage ponds dry out—but in some areas, especially arid Southern California, this isn’t always possible. The next best solution is to cover the liner with sand, gravel or earth to help shield it against UV rays. In some situations, sand and gravel may not be ideal, either, but it is one solution to consider if feasible and if you’d like to ensure your geomembranes last as long as possible.
Unstable Dam Walls
Wherever possible, it’s often best to create retention ponds in natural valleys so that you can minimize excavation and instead build dam walls to contain water. This is particularly advantageous if you want to increase the depth of retention ponds—simply increase the height of the dam walls.
However, this can be problematic in many regions in California. For one thing, some regions are flat enough that there aren’t great places to build dam enclosures, which makes excavation into the ground the only real way to build a pond. But even where dam and embankment building are possible, loose and sandy soils may make this a less than ideal choice.
That’s because these shifting soils can lead to embankment collapse. This is particularly true when they dry out, or when heavy seasonal rains—such as those California often experiences in November and December—cause erosion to the bank. The potential for collapse gets worse the deeper the pond is or the greater the volume of water that it holds. Wherever possible, select sites with lots of clay in the soil, or if you must build an embankment reservoir in an area where there is little clay in the soil, then it may serve you well to have clay shipped in to shore up the walls.
Most of these concerns are relatively easy to address one way or another. It all comes down to site selection, clearing, proper site preparation and finding ways to protect your geomembrane so that it lasts as long as possible. Considering each of these things will help you create a long-lasting rainwater harvesting or water storage and irrigation system to help supplement water supplies during California droughts.
