There’s a common misconception that lakes are little more than holes dug into the ground to gather water. While some small ponds are still made that way, trying to fill a depression of raw dirt with water usually only results in mud and rapid draining over the course of a few days or weeks. It’s much easier to start with an existing depression like a small valley or even a large drainage ditch. By building a dam at an existing end of the depression, you’re dramatically reducing the water necessary to establish the lake. Even if further excavation is needed for a better depth or to shape the future shorelines, it’s a fraction of the work necessary for digging a lake into a flat section of ground. In the following section, you can learn why damming is a better choice than digging and how even fully excavated lakes will need a dam.
Damming vs Digging
Most lakes will require both digging and damming to function properly. However, there is a big difference between primarily shaping the pond through excavation or relying on existing depressions and valleys. Excavation costs range from $2 to $200 per cubic foot of soil moved depending on its composition, vegetation or forests on the site, height of the water table, bedrock layers and much more. Every extra foot of depth you require can add tens of thousands of dollars to costs, when you’re digging that depth over multiple acres of soil. If you can start with a dry riverbed or gully, that is already 10 to 15 feet deep, you could cut your excavation costs by 30% or more. Cost will also be lower if the existing banks only need minimal stabilization. Any time you can minimize soil disturbances by reducing excavation, you also keep silt and mud out of the water and reduce chances for erosion.
Using Natural Topography
Of course, not all regions lend themselves to the land features that make good man-made lakes. Filling a gully or drainage basin that has severe erosion or undercut banks will only result in collapses that fill the lake with sediment and create leaks. In these areas, it’s often necessary to excavate lakes on flat or even raised ground and use dams primarily for power generation or water reservoirs. This allows for the accumulation of heavy rainfall or pumped water from seasonal wells that run dry during certain parts of the year. Yet, even in the flattest areas, there are still minor depressions that gather more water during a storm that others. Using whatever amount of natural topography is available to guide water to the lake can make the difference between a successful or partially functioning reservoir.
Location for the Dam
Dams are built in the narrowest part of the natural valley or depression used for the lake. If you’re impounding a stream or river as a water supply, the dam is usually located on a narrow part of the existing body of water. Wide dams are weaker and more prone to ruptures in the center where there’s little support. If you’re building the lake completely from scratch, with extensive excavation, you can technically site the dam at any narrow end of the feature. However, you’ll likely have a lot of input from your permit-issuing authorities at the federal, state, and local level on where and how to build your dam. Your lake engineer will also help you determine the optimal placement for the dam and which drainage system to install to safely handle overflow water.
Building a Strong Dam Core
Earthen dams are popular for small and large lakes alike because they’re often built with the soil removed during excavation. This significantly reduces the costs of building the dam when compared to full concrete construction. A lake’s dam is only as strong as its core. The core is an impermeable layer of highly compacted soil and clay. It is often mixed with concrete or even asphalt as a binder to prevent water from seeping or leaking through the dam. The compacted soil on top makes up the bulk of the dam. This soil may allow a little water to seep through it, but the flow stops when it hits the core. A poorly built, or weak dam core, reduces the strength of the entire lake. Dams are subject to millions of pounds of force pressing from multiple directions due to the weight of the water trapped behind them. Without the proper angle, height, or compaction techniques, earthen dams can fail as soon as the lake first fills. Soil testing is an essential step to ensure the fill you’re using for dam construction is appropriate for building a strong earthen structure.
Dam Maintenance
Concrete dams require daily and monthly maintenance, but earthen dams only need inspecting that often rules out problems. They’re significantly lower in maintenance, yet attention is crucial on specific occasions. Trees, shrubs, and brush must be removed from earthen dams seasonally to prevent the large root systems from breaking up the compacted soil and creating leaks. Grass and small plants are fine and help prevent erosion, but they do require occasional mowing to keep the surface visible enough to spot leaks when they first occur. Trash racks installed on the dam should be cleaned monthly unless there’s so much recreational traffic in the summer that weekly cleanings are needed. Livestock should be kept away and foot traffic on the dam itself should be minimal to protect the structure from erosion year-round. Covering It is best to cover at least part of the dam with rip rap. This assists the maintenance process since medium-sized rocks are easier to remove and replace than soil.
Dams are best designed by an experienced engineer with hydrology training. All earthen dams will lose a small amount of water through seepage if they aren’t lined with an impermeable barrier. If you’re interested in building an earthen dam, consider adding a liner from BTL Liners. Lining the dam with a geomembrane product like ArmorPro will stop water seepage, stabilize the banks and prevent erosion from reducing its height over time.
