Physical Structure
Dams, embankments, excavations
There are two basic types of dams, and either can be used when creating an irrigation pond. Generally, the location of the dam and the nature of the terrain will determine which type works better for you.
Excavated Ponds
If you’re working on relatively flat terrain, excavated ponds are by far the simplest to build. However, with no significant natural depressions to take advantage of, the capacity of an excavated pond is necessarily limited. The depth of an excavated pond is unlikely to exceed 20 feet, compared to a dammed reservoir that may be 50 feet deep or more. Because of this limited capacity, many farmers prefer to use these in situations where the overall water demand is small, but others choose to build multiple ponds, each tied to specific fields. In the short term, this is obviously more expensive, but an investment in water security in at-risk areas can quickly pay off in trying times.
Where evaporation is an issue, excavated ponds have an advantage, since the profile of the pond can be designed for greater depth and a smaller footprint. This approach minimizes the water surface are where evaporation occurs. Other advantages include ease in construction, a lower risk of flood damage, and comparatively low maintenance requirements.
While excavated ponds can be fed by flow from aquifers or ground water, that’s not an ideal solution when their fundamental purpose is to address problems of water scarcity and excessive aquifer withdrawals. In contrast, surface runoff ponds take advantage of overflow from heavy storms that dump too much rain for the soil to immediately absorb. This represents a flexible and effective response to changing precipitation patterns arising from climate change.
Excavated ponds have the advantage of very flexible placement, depending primarily on whether they’re fed by surface runoff, irrigation tailwater, or some combination of the two. Surface runoff ponds should be constructed in terrain that is relatively flat but well drained. Natural depressions are excellent locations since water will typically flow in that direction already. Locating a pond in the middle of a broad drainage field is an excellent option, but even placement on the border of a drainage field can work well if capture and diversion practices are implemented to direct flow to the pond.
Embankment Ponds
There are two basic types of embankment dams: earth-filled and rock-filled. Compared to more highly engineered examples, embankment dams are generally built from natural materials and are best suited to shallow slopes where the hydraulic pressure at the base of the dam isn’t excessive. Nevertheless, since embankment dams, as a class, are particularly vulnerable to erosion, spillways are a critical feature. Spillways are designed to release excess water if it rises too high, as in an extreme storm event, in order to protect the dam’s structural integrity.
Earthen dams are fairly basic constructs of compacted earth and are best suited to low dams with relatively impervious soil. The dam is constructed in stages and compacted as each layer of earth is added. Earthen dams can be designed and constructed to compensate for a weak foundation.
Rock-fill dams have a more complex profile and feature an impervious core (typically compacted earth), covered by compacted transition layers of sand, gravel, and stone. Rock-fill dams are often somewhat steeper and may be taller than earthen dams.
Spillways
Any embankment dam is vulnerable to destruction if it lacks adequate spillways. When a pond or reservoir is overfilled, the excess water can overtop the dam, which will quickly lead to significant damage and even total collapse. In these circumstances, even solidly built dams can be quickly destroyed.
When designing an embankment pond, ensure that your dam allows an egress point to maintain proper freeboard space in seasons where it is at capacity, plus auxiliary spillways to handle the rapid inflow of water during extreme storms. To protect this (hopefully) rarely used structures from devastating erosion during severe storms, be sure to establish healthy, dense plant cover as soon as possible after construction is completed. Perennial grasses are an ideal option that will minimize the need for constant renewal.
Both primary and auxiliary spillways must convey water safely past the dam’s downstream slope to prevent rapid and destructive erosive forces from weakening the dam’s structure. Consult with a qualified engineer and local authorities to ensure that your dam meets all requirements.
Permeability
Since the whole point of an irrigation pond is to supply water during times when natural rainfall is inadequate or unreliable, the loss of excessive volumes of stored water is unacceptable. We’ve already mentioned how evaporative losses can be minimized in excavated ponds by building them deep rather than broad. Another common source of water loss is seepage. Especially in excavated ponds, seepage can be a significant problem when the soil isn’t able to form an impermeable layer.
Soil Liners
Clay and silty clays are the best soils for this purpose, but the clay layer must be thick enough to form a reliably impermeable seal, and compaction along the sides and bottom of the excavation may be required. The deeper the pond, the more stress will be placed on the bottom seal, and the weight of the water, over time, can potentially force itself through the clay (or sand or gravel) layer and result in significant water loss.
It’s important to carefully consider the type of soil present in your site, since areas that have lots of rock or sand can make it extremely difficult to establish an effective and durable seal. If your soil requires additional clay, it is likely to be more economical in the long run to use a durable, highly impermeable geotextile liner.
Geotextile liners
Whether your soil is coarse-grained or fine, woven, impermeable geotextile liners are an excellent option for irrigation ponds. Reinforced polyethylene (RPE) and artificial rubber (EPDM) are the most widely used geotextile materials for irrigation ponds because they offer extremely high reliability, excellent puncture and tear resistance and virtually 100% protection against seepage. These liners are quite thin. A heavy-duty liner of 60 mils is only 1.5 mm or 0.06 inches. Compared to the several feet thickness required for a soil/clay liner, the geotextile options will leave you with substantially more storage space.
RPE liners are exceptionally well suited for use in irrigation ponds, even over EPDM. RPE liners are manufactured with a reinforcing scrim that not only provides the same level of impermeability but offers substantially higher strength and puncture resistance than EPDM. It is also substantially lighter. This means it requires no heavy equipment to move or install, it is available in vastly larger panel sizes, and if your installation requires the use of multiple panels, they can be rapidly and permanently joined on-site.
Regardless which type of liner material you choose, it’s important to remove all rocks, roots, and other protuberances from the excavation. In some cases an underlayment may be required to protect the liner material from punctures or deformation that can weaken the material over time.
