Selecting a Site
When choosing a site for an irrigation reservoir, you’ll want to work in concert with the designer to select several site options and weigh their relative strengths and weaknesses before you make your final decision.
Storage ponds can be located nearly anywhere along the irrigation route, and design specifics might be slightly adjusted in each case. These specifics might lead to changes in the irrigation system, power supply, and management requirements. The main factors you’ll be focused on include soil properties, the ability to provide necessary storage volume, and overall cost. Don’t forget to consider conservation values and how different sites will affect your ability to meet regulations regarding runoff or employ conservation measures that may qualify you for state and federal funding.
The location of your water supply with relation to your fields will provide various benefits and drawbacks, whether it’s at the highest point, the midpoint, or the lowest point.
High Point Storage:
Locating your pond at the highest point relative to your irrigated fields allows you to take advantage of the force of gravity, allowing it to travel at relatively high flow rates without the need for a pump. However, captured drainage and stormwater will need to be pumped up into the storage pond. Given this need, the pump may need to be fairly large in order to efficiently handle large volumes of stormwater runoff. Another consideration is that the soil qualities located on high ground may be lighter compared to lower ground, making it less than ideal for construction.
Low Point Storage:
Locations at the low end of an irrigation system are naturally suited for collecting operational water and stormwater without the need for a pump. Structural diversions can be used to direct runoff toward the pond without mechanical aid, and even potentially heavy storms won’t necessarily influence your choice of pumping equipment. Soils located lower down in a drainage field are also likely to have heavier soils which may mean greater impermeability. However, water from a low point storage pond will need to be pumped to the top of the irrigation system, and that is sometimes a considerable distance. Groundwater depth also needs to be considered in low points, particularly for an excavated pond, since a shallow water table will absolutely limit the depth of your pond.
Midpoint Storage:
This is, obviously, a compromise between the other choices and you’ll see some of the limitations of both, but there may be specific strengths of a midpoint site that outweighs added complexities. For example, such a placement will usually require a complex pump-well and valve arrangement for proper performance.
A Stitch in Time: Irrigation Pond Management
Silt
Storage ponds that accept operational water from fields or stormwater flowing from a natural drainage basin will always be challenged by the accumulation of silt picked up and carried by water as it flows over the ground. Disturbed soil, especially tilled soil without cover crops, is vulnerable to higher levels of erosion, as are bare or disturbed areas in drainage fields. Silt carried in from either source will be deposited in the storage pond as the water flow slows, sometimes at the entry point and sometimes spread throughout the pond.
In order to prevent clogging at inlets and general loss of storage capacity over time, there are several important steps to take when installing your pond, starting with adopting erosion control practices in the drainage area. A drainage area of undisturbed land covered by a variety of well-established trees, grasses, and perennial wildflowers creates the very best protection against erosion, of course. Cultivated areas where conservation practices are employed, such as the use of terraces, conservation tillage, cover crops, field-edge filter strips, and grassed waterways, offer the next best level of erosion protection.
Damage from incoming silt can compromise the function of a storage pond so quickly that farmers should seriously consider delaying the construction of the pond altogether until recommended conservation practices are installed and well established. Even once the irrigation reservoir has been built, the project shouldn’t be considered finished until erosion protection has been established around the pond itself. Livestock access, water movement at the ingress point, and even wave action in large reservoirs can degrade both the borders of the pond and the upstream face of an embankment dam. Again, the establishment of native plants along the shoreline provides not only protection against erosion, but natural silt removal and decontamination.
Embankment Dams
If you have an embankment pond, it’s important to check the dam regularly to ensure there are no leaks or structural problems. A leaky dam can quickly escalate to a collapsed dam and complete loss of your irrigation supply. When inspecting the pond, look for cracks, slides and depressions on the dam faces.
Cracks wider than 1” or a series of cracks traveling parallel to the crest of the embankment may precede a slide on either side of the dam. Cracks like these allow runoff water to enter the cracks and saturate the embankment, which can cause instability. Cracks traveling perpendicular to the dam’s crest often indicate differential settlement within the embankment structure. These cracks allow seepage to enter the structure and can quickly lead to piping, which indicates a probable dam failure.
These conditions indicate an urgent need for repair. Moreover, if any of these signs exhibit rapid development or expansion, contact your state’s dam safety agency immediately, since the structural integrity of the dam is likely compromised.
Slides and depressions are also structural safety concerns for embankment dams that should be documented and quickly repaired. Dam failure can easily cause damage to downstream structures and even result in loss of life.
Moderately sized reservoirs and ponds can sometimes experience embankment damage due to wave action. Ceaseless movement against the dam’s upstream side over time can weaken the underpinnings of the dam and render it unstable. Floating log booms secured a few feet in front of the dam will dampen wave activity and are an inexpensive solution for smaller reservoirs. In cases where more protection is needed, riprap installed along the upstream face of the dam can be very effective.
Excavated Ponds
Excavated ponds should be inspected for problems with shore erosion. Whether that’s crumbling edges resulting from livestock or wildlife activity, or undercuts resulting from rapid water movement, these need to be resolved and the cause eliminated. Fencing is often an effective option for animals, while berms or even riprap can help with the speed of water at the ingress. The key in this case is to slow the water down. Changing the direction of water flow or providing a surface that breaks it up is the most effective strategy to prevent erosion damage.
