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The ABC’s of irrigation reservoir planning and construction

of irrigation reservoir planning and construction


March 31, 2008
By Marg Land


Topics

So, you’re thinking about building
a new irrigation pond. Or maybe you just want to improve the reservoir
you’re currently using.

So, you’re thinking about building a new irrigation pond. Or maybe you just want to improve the reservoir you’re currently using.

According to Ontario Ministry of Agriculture, Food and Rural Affairs Water Quantity Engineer, Rebecca Shortt, you should be considering several factors, including where and how you plan to obtain water to fill your reservoir, well before you start digging.

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Rebecca Shortt, Water Quantity Engineer with OMAFRA.

In Ontario, the average annual rainfall hovers around the one metre mark, which, when taken from a small surface area, is definitely not enough water for irrigation purposes, explains Shortt. Instead, producers need to consider other water sources, such as runoff (rain and melt water gathered from a wider area), gravity flow from a creek or ditch, pumped water from a creek, ditch or well, and possibly a ground water source.

The source of water will help dictate the structure of your reservoir, she adds. If you are planning on filling the reservoir using gravity flow or runoff water, a below-grade reservoir, which is built into the ground, is required. If you plan to pump the water needed to fill the reservoir, an above-grade structure, built on top of the ground, can be used.

According to Shortt, the best and most versatile style of reservoir is a combination of both below- and above-grade construction, providing the grower with the ability to use multiple water sources to fill the storage area.

Design
When it comes to designing and constructing your water storage area, one of the most important elements to deal with is the lining, which needs to be composed of at least 15 per cent clay in order to prevent leaking.

Shortt explains that in Manitoba, farmers interested in putting in an irrigation reservoir use an electronic device called an EM38 which, when mounted behind a tractor or ATV, can help map a farm for clay deposits. The areas mapped can then be test drilled to see how deep the clay layer goes.

She adds that if clay is not readily available at the construction site, it needs to be trucked in or a commercial liner used instead. The lining also needs to be well compacted in order to avoid possible leaks.

Another important factor of reservoir design and construction is wall slope. In Manitoba, the majority of irrigation reservoirs are constructed with a 4:1 outside slope and a 5:1 inside slope, says Shortt. The inside slope is more gradual due to the fact it’s very hard to find armour stone in Manitoba to line the reservoir at the water level, thus protecting against the possibility of wave erosion. Instead, the slope is constructed on a more gradual pitch, making the reservoir wall thicker and stronger to protect against erosion. The slopes are also gradual enough to allow for mowing along the top and edges.

Shortt recommends at least a 2:1 inside slope, unless the reservoir walls are being constructed with sand, when it should be increased to a minimum of 3:1. The outside slope should always be at least 3:1.

The top of the bank should have a width of between 10 and 16 feet to allow for the use of mowing equipment and access by motor vehicles, just in case repairs need to be made later.

Erosion control:
When the reservoir is being constructed, all topsoil should be removed and used to help cover the outside banks of the structure. Once the banks have been constructed and formed, they need to be seeded with a mixed grass variety that is also drought tolerant. This helps keep rain runoff from eroding the outside banks. Armour stone will need to be placed around the inside of the reservoir at the proposed water level to help provide protection for wave erosion.

There are two areas of the reservoir at risk for failure – deep failure, which can occur well below the surface of the water in an eroded area, and toe failure, which can occur at the water’s surface and usually involves some wave and/or runoff erosion.

In order to provide some insurance against possible reservoir containment failure, a grower may want to consider hiring a consulting engineer to help with the reservoir construction, suggests Shortt, especially if the project has banks more than five feet high, is being constructed anywhere close to roads or buildings, and any other high-risk factors. The higher the banks, the greater the potential for widespread effects in the case of a failure, she adds.

As further insurance against catastrophe, the reservoir should be constructed at least 50 feet away from a municipal drain and as far away from roads and neighbours’ fields and buildings as possible. You may want to consider fencing the reservoir if there are young children living nearby. Construction should also take into consideration avoiding fertilizer, fuel and chemical runoff making it into the water.

Permits:
Shortt advises that growers should plan on spending at least six months getting through the water permit application process. A water permit is required if more than 50,000 L of water is pumped from the reservoir or another water source per day. While the permit is still free for farmers, the application process has become more complicated in recent years and longer forms providing more extensive information to government officials are required. Information about water permits and the application process is available by calling 1-800-265-7672.

If the reservoir has been located close to a creek or building in a flood plain, you will also need a permit from your local conservation authority. And, if you plan to remove the fill or topsoil excavated during the reservoir construction from your property, you will require an exemption from the Ministry of Natural Resources’ Aggregate Resources Act.

If you are pumping water from a municipal source and are hooked up to the municipal water system, a water
permit is not required as the municipality already holds a water permit and is responsible for recording the water amounts pumped.

Financial assistance
Cost sharing opportunities for the construction of a pond to hold water and to offset consulting fees are available through the Environmental Farm Plan, which is administered through the Ontario Soil and Crop Improvement Association (1-800-265-9751).


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