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1. Overview of drainage under-performance
Subsurface (tile) drainage removes excess water from the soil to create good fieldwork and crop growth conditions on poorly drained soils. When properly designed, installed, and maintained, subsurface drainage systems can perform well for decades. However, many design, installation, and management considerations can lead to the under-performance of subsurface drainage systems.
One measure of drainage system performance is that the system should be able to lower the water table from the soil surface to 1-ft depth in less than 48 hours following a heavy rainfall. Additionally, the drainage system should be able to maintain the water table below 2-ft depth over at least 90% of the drained area during at least 90% of the growing season. If the performance of the drainage system falls below these criteria, it is likely to be underperforming and may subsequently reduce profitability.
In this bulletin, three common causes for the under-performance of subsurface drainage systems are described (Section 2). Then, the impacts of these common causes of underperformance are presented (sections 3-5).
2. Causes of under-performance
This section describes design, installation, and maintenance issues as common causes for the under-performance of drainage systems.
2.1. Design issues
Design issues may cause the drainage system to underperform. Most design issues are avoidable by increasing knowledge through education and training opportunities. Drainage system users are also encouraged to use the Drain Spacing Tool to maximize system performance and profit. To learn about the Drain Spacing Tool, see Ghane (2021e).
Common design issues include pipe grade less than the minimum requirement (Section 2.2.3), installation in a very poorly permeable deep soil layer (Section 1.1.2), installation in sand layers without an envelope or sand-slot pipes (Section 2.2.3), and installation in high-risk iron ochre areas without special design considerations (Section 2.3).
2.1.1. Under-design issues
Under-design occurs when choosing a drainage coefficient less than the drainage intensity (that is, sizing the main collector diameter too small). In this situation, water enters the main pipe faster than the main can carry it away. The solution is upsizing the main.
Another under-design issue is spacing the drain pipes too wide. In this situation, water removal is slower than is necessary for profitable crop production. The solution is adding additional lateral drain pipes.
2.1.2. Drain depth issues
If a shallow permeable upper soil layer sits on top of a very poorly permeable restrictive layer, drain pipes should be installed in the more upper layer at a shallower depth while maintaining minimum pipe cover. This is because drain pipes are always more effective when installed in more permeable soils. For more information about shallow drains, see Ghane (2021h). Avoid installing deep drains in muck or peat soils because it may enhance subsidence from the oxidation of organic matter. Avoid too deep (>4 ft) of a drain depth because the drain pipes may draw excessive seepage from nearby areas.
Most commercial tile plows have enough power to install deep drains without difficulty. However, a pull-behind plow is more susceptible to difficulty pulling the plow at deeper drain depths, and it may require a second tractor, especially in heavy soils with stones.
2.1.3. Compromised outlet
Another common design issue is a compromised outlet. The outlet may be off the ditch bottom, but the ditch water level frequently rises high enough to submerge the outlet and reduce flow (Figure 1). In this situation, either clean and deepen the ditch, or raise the outlet to allow normal free flow. Otherwise, a lift station may be necessary.
Figure 1- Left: The outlet is submerged because it is too close to the ditch bottom. Right: The same ditch during the process of being cleaned and deepened to allow free flow from the outlet (photo credit: Jason R. Piwarski).
2.1.4. Lack of breathers
Lack of breathers in the system is another example of a common design issue. Breathers allow air to enter the drain pipe to vent the pipe. Breathers are usually installed when going from a relatively flat minimum grade to a steep grade, thereby preventing piping of the soil into the drain pipe. Also, if there is no breather and the water table is above the top of the lateral drain pipe, flow may be restricted when going from relatively flat minimum grade to a steep grade. Breathers may also be needed when the lateral drain pipe is very long.
Inadequate venting can cause piping of the soil into the pipe when a steep grade results in high water velocity (low pressure) in the pipe. Inadequate venting can cause blowouts when a sudden change of grade from steep to a relatively flat minimum grade results in high pressure in the pipe. Tile blowouts are also caused by blockage of the system outlet, or a broken or cut drain pipe.
2.2. Installation problems
Poor installation may cause the drainage system to underperform. Installation quality should be assured using proper machinery, using suitable methods, following correct specifications and standards, and generally providing good quality control during installation. Installation problems are avoidable by increasing knowledge and gaining experience.
2.2.1. Wet conditions
Avoid installation or any other fieldwork during wet soil surface conditions because it leads to soil compaction, which is one of the causes of impeded infiltration and percolation (Section 4.1).
When the water table is above the drain installation depth, there is a higher risk of smearing the soil adjacent to the drain pipe, especially in heavy clay soil. When the water table is near the drain installation depth, capillary rise can still create wet conditions for the soil at the drain depth, thereby increasing the risk of smearing. Smearing slows down water entry into the drain pipes, so the system will not work well at first. It may take up to 3 years for the drainage system to work efficiently as the ground goes through cycles of drying, wetting, and frost to break up the compaction
The ideal installation condition is when the ground is driest and the water table is as deep as possible, usually during summer. Typically, there is limited control over this because the condition may be less than ideal when the drainage contractor is available to work during normal installation windows (before and after planting). To reduce the risk of under-performance due to installation under wet conditions, one option is growing crops with an early harvest (wheat, barley, oats, rye, and corn silage) during the year planned for installation to provide ideal installation conditions after harvest. Another option is installing through a standing crop when the ground is driest.
2.2.2. Examples of installation problems
Examples of avoidable installation problems include installing perforated drain pipes through trees or shrubs (Section 3.1.1), improper connections causing root clogging (Section 3.1.1), sediment clogging of the drain pipe (Section 3.2.1), off-grade dips or humps in the drain pipe causing root clogging (Section 3.1.1), improper installation speed, improper use of pull-behind or mounted plows, mishandling of pipes, and lack of outlet protection.
Improper installation speed can cause off-grade outcomes. Flatter grades require a slower installation speed than steeper grades.
An inexperienced operator using a pull-behind plow may cause poor installation because it is harder to maintain a consistent grade. These machines can cause installation challenges when encountering dead furrows, fence lines, ridges, swales, and rocks. Thus, experience is essential with pull-behind plows.
Two examples of pipe mishandling are exposing the pipe (with or without sock) to the sun for too long and over-stretching the pipe when not using a power feeder. Exposing black sand-slot pipes to the sun for too long may cause the narrow perforation widths to stretch during installation and become wider perforations, which allow fine sand or silt to enter the pipe, causing sediment-clogged pipes (Section 3.2).
When installing lateral drains in the downslope direction, water can enter the laterals. Then, laterals may need to be connected to the main pipe under water, leading to a poor connection.
In some situations, air-locking can occur when there is a severe negative grade in a section of the drain pipe and the water table is above the top of the pipe. In this case, air generally cannot exit the pipe and it restricts drain flow.
2.3. Inadequate maintenance
Drainage systems need maintenance to keep them in good working condition. When proper maintenance is lacking, the system will not work as designed.
For example, the drainage system should be regularly maintained to prevent blockage of the outlet. The field should be checked for blowouts and wet spots. If a hole appears over a drain pipe due to a pipe blowout, soil could enter the drain pipe and clog the drainage system. In that case, replace the pipe in the blowout area as soon as possible. If a wet spot appears in the field under normal rainfall that was not there before, there may be a drain pipe clogging issue (Section 3) or impeded infiltration issue (Section 4). Overall, inadequate maintenance can result in under-performance of the drainage system.