Drainage systems need to be maintained. A maintenance plan should be put in place for both in-field and open drains, focusing on areas susceptible to blockages, writes Patrick Tuohy and Owen Fenton, Teagasc research officers.
Every drainage scheme is only as good as its outfall. Maintenance vastly improves the capacity and the lifespan of the drainage system, but also helps with water storage, sediment trapping and remediation of nutrient losses.
Drainage systems are poorly maintained in most cases. A maintenance plan should be adopted for both in-field and open drains, focusing on areas susceptible to blockages.
This provides a cheap and effective means of improving drainage by maximising the effectiveness of existing drainage infrastructure.
Fine soil particles are many times smaller than the aggregate (e.g. stone) around a pipe or the slits in the actual drainage pipe. This means they can get washed from the soil and ultimately settle in field drains and impede flow.
Iron (ochre) can also block drains where it accumulates after being washed out of the soil. Plants and their roots can thrive in open channels, at the pipe outlet and deep within the pipe system causing blockages.
Collapse/sedimentation of open drains, due to flow conditions, undercutting of banks or livestock damage can also cause impediments.
- Drainage systems will deteriorate at a fairly steady rate until blockages become established and “self-cleaning” is inhibited;
- If flow is slowed or stopped entirely then large volumes of sediment in the system will be deposited. Relatively minor blockages can quickly undermine the whole system;
- Regular inspection, cleaning and maintenance is required;
- During wet years, excessive soil damage by machinery and livestock can reduce the natural drainage capacity of the soil, handicapping the drainage system.
Open drains, culverts and outfalls must be cleaned regularly to remove any obstructions while they should be established to as great a depth as possible to aid flows. Exclude livestock access to open drains.
Field drainpipes and outlets should be jetted/flushed or rodded regularly to maintain flow, and their outlets should be well marked and protected during the cleaning of open drains
To protect fish eggs and small salmonids, drainage works and the maintenance of drainage systems in areas likely to contain these species should be carried out between mid-May and mid-September.
When planning any new drainage works, the potential of the land to be drained should be assessed to determine if the costs incurred will result in an economic return through additional yield and utilisation of the grass or other crops grown.
Collect all the information at hand, over an extended period to establish where and what the root causes are:
- Where does the water gather or pond?
- Where does overland flow occur?
- Where are the worst underfoot conditions?
- Where are the poorest areas of grass growth?
- Are there weeded areas?
This information will help in deciding where best to invest in drainage works.
Soil type evaluation
A number of test pits (at least 2.5 m deep) should be dug within the area to be drained. The test pits should be in areas that are representative of area as a whole. The depth and type of the drain to be installed will depend on the interpretation of the characteristics revealed by the test pits as described in the video above.
Video source: Teagasc YouTube