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Drainage Maintenance to protect Water Quality

Ireland lies in a temperate zone where the main role of drainage is the removal of excess water in the root zone of crops from surplus rainfall, improving land trafficability and increasing productivity. Meabh O'Hagan, ASSAP Advisor and Niall McLoughlin, Lakeland Dairies has some advice.

When soils are saturated or waterlogged, water will flow over the surface moving sediment and nutrients into our waterbodies. This is known as Overland Flow. The function of a main drainage system is to collect, transport and dispose of water collected from the field drainage system through an outlet.  Main drains may also collect excess water directly from the fields themselves (surface and groundwater).

Appropriately designed and constructed land drainage will lead to improved nutrient uptake by the plant. This can lower losses of nutrient and sediment to watercourses, however great care must be taken to ensure that drainage channels do not become pathways for losses of nutrients. 

Water quality in Ireland has been falling over the last number of years. Hydromorphology which includes land drainage and channel straightening is the second biggest pressure on water quality in Ireland. 

To minimise sediment and nutrient losses when carrying out drain maintenance, adhere to the following practices:

  • Any maintenance to surface water drains should only be carried out during the months July to September. Fish and their spawning grounds are protected under the Fisheries Acts (1959 – 2010). In-stream works should not be carried out without prior consultation and approval of Inland Fisheries Ireland (fisheriesireland.ie)
  • When cleaning drains it is recommended that only one side of a drain is cleaned at a time leaving the opposite bank undisturbed. Do not clean the opposite side until sufficient growth has regenerated on the first to ensure the bank has stabilised. 
  • Drains should not be over-cleaned, retain as much vegetation as possible only removing material from the bottom of the drain. Only clean the lengths where flow is impeded leaving the stone and gravel in-situ. 
  • Ensure the bank is sloped afterwards to prevent collapse. Unstable banks can increase sediment and nutrient loss to the watercourse. The following table outlines recommended bank slopes.
SoilChannel < 1.3m deepChannel >1.3m deepMax water velocity m/s 

Heavy Clay 0.5:1 1:1  1.5 
Clay or silt loam 1:1  1.5:1  1.0 
  • In order to avoid sediments entering a waterbody a 20m length of uncleaned drain should be kept at the lowest length of the drain to act as a sediment trap.
  • Spread the spoil as soon as possible after cleaning, ensuring adequate distance between the spreading area and the open drain and spread it as thinly as possible.
  • In the course of carrying out maintenance leave as much vegetation as possible in place along the banks and margins. Native hedgerows along watercourses can stabilise the bank and act as a natural buffer zone while providing an important habitat.
  • Fence off the drain afterwards leaving a minimum of 1.5m fenced buffer. Allowing vegetation to develop along this riparian margin can help trap nutrients and sediment preventing them from entering drains.
  • There is a requirement for a 5m buffer when spreading organic manures along all surface water drains and this expands to 10m for the two weeks either side of the closed period. This will help prevent drains from becoming a pathway for nutrients to enter rivers and streams.

Due consideration should be given to impacts on wildlife, water and biodiversity prior to undertaking any new drainage works. Wetlands have the highest biodiversity value of any farm habitats. Fish bearing or spawning streams should never be cleaned – refer to Inland Fisheries Ireland if you have any concerns about waterbodies in your locality.

Speak to your agricultural advisor before carrying out any maintenance on surface water drains to ensure you are not impacting water quality.

In the below video, Meabh O'Hagan, ASSAP Advisor and Niall McLoughlin, Lakeland Dairies go through some steps you can take to minimise sediment and nutrient losses when carrying out drain maintenance.

More information on Water Quality Week is available here 

For more information on protecting water quality visit the water quality section of the website here