Better soil productivity with field drainage

DRAINAGE RI Farm
CHIPS: 10-15mm clean quarry chips are suitable for gravel tunnelling.

THE objective of a field drainage system is to remove excess water from the soil therefore improving the conditions for root development and grass growth. Improved drainage will also make field operations easier, including the harvesting of silage, earlier turnout of livestock and improved grass yields.

Although an investment which will reap many benefits, budget carefully and avoid taking on too much expenditure during this difficult financial time.

FILLED: A conventional collector drain filled to the top with clean broken stone.

Trevor Alcorn, a dairying dev-elopment adviser at the College of Agriculture, Food and Rural Enterprise (CAFRE), said: “Many of the drainage schemes carried out with grant aid in the 1970s and 1980s are at the stage where they need a lot of maintenance to remain effective.

“If neglected, the whole drainage system can break down in a very short time. In the current spell of dry weather and with silage in, now is an ideal time to carry out drainage works. It is financially better to maintain and repair than to start a new drainage system.”

As a result, the CAFRE expert says that the following steps should be followed.

“Sheughs need to be inspected to ensure water is flowing freely and to allow a sufficient outfall from the drainage pipes and avoiding build-up of silt in the pipes. However, it should also be noted that infilling or laying drainage pipes in open sheughs is not permitted except by prior permission from DAERA.

“Only vegetation and silt should be cleared from the sheugh to avoid deepening or widening it. You should also try to leave the vegetation untouched along one side of the sheugh at cleaning time. Fencing off sheughs should also be considered to prevent cattle from tramping banks and drainage outlets – although take care to position fences to allow for regular cleaning out of sheughs. And, the use of occasional inspection chambers in suitable locations is beneficial to monitor water flow and clear obstructions in areas prone to silting.

“In addition, inspection holes should be opened and checks made to see if the old pipes are open and stones still clean. Overall, plan your repair work of existing drainage as necessary.

“When designing a new drainage system, the starting point on any field is to investigate the site to find the exact cause of the problem. Visible indications may be rising springs, soft ground conditions in dry weather and wet patches with rushes.

“Once these areas have been identified the type of drainage system will depend on the following categories: High water table – low lying, flat ground with little or no fall; Cleaning existing sheughs may help, but a series of equally spaced parallel piped field drains will also be required.

“Impervious subsoil is common in many areas of Northern Ireland, especially in counties Tyrone and Fermanagh. It is characterised by shallow topsoil overlying a poorly structured ‘heavy’ clay subsoil which will not let water move freely through it. The design of this drainage system must allow for the use of either mole drains or gravel tunnels over a series of main piped collector drains.

“In the case of springs or seepage lines, water moves through the free draining subsoil and runs along the top of the permeable layer bursting out on the side of the hill. Drains will have to be constructed deep enough to intercept the water in the soil layer in which it is moving and in some instances can be up to two metres deep.

“Within conventional drainage there are two options. Wide spacing or deep drains are most cost effective, but it depends on permeability of soil and the amount of fall available. On free draining soils with a high water table, drains could be 12-15 metres apart and up to a metre deep (rarely beneficial to go below a metre deep).

“The second option – narrow spaced and shallower drains – are more suited to clay soils with drains spaced six to seven metres apart and 900mm deep. If considering mole draining or gravel tunnelling, then ensure collector drains are at the correct depth.

“When considering new drainage works, all farmers and landowners claiming under direct aid schemes, including Single Farm Payment, must meet the cross-compliance conditions. Prior permission from DAERA is required if you have land designated as permanent grassland on your farm. And, a further requirement of cross-compliance is the Environmental Impact Ass-essment Regulation and this includes drainage on uncultivated land. However, the drainage of cultivated land is permitted without notification.

“You should also aim for a minimum number of outfalls, whilst ensuring that there is sufficient fall. Due to the undulating and variable landscape in Northern Ireland, the herringbone design has been popular, with the side drain running across the slope at a gradient not exceeding one in 20.

“When working around existing drains try and connect them into the new layout as it will give better drainage and reduce the likelihood of ‘boil ups’ at a later date. Carefully connect pipe to pipe at the joins and ensure stones are manually placed on top of the joins to keep them stable while the rest of the stones are added.

“Also, stay with your drainage contractor to ensure the drains are positioned for best effect as only the farmer will be familiar with the field in a wet time. Choose to work in a dry time, as working on damp ground will do more harm than good to soils.

“You should also consider the cost of stones, pipework and contractors charges and budget accordingly. Plan to do just as much as you can afford. These are difficult times and proceed with financial caution.

“As far as drainage stones are concerned, the stones should act as a filter to the drainage pipes as well as a connector if mole drains or gravel tunnels are used. Drains should be filled to plough depth as there is little benefit in filling them to the top unless it is to solve the problem of ponded water on the surface. The choice of stone will depend largely on what is available locally. It can be clean broken stone or gravel and range in size from

12-36mm.

“In heavy clay soils with poor permeability, conventional drains may not be adequate to provide satisfactory drainage even at the closer spacing. In this case it is necessary to speed up water movement by both loosening the subsoil and providing closely spaced channels to carry the water to the drain. This can be done by either mole drainage or gravel tunnels.”

Both provide comparable results in stable soils and the mole drainage would be preferable because it is much cheaper to install. However mole drainage is not permanent and will only last in the region of five years. They are not suited to steep slopes or unstable ground as channels will collapse, nor is it satisfactory in stoney subsoils.

The gravel tunnel, whilst more expensive, has a number of advantages over mole drains: It is a permanent system; Can cope with unstable soils – stone infill stops the collapse of the channel; Can cope with stones in the sub soil; Can work on steep slopes and has greater flexibility on soil moisture content at installation.

“Both systems will require coll-ector drains spaced every 20-30m, which they will run over the top of. Spacing between each will also be similar at approximately 1.5m apart and a depth of 450-500mm.”

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