• Open trench drains are the arteries of a farm drainage system
• Trench drains collect water from underground pipes, lower the water table or intercept seepage
• Raised beds can be effective on duplex soils
• Shallow spoon drains do not drain soils
• Hump and hollow drainage works in flat swamp areas with a high water table

These are the first component of a farm drainage system to be installed. This is because they are the arteries of the drainage system or the means by which water is removed from paddocks (Figure 35). Often trench drains can be installed along fence lines, laneways and in natural depressions.

These drains can also collect water from a pipe drainage system (Figure 36), act directly as a land drain to lower the water table (Figure 37), or intercept surface or groundwater flow in cut-off drains (Figure 38). Intercepting surface water flowing off roads and hard surface areas around sheds is critically important around paddocks used for annual cropping in order to protect the soil in paddocks from erosion. The volume of water coming off these areas is often underestimated, particularly in heavy rainfall events.

Open ditches are usually installed with an excavator. Ensure there is an outfall for ditches so that there is sufficient gradient on the ditches to keep water flowing. Open ditches have flat bottoms and are not V-shaped, to prevent scouring. A ditch with a bottom width of 40-50 cm requires a gradient of 0.15 -0.25% (15 mm in 100 m to 25 mm in 100 m) to maintain sufficient velocity to prevent weed establishment. Ditches are normally 1 - 2 m deep but need to be at least 1.2 m deep.

Side batter slopes of ditches should be sufficient to prevent the sides collapsing. The batter (vertical: horizontal distance) depends on soil texture. For ditches less than 1.3m deep the batters required are: heavy clay 1:1 (vertical : horizontal); clay or silt loam 1:1.5; sandy loam 1:1.5 to 1:2; sand 1: 2 to 1:3 or more. Where drains are deeper than 1.3 m, the side batters will need to be greater than these. Where unstable soils are present e.g. very fine sand, establish grass on the banks as soon as possible to minimise bank erosion. Severe cases may require lining with stone or protective matting.

Trench drains need to be built in such a way that enables regular maintenance and should all be fenced on both sides to prevent stock access. Without fencing to prevent stock access, open trenches will clog up due to side wall collapse and may only effectively operate for two to three years rather than 10-20 years or longer with fencing.

If spoil is stored near the trench, gaps must be left at short intervals to allow for surface flow off the land (Figure 39).

Surface trench drains can interrupt the flow of farming operations (Chapter 3) and may need to have culverts installed for stock crossing at gateways (Figure 40). Where centre pivot irrigators are required to cross open trench drains, each irrigator wheel will require a drain crossing. These crossing may be made of logs (picture at front of this chapter), concrete culverts, container bases or other novel hardware but they all add considerable cost to the installation of a farm drainage system.

Shallow spoon drains or grassed waterways promote surface water removal along natural drainage lines and should be used as drainage lines which link up hollows and depressions, particularly on undulating paddocks of duplex soils in the Midlands. Grassed waterways are usually 2.5 – 3.0 m wide, of 200-300 mm minimum depth and should run along the natural water pathway. If they are on a side slope, they will need to be deeper and more carefully constructed. Shallow spoon drains are installed with a rotary spinner drainer, road grader or excavator with a wide bucket. It is essential that the spoil is flung well into the paddock away from the drain so that surface water is not impeded in getting to the drain by an artificial levee (Figure 41). A machine such as the Wolverine drainer can throw spoil either side of the machine and up to 50 m sideways. One pass can usually remove up to 10 cm depth of soil, but greater depth will require more passes. The side batters will require extra passes. Shallow spoon drains should be grassed with perennial grasses and not cultivated during cropping rotations to prevent erosion of the channel. They should cause minimal disruption to cultivation operations as they are wide and shallow enough to drive across once established.

Shallow surface drains (Figure 42) will remove flooding or surface water during heavy rainfall but do not provide for through soil drainage or drain seepage water. One can not fix a subsurface water problem with a surface drain.

The spoon drain draws water over the soil surface to get rid of surface flooding but once this is achieved these drains have very limited effect on lowering the water table. A trench drain by comparison draws water through the soil and results in an overall lowering of the water table (Figure 43). The objective of installing trench drains is to provide approximately 40 cm of drained soil for plants to grow in.

Texture contrast soils in Tasmania are used for cereal, pea, poppy and potato cropping but these soils (Chromosols, Kurosols, Sodosols) are difficult to crop as they usually become wet and unworkable in winter and occasionally flood. The clay-textured B horizon impedes water movement and root development, and the lighter textured A horizons suffer from waterlogging or water deficit depending on the season and are prone to crust formation and hard setting. Cultivation for crop sowing and harvesting is often carried out when soil moisture content is greater than ideal, which results in soil structure problems such as compaction and hard setting. 

Raised bed cropping is a management strategy for removing excess surface water during plant growth. Properly planned and constructed raised beds maintain a seed bed that promotes optimum root growth and maximum aeration, infiltration and drainage. Raised beds (Figure 44) are a means of reducing waterlogging by creating:

• A deepened seedbed that is not dense and does not constrain root growth, with compaction limited to the furrows.
• A seedbed with more roots and sufficient large pores for good aeration, infiltration and drainage.
• A short distance and a reasonable height from the bed centres to the furrow base for a substantial hydraulic gradient to stimulate lateral drainage.

However, disadvantages are that construction of raised beds involves considerable cultivation during formation and they need to be flattened after the cropping phase for the pasture phase in the rotation and both of these mechanical operations can destroy soil structure.

n many years raised beds allow for spring planting of crops on duplex soils where otherwise waterlogging would have prevented machinery access and successful crop planting and establishment. Yield improvements can be expected from the raised beds but depend on the season and locality. Research in Tasmania found raised beds had greater infiltration, lower bulk density, lower shear strength at 100-200 mm depth and lower penetration resistance than neighbouring undrained paddocks (Cotching and Dean 2003). These improved soil physical conditions result in better drained and aerated soil for plant growth than conventionally managed sites.

All raised bed installations must include appropriate surface drains to act as an effective outfall. The direction of beds must enable water to exit the paddock. Bed hollows, or trenches, must be able to drain into field drains in low lying areas. The amount of extra water running from raised beds is not trivial, and there must be sufficient fall to allow water to exit the paddock within 24 – 48 hours after significant rainfall (Figure 45). Most land with shallow texture contrast soils that are prone to waterlogging has a low slope and so precision contour surveys are essential to detect the dominant direction of slope for planning the orientation of raised beds, to identify any humps and hollows that might change the direction of surface flows, to identify the need for local drains or ‘cross drains’ to remove water from small depressions, and the best location to dispose of water from the drains at the lower end of the beds. Raised beds are constructed with a special-purpose implement called a bed-former, that comes in 3-point linkage and trailing models. Some makes can alter bed width. Beds are usually installed 1.8m - 2m wide. Planning is important when considering installing raised beds so that the widths of tractors, sprayers and harvesting equipment can use the furrows, creating a controlled traffic opportunity. The best soil moisture conditions in which to construct beds is the same as that for deep cultivation, i.e. moist, not dry or wet & plastic.

Land planing is done to improve the uniformity of surface drainage and water infiltration by removing high points and in-filling shallow depressions on an otherwise relatively flat paddock. Water runs off the paddock faster resulting in less waterlogging and better soil health. Land planing fills in minor depressions, but larger depressions are likely to need installation of surface or underground drainage.

The process starts with an accurate elevation survey as accurate elevation data ensures the minimum amount of soil is moved to move the greatest volume of water. Moving excessive amounts of soil costs money and wastes time. The land plane operates in 5 mm height increments and so accurate elevation data is critical to the operation. Software is used to assess the best practical drainage outcomes by dividing the paddock into multiple small catchment areas each feeding into drainage lines. It works with the natural topography of the paddock to minimise the amount of cut and fill. RTK guidance using in-cab T3RRA Cutta™ design software and iGrade™ software that automates movement of the land plane, allows the driver to follow predesigned curved surfaces that are closely aligned to the natural topography with automated control of the blade height of the earth moving equipment (Figure 46).

Hump and hollow drainage is where major land surface reshaping creates parallel ridges with even side slope to shallow drains. This form of surface drainage is appropriate when water either perches on the soil surface or winter water tables are at or near the surface and subsoil drainage is limited by restricted outfall. There is a need to either shed water off the surface by creating a slope on the ground or elevating the soil above the water table. Hump and hollow drainage (Figure 47) is most appropriate in swamp areas with large flat areas having a regionally high water table. It works best on soils without contrasting subsoil layers. It is also used on sandy soils with surface water perching. Sandy soils cannot normally be subsurface drained because the pipes become blocked with inflowing sand. Hump and hollow drains only work in conjunction with a good system of arterial drains with suitable outfall (Figure 47).

The first step installing hump and hollow drainage is undertaking an accurate survey of the area to be drained, as in any drainage planning.

“Don’t assume you can eyeball the gradient accurately as excavator time is expensive. Don’t be lazy when it comes to doing the initial survey” James Gourley.

Hump and hollow drainage can be installed using an excavator with a 3 m wide bucket (Figure 48) or a road grader. Drains are spaced mostly at approximately 25 m apart, but heavier soils (clay loams and clays) often operate better at narrower 15 – 20 m spacing. The wider the drain spacing, the deeper the drain must be to create an even side gradient on the hump. Deeper drains often mean that the gradient to the outfall becomes more critical and it can be harder to construct the hollows with sufficient grade to the outfall. Cuts of 350-450 mm are common with the final height from the top of the hump to the bottom of the hollow being 700 – 800 mm.

A good sequence of operations is to cultivate in spring, sow a crop of turnips, feed off the crop in February, install humps and hollows in February/March when soils are dry, and then sow down new pasture in the autumn. Alternatively, spray off and cultivate in spring, install humps and hollows, sow a crop of turnips, feed off the crop over late summer to gain consolidation and then sow down to new pasture in the autumn. An excavator can operate when soil conditions are moist to wet, but due to losing wheel traction in the wet, a road grader only works efficiently when soils are dry. The costs of installing hump and hollow drainage go beyond just the machinery costs. Budgets must allow for lime, fertiliser, grass seed and fencing.

Redistribution of soil materials during construction can alter the nutrient levels available for pasture growth. Measurements have shown that after several years of fertiliser application, the side slope sections of humps and hollows have lower phosphorus levels than either the top or bottom sections and potassium levels increase progressively from top to bottom. These results may be due to more fertiliser being applied to the tops of humps than side slopes and movement of potassium down slope with runoff as potassium is readily soluble (Cotching 2000).

The concentration of nutrients in runoff from dairy pastures on land drained with a hump and hollow system was found to be high and 10 – 100 times greater than those recorded in other parts of the catchment (Cotching 2000; Holz 2010). Applying phosphorus fertiliser between November and March, when rainfall builds up soil water reserves rather than running off paddocks, and applying less phosphorus to paddocks which have high soil phosphorus levels results in less phosphorus being lost from farms. Both management changes would lower farm input costs and help profitability.

Installing hump and hollow drainage should be seen as part of a package of drainage and pasture improvement. It also smooths out depressions left from previous land clearing. Even with hump and hollow drainage installed, you should still take care to prevent pugging and soil compaction by heavy mobs of stock. Maintenance of hump and hollow drains may require rolling the soil surface if cattle have pugged the ground, and the base of the hollows may need to be cleaned out with a spinner drainer every one or two years to keep water flowing.

Outlets for water from the hollows are as critical as in any other drainage system. If headlands are formed at the outlet end of paddocks, then pipes must be installed beneath the constructed headland to allow water to drain away (Figure 49). These outlets will need to be checked for any blockage resulting from stock trampling, breakage by machinery and excessive grass growth.

On land with slopes greater than 5 % that is used for annual cropping, permanently grassed waterways are needed in depression floors to contain surface water flows and to prevent serious soil erosion (Figure 50).

These waterways need to be wide and shallow to dissipate the flowing water energy. The soil from the drain needs to be spread well away from the drain edge to prevent formation of a lip, which prevents water entering the side of the drain. They can be installed with a rotary drainer (Figure 51), road grader or excavator with a wide flat bucket. The base of the slope where the water is discharged may need to be protected with a rock-lined sump to dissipate the energy in the flowing water.