• Draining one paddock can have impacts on other parts of the farm
• Diagnose in the wet; Install drains when dry
• Check the outfall
• Drain the landscape; drain the soil
• Undertake drainage installation in stages
• Consider local, State and Federal planning and environmental regulations

Every drainage situation is unique. Careful investigation of the site with correct diagnosis and planning of a farm drainage system can deliver benefits to productivity, ease of operations and profitability. When considering the costs and likely benefits to be gained from improving land drainage, both the effects on the current enterprise and the potential of being able to undertake more profitable enterprises should be assessed. A drainage plan must consider what the ground is going to be used for in the future. The ability to grow higher crop yields, more grass and carry more cows or sheep all need to be factored into planning and future budgets. The cost of investing in a farm drainage system can appear to be high to some farmers. However, many Tasmanian farmers have found that the costs of investing in drainage can be paid back in a relatively short time frame, sometimes as little as one year, through improved productivity and profitability. Any drainage should be installed correctly and with due consideration of environmental and off-site impacts (see Chapter 11). Drainage planning should consider our changing climate that may result in changing rainfall patterns or more frequent/more intense storms leading to flooding or prolonged waterlogging of our soils with an increasing number of wet days in winter and early spring when crops are sensitive to waterlogging (ACE CRC 2010; Liu et al. 2023).

A drainage system can dominate the layout of farm infrastructure including paddock shapes and sizes, fences and laneways and irrigator locations. It is rare that draining a paddock or farm can be achieved without having an impact on other parts of the farm. Drained water must flow somewhere. Draining water off one part of your farm can flood lower lying areas or cause serious problems for your neighbours. This means that any drainage plan should involve some degree of farm planning or a drainage plan that covers the whole farm and may even need to cover some of a neighbour’s farm. Marking out the complete drainage design on a farm map, Google image or digital image obtained by drone can help in getting the integration of farm layout and drainage. Don’t just ‘half drain’ an area but put planned drains into a bigger whole farm context. It may be better to design farm layout for future drainage installation rather than trying to retrofit drainage to an existing farm plan, as land gradients and outfalls are natural landscape features that cannot be changed. Blocking natural surface flows of water with farm infrastructure such as laneways can lead to ponding of water or flooding. When installing a drainage plan, it may need to be adapted as the drainage work proceeds due to changing soil conditions or undiagnosed conditions, e.g. buried rock.

It is important to determine the extent of any waterlogging problems and satellite images of the paddock or farm are a valuable aid. Images can be obtained using Google Earth (Figure 12) that also has historic images that often allow for comparison between years and seasons. Detailed high-quality elevation maps are useful to help guide potential drain installation locations. High resolution elevation mapping can be used to model drainage designs at the paddock or small catchment scale to inform drainage activities. Detailed crop imagery showing variability of crop growth across paddocks can also be helpful (Chapter 4).

The author has come up with a set of ‘Rules’ for drainage in Tasmania. These are:

It is essential that waterlogging problems are correctly diagnosed so that any drainage addresses the cause of the waterlogging (Chapter 5). Late winter or early spring (August/September) is the optimum time to investigate waterlogging problems in Tasmania. Poorly drained soils can be identified in the summer but seepage, ponding areas and any perched water can only be identified when conditions are wet.

Check to see if there are any existing drains that may or may not be functioning as designed. Many farms have older trench drains that have not been maintained over the years and the first step in this case is likely to be cleaning out the existing drains. Once diagnosis of the problem has been completed, a drainage plan can be developed. This may be a comprehensive system with surface and subsurface drains, or it may be installation of a few strategically placed drains. Creating a good farm drainage plan can take time, as much as or more time than is spent on installation, and this may mean investing in the services of an experienced consultant.

Most drainage installation should be done when the soil is moist, not wet, at depth but dry on the surface. Dry surface conditions are required for increased bearing strength to hold up machinery and prevent machinery getting bogged (Figure 13). Moist subsoils are optimum for excavation or trenching as the soil is friable rather than hard or too soft. Moling requires the soil to be drying with a firm topsoil and plastic subsoil (Chapter 8).

Check to ensure that land gradients allow for a suitable outfall for any drainage water whether this be from areas of raised beds, from the paddock, or off the whole farm. Existing outfalls may not have been designed or installed for current or future drainage needs (Figure 14) so that drainage can create flooding on the drained farm or perhaps the neighbour’s farm. Negotiations with neighbours may be required to ensure outfalls are going to work and there is sufficient capacity in drains to take increased water flows. Outfalls into streams or large drainage trust drains may need to be oriented in a downstream direction to obtain a venturi or suction effect (Figure 15) that minimises backflow up the drain in high flow conditions. Outfalls into major rivers or into tidal areas may need to have flaps over the end of the pipe so that high flows or high tides do not back water up the drains.

When constructing a drainage system it is important to start construction at the lowest point – the outfall. This will ensure that drains constructed upslope from here with an even gradient will flow and water will have an outlet. It is critical to the functioning of a drainage system that the outfall/outlet continues to operate. Any collapse, breakage or blockage of the outfall will jeopardise all the investment in a drainage system and so checking outfalls must be a regular part of drain maintenance (Chapter 10).

If water cannot be drained by gravity to an outfall, then it must be delivered to a sump and then pumped to a main drain. Sump construction can add additional cost, complexity and maintenance to the drainage system. The sump must be large enough so that the pump doesn’t excessively start and stop. Storage below the minimum water level serves as sediment storage and minimum clearance for the suction pipe. The sump can be a pit, tank, section of a trench or a low area in the landscape that serves as a collection point for the drainage system. The sides can be protected with rocks or be a wide diameter concrete pipe. The level of the pump intake depends on the drain inlet position. The highest water level in the sump shouldn’t exceed the bottom of the inlet drainpipe. Although higher water levels may be possible in some cases, this will usually compromise the drain system’s effectiveness. If water levels in the main drain or stream outlet are relatively stable, you can set the level of the discharge pipe just above the maximum anticipated water level, so water can freely discharge into the outlet. If the outlet water elevation fluctuates, such as in stream floods, a lower outlet elevation can be used, reducing operating costs, but a flap or valve should be installed to prevent backflow when the discharge pipe is submerged.

When diagnosing a waterlogging problem, one needs to know the source of the water and where it is moving in the soil. The first thing to do is to look toward the top of the local catchment area, i.e. look upslope. Rather than looking downslope where water will flow to (the outfall), there is a need to identify where in the landscape the water is coming from. This step is a critical one in diagnosing the problem, so that this water can be drained away before it causes a waterlogging problem. Determine if lower lying areas can be isolated from up-slope water with interception or cut-off drains. An interception drain may be able to collect water from the upslope geomorphic unit (hills or terraces), thus alleviating seepage into the lower geomorphic unit (flats). It is much more efficient to divert or intercept water than it is to drain it through long distances in the soil. However, on many flat land areas, rain fall on its own is sufficient to cause waterlogging. It is often the landscape topography that will determine the location of shallow surface drains. Always consider strategic placement of underground pipe drains in the landscape before recommending a grid pattern of drains. Regular spacing or grid pattern drains in areas with elevation differences, i.e. topography, usually does not optimise opportunities for placement of intercept drains in seepage areas resulting in some areas not being drained and potentially an over investment in the number of drains.

There are several characteristics to look out for both on the soil surface and in a soil pit to correctly diagnose how best to drain the soil (Chapter 5).

There can be many advantages to installing drainage in stages, both in terms of effectiveness and funding the costs over time. If a drain doesn’t work, then an alternative can be tried. If improvements to waterlogging are sufficient from a small initial investment in drainage, then a decision may be made not to proceed with any further drainage. Any drainage is costly and so staged installation allows for this cost to be funded from cash flow rather than a large capital investment.

Surface water problems should be addressed before undertaking any subsurface drainage.

• Generally, constructing any arterial trench drains is the first step in order to allow for flows of water off the property and to initiate soil drainage adjacent to the trench drains. Together with these main drains, outfalls that work must be installed in the first stage of constructing a drainage system.
• Land planning may be the next option to fill in depressions. These may need to be touched up in following years as the surface can sink.
• If diagnosis has revealed that interception drains are appropriate, either open trench or underground, then these drains are next on the priority list. The effectiveness of these interception drains should be monitored over a winter season to make sure that they are appropriately placed, intercept sufficient water to minimise any waterlogging problem, and to see if they are having an effect downslope that may reduce the requirement for further drainage.
• The next step is installation of paddock drains, either trench drains or surface waterways.
• Next to be installed are the underground pipes and mole drains.
• The last step would be installing any hump & hollow drainage if appropriate.

Poor planning and design can lead to detrimental agricultural and environmental impacts and even breach of legislation. Prior to undertaking any drainage work in Tasmania, consideration must be given to local, State and Federal planning and environmental regulations.

Prior to undertaking any drainage works, the following should be considered:

• Ensure proposed works are on your property. Works on Crown Land require approval under the Crown Lands Act 1976 in the form of a lease or licence. Property boundaries and private or public ownership of titles can be checked on LISTMap.
• Check with your local Council. Works within or near wetlands and waterways may need local government approval under the Land Use Planning and Approvals Act 1993 (LUPAA 1993).
• Talk to your neighbours. As identified on page 24, drainage works may impact your neighbours. Getting consent may avoid potential legal action relating to water availability.
• Creating channels and diverting surface water in most instances is regulated under the Water Management Act 1999. Contact DNRET for further information.
• Drainage works must not impact on fish passage or cause harm to fish habitat under the Inland Fisheries Act 1995. For further information contact inland fisheries.
• Drainage works have potential to disturb Aboriginal relics. All relics are protected under the Aboriginal Heritage Act 1975. Use the Aboriginal Heritage Property Search (https://www. aboriginalheritage.tas.gov.au/propertysearch/) or Dial Before you Dig to determine if your project may impact Aboriginal relics.

Due diligence also includes knowing your property’s natural assets and understanding the natural landscape. LISTMap provides several useful layers to get to know your property’s natural assets, particularly those of high ecological importance that may be protected under legislation (Figure 16). These layers include:

• Threatened Native Vegetation Communities 2020 (TNVC 2020). Threatened Native Vegetation Communities 2020 (TNVC 2020) is a state-wide mapping layer produced by the Tasmanian Vegetation Monitoring and Mapping Program (TVMMP) showing the indicative extent of threatened native vegetation communities across Tasmania. It estimates the mapped extent of 39 communities listed under Schedule 3A of Threatened native vegetation communities of the Nature Conservation Act (2002).
• TASVEG 4.0. TASVEG is a Tasmania-wide vegetation map produced by the Tasmanian Vegetation Monitoring and Mapping Program (TVMMP). While not all vegetation communities are threatened, clearing of any vegetation is regulated under the Land Use Planning and Approvals Act 1993 and the Forest Practices Act 1985.
• Conservation of Freshwater Ecosystem Values (CFEV) Wetlands. The CFEV Wetlands data layer shows a statewide coverage of wetlands in Tasmania. Wetlands provide critical habitat for wetland species and play an important ecologic and hydrological role in the landscape. Drainage design should ensure that ecologically important wetlands are not drained.
• RAMSAR Wetlands. The Convention on Wetlands is an intergovernmental treaty which provides the framework for national action and international cooperation for the conservation and wise use of wetlands and their resources. It is commonly referred to as the Ramsar Convention.
• Threatened Flora and Threatened Fauna. Threatened species are protected under the Threatened Species Protection Act 1995. Under this Act, a permit is required to knowingly “take” (which includes kill, injure, catch, damage, destroy and collect), keep, trade in or process any specimen of a listed species. Species may also be listed as threatened under the Commonwealth Environment Protection and Biodiversity Conservation Act 1999 (EPBC Act).
• CFEV Karst – Integrated Conservation Values. Karst systems are sensitive to changes in surface and groundwater flows. Drainage in a karst environment requires special consideration and expert advice.

While well planned and designed drainage can improve environmental outcomes, it also creates connections in the landscape for the spread of pathogens, pests, weeds and nutrients.

Existing Infrastructure

Before installing any drains make sure that any underground services have been located. Be aware of telecom cables, water supply pipes, effluent disposal pipes and underground water mains for irrigators. When operating machinery be aware of any electric fences and overhead power lines. Contact should be made with Before You Dig Australia (BYDA): https://www.byda.com.au/contact/ . Dial Before You Dig is a free national referral service designed to prevent damage and disruption to the vast pipe and cable networks which provides Australia with essential services. Lodge your free enquiry by going online atwww.1100.com.au, downloading the iPhone app or ringing the national call centre on 1100.

Sodicity and Salinity

Sodicity and salinity are soil specific issues that need to be correctly diagnosed in the planning stages of drainage (Chapter 9). An incorrect diagnosis can lead to drainage failure and the loss of any investment in drainage as well as the potential for severe land degradation.

Acid sulfate soils (ASS)

Most of the ASS likely to be of concern occur along the north coast of Tasmania. Drainage in areas of the coastal zone that are at or below 20 metres Australian Height Datum (AHD) that will disturb soil or nearby ground water hydrology should be investigated for potential ASS. Maps of the areas of concern can be accessed online at: https://maps.thelist.tas.gov.au

Acid sulfate soils contain iron pyrite or iron sulfide (jarosite). In an undisturbed and waterlogged state these soils are harmless. Drainage of these areas is designed to lower the water table that aerates the soil leading to oxidation of the sulfuric rich sediments in the subsoils producing sulfuric acid in large quantities. When rain occurs following prolonged dry periods, the sulfuric acid in these soils moves through the soil profile and may release toxic heavy metals which eventually flow into surrounding waterways. Toxic “slugs” of concentrated acid runoff can move downstream and flow into estuaries, reducing oxygen levels in the water, decreasing water quality to such an extent that fish are killed. One of the first signs of acid sulfate soils is reddish coloured iron oxides in surface drains (Figure 17). The water in these drains can be extremely acid with the water acidity as low pH 2 being measured in drains in Northwest Tasmania.

While management options are available for minimising environmental damage from ASS, the first option should always be to avoid disturbing these sediments. Only if this option is unavailable should other options be considered.

In zones at risk of containing ASS only some soil types will contain ASS, and it will only be necessary to conduct further sampling if the proposed drainage is likely to disturb significant amounts of ASS. Hump and hollow drainage is unlikely to disturb subsoil ASS and so acid sulfate soils do not need to be factored into the design. Deep trench drains may disturb ASS and so the drainage should be designed so that the ASS materials can be avoided. If this is not possible, a management plan should be developed to minimise environmental harm from the project and justification provided as to why the ASS cannot be avoided. If disturbance is unavoidable, field tests with laboratory analyses should be conducted. Once the location, depth and concentration of any ASS present on the site have been determined, options to manage the risk can be developed in accordance with the Tasmanian Acid Sulfate Soil Management Guidelines (DPIPWE 2009).

An example of a paddock drainage plan where different drainage solutions are required, depending on soil type and landscape position, is shown in Figure 17. The paddock is used for intensive crop production and so considerable investment in drainage is warranted. This includes a main open trench drain (A) as the main drainage outlet, subsurface pipe drains, mole drains and shallow surface drains. The main open drain required cleaning out with an excavator. Deep open drains should be constructed before underground or mole drains as the deep drains provide the ‘arteries’ for drained water to exit the landscape. Culverts and stock crossings are required where open drains cross paddock gateways, across constructed laneways and irrigator wheel tracks. This will incur considerable expense.

Underground pipe drains (areas B & C) are located at the base of slope where seepage occurs in areas with moderately well drained Cressy clay loam soils on the elevated ground and poorly drained Kinburn clay soils in depressions and on low-lying flats. Kinburn soils have clay textured topsoils with strong structure overlying grey massive clays. Underground drains should be installed with a trenchless machine with 100 mm diameter perforated drainage pipe installed. Many of the recommended underground drains are placed at approximately 60 m apart in order to optimise return on investment. However, following installation and a winter season, this spacing should be reviewed to determine the degree of improved drainage and if supplementary drains at 30 m spacing should be installed using either 100 mm or 65 mm diameter pipe.

Mole drains are recommended in areas with Kinburn soils (areas C) that have heavy clay subsoils. The mole drains should be installed at 2 m spacing. These mole drains should be installed after the underground pipe drains and should be drawn through the gravel overlying the underground drains.

Grassed waterways are recommended on duplex Brumby sandy loam soils (area D) as the area has minimal fall and these shallow surface drains will join up low depressions in the landscape and allow for the most effective removal of excess water. Saturated hydraulic conductivity is moderate in the topsoil and high in the A2 horizon of these duplex soils, but minimal in the subsoil heavy clays, resulting in seasonally perched water tables that sit on top of the heavy clay subsoils.

Dairying is undertaken on the property with over 500 cows milked under irrigation via a centre pivot and hard hose irrigators. The soils are derived from alluvium deposited on a low-lying flood plain. Soils are predominantly clay loams overlying light to medium massive clays classified as hydrosols and dermosols. There are some areas of loam topsoils overlying sandy clay loam subsoils on higher areas. The soils on the flats are normally poorly drained due to their low-lying position in the landscape. The clay textures and massive structure mean that these soils are slowly permeable, but field evidence indicates that underground drains will work well.

Drainage is critical in avoiding wet traffic (cows and tractors) operations. Deep open trenches are required to take large volumes of water away and mole drains are recommended when soils do not crack deeply in summer. Some of these soils have a self-mulching ability (the formation of a surface tilth comprising 0.5-1.0 cm aggregates created by repeated cycles of shrink/swell activity). Stocking at high moisture contents on these soils rapidly destroys the self-mulching characteristic and forms clods with very high soil strength. Stocking of winter forage crops is highly detrimental to the structure and future drainage of these soils.

The placement of many of the existing trench drains is appropriate as they occupy low parts of the landscape. Ensure that these drains remain clean in order to allow for free drainage of the large volumes of water that will be collected in the new underground pipe and mole drains. Outfalls for the drains are mainly to a local creek that can flood. The new open trench drains should be constructed before underground or mole drains as the deep drains provide the ‘arteries’ for drained water to exit the landscape. Culverts and stock crossing will be required where open drains cross paddock gateways, across constructed laneways and irrigator wheel tracks.

Underground pipe drains are often located at the base of slopes in areas with moderately well drained soils on the elevated river terraces and poorly drained clay soils in depressions and on low-lying flats. Many of these underground drain lines are curved as they follow the edges of naturally elevated terrace edges. Underground pipe drains should be installed with a trenchless machine with 100 mm diameter perforated drainage pipe installed. The recommended underground drains are placed at approximately 30 - 60 m apart, depending on the degree of waterlogging and the opportunity to combine with mole drains. Following installation and a winter season, this spacing should be reviewed to determine the degree of improved drainage and if supplementary drains should be installed.

Mole drains are recommended in flat areas with heavy clay soils that occur mainly on the right half of the plan. Soils on the left half of the plan have lighter textured subsoils with greater water conductivity. The soils with lighter subsoils will respond well to the underground pipe drains, but their lighter textures (lower clay content) make them unsuitable for mole drains. The mole drains should be installed at 2 m spacing. The mole drains should be installed after the underground pipe drains and should be drawn through the gravel overlying the underground drains.