Land Drainage

Colour photograph showing a 'hump and hollow' pipe sticking out of a raised area in a large green field.Drains are constructed and are important for carrying excess water off agricultural land. Without drains to remove water, the soil becomes waterlogged hampering the growth of crops and pasture. Drainage is essential in establishing good productive land.

There are various types of drainage systems. Before designing the drainage system it is important to consider what will be appropriate for your needs and where the drain is carrying the water to.

Proper planning, design, construction and maintenance of both surface and subsurface drains and channels will minimise the likelihood that they will cause environmental harm and alleviate some of their adverse effects. See Environmental Best Practice Guidelines

When to construct (or dig) drains

It is preferable to drain in the dry months to minimise muddying of downstream waterways. This will also increase the chance that the drains can be revegetated before water starts running again.

Drain Planning

Careful planning should be undertaken when planning drainage.

Things to consider:
  • Ensure the land can be drained.
  • Determine whether the soil types are able to sustain drainage.
  • Check the possibility of acid sulfate soils or salinity occurring.
  • Know where the drains will move water to. Will this impact on the downstream environment?
  • Know the land capabilities. Will the natural slope of the land sustain drainage? If so, can the drain be changed or modified to suit the application?
  • Gain knowledge of the likely flood events or extent of waterlogging. Will your drain have the capacity to pass the size of the flood or amount of water concerned? Find out about Floodplain Mapping for your area.
  • Don't over-design the drain.
  • Seek appropriate advice according to the prevailing circumstances. This may, in many circumstances, only require some levels to be taken. For larger drainage systems, in-depth soil analysis and design engineering might be required.

Drain Care and Environmental Considerations

Caring for drains is similar to caring for streams. Activities that occur in the catchments affect the stream once water discharges from the catchment.

Effective drain management is important for preventing environmental problems. Sediments, nutrients (fertilisers), herbicides, pesticides, organic wastes or pollutants washed into the drains flow into the stream and impact on water quality downstream.

Grass cover or other effective plant coverage of the drain should be encouraged to prevent eroding soil moving into the waterway. Grass assists in holding the banks together.

When constructing the drain, it is important to analyse the soil type, as some soils are more prone to erosion than others.

Steep, severe drain banks (batters) should be avoided in most situations. Banks at too steep an angle (batters) are more likely to erode than gentle gradients.

Runoff Speed and Erosion

Drains constructed in sloping country also have the potential to increase water velocity, thus an increase in energy is produced. Energy is caused by increasing the "speed" of the water flow and this has the potential to cause erosive affects, where "head cutting" (gouging out of the bottom and sides of the drain) and drain bank erosion can occur, resulting in sediment transportation and downstream siltation.

There is a tendency among those wishing to drain land to remove floodwaters as quickly as possible from the flood prone areas. Sometimes the "slowing down" of water being drained from an area is one of the most useful and effective methods of minimising the above-mentioned erosive affects. There are several methods that can be considered and applied, such as:
  • Retaining areas of bushland, particularly alongside drains, to slow runoff.
  • Increase the drain length by NOT straightening existing waterways, drains or creeks. This reduces the steepness of the drainage system and stops water running off too quickly, thus minimising erosion.
  • Construct paddock drains to achieve effective drainage, but on minimal gradients where possible.
  • Installation of the smallest but most effective possible outlets from drainage zones. This has the effect of reducing sediment and nutrient transportation by allowing a lesser discharge of water. This method is often used in the Hump and Hollow drainage application, but can be used with the more conventional open ditch applications.
  • Installation of simple rock riffles and drop structures. These can be placed in areas where erosion has or is likely to occur. Advice on these structures, particularly for larger applications, should be sought from technical experts. Engineers such as Department of Primary Industries, Water and Environment River Engineering staff and Regional Water Management Officers are a good first point of contact.

What Happens When You Drain Land?

The purpose of the drain must be ascertained so excess water can be moved without damaging the downstream environment. It is possible that removing a flooding problem by building a drainage system in an upstream area could cause, or add to, the severity of flooding downstream. This means the flooding problem has not been rectified, only moved.

Drains and Land Management

Stock
Stock access to the drains should also be controlled, just as stock access to riparian land needs to be managed, to prevent stream bank damage. Fencing drains prevents stock from damaging and depositing organic matter into the drain, and subsequently the catchment. This has the benefit of cutting down on maintenance costs associated with desilting the drain.

Weeds
Weed control methods should be undertaken as a yearly event and should be carefully planned. Failure to do so could result in drains becoming ineffective, and selection of the wrong methods for weed control can be an expensive mistake. If using chemical sprays, selection of the correct chemicals is critical to ensure effective weed control and is critical to ensure animals living in the drain are not killed. Some drains sustain life such as frogs and fish, some of which may be threatened species.

Maintenance
There is a need to control weed growth, other foliage growth and siltation in drains to maximise the effectiveness of the drainage system and to reduce further weed spread and flooding occurring.

Drains must be regularly inspected and maintained to achieve long-term effectiveness in performance for removing water.

A typical maintenance program should include:
  • spraying
  • removal of weeds
  • reduction of nutrient sources (that some weeds thrive on!)
  • reduction of sunlight by retaining tree growth alongside drains thus minimising weed infestations
  • inspection to see if stock have broken, and gone through, fences thus damaging drains.
Note: Carefully select herbicides if using chemical sprays, as many herbicides can kill in-stream life. There are several products that have now been produced to assist in these applications. See Rivercare Guideline for Herbicide use near water for details.

Environmental Best Practice Guidelines

Download the following environmental best practice guidelines before constructing new drainage systems.

Contact

RWMO Arthur-Pieman
Sam Morgan
GPO Box 44
HOBART TAS 7001
Phone: 03 6443 8608
Mobile: 0418 993 823
Fax: 03 6442 4569
Email: Sam.Morgan@dpipwe.tas.gov.au

Land Resource Assessment
Rob Moreton
171 Westbury Road
PROSPECT TAS 7250
Phone: 03 6777 2240
Fax: 03 6336 5365
Email: Rob.Moreton@dpipwe.tas.gov.au

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