Deep drainage (groundwater drains) for salinity management in Western Australia

Page last updated: Friday, 9 September 2022 - 11:45am

Please note: This content may be out of date and is currently under review.

This page outlines some simple guidelines and investigations that should be undertaken before designing a groundwater drainage system and submitting a Notice of Intent to Drain: where to put them; site assessment, design and construction guidelines; working with contractors.

The Department of Primary Industries and Regional Development provides this information to help managers to use best practice in groundwater drainage, and comply with the Soil and Land Conservation Regulations 1992.

About deep drainage

For a start, deep drains that intersect and drain groundwater (fresh or saline) are called groundwater drains. The rest of this page is about using deep drains for salinity management, and will only refer to groundwater drains.

Please note that the term 'deep drains' is not necessarily accurate – drains do not need to be deep to drain groundwater. If groundwater is very shallow, then the drains do not need to be very deep. This is one reason we should be using the term groundwater drains instead of deep drains.

Before planning groundwater drainage works, all of the options. Landowners proposing to drain or pump groundwater for the purpose of controlling salinity are required to lodge a Notice of Intent to Drain with the Commissioner of Soil and Land Conservation at least 90 days before discharging water. This is to ensure that drains are well designed and salinity, waterlogging, sedimentation and other onsite and/or offsite impacts are minimised. (Soil and Land Conservation Regulations 1992)

The views of your downstream neighbours are important and a clearing permit may also be required if native vegetation is to be cleared.

Sub-surface, or groundwater drains, can be very costly to install and to maintain. The information below will help to assess which water management options are best suited for you and your farm.

Water management options include:

  • managing surface water
  • using perennials that can take advantage of shallow water (including saltbush and saltland pastures)
  • groundwater drains
  • do nothing – not recommended.

We recommend managing surface water before constructing groundwater drainage, as surface water can be a valuable resource. Uncontrolled surface water can lead to waterlogging, recharge of groundwater, and damage groundwater drains.

The benefits of surface water management over groundwater drains are:

  • relatively low cost of construction
  • surface run-off water is usually good quality and can be stored in farm dams or directed into natural watercourses
  • groundwater recharge is reduced
  • peak stream flow in waterways is reduced, and a possible reduction in erosion, flooding and sedimentation
  • minimal maintenance
  • a notification of intent to drain is not required
  • reduced waterlogging can decrease the effect of salt in the soil profile.

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Characteristics of groundwater drains

A groundwater drain is a channel which intercepts and drains groundwater. It can be 'open' to allow the inflow of surface water or 'leveed' to exclude surface water. Leveed groundwater drains are usually preferred because they reduce erosion, prevent surface water entering the drain, and the risk of drain batters collapsing is lessened.

Purpose of groundwater drains

  • to capture and move sub-surface seepage and groundwater
  • to lower the watertable
  • to reduce waterlogging and surface salinity levels
  • to ensure discharge is sent to a safe disposal point.

Where are they put in?

  • They are constructed in lower parts of the landscape such as valley floors and footslopes and on the coastal plain (where the slope is less than 0.5%).
  • They are best constructed in soils with relatively high permeability. 
  • They can also follow the alignment of natural drainage lines, but not in the actual drainage line, as large flows can damage the drain.

How do they work?

  • When excavated to below the watertable, groundwater seeps into the drain. The watertable adjacent to the drain will fall. This effect decreases with distance from the drain. This is the drawdown effect.
  • The area of land adjacent to the drain which is affected, and the amount the watertable is lowered depends on:
    • transmissivity of soil in the intersected watertable
    • the depth the drain is excavated below the watertable
    • the rate that the groundwater is being replenished.
  • The land being drained needs rainfall to leach salt from the upper soil profile.
  • A drawback to removal of salts from the upper profile is that it may lead to erosion of dispersive soils. Groundwater drains in dispersive soils should always be leveed, to prevent fresh surface water eroding the drain banks. See managing dispersive soils for more information.

​Factors to consider before construction

  • existing drainage lines
  • property infrastructure
  • services (power, phone, water lines) both on-farm and off-farm if the drain will cross roads
  • intended disposal point
  • other drains in the area — yours may be linked
  • downstream assets including those dependent upon surface water flows
  • possibility of acid drainage water and acid sulphate soils.

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Site assessment

Site assessment is the most important part in planning a groundwater drain.

With a backhoe, put in approximately 5 pits per kilometre of the intended drain alignment, to 2.5–3 m depth. In each pit observe:

  • Soil horizons and soil type
    • is the soil sandy or clayey?
    • soil characteristics will give you an idea of how effective your drain will be.
  • Possible barriers:
    • dolerite dykes, coffee rock or tight clay horizons.
  • Which horizon does the water flow from?
    • this will let you know how deep your drain may have to be.
  • What is the flow rate of the water into the pits?
    • in one hour the soil has good permeability
    • in 12 hours the soil has average permeability
    • in 24 hours the soil has poor permeability
    • if the soil has poor permeability, flow rates will be low and it is unlikely that a groundwater drain will be effective.
  • EC and pH of the water.
    • a high EC reading (indicating saline water) can affect where the drain water can be discharged.
    • some groundwater can be highly acidic which may also impact on where or how the water can be discharged.​ Highly acidic water may also contain high levels of toxic heavy metals. We recommend having very acid water tested for these contaminants. 

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Design considerations and costs

The Department of Primary Industries and Regional Development (DPIRD) has developed guidelines for construction of leveed groundwater drains and open groundwater drains. These provide information on all aspects of groundwater drainage planning and construction.

Some of the more important points to consider are:

  • Seek more than one quote as construction rates can vary from $3500 to $8000/km, depending upon the design of the drain and presence of hardpans.
  • Do not construct groundwater drains in the natural drainage line.
  • Drain must be deep enough to intersect the watertable.
  • Gradient of the drain should be no more than 0.5%.
  • Spoil banks should be shaped so that surface run-off flows to outer sides of banks and not into the drain.
  • Ponded water outside the spoil bank can be discharged into the drain by installing pipes that extend through the spoil banks.
  • Groundwater drains must be desilted after significant storm events because of sedimentation and slumping of banks. The cost of desilting a deep drain is about $1500/km every five years.
  • Is there any local evidence that the type of saltland to be drained has been reclaimed? Severely salt affected land, and land that has been salt affected for more than 20 years, is unlikely to show rapid recovery after drainage construction.

Working with contractors

The information on this page is a guideline for good practice in construction of groundwater drains. More detailed guidelines are given in the leveed groundwater drains and open groundwater drains web pages. We recommend that land managers and contractors are familiar with these guidelines, and adhere to them as much as possible in the design and construction of groundwater drains.

We recommend that land managers discuss their requirements and standards with contractors, document those, and that both parties are aware of the consequences of:

DPIRD provides a pre-Notice of Intent to Drain service, in which an experienced officer will meet on-site to assess the proposed drainage area and discuss your drainage options with you. They will also run through the Notice of Intent to Drain process with you.

Contact information

Commissioner Soil and Land
John Simons
+61 (0)8 9083 1128