Open deep drains

Page last updated: Monday, 20 August 2018 - 3:14pm

Open deep drains are deep excavated channels with sloping floors and sides, designed to intercept and drain surface (usually fresh) and subsurface water (usually saline in Western Australia) from agricultural land. These drains are most effective where the channel is dug into a watertable in soils that allow good lateral drainage.

We recommend that deep drains are part of an integrated water and salinity management program.

Note

An owner or occupier of land must give at least 90 days notice — to the Commissioner of Soil and Land Conservation — of an intent to drain subsurface water to control salinity and discharging that water onto other land, into other water or into a watercourse. The notice must be in writing using the notice of intent to drain or pump (NOID) form. Principles and guidelines for inland drainage are covered in the policy framework for inland drainage.

Drainage principles

These principles from the policy framework for inland drainage are used during the NOID process.

  • Drainage should be considered within an integrated catchment management framework, as part of the total water cycle, and quality and quantity of drainage water is managed.
  • Environmental impacts, positive and negative, should be identified and described at a level appropriate to the scale of the drainage proposal. Proposals should demonstrate an overall environmental benefit.
  • Public good from the proposal should be identified and be relative to the scale and risk of the proposal.
  • Best practice should be used, appropriate to the scale and risks of the proposal, for planning, design, consultation, construction and management.
  • Consultation and participation is expected for stakeholders affected by drainage proposals.
  • Costs for design, construction, operation and maintenance shall be identified, allocated and agreed for the life of the drainage proposal.
  • Where necessary (depends on scale of the proposal), there should be documentation of governance, financial arrangements, and intentions for detailed design, access, construction, operation, maintenance, monitoring and evaluation, and allocation of liabilities.

Where open deep drains can be used

  • on agricultural land where there are areas with suitable soils, low slope, high watertables, and waterlogging and/or surface salinity problems
  • where open deep drains can be used alone or with other practices
  • clear of flow lines, streams, creeks and rivers
  • constructed where a small quantity of surface water, that is not a channelled flow, enters the structure over the sideslopes, but is not diverted, concentrated or confined by the structure
  • where a suitable outlet is available to dispose of the quantity and quality of water collected.

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Planning considerations

Identify services that are likely to be affected by the construction of the planned works. Plans are discussed with the service provider to confirm ways of avoiding disturbance or damage.

  • Make allowance for vehicle, livestock and wildlife crossings. For very deep and steep-sided drains, consider escape structures for trapped livestock and wildlife.
  • Determine the depth, quality and quantity of the water to be intercepted where an open deep drain is to be installed to control subsurface flows. The texture of the water-bearing subsoil will determine permeability and flow rates.
  • To create sufficient drawdown over a large area, multiple drains may be required at a calculated spacing.
  • Establish the stability of the inlet, contained flow and outlet where an open deep drain is installed to control flooding and waterlogging.
  • The structure should not interfere with, divert or diminish the natural flow characteristics of any flow path.

Site assessment is critical for successful deep drains

The site assessment is the most important part in planning a deep drain.

With a backhoe, put in about 5 pits per kilometre of the intended drain alignment to 2.5–3m depth.

In each pit observe:

  • soil horizons and soil type:
    • Is the soil sandy or clayey? Heavy clay is less permeable and less suitable for deep drainage.
    • From which horizon does the water flow? This will let you know how deep your drain may have to be.
  • soil characteristics, which will give you an idea of the drain's effectiveness:
    • possible barriers across the drain — dolerite dykes, coffee rock or tight clay horizons will make construction difficult
  • time taken to establish a steady flow of water into the pit:
    • in 1 hour, the soil has good permeability
    • in 12 hours, the soil has average permeability
    • in 24 hours, the soil has poor permeability and it is unlikely that a deep drain will be effective because of the low flow rates
  • water quality in the drain:
    • salinity level — a high electrical conductivity (EC) reading will determine where the drain water can be safely discharged
    • acidity and heavy metal contamination — some groundwater can be highly acidic and can be naturally contaminated with heavy metals. This will determine where the drain water can be safely discharged.​

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Design criteria

Site characteristics are accurately measured.

Soil types are determined and tested at the construction site to ensure the stability of the proposed structure.

Catchment peak flow run-off is determined using a recognised method, such as the Flood Index Method or the Rational Method (see relevant chapters in the Australian Rainfall and Runoff site). Knowing the peak flow will confirm the likelihood of the structure overflowing safely and the frequency of filling.

Roughness coefficient — the choice of roughness coefficient must be appropriate for the section of channel being planned. Channel depth/width ratios must be calculated for each change in the channel condition.

Velocity — please refer to the suggested maximum permissable velocities of flow table because design should not exceed these values.

Grade of channel is constant and no greater than 0.2%. As grade increases, the velocity increases proportionally.

Planning methodology — use the Manning formula and roughness coefficients to calculate channel flow depth for channels with fixed widths. Also use this method where channel width is variable.

If required, test holes or drilling will determine the hydraulic conductivity and quality of subsurface flow. Darcy’s Law is used to determine the flow rate.

Capacity is sufficient to contain the peak flow run-off from a 20-year average recurrence interval (ARI) storm. Once the design ARI is exceeded, the channel will overflow. Greater capacity can only be built into the channel by replanning for increased flows from a greater ARI. This may be necessary if the overflow from a planned channel will cause other than superficial damage.

Drain spacing is determined by a recognised method, such as the steady state ellipse equation, if drains are to be installed to lower a watertable over a large area. See the Department of Water and Environmental Regulation's cone of depression online calculator.

Channel cross-sectional shape is trapezoidal or square. The designed cross-sectional area will vary with the velocity and quantity of water to be contained.

Diagram of typical cross-section of an open deep drain channel
Typical cross-section of an open deep drain channel

Maximum drain length within a layout is not to exceed 5000 metres for an individual drain or the longest drain. Bends in the levee should be avoided. Where this is not practical, bends must not be acute. Acute bends lead to undue hydraulic stress on the side slopes of the channel.

Channel base width is determined by design criteria and may be single excavator bucket width, or multiples of bucket width.

Sideslope ratio to be determined from soil type and drain depth, as suggested in the following table.

Soil type

Shallow channels up to 1.2m deep

(horizontal : vertical)

Deep channels 1.2m and deeper

(horizontal : vertical)

Sand – clayey sand

2 : 1

3 : 1

Sandy loam – silt load

1.5 : 1 2 : 1

Sandy clay loam – light clay

1 : 1 1.5 : 1

Light medium clay – heavy clay

0.5 : 1 1 : 1

Depth is to about 2 or 3 metres.

Cross fall of channel floor is zero ideally.

Spoil banks are placed clear of the excavation, creating a berm or ledge between the channel and spoil. Spoil banks should not be used as levees on channels that contain overland flows unless they have been adequately engineered to overflow safely. Flows greater than the ARI design contained by those levees will have a greater depth and proportionally greater velocity. This could exceed the original design criteria and cause channel failure. Berms allow access for machinery maintaining the channel and help prevent spoil from washing or rolling into the channel. To eliminate slumping, spoil is shaped into a bank with sideslope ratios as suggested below.

Cross-section of an open deep drain showing channel, berm and spoil
Cross-section of an open deep drain showing channel, berm and spoil

Outlets shall be stable and of sufficient capacity to prevent ponding or erosion. The outlet can be another channel or natural waterway.

Vegetation cover should be encouraged on spoil, berm and channel sideslopes. Annual or perennial pasture grass species or a mixture of both can be scattered onto these areas. If trees and shrubs are to be incorporated, they should be planted outside any disturbed area, not compromise surface water earthworks, and allow sufficient space for maintenance access in the long term.

Fencing, where necessary, should be outside the spoil banks, leaving enough room for maintenance machinery.

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Legal and environmental aspects

Legal aspects: common and statute

Common law rules govern the flow of surface water from artificial drains and underground water that is discharged into watercourses. Artificial drainage must be constructed so it does not have a detrimental effect on streams further down the catchment.

Take reasonable steps to ensure the safety of another person and another person's property.

Consider the effect that planned earthworks will have on other people and seek consent from any person that may be affected.

Due care must be taken during construction and maintenance to stop the loss of disturbed material from the site.

Under Regulation 5 of the Soil and Land Conservation Act 1945, the land owner or occupier proposing to 'drain or pump water from under the land surface because of salinity of the land or the water, and to discharge that water onto other land, into other water or into a watercourse' must notify the Commissioner of Soil and Land Conservation (the Commissioner) of that intent at least 90 days before discharging the water, using the notice of intent to drain or pump water (NOID) form.

For proposals to drain land within the Peel–Harvey Catchment, proponents must submit a notice of intent to drain or pump water (NOID) to the Commissioner at least 90 days before draining surface or subsurface water onto other land, into other water or into a watercourse

Interference with a watercourse in a proclaimed surface water management area is controlled under the provisions of the Rights in Water and Irrigation Act 1914 (WA).

The Occupational Safety and Health Act 1984 sets objectives to promote and improve occupational safety and health standards. This Act sets out broad duties and is supported by more detailed requirements in the Occupational Safety and Health Regulations 1996.

The legislation is further supported by guidance material such as approved Codes of Practice through WorkSafe Western Australia. ‘Code of Practice – Excavations’ applies to all workplaces where excavation occurs, and particularly when 'a person is required to work in an excavated area or other opening in the ground that is at least 1.5 metres deep'.

Environmental aspects

Flows of poor quality water (saline, acid, heavy metals) can degrade channels, watercourses and wetlands for many kilometres downstream of the discharge point. Refer to the Department of Water and Environmental Regulation's acid sulfate soils website.

Eroded material from poorly planned, constructed or maintained drainage systems can reduce flow capacities when deposited in downstream channels.

Deep drains are effective barriers for livestock and a lot of wildlife, and a trap for animals that may fall into the drain. Fencing can keep larger animals out, but smaller animals falling into the drain will need some form of escape chute at regular intervals. This is especially important if deep drains cross areas of bush or vegetation corridors.

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Construction

  • The drains are usually constructed with an excavator. A bulldozer or grader may be used to shape the spoil into levees and provide a channel for surface flows.
  • The centre of the channel will be pegged for alignment as indicated on the plan. Cut and/or slope stake will be surveyed to indicate the depth to the channel floor.
  • Channel will be excavated to line and on grade as detailed on the plan. Confirm and/or check and maintain the channel grade by using a level, laser level or boning rods.
  • Sideslope ratios can be confirmed by using an electronic builders slope finder or battometer.
  • Spoil shall be placed and shaped as indicated on the plan.
  • Design and implement suitable crossings and culverts, especially if the drains cross roads.

Operation and maintenance

  • Follow the agreed monitoring and evaluation schedule, in accordance with any approvals or conditions.
  • Reconstruct damaged channel sideslopes or spoils banks to original specifications.
  • Clean deep drains after about 5 years to remove mud and chemical seals on the side walls which block discharge into the drain.
  • Remove silt or eroded material from the channel and incorporate into spoils banks to original specifications.
  • Regularly inspect piped inlets, if incorporated, to remove debris and to check for damage.

Reference material

  • Bligh, KJ 1989, Soil conservation earthworks design manual, Department of Agriculture, Western Australia, Perth.
  • Clement, J, Bennett, M, Kwaymullina, A & Gardner A 2001, The law of landcare in Western Australia, 2nd edn, Environmental Defender’s Office WA (Inc), Perth.
  • Keen, MG 1998, Common conservation works used in Western Australia, Agriculture Western Australia, Perth.
  • Keen, MG 2001, Field pocket book of conservation earthworks formulae and tables, Department of Agriculture, Western Australia, Perth.
  • McCuen, RH 1989, Hydrologic analysis and design, 2nd edn, Department of Engineering, University of Maryland, Prentice Hall, Upper Saddle River, New Jersey, USA.
  • Moore, GA 2001, Soil guide: a handbook for understanding and managing agricultural soils, Bulletin 4343, Department of Agriculture and Food, Western Australia, Perth.
  • Schwab, GO, Fangmeier, D, Elliot, W, & Frevert, R 1992, Soil and water conservation engineering, 4th edn, John Wiley and Sons Inc., Toronto, Canada.

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Planning, pegging and construction skills

National competencies exist for the conservation earthworks industry and are detailed as part of the Agriculture, Horticulture and Conservation and Land Management Training Package.

The relevant qualification is:

  • AHC32316: Certificate III in Conservation Earthworks

Contact information

Richard George
+61 (0)8 9780 6296
Commissioner Soil and Land