Shallow relief drains

Page last updated: Wednesday, 1 May 2019 - 2:52pm

Shallow relief drains are effective at removing surface water from flooded land. Removing flood water reduces waterlogging and groundwater recharge and improves productivity of the soil. This page provides guidelines and design criteria for effective shallow relief drains.

We recommend that surface water management is part of an integrated water and salinity management program.

What is a shallow relief drain?

A shallow relief drain is a shallow channel designed to manage run-off on flat, low-lying farmland and to remove water from areas affected by inundation, waterlogging or flooding. There are different types of shallow relief drains and they are defined by the shape of their channel. These are the U- (or spoon) drain, W-drain, and V-drain, as well as trapezoidal-shaped channels (see Figures 1, 2 and 3 below). For cropping areas, shallow relief drains can be used in combination with raised beds.

Why use shallow relief drains?

Shallow relief drains are easy to construct and a cost-effective means to:

  • remove surface water to a safe disposal area, to reduce the length of time the land is inundated or flooded
  • reduce the potential for localised recharge and the development of secondary salinity
  • remove accumulated surface water from depressions and low-lying land
  • improve the continuity of flow within the catchment along the valley floor
  • divert run-off from land that is prone to inundation or flooding.

W-drains

A W-drain has 2 parallel, flat-bottomed channels, with the spoil from the channels placed between them to form a bank, giving the structure a ‘W’ shape (Figure 1). Surface water can enter the 2 channels on either side of the common spoil bank. The spoil bank can be formed into an access road or track and in some cases cropped. W-drains have been widely used across the agricultural areas with good results.

Diagram of a W-drain cross-section showing channels and central mound
Figure 1 W-drains collect water from both sides and remove the problem of spoil blocking water movement into the channel

U- (or spoon) and V-drains

U- (Figure 2) and V- (Figure 3) shaped drains have a single channel. The excavated soil is spread on either side of the channel in as thin a layer as possible during construction to ensure that shallow surface flows have the best access to the channel.

Diagram of U- (or spoon) drain cross-section showing the broad, rounded channel with spoil on both sides
Figure 2 U- (or spoon) drain cross-section showing the broad, rounded channel with spoil on both sides
Diagram of a V-drain cross-section showing a single channel and spoil on either side
Figure 3 V-drain cross-section showing a single channel and spoil on either side. The spoil heaps are not continuous to allow water easy access to the channel from either side

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Best time to mark channel lines

The best time to plan shallow relief drains is after heavy rain, when inundated areas are easily seen and the lowest points in the landscape can be located. These points can be pegged and levels surveyed later to work out the best layout and drain depth for the scheme.

Best time to construct shallow relief drains

The best time is:

  • when you can get machinery access and not get bogged
  • when the soil is moist but not wet
  • when there is still good groundcover on undisturbed soil to prevent erosion
  • when the risk of damage from summer storms is less.

This generally means the most suitable time is late spring or early summer (depending on the site). Many shallow drains are also constructed in late autumn to early winter when the soil is slightly moist and some groundcover has developed.

Conditions and considerations

Conditions where shallow relief drains can be applied

Shallow relief drains can be applied:

  • in the lower portion of the landscape
  • clear of floodplains, where peak flows would compromise the stability of the structure or where the structure would interfere with natural flows
  • where suitable soils are available (Note: sodic-dispersive soils will erode with fresh water)
  • where a stable outlet is available to dispose of the quantity of water collected.

Caution: Where dispersive (sodic) soils are exposed in shallow relief drains, there is an increased risk of erosion. Please seek local advice on management options, and see Managing waterlogged dispersive (sodic) soils for more information.

Planning considerations

  • Use surface water management structures upslope to prevent flooding of the valley floor.
  • For large drains and systems, we recommend that a trained and accredited person be employed for planning and construction supervision.
  • The route selected for the drain should be through the lowest portion of the landscape.
  • Identify services that could be disturbed or damaged by the construction of the planned works. Discuss the plan with your service provider to confirm ways of avoiding disturbance or damage.
  • To create a sufficient drainage effect over a large area, multiple drains may be required to link all flooded areas.
  • Heavy-rainfall summer storms are forecast to increase with climate change in Western Australia's agricultural areas. Design should account for this, especially where drains converge.

Construction

Shallow drains are usually constructed with a road grader or scraper.

  • Scrapers are generally used to construct shallow trapezoidal shaped drains. The advantage of using a scraper is that it can transport the spoil from the drainage site and use it to fill in depressions elsewhere in the paddock.
  • Road graders are used mainly to build relatively shallow W-drains or U-drains. They can also be used to spread the spoil in a thin layer on either side of a U-drain.
  • The centre of the channel/structure will be pegged for alignment as indicated on the plan.
  • The levels of the alignment will be checked to identify ‘highs’ and ‘lows’. Cut stakes or slope stakes to be used as necessary.
  • Channel shall be excavated to line and on grade as detailed on the plan. Confirm and maintain the channel grade by using a laser level.
  • Sideslope ratios can be confirmed by using an electronic builders slope finder or battometer.
  • Place and shape spoil as indicated on the plan.

Operation and maintenance

  • Reconstruct damaged channel sideslopes or spoil banks (W-drains) to the original specifications.
  • Remove or incorporate silt or eroded material removed from the channel into the spoil bank (W-drain) to the original specifications.

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Design guidelines

These guidelines will help you plan and construct effective shallow relief drains.

Site characteristics: are accurately measured.

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

Planning methodology: use Manning’s formula and roughness coefficients to confirm the channel flow depth to velocity ratio will not cause erosion of the structure.

Catchment peak flow run-off: to be 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 frequency of filling.

Capacity: sufficient to contain the peak flow run-off from a 5 to 10-year average return interval (ARI) storm; once the design ARI is exceeded, the channel will overflow.

Roughness coefficient: the choice of roughness coefficient must be appropriate for the section of channel being planned; channel depth to width ratios must be calculated for each change in channel condition.

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

Grade of channel: no greater than 0.2%; as grade increases, velocity increases proportionally.

Channel cross-section: shallow trapezoidal, W- or U- (parabolic) shaped; the 'V' shape is only used for very small flows.

Sideslope ratio: for trapezoidal, 'W' and 'V' channel shapes should be 6 in 1.

Cross fall of channel floor: ideally to be zero; for W-drains, both channels should be at the same elevation and have no cross fall.

Maximum drain length: within a layout is not to exceed 5000 metres for an individual drain or the longest drain.

Depth: usually about 0.3 metres, but no greater than 0.5 metres.

Channels base widths: may be single grader blade cutting width or scraper bowl width, or multiples of blade/bowl widths; W-drain channels should be the same width.

Spoil banks: for channel cross-sectional shapes other than ‘W’, spoil should be removed from the drainage site or ‘run off’ so they do not interfere with flows; W-drain spoil is placed and shaped between the channels.

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 and sideslopes.

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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 discharged into watercourses. Artificial drainage must be constructed so it does not have a detrimental effect on streams further down the catchment.

Environmental aspects

Flows of poor quality water can degrade downstream channels, watercourses and wetlands. Eroded material from poorly planned, constructed or maintained drainage systems can reduce flow capacities when deposited in downstream channels.

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.
  • Houghton, PD & Charman, PEV 1986, Glossary of terms used in soil conservation, Soil Conservation Service of New South Wales.
  • Keen, MG 1998, Common conservation works used in Western Australia, Agriculture Western Australia, Western Australia, Perth.
  • Keen, MG 2001, Field pocket book of conservation earthworks formulae and tables, Department of Agriculture, Western Australia, Perth.
  • 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.

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

Tilwin Westrup
+61 (0)8 9780 6165