Grade banks

Page last updated: Thursday, 1 November 2018 - 2:43pm

We recommend constructing grade banks to intercept and divert surface water run-off into storage or waterways, to limit soil erosion. Grade banks are usually designed as a set in the middle and upper slopes of hills that have a slope of 2% to 10%. Where possible, broad-based banks are preferred, to allow cropping and other machinery to work across them.

We recommend that grade banks are part of an integrated water and salinity management program.

Why use grade banks?

Grade banks are an effective and efficient ways of reducing the risk of surface water run-off causing erosion, flooding and waterlogging. Grade banks can also direct surface water into farm dams for farm-water self-sufficiency. Minimum-tillage systems and reduced rainfall in the agricultural areas of Western Australia have decreased surface water run-off and the likelihood of erosion from winter rainfall.

However, summer storms are a major cause of serious soil erosion, and conventional or broad-based banks are recommended on susceptible sites.

To collect reliable water supplies for storage in farm dams, we recommend using roaded catchments rather than grade banks.

Benefits of grade banks

Grade banks can:

  • reduce erosion from surface water run-off
  • reduce sediment content in run-off
  • reduce waterlogging at the base of slopes
  • allow collection and re-use of surface water
  • lead to increased crop and pasture yields where seasonal waterlogging occurs
  • reduce flood peak flows.

Concerns with grade banks

Grade banks can have some problems, including:

  • obstructing traffic movement (broad-based banks may overcome this)
  • limiting the efficiency of precision cropping (odd-shaped cropping areas may increase overlaps or gaps)
  • increased maintenance requirements, especially where livestock of machinery cross, to prevent overflow
  • finding safe disposal points.

How grade banks work

In periods of heavy rainfall, there is often water running downhill on the surface which can cause soil erosion. As water runs down the slope, the area contributing run-off increases, leading to an increase in run-off depth and speed and erosivity of the water. Grade banks are designed to intercept this run-off before it reaches a depth and speed likely to cause erosion.

Because the slope of a grade bank is less than the slope of a hill (banks run across the slope), run-off takes longer to move downslope and the peak flow is reduced.

Grade banks need a safe disposal system, which can be a natural waterway, a constructed grassed waterway, or into a dam.

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Planning a system of grade banks

Conditions suiting grade banks

Grade banks can be constructed in the middle and upper parts of the landscape, on any agricultural land with a slope between 2% and 10%, where:

  • suitable soils exist
  • erosion is a problem
  • flood peak flows need to be reduced
  • topography allows the practice to be implemented
  • a stable outlet can be provided.

Planning considerations

Grade banks are most effective when they are part of a whole-farm plan. Points to include in the plan are:

  • the area and slope of land at risk from erosion; assess local rainfall records and local experience
  • services and infrastructure likely to be affected by the construction of planned works (vehicle movement, tracks, fences, power, pipes and cables)
  • existing waterways and dams for safe run-off disposal
  • integration with other surface water management and earthworks
  • integration with machinery movement and land use; precision agriculture and autosteer systems often require parallel borders.

Grade banks may not be suitable:

  • on land sloping less than 2%; using a flat grade in these areas may allow the construction of banks on land sloping as little as 1%
  • on slopes greater than 10% because of the depth of cut of the uphill side slopes of the channel; stable construction of the bank is also difficult to achieve on these slopes.

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Main design characteristics

  • Grade banks on long slopes are usually spaced 50 to 220m apart. Banks are closer on steeper slopes, where storm events are more common, and if the soil is more erodible. We recommend you consult a trained operator or adviser using published charts and tables to calculate spacing.
  • For general erosion control, the capacity of the bank is to carry the run-off expected from the most severe storm in an average 10–year period. This is referred to as the 1 in 10 average recurrence interval (ARI). For control of serious peak flooding, an ARI of 20 years or greater may be needed.
  • Grade banks should have a flat channel floor up to 3.5m wide, with sloping batters between 1:3 and 1:6, depending on slope and soil type. Broad-based grade banks allow cropping and vehicle movement across the banks which may mean that more maintenance is required.
  • On land slopes of about 2%, channel depth of the bank should be 0.4m from the water level (when the bank is full) to the floor of the channel. For slopes between 2% and 5%, the depth should be 0.5m, and for slopes between 5 and 10%, the depth should be 0.6m.
  • The bank (on the lower side) should have a freeboard of 0.2m above water level when the channel is full.
  • The recommended channel gradient is between 0.2% and 0.5% (20cm to 50cm fall in 100m). A standard grade is 0.4%, while 0.5% is steep and is used on steep slopes. Use a grade of 0.2% for land with slopes flatter than 2%.
  • Channel length should not exceed 1000m. A single, 1000m channel can capture run-off from a catchment area of up to 25 hectares, depending on rainfall, run-off and erosion risk. The higher the rainfall, run-off and erosion risk, the smaller the area of catchment protected.
  • Outlets from banks must be stable and able to handle peak discharge. Waterways used as outlets must be vegetated; grassed waterways protected from grazing are recommended. Dams can be used to hold discharge, if they have sufficient storage and a safe overflow. Level sills at the outlet end of the bank can be used where discharge is required to disperse across land before entering a stream. Average sill length is 10m. Longer sills of up to 15m are required for steeper slopes or where outlet flow velocities need to be reduced.

Legal aspects and downstream properties

Landholders and contractors constructing or operating grade banks have a duty of care to ensure that reasonable steps are taken to prevent harm being caused to another person, property or environment.

Seek consent from any person, agency or local government authority that may be affected by the construction of grade banks and discharge of run-off.

Grade banks must not divert flows from one catchment to another catchment that would not naturally receive that flow.

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

Environmental aspects

Flows of poor quality water can degrade downstream channels, watercourses and wetlands.

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

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Construction

These guidelines apply to grade banks constructed by bulldozers or graders, and single and multiple banks planned and constructed as a system.

Use optical or laser levelling to survey and align the bank on a correct grade. Level to an accuracy of plus or minus 5cm or better, and place alignment marks (pins or pegs).

The survey interval, to establish alignment marks, should be no greater than 25m. The survey hand can pace horizontal distances as long as the intervals of a person’s pace are predetermined for each survey and checked at least twice daily.

Extra alignment marks are required on tight bends, particularly when crossing ridges and small depressions. These aid machinery operations.

When no level sill is constructed, extra grade is included at the bank outlet to allow the grade bank channel to return to ground level. This reduces channel ponding at the outlet. As a guide, grade is doubled for 60m at the outlet end of the grade bank.

Construction machinery should have suitable capacity, horsepower and appropriate attachments. Bulldozers and graders are suitable. Smaller bulldozers (equivalent to Caterpillar D5 or D6) with multiple shank rippers are more efficient. Graders of medium size (approximately equivalent to Caterpillar 120G or 12G to 140 or 140G) with rear-mounted multiple shank rippers are most efficient.

Construction technique using a bulldozer

  1. Mark alignment with ripper shank.
  2. If any of the channel is to be ripped, rip whole alignment where channel is to be excavated.
  3. Form uphill sideslope with one push and transport spoil beyond survey line.
  4. Run uphill sideslope off.
  5. Rip channel again if necessary.
  6. Two or three cross-sectional pushes to create flat-bottomed channel and bank.
  7. Roll bulldozer over excavated earth to compact bank.
  8. Run-off to clear excavation and form channel and bank sideslopes.

Construction technique using a grader

  1. Mark alignment with ripper shank.
  2. Remove topsoil.
  3. If any of the channel is to be ripped, rip whole alignment where channel is to be excavated.
  4. Excavate channel and channel's uphill sideslope, battering each windrow into and up bank to form bank's uphill sideslope (grader compacts bank with battering of each windrow).
  5. Form downhill sideslope of bank.

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Operation and maintenance

Grade bank channel, bank and/or sideslopes damaged by livestock or run-off should be repaired to original construction standards as soon as practicable.

Breached banks should be repaired as soon as possible after the damage occurs. Breaches should be filled with compacted material of the same quality or better than that used in the original bank construction.

Reconstructive maintenance may only be needed at 5 or more year intervals for correctly constructed grade banks, with adequate channel capacity, correct sideslopes and adequate bank compacted freeboard.

Plans and specifications

Plans and specifications shall conform to this standard and describe what is required to ensure the practice achieves its intended purpose. Copies of the plan are to be presented to landholder and construction contractor.

Sources of information

  • Ball, J, Babister, M, Nathan, R, Weeks, W, Weinmann, E, Retallick, M & Testoni, I (Editors) 2016, Australian Rainfall and Runoff: A Guide to Flood Estimation, Commonwealth of Australia, arr.ga.gov.au/arr-guideline.
  • 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.
  • Hudson, N 1992, Soil conservation, BT Batsford Ltd., London, England.
  • Schwab, GO, Fangmeier, D & Elliot, W 1995, Soil and water management systems, 4th edn, John Wiley and Sons Inc., Toronto, Canada.
  • Schwab, GO, Fangmeier, D, Elliot, W, & Frevert, R 1992, Soil and water conservation engineering, 4th edn, John Wiley and Sons Inc., Toronto, Canada.

Contact information

Paul Galloway
+61 (0)8 9083 1127
Tim Overheu
+61 (0)8 9892 8533