Why use a grade bank?
Minimum tillage and reduced rainfall in winter – amount and intensity – have led some people to think that grade banks are not necessary anymore: this is not the case.
Erosive rainfall from summer storms is more likely to cause erosion than is winter rainfall, and the forecast changing climate is for more summer storms. Summer storms have higher intensity rainfall, with more run-off and a higher risk of erosion.
Grade banks are a practical way of reducing the risk of water erosion. Broad-based banks have advantages for modern cropping systems.
What is a broad-based bank?
A broad-based bank is an earth embankment with an upslope channel that is constructed on a grade. It is designed to intercept run-off from sloping land and divert the water into a farm dam or waterway. A broad-based bank has a wide, flat-bottomed channel, a low downslope spoil bank, and gentle batters. This gives the structure a smooth cross-section with a low profile and a large capacity channel. The low profile allows for stock, farm machinery and vehicle passage across the bank and allows tillage over the bank and harvesting along the length of the bank batters and channel.
Broad-based banks are designed to safely overflow in very wet years, and should be part of an integrated surface water management plan.
Where to use broad-based banks
Broad-based banks are especially suited to medium to low rainfall cropping systems using minimum tillage and controlled traffic farming (CTF).
Broad-based banks are best used in the upper and middle parts of the landscape where the land slopes between 2% and 6%. The banks are more easily constructed in soils with uniform or gradational profiles, because removing topsoil during construction then replacing is often not required in these soil types. Broad-based banks are more suitable than conventional grade banks on dispersive soils because the wider batter and consolidated bank is more resistant to tunnel erosion.
Benefits of broad-based banks
Broad-based banks have the advantages of conventional banks in controlling surface water flow, plus allow machinery to work across the banks for cropping systems. Broad-based banks can:
- control run-off on sloping land to reduce the potential for soil erosion
- decrease the extent of flooding and inundation in the lower landscape by reducing the rate of run-off accumulation (or peak flow) from the slopes on which they are constructed
- reduce the potential for salinity and waterlogging downslope by diverting run-off away from affected areas
- convey larger quantities of water in the larger capacity channel
- allow no-till or minimum-till cropping over the bank batters and channel
- improve ease of weed control on the banks
- allow machinery, vehicle and livestock access over the bank
- harvest water if compacted banks are used
- improve the appearance of paddocks and farm landscapes as an alternative to more incised grade banks.
Risks with broad-based banks
- The channel will be prone to erosion if the base is not flat, too narrow or the grade is too steep.
- Poor planning and design of a broad-based bank can lead to overtopping and possible failure of the structure, as well as soil erosion between banks.
- Machinery and livestock tracks across the bank can lead to low points and bank failure (erosion). This is of particular concern in controlled traffic farming systems.
- Recharge to groundwater will occur in sandy or gravelly soils, or where water can pond within the structure.
How they work
As water runs downslope it increases in depth and velocity as it gathers more flow. Once the velocity of flow becomes high enough to detach and move soil particles, erosion occurs. Broad-based banks intercept run-off water before it becomes erosive and diverts it to a safe waterway or retention structure to prevent erosion.
Broad-based banks slow run-off and reduce peak flows. As run-off is channelled through a system of broad-based banks, it takes longer for water to move downslope. By retarding the flow, broad-based banks extend run-off duration and reduce flash flooding in the lower landscape.
Run-off from slopes can inundate low-lying land where it can infiltrate and cause localised groundwater recharge. This process is referred to as ‘run-off recharge’ and can result in waterlogging and salinity. Broad-based banks can be placed above affected areas to divert run-off before it has a chance to flow onto the low-lying land and infiltrate.
Broad-based banks can also be used to harvest run-off from sloping land by intercepting and diverting the water into dams.
Planning for a system of broad-based banks
Broad-based banks are most effective when they are part of a whole-of-farm water management plan. The whole-farm plan should integrate with any catchment plans and works, so you may need to discuss the works with neighbours before starting.
Factors to consider when planning broad-based banks:
- the location of above and below ground services (such as power lines, telephone lines, water pipes) likely to be affected by the construction of the banks
- the size of area affected by erosion or requiring surface water management
- banks on dispersive surface soils are very susceptible to erosion, and need special management. Outlets for all broad-based banks must have the capacity to handle the expected discharge and must be designed to minimise erosion
- waterways used as disposal points must be stable and vegetated
- dams used to receive discharge must have adequate spillways to ensure that the overflow from the dam itself does not cause erosion
- level sills at the outlet end of broad-based banks can be used to disperse discharge across land before it enters a stream; level sills are likely to need regular maintenance to perform as intended
- the layout of the broad-based banks may need to be incorporated with existing and proposed dams to ensure they retain sufficient catchment area
- fences may need to be realigned or removed to suit the layout of the banks
- access tracks may need to be realigned or crossings built into the banks
- machinery traffic and cultivation requirements. This may need a review of GPS guidance runline direction for cropping operations, management of the wheel tracks in controlled traffic farming, and safe disposal of water.
The design characteristics for broad-based banks include (see Figure 1):
- a capacity to carry run-off generated by at least a 1-in-20 year average recurrence interval (ARI) rainfall event. ARI is the average interval in which a rainfall event will be equalled or exceeded and provides a relationship between large rainfall events and their approximate frequency. The ARI can assist in design and planning for excess run-off associated with larger rainfall events. ARI is a measure of probability only. For example, an ARI of 20 years will represent the design event that will typically need to be managed at least once in 20 years.
- a flat channel floor with a base width of about 5 metres (m); the base width can depend on the width of the construction machinery
- a freeboard (the vertical distance between the water level when the channel is full and the top of the bank) of at least 0.2m when the channel is full
- batter slopes of 1:6 so they can have topsoil retained and be cropped
- a channel depth of 0.3m when the land slope is at 2%, increasing to 0.5m for land slopes over 5%
- a channel grade that depends on the topography, soil types and plant cover. In general, grades are between 0.15% and 0.3%. Sandy surface soils will need slower velocities than loamy or clayey soils. Where channel erosion is likely to be a problem, flow velocity can be reduced by using a broader channel or reducing the channel gradient
- a standard broad-based bank length of 1000m to cater for the run-off from a catchment area of up to 25 hectares.
Broad-based banks are spaced to minimise rill erosion between banks. Bank spacing depends on rainfall, soil type, erodibility and land slope. Seek advice from a trained adviser or contractor.
Spacing should be adjusted to allow for land that has existing erosion because filled rills and gullies are more likely to re-erode. Bank spacing designed for flood mitigation differs from that for erosion control.
The broad-based bank is surveyed at the correct grade and marked at 10 to 20m intervals, depending on undulations along the alignment. The bank is normally constructed with a large road grader or bulldozer. In many cases a combination of the 2 machines produces a better result.
In duplex soils the topsoil is removed before construction and then replaced over the spoil bank to allow for easier establishment of vegetation cover. Ideally, construct banks before the seeding season so that plant cover can be established to protect the new structure.
Maintaining banks will prevent erosion in most years and retain the bank's capacity to move water.
- Maintain the original bank height and remove any vegetation or sediment from the channel to maintain its capacity. Where a broad-based bank is breached, the damage should be repaired immediately by filling, compacting and re-grading.
- Fill and maintain wheel tracks – depressed from regular traffic – that lead into broad-based banks or cause low points on the bank batter.
- Protect and maintain discharge points – including level sills and grassed waterways – to cope with water leaving the broad-based banks.
Correctly constructed and compacted broad-based banks – with adequate channel capacity, batters and freeboard – should only need maintenance every 5 to 10 years, depending on soil type, erodibility and rainfall. However, wheel tracks across broad-based banks will need to filled level and maintained more often than the main bank channel.
Some local governments require development approval for certain types of earthworks, which may include broad-based banks. Check with your local government authority to see if any approval is required before starting work.
If your broad-based bank is likely to adversely affect public land (such as road reserves, river reserves and conservation reserves) you must seek approval from the public authority responsible for that land before starting work. Activities which are likely to require approval include earthworks on public land or damaging or clearing vegetation growing on public land.
The landholder and contractor both have a duty of care to ensure that reasonable steps are taken to prevent harm being caused to another person, property or the environment as a result of the construction or operation of a broad-based bank. Significant criminal penalties apply to causing certain forms of environmental harm.
You may also be liable under common law for damage caused to a neighbouring land property by the construction or operation of a broad-based bank. To minimise this risk, identify the likely adverse impacts and plan appropriate safeguards as part of the implementation and operation of the works.