Grade banks for managing surface water

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. 

• Grade banks on long slopes are usually spaced 50 to 220 m 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 3.5 m 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 0.4 m 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 0.5 m, and for slopes between 5 and 10%, the depth should be 0.6 m. 
• 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% (20 cm to 50 cm fall in 100 m). 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 1000 m. A single, 1000 m 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 10 m. Longer sills of up to 15 m are required for steeper slopes or where outlet flow velocities need to be reduced. 

• 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. 

• 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 25 m. 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 60 m 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. 

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 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.