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Soil Conservation Guidelines for Queensland Chapter 7 Contour banks

Chapter 7

Contour banks

• Contour banks intercept runoff before it concentrates and starts to cause erosion, and then safely channel it into stable grassed waterways, natural depressions, or grassed areas adjacent to a paddock.

• When designing contour banks the key objective is to ensure that under the design conditions the velocity of flow remains low enough to avoid erosion. The velocity of flow depends on the gradient, length, spacing, and cross-section (or depth of flow) as well as the vegetation cover between, in, and on the banks.

• The major considerations when designing contour banks are the land slope, land use/cover, soil type and rainfall of the catchment. It is also important to consider practical farm management requirements such as trafficability, especially in choosing the spacing and alignment of the banks.

• Contour banks are usually designed to a standard that will safely carry runoff resulting from a rainfall event with a 10 year average recurrence interval. To ensure a safe operating margin, additional allowance is required for freeboard and settlement following construction.

• Contour banks can be built in a range of different shapes (narrow-, broad-based, broad top- or bottom-side) depending on the slope, rainfall, soil conditions, land use, and equipment available. Particular attention should be given to the design and maintenance of the channel outlets as these are weak points where erosion can often occur.

Key points

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Contents

7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

7.2 Contour bank types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

7.3 Design criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

7.3.1 Design velocity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8

7.3.2 Gradients . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9

7.3.3 Contour bank length . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

7.3.4 Bank spacing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

7.3.5 Parallel layouts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .18

7.3.6 Contour bank cross-sections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

7.3.7 Freeboard and settlement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .22

7.4 Design approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

7.4.1 Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .24

7.4.2 Contour bank design charts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .25

7.5 Farming with contour banks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

7.5.1 Controlled traffic farming and ‘tramlining’ . . . . . . . . . . . . . . . . . . . . . . . . . . . . .27

7.5.2 Contour banks and CTF layouts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .27

7.5.3 CTF layouts on sloping land . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .28

7.5.4 Contour bank maintenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .30

7.6 Further information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

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Glossary

average recurrence interval (ARI): the average period in years between the occurrence of an event (usually a storm or a flood) of specified magnitude and an event of equal or greater magnitude.

contour bank: a constructed earth embankment, incorporating a channel on the upslope side, typically traversing a slope on or close to the contour to control and/or prevent the erosion of that slope. Also referred to as graded banks, terraces, or bunds.

controlled traffic farming (CTF): a cropping system in which the crop zone and the machinery traffic lanes are distinctly and permanently separated. In practice it means that all implements have a particular span, or multiple of it, and all wheel tracks are confined to specific traffic lanes. Also referred to as tramlining.

flat ridge pattern: a variation in contour bank spacing that arises when contour banks carry runoff across ridgelines with low slopes or saddles.

freeboard: the vertical distance between the top water level and the crest of a bank, dam, or similar structure. Freeboard should include an allowance for settlement, and is provided for in designing such structures to prevent overtopping.

ferrosols (krasnozems): soils with B2 horizons that are high in free iron oxide, and which lack strong texture contrast between A and B horizons.

Manning’s roughness coefficient: see retardance.

Rational Method: a formula for estimating peak discharge of runoff from a catchment above a specific point calculated using the peak discharge, rainfall intensity for the selected period, runoff coefficient, and catchment area—see Chapter 4 of these guidelines.

retardance: a measure of resistance to flow in a channel; the greater the resistance the higher the retardance. It is calculated using the Manning’s formula and has the symbol ‘n’. Retardance is influenced by the physical roughness of the internal surface of the channel (e.g. the vegetation that lines it), channel cross-section, alignment, and obstructions.

stream power: the rate the energy of flowing water is expended on the bed and banks of a channel.

Universal Soil Loss Equation (USLE): a mathematical relationship developed in the USA to predict long-term average soil losses in runoff under specified environmental and management systems.

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7.1 IntroductionContour banks are earthen structures constructed across cultivated slopes at intervals down the slope. In some countries and in some Australian states other than Queensland, contour banks are referred to as ‘graded banks’, ‘terraces’, or ‘bunds’. The function of contour banks is to intercept runoff and safely channel it into stable grassed waterways, natural depressions, or grassed areas adjacent to a paddock. They reduce slope length and intercept runoff before it concentrates and starts to cause erosion. Contour banks also trap sediment from overland flow, especially from rills and old gully lines. Any crop or stubble in a contour bank channel acts as a filter as runoff moves slowly along the contour bank channel.

These days, there is a tendency for people to downplay the importance of contour banks as a soil control measure in cropping lands. Best management practices for cropping land will often exclude contour banks, but will refer to controlled traffic farming as being an alternative. In fact, the need for contour banks, used in association with practices like zero tillage, is as great as ever. The use of controlled traffic farming in association with contour banks is discussed in section 7.5.

Contour bank layouts require careful planning to ensure that runoff is well coordinated between properties within a catchment and across public utilities such as roads and railway lines (see Figure 7.1). More information on how to coordinate the planning of contour banks is provided in Chapter 2.

Figure 7.1: Plan of a contour bank and waterway layout

Contour banks are not built strictly ‘on the contour’. They need to have a low gradient (usually between 0.1% and 0.4%) to allow water to flow but to minimise the chance of channel flow reaching erosive velocities when the channel is in a bare condition. In some intensive farming situations (e.g. horticulture or sugarcane cultivation) where pondage must be avoided or where parallel layouts are required, contour banks may be constructed to steeper gradients for limited distances. If the channel is protected from erosion, for example, by maintaining permanent cover, contour banks can be safely constructed at steeper gradients. The spacing between contour banks depends mainly on the slope of the land but is also influenced by soil type, cropping practices, and previous occurrence of erosion at the site.

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Contour banks are usually designed in theory to a standard that will safely carry water resulting from a runoff event with a 10 year average recurrence interval. However, in reality, the ability of a contour bank to carry this design runoff is largely dependent on the condition of the channel at the time the runoff event occurs. A contour bank with a smooth, bare channel can carry around five times more runoff than one where the channel supports a close growing crop or dense stubble.

Cropping practices that maintain good surface cover of vegetation will greatly reduce the incidence and amount of erosion between contour banks. This will enhance the effectiveness of contour banks and greatly reduce their maintenance costs.

Contour banks also play an important additional soil conservation role by acting as sediment traps. Up to 80% of the soil moved from a contour bay may be deposited in the contour bank channel (Freebairn and Wockner 1986). The rate of deposition and filtration of nutrients and pesticides is greatest when the channel contains a close growing crop or standing stubble.

In intensively cropped areas, such as where horticulture crops or sugar cane are grown, contour banks are usually constructed parallel to each other. This is for ease of inter-row cultivation, pesticide application, irrigation and harvesting practices. However in broadacre cropping areas contour banks are usually not laid out in parallel because of the irregular nature of the topography. The introduction of controlled traffic farming over recent decades has also had significant implications for contour bank systems. More information on contour banks in horticulture is provided in Chapter 12.

Special techniques are required to construct contour banks in areas with shallow dispersible subsoils (e.g. at depths of less than 30 cm). This is because when such subsoils are exposed to rainfall and runoff in the channel, the contour bank will be prone to failure by tunnel erosion.

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7.2 Contour bank typesContour banks can be constructed in four design styles:

Narrow-based contour banks (Figure 7.2) feature batters that are too steep to cultivate. The batters of narrow-based contour banks are instead normally planted to grass and hence require weed control, especially during the first two years. The channels of narrow-based contour banks are usually treated as part of the contour bay, which normally means that they are cultivated and cropped. However, some farmers choose to leave the channels grassed. The channel and batters of narrow-based contour banks may take up to 10% of the total area of a paddock.

Figure 7.2: Narrow-based contour bank

original ground level

Narrow-based contour banks are commonly used on cultivated land that is steep (i.e. 5–12% slopes) or only occasionally cultivated. They are not suited to cracking clay soils because the deep cracks formed by these soils during the dry season weaken the banks and may lead to subsequent failure. Narrow-based contour banks are also prone to failure due to burrowing by animals where such animals (e.g. rabbits) occur.

Broad-based contour banks (Figure 7.3) feature batters that can be easily worked with tillage and planting machinery. This allows the whole of the paddock to be cropped including the channel. Broad-based contour banks are generally used on deep soils and on land with low slope (<5%). They can be safely crossed by farming equipment under a controlled traffic system at a range of angles depending on the slopes of their batters. Because the batters are cultivated, the risk of failure of broad-based banks by cracking is reduced.

Figure 7.3: Broad-based contour bank

h


Broad-based banks are more costly to build and maintain than narrow-based banks, and become impractical to construct when slopes exceed about 5%.

Semi-broad-based banks may have a broad base either on the top side (Figure 7.4a) or the bottom side (Figure 7.4b). Broad-based top side banks are used on steeper slopes and/or on cracking clay soils, where the up-slope batter of the bank is broadened to suit the width of the most commonly used machinery. Broad-based bottom side banks are commonly used on lower slopes, particularly in irrigation areas—sugar cane—where the channel of the bank may be grassed and used as an irrigator tow-path.

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(a) (b)

h


h


Figure 7.4: Semi-broad-based contour bank a) top side, b) bottom side

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7.3 Design criteriaContour banks are normally not designed individually. Rather, general specifications for contour banks are usually developed for a district and for particular situations. These specifications are usually based on the following parameters:

• gradient

• length

• spacing

• cross-section and depth of flow.

The capacity of contour banks generally declines over time. The height of banks will reduce due to settlement soon after construction or progressive wearing down due to tillage equipment, whilst the capacity of channels will also progressively reduce due to sedimentation. Since contour bank maintenance is normally infrequent (e.g. on a 5 to 10 year cycle) and irregular, it is best that contour banks are constructed initially to exceed the specified standards by a safe margin. That way, the banks will still function effectively, even as they progressively deteriorate between maintenance events. Notwithstanding this, it is also important to note that the dimensions of a newly constructed contour bank are often governed by the construction technique rather than by the prescribed specifications. For example, contour banks constructed with one push of a large bulldozer (a commonly used technique) may greatly exceed the standard specifications.

7.3.1 Design velocityIt is desirable that the flow velocity in a contour bank channel is low. This is to avoid the chance of erosion and to ensure maximum deposition or trapping of sediment. Low velocities also reduce design peak discharges in waterways by lengthening the time of concentration. Where cultivated and cropped, the aim of the contour bank design should be to keep velocities below 0.4 m/s for easily eroded soils; and below 0.6 m/s for erosion resistant soils. Contour banks must be designed with sufficient capacity to accommodate the design event at or below this velocity.

The velocity of flow in a contour bank channel is very dependent on the condition of the channel at the time that a runoff event occurs. If the channel is smooth and unvegetated (i.e. Manning’s n of 0.03) the bank potential to discharge runoff will be at a maximum. Under these conditions high velocities will occur if there is a significant depth of flow such as in a major runoff event. However, if flow is restricted by the presence of a cereal crop such as wheat or standing stubble after harvest in the channel (i.e. where Manning’s n may be 0.15), the velocity is not likely to exceed 0.2 m/s. Where controlled traffic farming systems are being used, the crop rows may sometimes be at right angles to the direction of flow in the channel. Under these circumstances, Manning’s n values could be expected to be greater than 0.15. Further research is needed to determine what Manning’s n values are likely to occur under these circumstances.

Because conditions that may occur in a contour bank channel are variable and impossible to predict with absolute certainty, developing a design is complex and requires acceptance of a level of risk. If a contour bank with a bare channel is flowing to capacity, it is likely to be handling an event much greater than that for which it was designed and erosive velocities will occur. This situation must be deliberately risked, as the only alternative is to build a smaller bank or to reduce the gradient. This would lead to regular failure if runoff events occur when the channel is restricted by crop or stubble.

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If a design indicates that velocities in the channel will be too high, then the following options should be considered:

• Use an alternative channel shape (e.g. a flat-bottomed trapezoidal shape will convey flows more safely than a triangular cross-section).

• Keep the channel permanently grassed.

• Use a lower gradient.

The equation to calculate stream power (see Chapter 6) may be used to determine the likelihood of erosion occurring in the channel. Table 7.1 provides values of stream power for a typical broad-based contour bank with a trapezoidal shape, a gradient of 0.2% and a Manning’s n of 0.03 (bare soil). For cracking clay soils it is recommended that values of stream power be below 3 (W/m2) (Titmarsh and Loch 1993). Table 7.1 indicates that this value will be exceeded for depths of flow of 0.4 metres or greater for this design.

Table 7.1: Stream power values for a typical broad-based contour bank under bare soil conditions

Depth of flow (m) Velocity (m/s) Discharge (m3/s) Stream power (W/m2)0.2 0.4 0.5 1.30.3 0.5 1.1 2.30.4 0.6 1.8 3.40.5 0.7 2.9 4.70.6 0.79 4.2 6.00.7 0.87 5.8 7.4

Based on the following parameters:Trapezoidal shape Grandient of 0.2% Manning’s n of 0.03

7.3.2 GradientsThe gradient of a contour bank design should be chosen to minimise the risk of erosion in the channel when it is in a bare condition and also to ensure that the channel has adequate capacity to carry the design runoff when flow in the channel is restricted by crop or standing stubble. Such a compromise can be difficult to achieve in practice because of the fivefold differences that can apply in the values of Manning’s n (0.03 to 0.15) for these two situations.

Contour bank gradients that are too high result in:

• erosion in the contour bank channel

• excessive rates of runoff into waterways.

On the other hand, contour bank gradients that are too low lead to:

• poor drainage—an important issue, especially for many horticultural crops

• low points in the bank that will pond runoff until they are filled with sediment

• ‘leakage’ into groundwater systems in locations where this is an issue

• failure by ‘piping’ (linked to tunnel erosion) where there are dispersible subsoils.

The impact of gradient on contour bank velocity and discharge is illustrated in Figures 7.5a and 7.5b respectively.

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Figure 7.5: Effect of gradient for two flow depths on a) contour bank velocity and b) contour bank discharge

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0.1 0.2 0.3 0.4

cont

our b

ank

velo

city

(met

res

/ se

cond

)

contour bank gradient (percentage)

Flow depth 0.5m

Flow depth 0.25 m

Parameters:Trapezoidal shape1:10 inlet and 1:6 bank slope4 m bed widthMannings n of 0.03

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

0.1 0.2 0.3 0.4

Cont

our b

ank

disc

harg

e (c

ubic

met

res

/ se

cond

)

Contour bank gradient (percentage)

Flow depth 0.5 mFlow depth 0.25 m

Trapezoidal shape1:10 inlet and 1:6 bank slopeBed width 4 m

(a)

(b)

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The gradient recommended for a contour bank depends on the steepness of the land and on the soil erodibility. This is in part because the capacity of a contour bank of a given height depends on the land slope. The lower the land slope the greater the storage capacity of the bank. Land slope also influences contour bank length. Where the landscape is steep, the distance across the slope between natural drainage lines is less. This means that the average distance that contour banks are required to span between drainage lines on steep slopes is likely to be much shorter than on low slopes.

Taking the above factors into account, there are good reasons for increasing contour bank gradients as land slope increases. Steeper gradients on higher slopes will compensate for the limited capacity of contour banks on such slopes. However, shorter contour banks on steeper slopes means that they are required to handle less runoff than longer banks, thus reducing the risk of erosion occurring in the channel.

On horticultural properties, higher gradients can be used where the channel is grassed, or where it is bare but not cultivated. If contour banks are used for access and are not cultivated, the risk of erosion in the channel is greatly reduced. In cane lands, gradients as high as 4% are used where green cane trash blanketing is used on erosion-resistant soils [for example, ferrosols (krasnozems)]. Contour bank channels in cane lands are only vulnerable to erosion for a relatively short period, when a new crop is planted after the removal of the ratoon crop (which occurs every 4 to 8 years). The use of minimum tillage practices or a cover crop can reduce the risk of erosion during the fallow period. Further information about managing contour bank gradients in cane lands is included in section 7.3.5, Parallel layouts.

It is normal practice for a contour bank to be constructed to the same height for its entire length. Since the amount of runoff to be carried increases with the length of the contour bank, variable gradients can be used along a contour bank channel, allowing for a greater discharge capacity. This will lengthen the time of concentration and reduce the peak discharge in the waterway.

Where contour banks are on low land slopes with maximum gradients of less than 0.2%, there is limited opportunity to use variable gradients. However, on steeper land slopes where the maximum gradient is higher, variable gradients can be used. As indicated in Table 7.2, this can mean that for a land slope of 3–5%, the gradient in the top third of the bank would be 0.2%, changing to 0.25% in the middle third and then to 0.3% in the lower third (outlet) section of the bank.

Table 7.2: Gradients for contour banks with cultivated channels based on land slope

Land slopeAppropriate contour bank gradients (%)

for average conditionsTop section Middle section Outlet section

1% 0.15 0.15 0.152% 0.2 0.2 0.23%–5% 0.2 0.25 0.35%–10% 0.3 0.4 0.5

In intensive cropping areas, parallel contour bank systems are often implemented (refer to section 7.3.5). This is for practicality in operating large cropping machinery, and often in conjunction with incorporating an irrigation layout. Some flexibility is required in setting contour bank gradients when using a parallel contour bank system. Nevertheless, gradients should always be managed to ensure that erosion in the channel is minimised.

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Gradients can be modified over short distances to improve workability of the layout. At the high end of a contour bank, it is quite acceptable to improve workability by using a high or low gradient to ensure that the bank meets a fence line at close to a right angle rather than an acute angle.

It is normal practice to ‘split’ contour banks on well-defined ridgelines so that they direct runoff away from the ridge. This ensures that runoff remains in its natural catchment and also provides an ideal position for a road or track to cross over contour banks. The exact location of the ‘split’ should be prominently marked during the surveying process so that the farmer is aware of its location and the significance of this position. The splits on a ridge of a series of banks down the slope should be aligned. This may require a readjustment of some levels at the completion of the surveying task to obtain the best alignment.

If contour banks carry runoff across ridgelines that have low slopes or even a saddle, considerable variation in the contour bank spacing (referred to as the ‘flat ridge pattern’) may result. This problem can be minimised by modifying the gradient where the bank crosses the ridge. Some zero grade sections in this situation would be acceptable as the low slope ensures maximum contour bank capacity and the convex nature of the topography ensures that there is less likelihood of concentrated flows discharging into this section of the contour bank.

The land slope in an individual contour bay in many inland cropping areas may vary from say 1% in the depression, to 0.5% on the ridge, to 2% in the area between the ridge and the depression. (If there is a saddle, the ridge slope will change from downhill to uphill). If this slope variation is not taken into account, the surveyor may end up with highly embarrassing variations in the vertical interval between contour banks. Consider the example below (see Figure 7.6 a–c).

Figure 7.6: Contour bank layout illustrating the flat ridge pattern: a) natural contour lines; b) banks flowing left to right; c) banks flowing right to left

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Figure 7.6a shows the true contour lines on a ridge with slopes varying from 0.8% to 2.0%. Figure 7.6b shows what would happen if banks with a 0.2% gradient were marked out in this paddock with the staffer moving in the direction B to D. Suppose the contour bank vertical interval (VI) at AB was an acceptable 1.5 metres with a horizontal interval (HI) of 75 metres. By the time the staffer reached point D, the VI at CD would be an unacceptable 0.96 metres (HI of only 48 metres).

The reason for this phenomenon is that the distance along the contour from B to D in Figure 7.6b is 370 metres (giving a height change of 0.74 metres at 0.2% gradient) while it is only 100 metres from A to C (giving a height change of 0.2 metres at 0.2% gradient).

Figure 7.6c shows what would happen if the staffer moved in the same direction as in Figure 7.6b, but where the direction of flow was reversed. The VI would change from 1.5 metres at AB to 2.53 metres at CD (HI at CD would be 127 metres). The change in VI is even greater than it was in Figure 7.6b because the distance from B to D is now 615 metres.

A similar situation (though not usually as severe) can occur in depressions.

What is the solution to this problem? The easiest solution, and maybe the preferred option, is to split the banks on the ridge. This is a common and sensible solution. However, splitting banks on every ridge may require more waterways, and paddocks will be more difficult to work on the contour because of the extra turning required.

Another solution is to reduce the gradient as the bank goes around the ridge. It should be acceptable to reduce the gradient down to .05% because the low slope on the ridge will mean that the contour bank will have a very high capacity. In addition, as the slope on the ridge is always a convex one, there is not likely to be any concentrated overland flow meeting the bank in this area. A gradient such as .05% will cause much less variation in the vertical interval.

Where a contour bank crosses a ‘sharp’ depression, resulting in a sharp bend in the bank, the gradient can be modified to smooth out the shape of the bank to improve workability. However, this will create a low point in the contour bank that will detain runoff until sufficient sedimentation occurs to remove the pond. If this procedure is adopted, the contour bank must be constructed with extra capacity where it crosses the drainage line. These points must also be checked after construction is completed to ensure they have adequate capacity.

Consideration should be given to increasing gradients where contour banks are to be built in land with prominent rilling and gullying. Alternatively, or in addition, in such circumstances contour banks should be constructed with additional height where they cross gully lines, bearing in mind that greater settlement of the bank is likely to occur at these points. Additional height at these points should reduce the need to increase the gradient. Ideally, gullies will have been filled in during the construction process, although some form of a depression is likely to remain. This depression will be subject to sedimentation and will disappear over time. Levelling of the contour bay between contour banks to remove old rill and gully lines is encouraged. If levelling is not carried out, rills will continue to concentrate runoff from the adjacent area leading to silt deposition in the contour bank channel.

Higher gradients can be considered for contour bays where zero tillage is adopted or where contour bank channels are not cultivated. As previously discussed the highest velocity likely to be achieved in a standard-size, broad-based contour bank with a wheat crop or standing wheat stubble is 0.2 m/s. There is a risk however, that should the property change hands, a new owner

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may adopt traditional cropping practices with lower levels of stubble and higher velocities. Therefore, it is preferable to use gradients applicable to a farming system that will have both bare and vegetated channels at different times.

From a hydraulic design aspect, level (0% gradient) contour banks, especially on low slopes, could accommodate the runoff they receive, provided they were built to an adequate specification. However, this is not recommended because such banks are subject to pondage along the bank, inhibiting crop growth and restricting tillage, planting and harvesting activities.

Gradients at contour bank outlets

Problems can occur at the point where contour banks discharge into waterways. In this situation, the gradient of the surface of the water in a channel is an important consideration as well as the gradient in the bed of the contour bank channel (Stephens 1987). Two different situations may occur: (1) where a bank discharges with a completely free outlet, and (2) where the outlet is obstructed in some way.

Where a bank discharges with a completely free outlet

Examples of where a bank discharges with a completely free outlet include where it discharges into:

• a wide deep hollow

• an adjacent grass paddock

• a subsurface waterway

• an eroding waterway.

In the above cases, the gradient of the water surface would be greater than that of the channel, and the velocity of the flow would increase as it discharges. This can cause erosion in bank outlets. In these situations there is no requirement for any extra gradient at the bank outlet.

Where contour banks are discharging into a grassed area, it is advisable to construct a spreader channel (Figure 7.7) at the outlet to ensure that discharge occurs over a wide section of the bank. Spreader channels are level channels created by pushing soil uphill rather than downhill as with conventional contour banks. They are used to reduce the concentration of water discharging at the end of a diversion or contour bank into an area of pasture or a watercourse.

Figure 7.7: Plan view of a spreader channel at the outlet of a contour bank

A spreader channel would normally require the last 20–50 metres of a contour bank to be level. This section would have an excavated channel from which soil has been pushed uphill. A hedge incorporating a species such as Monto vetiver grass along the spreading area will help ensure that runoff exits the sill over the entire length of the spreading area.

Where there is an overfall at the potential outlet of a contour bank, some adjustment to contour bank spacing may be required to find a more stable outlet

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for a contour bank. Normal gradients or even a level section should be used where there is an overfall. Such overfalls should be stabilised at the outlet by means of a structure such as a rock chute. Where the contour bank outlet is unstable, the last section of channel should be permanently grassed.

Where the outlet is obstructed in some way

Examples of where the discharge is obstructed in some way include:

• a bank outlet that is too narrow or choked with grass or stubble

• where the bank discharges into a waterway that is flowing at a similar height to the water in the contour bank.

In the above cases the gradient of the water surface in the channel of the contour bank will be less than that of the channel bed and the velocity will decrease. This can cause the bank to overflow near the outlet. The contour bank gradient will generally need to be increased in these situations to provide for both the estimated depth of excavation to construct the contour bank as well as the design depth of flow above ground level in the waterway.

In situations where the ground slope is low it may not be possible to sufficiently increase fall at the bank outlet. In such situations bank height should be increased for at least the last 200 metres of the contour bank. As an additional measure to reduce risk of overflow, the contour bank may be constructed to discharge into a secondary waterway running adjacent to the main waterway for about half a contour bay interval.

7.3.3 Contour bank lengthFarmers generally prefer contour banks to be as long as possible and waterways to be as few as possible to minimise loss of cropped area and to reduce interference with normal practices using large cropping machinery. However this can come at a cost, as bank length increases so does the risk of failure.

Contour bank length is related to land slope. On steeper landscapes, natural drainage lines are closer together, meaning that the distances to be spanned by contour banks are shorter. Whilst, on lower slopes the capacity of a bank of a given height will be greater than on a steeper slope (see Section 7.3.6). This means that longer banks and lower gradients can be used on low slopes. The longest bank lengths are implemented on low sloping extensive cropping areas of the Western Downs and the Central Highlands. Contour banks may however be shortened in more intensive cropping systems such as with the growing of sugar cane or other horticultural crops for ease of harvesting.

The amount of runoff discharged from a contour bay will be proportional to the area of the bay. Figure 7.8 predicts peak discharges for various contour bank lengths and different levels of cover on a 2% slope with a 90 metre contour bank spacing, at Pittsworth on the Darling Downs, using the Empirical version of the Rational Method—see Chapter 4. Significantly higher discharge is predicted under the low cover system due to the higher velocity (and hence shorter time of concentration) and the higher C value. A contour bank with bare soil in the channel will be able to accommodate considerably more runoff than a bank with a channel that is carrying a crop or standing stubble.

Table 7.3 provides a guide to selecting maximum bank lengths based on land slope. This table assumes contour bank capacities normally maintained by farmers on such slopes and the minimum contour bank spacing normally recommended on such slopes. It also assumes that the runoff is travelling in one direction in the contour bank channel.

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Figure 7.8: Peak discharge estimates based on contour bank length at Pittsworth

Table 7.3 : Recommended maximum bank lengths for various land slopes

Land slope % Recommended maximum bank length (m)1 25001.5 20002 17503 15004 10005 7506 6007 4508 4009 35010 300Based on the following parameters:Single spaced contour banksUse of cropping systems that provide high levels of coverHigh standard of contour bank maintenance

An alternative approach to designing of contour bank length is to use the KINCON model (Connolly et al. 1991).

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7.3.4 Bank spacingFarmers prefer wide spacing between contour banks as it ensures less interference with the operation of farm machinery and reduces the cost per hectare of construction. However there are a number of factors that limit the spacing between contour banks:

• Wider spacing increases erosion.

• Wider spacing allows overland flows to concentrate, forming gullies between the banks and building up deltas in the channel of the contour bank below.

• There are practical limitations on bank size and on a bank’s ability to handle runoff.

Various guides have been developed to determine contour bank spacings. These are based on land slope, soil erodibility, land use, and rainfall erosivity. These factors are not all equally significant. The Universal Soil Loss Equation (see Chapter 1) shows for instance that steepness of slope has a much more significant impact on erosion than does the length of the slope.

There are no strict rules that determine the ‘correct’ spacing for a particular situation. The concept of ‘single’ and ‘double’ spacing has been used to vary contour bank spacing depending on the average conditions expected in a paddock. Experience in Queensland has shown that the spacing listed in Table 7.4 is suitable for most cropping situations.

Table 7.4: Recommended contour bank spacing

Average land slope (%)

Single spacing Double spacingVertical interval (VI)(m) Horizontal interval (HI)(m) Vertical interval (VI)(m) Horizontal interval (HI)(m)

1 0.9 90 1.8 1802 1.2 60 2.4 1203 1.4 45 2.8 904 1.6 40 3.2 805 1.8 36 3.6 726 1.9 32 3.8 647 2.1 30 4.2 608 2.4 30 4.8 609 2.7 30 5.4 6010 3.0 30 6.0 60

Single spacing should be used where:

• bare fallow cropping systems are likely to be used

• a paddock is already seriously eroding

• soils are highly erodible

• contour bank length is close to the recommended maximum length

• contour banks are likely to be maintained only to a minimum standard

• parallel contour banks with higher than normal gradients are planned.

Double spacing may be used where:

• cropping systems that ensure high stubble levels during the fallow phase are used

• minimal erosion has occurred and soils are stable

• contour banks are likely to be built and maintained to a high standard.

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Intermediate spacing (between single and double) is used in some districts. An argument against this practice is that if the spacing were subsequently halved it would result in spacing that is unacceptably close for most farmers. Experience has shown that the normal single or double spacing is acceptable provided the conditions listed above are met.

Other factors may determine the spacing required for a particular situation. For example, parallel contour banks in irrigated cane are traditionally spaced 40 metres or 80 metres apart to match the spray width of water winches used for irrigation.

Where topography is irregular, the distance between banks will also vary with changes in the slope of the land. For this reason it is preferable to measure bank spacing using the vertical interval rather than the horizontal interval. To determine the vertical interval in areas of variable topography a compromise is required. The recommended approach is to use the average vertical interval for the contour bay.

7.3.5 Parallel layoutsParallel layouts are required for any situation where inter-row farming operations are practised or where crops are irrigated. Parallel layouts have been used traditionally in intensive cropping areas such as for sugar cane or horticulture.

Implementing parallel layouts requires additional detailed topographic information. Parallel layouts are most readily implemented where the topography is regular (i.e. slope varies minimally within each of the proposed contour bays). In intensive cropping areas, contour banks are deliberately shortened allowing for greater opportunities to alter gradients to ensure that the contour bank system is parallel.

The implementation of a parallel layout usually relies on using as many natural depressions as possible. This will result in many short contour banks with consequent negative impact on machinery manoeuvrability by requiring the operator to turn around at each waterway. One option to overcome this problem is to use subsurface waterways, (see Chapter 9). Subsurface waterways assist trafficability by allowing the tractor operator to lift an implement and travel across the waterway.

Using single spaced contour banks in parallel layouts will reduce the amount of runoff that the channels need to accommodate. This will also provide more options for varying gradients to implement the parallel system. The spacing should be modified to match the implement widths or the irrigation system in use on the farm.

Where higher than normal gradients are required, the use of a parabolic, or a flat-bottomed, contour bank channel rather than a triangular one should be considered. It may also be necessary that the channel be grassed. Designs should be tested thoroughly before completion to be sure that the expected velocities are not likely to cause erosion when the channels are in a bare condition.

Table 7.5 (Scarborough et al. 1992) provides examples of gradients recommended for use in parallel layouts in the Coastal Burnett region. This table applied to situations where contour bank channels are cultivated but could be used as a general guide for the whole of Queensland. If green cane trash blanketing is used and measures are taken to provide erosion protection after the removal of the ratoon crop (every 4 to 8 years), then higher gradients than shown in Table 7.5 could be used.

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Table 7.5: Recommended gradients for contour banks in parallel layouts where channels are cultivated (from Scarborough et al. 1992)

Soil erodibility Average grade (%) Max. grade for 50 m (%)Low 1.5 3.0

Medium 1.0 2.0High 0.5 1.0

A steep gradient of say 3–4% may be acceptable over a short distance, for example 50 metres, at the high end of a contour bank channel. This is because minimal flow is being carried in this section.

It is often a great advantage if contour banks are not only parallel to each other but also straight. This is especially the case in vineyards or with trellised tree crops. It is difficult and expensive to build trellises on curved alignments. Straight parallel banks are also preferred in canelands, as water winches used to irrigate sugar cane generally require straight rows to operate effectively.

Some sections of reverse grade may be unavoidable in a parallel contour bank system. Such sections will however cause ponding. The extent to which ponding may be a problem depends on the cropping system used and the soil types present. It may be possible to avoid having a reverse grade by constructing an additional cut in the elevated section of the channel. Alternatively, a low section leading to a reverse gradient can be corrected by constructing that section of the bank from the lower side. This will result in the channel at that point being higher than adjacent sections.

Parallel layouts have seldom been implemented on broadacre farming systems. This is because the rolling landscapes of such areas normally feature considerable slope variation. The lowest slopes are usually found on ridge lines while the maximum slopes occur between the ridge line and the drainage line. Contour banks used in broadacre cropping are generally long with low gradients. There is limited opportunity to increase the gradient unless the contour bank channel is to be permanently grassed.

Controlled traffic farming (CTF) systems that have been widely adopted in recent decades require land to be cultivated in parallel blocks. In broadacre systems, this has generally been achieved by cultivating whole paddocks, usually in a single direction, passing up and over contour banks (see Section 7.5). In the South Burnett region, some farmers have implemented parallel farming with non-parallel, broad-based contour banks by selecting a key bank and working parallel to it. Contour banks above and below the key bank are then crossed at a slight angle. This systems results in furrows that are close to the contour but which drain either into a waterway or a contour bank.

7.3.6 Contour bank cross-sectionsContour banks can take a range of shapes as discussed at the beginning of this chapter. While contour banks are commonly constructed with a trapezoidal shape, the cross-section usually reverts to a triangular shape after a few years of tillage operations.

The cross-sectional area of a contour bank is most influenced by the bank height and the land slope. Figure 7.9 illustrates the effect of bank height on the cross-sectional area of flow. The data is based on a triangular-shaped, broad-based contour bank on a 2% land slope and with a bank batter of 1:6 (V:H). It assumes that the excavated upslope batter conforms to normal land slope.

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Figure 7.9: Effect of bank height on the cross-sectional area of a contour bank

Figure 7.10 illustrates the effect that land slope has on contour bank capacity. As for Figure 7.9, the data is based on a triangular-shaped, broad-based bank with a flow depth of 0.5 metres and a bank batter of 1:6. It also assumes that the excavated batter conforms to normal land slope. On a land slope of 1% where a contour bank vertical interval of 0.9 metres would be used, half the contour bay would be under water if there was a flow depth of 0.45 metres. This illustrates the enormous amount of storage that contour banks can have on very low slopes.

Figure 7.10: Effect of land slope on contour bank cross-sectional area

Where land slopes are low, the excavated batter will often conform to the normal land slope after a few years of tillage operations. If the bank has been constructed with a bulldozer using a long length of travel in pushing up the bank, then the excavated bank batter will almost conform with normal land slope after construction is complete.

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Figure 7.11 also illustrates how land slope impacts on the cross- sectional area of contour banks. Five percent (5%) land slope is the normally accepted limit for the construction of broad based contour banks with 1:4 (V:H) batters.

Figure 7.11: Broad-based contour banks with 1:4 batters on land slopes 1%, 2%, and 5%

1:4

1:4

1:33

1% slope

5 m 5 m 5 m 45 m

0.63 m

1:4

1:4

1:17

2% slope

5 m 5 m 5 m 15 m

0.63 m

1:7

5% slope

1:4

1:4

5 m 5 m 5 m

0.63 m

(a) 1% slope

(b) 2% slope

(c) 5% slope

To provide protection against erosion of contour bank channels on steeper slopes, it is best to aim for a flat-bottomed channel (trapezoidal or parabolic). On steeper slopes there will be a distinct change in slope where the excavated batter meets the normal land slope. This point is referred to as the ‘nick point’ (Figure 7.12). It can contribute to rill erosion as overland flows meet the increased slope as they flow into the channel.

Figure 7.12: Contour bank cross-section illustrating nick point

nick point

The limitations (and requirements) of machinery must be taken into consideration when determining contour bank cross-sections. The length and grade of the batters of contour banks should be constructed to suit the equipment used to operate on them (especially planting machinery).

For cultivated banks, batters flatter than 1:4 (V:H) are recommended. Section 7.5 provides information on contour bank shapes suitable for traversing by machinery. If a trapezoidal channel is constructed then the base must also conform to machinery requirements. Tow paths for travelling irrigators located in the bank channel require a trapezoidal shape with at least a 2.0 metre bottom width to help tracking of the irrigator.

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7.3.7 Freeboard and settlementFollowing construction, contour bank capacities need to be checked to ensure they are adequate for the conditions. Special attention should be given to checking points where contour banks cross old gully lines. The bank should be built higher at these locations to ensure that it has adequate capacity to accommodate the design flow as it crosses the old gully line.

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7.4 Design approachAs stated at the beginning of this chapter, contour banks are normally constructed according to general specifications that are then applied to particular circumstances across a whole district. Such specifications have been developed after numerous field observations over many years. In some instances this information can be found in the relevant land management field manual.

When it is necessary to develop an individual design for a contour bank or to prepare or modify district specifications, the following approach is recommended.

In Chapter 6 the concept of combining equations 6.1 (Q=AV) and 6.2 (the Manning formula) was discussed—see Equation 7.1.

Because the surface conditions of a channel in a contour bank may vary from a bare condition (Manning’s n of 0.03) to a crop or standing stubble (Manning’s n of 0.15 in the case of a wheat crop or standing wheat stubble), it is necessary to consider both conditions in the design. This requires estimating two design discharges. The design peak discharge for a contour bank varies considerably for a high- and low-cover farming system (see Figure 7.8). The amount of cover varies between and across all paddocks and through time. A low-cover paddock, as would result from a fallow management farming system, will have a high cover level when it is growing a crop. A paddock where a high level of stubble management is used may have low cover during a period of drought when no crop has been planted or the crop has failed.

Since flow in contour banks is most restricted under crop or standing stubble it is best to design initially for this condition and then check to see what happens to the design discharge when the channel is bare. A limitation of this method of design is that it does not take the temporary storage capacity of the contour bank channel into account. The method therefore provides an overestimation of the actual capacity required. In addition, contour banks can act as temporary storage structures (Galletly 1980). Further research is required to develop a design method that incorporates storage capacity.

Factors in Equation 7.1 for which values are known can be directly substituted as:

• discharge, Q

• gradient, S

• roughness coefficient, n.

Since the design is initially for a high level of channel roughness, it can be assumed that the flow will be well below erosive velocities and therefore the value of V can be omitted from the equation. Solving the equation then resolves to finding a depth of flow in the contour bank channel that will give a hydraulic radius, R, and cross-sectional area, A, that will accommodate the required value of Q for a given gradient and value of Manning’s n. This can be solved using an iterative process.

The best way to undertake this iterative process is to prepare a spreadsheet based on the required cross sectional shape incorporating trial depths of flow and a high and low value of Manning’s n (Table 7.6). It can be seen that erodible velocities (>0.5 m/s) will occur once the depth of flow in a bare channel exceeds 0.25 m depth of flow. However when the channel is protected by standing stubble, a flow depth of 0.7 m will only be flowing at 0.17m/s.

Equation 7.1

Q A

= V = R0.66 S0.5

n Where

Q = the discharge or hydraulic capacity of the channel (m3/s) A = cross sectional area (m2) V = average velocity (m/s) R = hydraulic radius (m) S = channel slope (m/m) n = Manning’s coefficient of roughness.

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Table 7.6: Discharges and velocities for a range of flow depths for a trapezoidal-shaped contour bank with a gradient of 0.2%

Depth (m) Cross-sectional area (m2)

Hydraulic radius (m)

Manning’s n = 0.15 e.g. standing wheat stubble

Manning’s n = 0.03 e.g. bare cultivated channel

Velocity (m/s) Discharge (m3/s) Velocity (m/s) Discharge (m3/s)0.10 0.48 0.09 0.06 0.03 0.29 0.140.15 0.78 0.12 0.07 0.06 0.37 0.290.20 1.12 0.15 0.09 0.10 0.44 0.490.25 1.50 0.19 0.10 0.15 0.49 0.740.30 1.92 0.22 0.11 0.21 0.54 1.040.35 2.38 0.25 0.12 0.28 0.59 1.410.40 2.88 0.28 0.13 0.37 0.64 1.830.45 3.42 0.30 0.14 0.46 0.68 2.320.50 4.00 0.33 0.14 0.58 0.72 2.880.55 4.62 0.36 0.15 0.70 0.76 3.500.60 5.28 0.39 0.16 0.84 0.80 4.200.65 5.98 0.41 0.17 0.99 0.83 4.970.70 6.72 0.44 0.17 1.16 0.87 5.82

Bold colour indicates erosive velocities for a bare soil (>0.5 m/s)Parameters:Trapezoidal cross-section with inlet slope of 1:10, bank batter of 1:6 and bed width of 4 metresContour bank gradient of 0.2%

7.4.1 ExampleTo determine the constructed height for a contour bank to accommodate discharges of 0.4 m3/sec when a contour bay has a mature wheat crop (n = 0.15) and a discharge of 0.9 m3/sec when the contour bay is under bare fallow (n = 0.03). The contour bank is to have a trapezoidal cross-section with inlet batters of 1:10, bank batter of 1:6, bed width of 4 metres and a gradient of 0.2%. Assume that the bank will be built by a bulldozer and that it will settle by 50% after construction.

Solution:

Step 1. Use a spreadsheet to prepare a table similar to Table 7.6 showing velocities and discharges for the two values of n for a range of trial depths and for the assumed gradient, i.e. 0.2%.

Step 2. From Table 7.6 when n = 0.15 a flow depth of 0.4 m will have a discharge of 0.37 m3/sec with a velocity of 0.13 m/sec.

Step 3. From Table 7.6 when n = .03 a flow depth of 0.3 m will have a discharge of 1.04 m3/sec with a velocity of 0.54 m/sec. (Note that there is a risk of an erodible velocity for the bare soil condition).

Step 4. Table 7.6 shows that a depth of flow of 0.4 m would be sufficient to accommodate the required flow. (Note that if an alternative design was required an additional spreadsheet could be prepared based on a different gradient.)

Step 5. An allowance of 0.15 m for freeboard would give a recommended settled bank height of 0.55 m which can accommodate the mature crop condition.

Step 6. An additional 50% should be added to allow for settlement giving a constructed height of 1.1 m (Table 6.3, in Chapter 6).

Note: should the bank carry a design depth of flow of 0.4 m in a bare fallow condition, Table 7.6 shows that it would be carrying a discharge of 1.83 m3/sec at a velocity of 0.64 m/sec. Such a velocity is likely to be erosive but such an event

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would be rare as it is double the design discharge for bare fallow. Since bare fallow farming systems contribute to high rates of soil erosion in a contour bay, it is most desirable that a high-cover farming system is adopted rather than one that has bare fallows.

7.4.2 Contour bank design chartsDesign charts can be prepared to show how contour banks with a specified cross-section perform under a range of values of Manning’s n, gradient and flow depth. Figure 7.13 is an example of a contour bank design chart for a broad-based contour bank with a bottom width of 4 metres and batters of 1:6 and 1:10 (V:H). The three graphs illustrate the dramatic effect that surface roughness in the channel has on both velocity and discharge.

Figure 7.13: Contour bank design chart for a trapezoidal shape and a range of values for Manning’s n, channel gradient and flow depth

1.0 1.5 2.0 2.5 3.0 4.0 5.0 6.0 7.0 8.00.80.60.40.30.20.10.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

s = 0.1%

s = 0.2%

s = 0.3%

s = 0.4%

1.2

3

d = 0.1m

d = 0

.2m

0.1

0.3

0.2 0.3 0.4 0.6

0.4

0.5

0.6

0.7

5.0

3

0.8 1.0 1.5 3.02.52.0 4.0

0.8

0.06

0.06 0.1 0.2 0.3 0.4 0.6 0.8

3

1.0 1.5 2.0

0.2

0.1

0.0

0.1

0.2

0.3

d = 0.1m

s = 0.1%

s = 0.2%s = 0.3%

s = 0.4%

d = 0.2m d = 0.3m d = 0.4m d = 0.5md = 0.6m

d = 0.7m

d = 0.1m

s = 0.1%

d = 0.2m

CONTOUR BANKDESIGN CHARTS

26/7/99

10 611

CONTOUR BANK CROSS SECTION

d = 0

.5m

d = 0.3m

s = 0.3%

s = 0.2%

d = 0

.4m

s = 0.4%

d =

0.6m

d =

0.7m

d =

0.4m

d =

0.3m

d =

0.7m

d =

0.6m

d =

0.5m

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Figure 7.14 shows a graph for the same cross-section as Figure 7.13 but for a constant gradient of 0.2% and three values of Manning’s n.

Figure 7.14: Contour bank design chart for a trapezoidal shape and a range of values for Manning’s n and flow depth

1.0 1.5 2.0 2.5 3.0 4.0 5.0 6.0 7.0 8.00.80.60.40.30.20.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

3

0.3

0.4

0.5

0.6

0.7

0.8

0.2

CONTOUR BANKDESIGN CHARTS

6/1/99

10 611

CONTOUR BANK CROSS SECTION

0.9

1.2

1.1

1.0

s = 0.002m/m = 0.2%

0.06

0.1

0.0

0.1

0.0

n=0.15

n=0.05

n=0.03

Mannings nBare cultivated channel n = 0.03Channel with sparce grass cover n = 0.05Channel with standing wheat stubble n = 0.15

d=0.

2m

d=0.

3m

d=0.

4m

d=0.

5m

d=0.

6m

d=0.

7m

BANK GRADIENT = 0.2%

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7.5 Farming with contour banksMost contour bank systems in Queensland were constructed many years ago. They were designed to be cropped by working the bays along the contour parallel to the channels. This was comparatively simple with the smaller machinery and more flexible farming systems in common use 30 years or so ago. However, it has become increasingly difficult as agriculture has scaled up, become much more mechanised, and adopted new systems such as zero till and controlled traffic farming. These recent changes mean that in some instances we need to rethink how a paddock containing contour banks should be worked.

7.5.1 Controlled traffic farming and ‘tramlining’Controlled traffic farming (CTF) began in Queensland in the early 1990s. CTF is a system where all field traffic including the harvester is restricted to permanent wheel tracks referred to as traffic lanes. Global positioning systems (GPS) are often used to steer the machinery and keep it on the precise track. With ‘true CTF’ all machinery is modified to have the same wheel base so that the wheels of each piece of equipment travel over the exact same ground. ‘Tramlining’ is similar to CTF in that all pieces of equipment travel in the same lanes (i.e. parallel to a fence line) but with tramlining the wheel base of the various pieces of equipment varies so that their wheels do not necessarily travel over the exact same ground.

When a paddock is trafficked in the traditional manner by a variety of tractors, harvesters, implements and trucks with different wheel spacings, a considerable amount of soil compaction occurs in all areas of the paddock. With CTF (and to a lesser extent tramlining) this compaction is restricted to just the limited parallel wheel tracks. CTF has mostly been applied to broadacre cropping areas but is also being adopted in sugar cane and horticulture.

When combined with zero tillage, CTF can result in the following advantages:

• reduced overall compaction (especially when it is required that paddocks are trafficked when in a moist condition, as for example, when harvesting)

• more porous soils in the cropping bays, allowing movement of water, air, plant roots, and soil organisms and producing healthier plants ensuring higher yields

• more efficient farming operations, as minimal overlap and longer runs result in up to 25% reduction of fuel, seed, fertiliser, and chemical usage

• energy savings because of minimal overlap, tyres moving on ‘permanent’ compacted wheel tracks and tines working in uncompacted soil. This results in reduced greenhouse gas emissions.

7.5.2 Contour banks and CTF layoutsContour banks help to control erosion on sloping land by intercepting runoff before it concentrates and then channelling it to a safe disposal area such as a grassed waterway. The role of contour banks becomes particularly important in seasons when there is minimal ground cover.

Most contour banks are not parallel. This leads to inefficiencies and compaction when working along the contour between contour banks. It is now a common practice for farmers operating on slopes of less than 4% to work their paddocks perpendicular to, and crossing over, the contour banks, and often parallel to a fence line or block boundary—a practice referred to as ‘tramlining’.

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It should be noted that in most instances this practice has been adopted for convenience and is not necessarily part of a fully implemented CTF system.

When working a paddock parallel to a fence line as shown in Figure 7.15, the direction of wheel tracks and planting rows may vary considerably in relation to the slope. In some parts of the paddock, tracks will be virtually up and down slope whilst in other parts they will be generally across the slope. This means that any runoff flowing along and down the rows and tracks may concentrate and cause erosion in various parts of the paddock. If contour banks are spaced widely apart this problem will be accentuated. The risk of erosion will be particularly high where drainage lines have been eliminated by contour banks (‘contoured out drainage lines’). Figure 7.15 contains an example of such a drainage line.

Figure 7.15: Tramlines parallel to a fence line

7.5.3 CTF layouts on sloping land When implementing CTF, the aim is to keep runoff in the same wheel track or crop row as it flows downhill. If this can be achieved it will avoid runoff being concentrated and causing erosion. With traditional contour cultivation, runoff is often discharged into contour banks by rills. The resultant silt ‘slug’ can lead to waterlogging in the contour bank channel and bank failure may occur where banks are below specifications. With CTF, wheel tracks and crop rows will discharge their runoff evenly along the entire length of contour banks.

To avoid the situations represented in Figure 7.15, a CTF layout needs to be designed with the following principles in mind:

• Wheel tracks and crop rows should intersect contour banks at as near as possible to right angles (90°). Lower angles of intersection (down to as low as 45°) may be acceptable provided there is no significant rilling in the paddock.

• A herringbone pattern (see Figure 7.16) should be used on water-spreading areas such as ridges (divergent drainage).

• Additional waterways or drains may be required where runoff concentrates (convergent drainage), as shown in Figure 7.16.

Figure 7.16 shows an example of a CTF layout on a relatively flat (2%) slope. Contour banks that directed all runoff to the waterway on the edge of the paddock had been constructed prior to adoption of CTF. For the CTF layout, two additional waterways have been added to accommodate convergent drainage.

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Figure 7.16: A CTF layout with two herringbone patterns

500 metres

Original grassed waterway

Additional grassed waterway

Contour bank

Ridge line

Wheel track direction

The layout has two herringbone patterns and the layouts pivot on headlands on ridge lines and on either side of waterways. Access tracks could be incorporated along the ridgeline headlands, below the contour, or beside the watercourse, as shown in Figure 7.17.

Runoff from wheel tracks and furrows must be able to flow into either a contour bank or a waterway. In some cases it may be necessary to construct spur banks between contour banks as shown in Figure 7.17. These banks direct runoff into waterways through gaps in the waterway bank. Runoff directed to the waterway would cause erosion if it was not allowed to enter the waterway via the spur bank.

Figure 7.17: Spur banks allowing runoff from wheel tracks and furrows to enter the waterway

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Most CTF layouts in grain-growing areas have been applied to land with slopes that are 3% or less. On steeper slopes, it is progressively more difficult to construct and maintain the broad batters on contour banks that enable tractors, implements, and harvesters to traverse them. Figure 7.18 shows how direction of working can be varied between contour bays depending on the land slope. In this example, bays 1 and 2 with a 2% slope are worked up and down hill. Bays 3 and 4 with a 3% slope are worked parallel to the contour banks.

Figure 7.18: Parallel strips of cultivation between non-parallel contour banks on 3% slopes

As machinery traversing contour banks can reduce their capacity, it is important to monitor them regularly and to carry out maintenance when required.

7.5.4 Contour bank maintenancePoorly maintained contour banks are a liability and are likely to cause rather than prevent erosion. When a bank breaks, the outflow may cause severe erosion in the contour bay below and contribute to the failure of subsequent contour banks.

Erosion occurs spasmodically. It is not possible to predict when a ‘rogue’ storm is likely to occur, so contour banks must always be maintained to the correct capacity. A property manager should not get complacent about contour bank maintenance during a period of dry years when there is little runoff. In any 10-year period a few of the biggest storms—even in one year—may cause 80 to 90% of the total soil loss for that 10-year period.

Sediment deposited in contour banks may contribute to ponding at various points along a contour bank channel. Such ponding, as well as increasing the risk of the bank overtopping, can be costly to farmers as it inhibits planting, weed control and harvesting operations.

Contour bank capacity

Recommended contour bank capacity depends on many factors. The amount of runoff a contour bank has to carry depends on the length of the contour bank and the bank spacing (see sections 7.3.3 and 7.3.4).

Land slope has a big influence on bank capacity. A bank on low sloping land will be able to store much more runoff than a bank of the same height on steeper land. Contour bank capacity will gradually reduce over time. Although they may appear to be suitable, contour banks could be dangerously lacking in capacity if they have not been maintained for several years.

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Table 7.7 shows the effect of depth on the capacity of a typical broad-based contour bank channel with a small amount of stubble in the channel. As depth of flow increases, there is a marked increase in the velocity of flow in the channel and the contour bank is able to carry much higher amounts of runoff.

Table 7.7: Effect of depth of flow on the capacity of a contour bank

Depth of flow in the contour bank channel

(m)

Conditions in the contour bank channel

Crop or standing stubble Bare fallow

Discharge (m3/sec)

0.3 0.2 1.0

0.4 0.4 1.8

0.5 0.6 2.9

0.6 0.8 4.2

These results are based on a contour bank cross section and a contour bank gradient of 0.2%:

Factors that reduce bank capacity

Bank capacity may be reduced in the following ways:

• Sheet and rill erosion in the interbank area deposits sediment in the channel. Soil loss experiments on the Darling Downs have shown that paddocks with inadequate levels of surface cover may average soil losses of around 50 tonnes of soil per year. However around 80% of this soil may be deposited in the contour bank channel.

• Soil tends to move downhill during cultivating and ploughing.

• Tillage operations move soil downhill and flatten bank crests.

• Vehicle and animal tracks reduce bank height.

How to check capacity

Contour bank capacity can be checked by using a length of string and a line level as shown in Figure 7.19. Alternatively, a builder’s level can be used on a straight piece of timber.

Figure 7.19: Checking contour bank capacity with a line level

Cross sectional area, A(m2), is calculated by multiplying one half of length XY (metres) by bank height (h) (metres)—Equation 7.2.

Under opportunity cropping there may be minimal chances to carry out maintenance works, so the opportunity should be taken when it arises.

The capacity of contour banks can also be captured by traversing the bank using an RTK (real-time kinematic) GPS receiver on a tractor or vehicle. The logging interval needs to be set very short (e.g. 1 point/sec), and driving done slowly up and over the banks.

RulerLine level String lineX Y

h

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The resultant data gives a very good profile of the bank, from the comfort of the vehicle. With a small amount of processing, the wetted perimeter and capacity can then be calculated.

The first point for banks to fail will be any low spots in the contour bank (comparable to a small dam). The weakest link in a contour bank is often a point where the bank has been constructed across an old rill or gully line. Higher rates of settlement can occur at such points and the effective height of the bank may be much less than the average bank height. Such low points can be observed visually by looking upwards at the bank from the middle of the contour bay below.

While maintenance is being carried out, landholders should check for low sections in contour or diversion banks. Low spots are easy to identify when looking uphill so that banks can be seen silhouetted over the top of the ones below, when low places are immediately evident. These should be filled and made higher to allow for settling.

Figure 7.20: Checking for low sections by looking uphill

Special attention should be paid to bank outlets as it is these sections that carry the most water. Bank outlets are especially susceptible to failure if vehicles or animals cross the bank at this point.

Contour banks should also be checked for any piping or tunnelling that may be associated with cracks during a dry season, or animal burrows, and repairs undertaken as soon as practical.

Reducing maintenance costs

The following measures have been found to reduce maintenance costs:

• Maintain high levels of surface cover (at least 30%).

• Level out rills and gully lines in the contour bay.

• If crossing over contour banks in a controlled traffic system, use implements with adequate flexibility and ground clearance.

Maintaining banks

Sediment deposited in contour bank channels following high-intensity storms should be removed as soon as practical. Scrapers are ideal for collecting sediment from the channel and using it to fill the rilled areas above the channel.

For regular maintenance work, bank heights may be maintained by moving the sediment out of the channel and onto the contour bank. This maintenance is commonly carried out with a grader or dozer. A grader will work more efficiently if the soil has been previously ripped or cultivated; it is also useful for cleaning bank channels. A dozer blade or scraper is useful in repairing broken banks, which should be repaired as soon as possible to prevent further damage.

Equation 7.2

A (m2) = ½ X Y x h

For example: h = 60 cm (0.6 m) XY = 14 m A = 4.2 m2

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While many contour banks are constructed with a semi-trapezoidal shape, they tend to become triangular in cross-section after a few years of cultivation, as the upslope batter tends to be indistinguishable from the natural slope. The triangular cross-section appears to be the easiest shape for farmers to maintain and is most compatible with tillage and planting implements.

Bank capacity and maintenance in relation to CTF

Because of the need to traverse contour banks with tractors and harvesters with controlled traffic farming, there is a tendency for farmers to have their contour banks low as they believe it will be more practicable. However, the late 1990s and early 2000s have generally been periods of below average runoff with limited runoff events, and many farmers have become complacent about contour bank maintenance. Many farmers who believe that their contour banks are suitable for traversing actually have banks with inadequate capacity.

Tillage equipment traversing contour banks will reduce bank capacity by ‘dragging down’ the crest of the contour bank. Additionally, wheel tracks in wet weather can be up to 15 cm deep. This effectively reduces contour bank height, resulting in lowering the capacity of a contour bank. Table 7.8 indicates that for a triangular-shaped contour bank on a 2% slope, the reduction in effective height from 60 cm to 45 cm would reduce cross-sectional capacity by 44%, that is, from 10.1 m2 to 5.7 m2.

Table 7.8: Effect of contour bank height on cross-sectional area

Contour bank height (m) Channel cross-sectional area (m2)0.4 4.5

0.45 5.70.5 7.0

0.55 8.50.6 10.1

Parameters:Land slope 2%Bank batter 1:6Upper bank batter conforms to the 2% land slope

Wheel tracks and press wheels on planters can also lead to the development of cracks as soils dry out. Such cracks can lead to bank failure if they fail to seal up before a runoff event occurs.

The risk of failure of contour banks under a controlled traffic layout will be greater on steeper slopes where a contour bank of a given height will have much less capacity than it will on a low slope. If banks are very low, it may be beneficial to hire a contractor to bring them up to specifications, but usually, a few extra runs with a disc plough along the bank will do the job.

Emergency maintenance

Any breaks that occur should be mended as soon as possible, even if it means running over an area of crop. Equally important are deposits of silt that sometimes occur when rows break over in cultivated crops. If not removed, these will partially block channels or outlets and could cause failure of the whole system. A dozer, bucket loader or grader blade is the best type of equipment to use for this job.

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References

Connolly, RD, Silburn, DM, and Barton, N (1991) Design of long contour banks in south-west Queensland. Information series No: QI91014, Department of Primary Industries, Queensland.

Freebairn, DM and Wockner, GH (1986) A study of soil erosion on vertisols of the eastern Darling Downs, Queensland, II , The effect of Soil, Rainfall, and Flow Conditions on Suspended Losses. Australian Journal of Soil Research 24, 135–158.

Galletly, JC (1980) Design and efficiency of contour banks. Agricultural Engineering Conference, Geelong, 1980.

Scarborough, R, Stone, B, and Glanville, T (1992) Specifications for erosion control measures. In: Land Management Manual, Coastal Burnett Districts, Queensland Department of Primary Industries, Brisbane.

Stephens, RM (1987) Water flow at the outlets of contour banks, Division of Land Utilisation, Department of Primary Industries, Brisbane.

Titmarsh, G and Loch, R (1993) Towards more efficient soil conservation layouts, report to the Cotton Research and Development Corporation, Department of Primary Industries, Queensland.

Other information

For further information on contour banks and managing erosion on cropping lands consult the Queensland Government website <qld.gov.au/environment/land/soil/erosion/management/> and the following fact sheets:

• Erosion control in cropping land

• Runoff control measures for erosion control in cropping land

• Controlled traffic farming—Soil conservation considerations for extensive cropping

• Maintaining contour banks

• Contour bank specifications.

7.6 Further information

http://www.qld.gov.au/environment/land/soil/erosion/management/

http://www.qld.gov.au/environment/land/soil/erosion/management/

chapter 7 contour banks - publications.qld.gov.au€¦ · 7–8 7.3 design criteria contour banks...

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