Note Number: AG0946
Published: December 2005
Updated: April 2010
Surface drainage is very useful in removing excess water from land in a controlled manner and as quickly as possible, to a artificial drainage system or a natural watercourse. This should be done with no damage to the environment.
Wet soil problems on dairy pastures are usually due to both excess surface and sub surface water. It is important to realise that surface drains will only remove surface water, and in most soil types, are almost useless for draining the soil profile.
Never-the-less surface drains are essential and very useful tool to manage wet soils when used in the right location for the right reasons. Typical applications for using surface drains include:
- draining large quantities of surface water off land quickly
intercepting water flowing down from upper slopes to lower lying land.
- intercepting run off water from other sources (eg. road side drains, neighbouring properties, dam overflows, etc.
- acting as collector system for sub surface (mole or tile drains) drainage systems.
Types of surface drainage systems
1. Ditches or open drains
These vary in size and length and can be simply categorised as follows:
- Shallow surface drains (up to ~0.3m deep) formed by hand shovel or "spinner cuts" are most useful for removing ponded water from shallow depressions to larger drains or streams. They are generally not suitable for draining large flat areas. They tend to be temporary in nature since they are often pugged by animals, silt up quickly, over grow with less desirable species, wrecked by machinery in wet weather and so on. They are very inconvenient for feedout or passenger vehicles and fodder conservation equipment.
- Medium sized drains (0.3m to ~ 1m deep) made by excavators or specific drainage machines are usually "V" shaped with a flat bottom with enough slope (gradient) to ensure that the water is removed quickly but not cause scouring of the drain walls or bottom. This is usually not a problem because they are used on the flatter areas. The velocity of water flow should be slow on the sandy and sandy clay loam soils and can be quicker on the clays and clay loams.
The slope or batter of the drain banks should be such that the banks do not collapse when wet. Each soil type has its own natural slope that will not collapse and machine formed banks should be similar or even less steep. Clays may have a batter of 1:1, silty and clay loams 1:1 to 1:1.5, and sandy loams 1:1.5 to 1:2, while looser sandier soils should be 1:2 to 1:3+. Figure 2 shows what happens to drains with insufficient batter angle.
The size of open drains depends on the amount of water to be removed or intercepted and formulae are available to calculate the required design parameters.
The time to construct open drains is when the topsoil and sub soil is soft enough to dig but water presence is not a problem, usually in late spring early summer and autumn.
Caution: be very wary of constructing open drains in dispersive soil as they are highly prone to erosion.
- large open drains or canals ( several meters deep and wide) made by dragline excavators, scrapers or bulldozers carry very large volumes of water and generally not applicable to most dairy farms apart from possibly delivering irrigation water or its drainage so will not be covered here.
Disadvantages of open drains
- They need regular cleaning (sediment and weeds)
- Can be dangerous to animals, machinery, and people (if unfenced)
- If fenced, cleaning is more difficult
- Fencing adds extra cost and maintenance
- Tend to become deeper and wider if not maintained by professional machinery operators
- Not suitable for sub-surface drainage
- Loss of ground
2. Grassed waterways
Grassed waterways are usually shallow, varying in width from narrow to meters wide, but are constructed such that they are often grazed as part of the paddock. They are sometimes used to bring drain outflows down slopes to prevent erosion without considerable expense.
Their steepness should be less than 1:4 (vertical : horizontal). A dense and unbroken grass sward is essential and should be established before it is used to discharge water. Avoid grazing the waterway while it is wet or draining water, particularly if the pasture is not well established.
The amount of water to be removed and the steepness of the slope will determine the size and shape of the waterway. A good rule of thumb is that the steeper the fall the wider the waterway. The bottom of the waterway should be as near to horizontal in cross section as possible so that the water spreads out evenly over the base.
The waterway must not be constructed in soils prone to erosion. Water passing down slopes which have clay sub soils should avoid saturation of large areas of this clay as it may result in landslips. Water passing down slopes should do so quickly in as narrow a channel as possible but avoiding the possibility of erosion. Dense pasture swards assist greatly in soil stabilisation.
3. Humps and hollows (bedding)
Hump and hollowing (see Figure 3) is the practice of forming (usually while renovating pastures) the ground surface into parallel convex (humps) surfaces separated by hollows. The humped shape sheds excess moisture relatively quickly while the hollows act as shallow surface drains.
Humps and hollows are useful in areas or on soil types that are not suitable for tile or mole drainage. They are also useful where the lack of suitable outfalls prohibit the use of tile drains, usually due to insufficient depth or fall.
The humps and hollows system comprises lateral surface drains which discharge into headland drains which in turn discharge via short open drains or shallow pipes into natural watercourses or open drains (see figure 4). If possible these areas should not be grazed while the drains are still wet because they will become pugged. If this does occur then end of season maintenance must ensure that all the drains are reshaped or cleared to allow quick drainage.
The size of this drain type depends on the amount of water to be removed or intercepted and formulae are available to calculate the required design parameters.
Humps may vary from 10 – 20 m spacings but the wider the hump, the more soil that must be moved, and the water run off may be substantially slower. Humps wider than this may not drain at all in their centres. Gippsland experiences suggest a spacing of approximately 15 m width and approximately 0.4 – 0.5 m depth. For drains discharging in one direction, lengths should not exceed about 150 m, but can be longer if discharging in two directions.
Humps and hollows make it more difficult for machinery activity such as fodder conservation. The humps may be overgrazed while the hollows may become pugged. The depth of topsoil must be enough to allow the humps and hollows to be formed without exposing the poorer and nutrient deficient subsoil. Alternatively more soil will be required to be moved to ensure a covering of topsoil in the hollows for resowing pasture. If the edge of the hump is too steep cattle trampling could cause soil movement into the hollows and possibly into watercourses.
4. Levees or graded banks
These are surface channels, often formed on sloping land, in such a way that the earth removed forms a bank or levee on the downslope of the channel. Where used on slopes, the main purpose of graded banks is to prevent surface runoff from building up volumes and velocity that may cause erosion.
The banks must follow the contour of the slope with a gradual decline so that water flows constantly and does not build up in depressions, thus rendering land below susceptible to "landslips ". The outfalls of the graded banks may possibly lead to dams for water collection for irrigation, stock water, etc. Bank spacing will depend on slope gradient, amount of water to be collected, outfall locations, slope topography and amount of rainfall.
Spacings of about 30 - 50 m for slopes with gradients of 50 - 120 have proven acceptable in most areas. Each bank should service a total area of up to about 3 hectares, with length not exceeding about 400m.
The channel will be at risk to erosion immediately after construction until a grass cover is established. Care must be taken on slopes that may have shallow topsoil over dispersive subsoil, as severe erosion is possible in these soil types.
Levee banks may be used to confine large flows of water to certain areas and are often used to control floodwaters along streams. However approval for use of levee banks in this situation is required from the local or regional water authorities.
5. Laser levelling (grading)
Laser levelling is a relatively new practise used on irrigation farms to even out and speed up the water flow off flood irrigation bays. Although not common laser levelling could be used on the "flats " areas in high rainfall zones to ensure water moves off quickly and does not remain in depressions.
The Water Act
The Water Act (1989) provides guidance for the management of waterways and swamps. Before considering draining a wet area you should contact your local Catchment Management Authority for advice, as a permit may be required.
This Information Note was originally written by Frank Mickan, Ellinbank
Published and Authorised by:
Department of Environment and Primary Industries
1 Spring Street
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