This Agnote explains the options available for draining excess subsurface water to a specific depth from the soil profile. The most suited drainage system is influenced by topography, rainfall, outfall type and location and especially soil type. More detailed explanations of each drainage type and how to determine which drainage system to use are covered in greater detail in other Agnotes in the "Managing Wet Soils" series. Many soils in southern Victoria used for dairy farming have had their drainage characteristics assessed.
Lecture 46: Subsurface drainage design-1
Many of these soils are subjected to waterlogging for several weeks or months each year, in the wetter years. Many farmers are often unsure of the physical and chemical characterisation of their soil types. This must be determined to ensure the correct drainage system is installed and is best suited to their soils.
Drainage systems are very expensive to install but their short and long term benefits generally outweigh their initial cost, provided they are well maintained. Most wet soils problems are the result of waterlogged soil profiles not just surface water. However, surface drains should be installed to drain excess surface water off paddocks before installing subsurface drains See Agriculture note AGO M anaging wet soils: surface drainage.
This will reduce the load on the underground drainage system thereby reducing its cost as an effective system can be installed at less cost.
Types of subsurface drainage pdf editor
Costing of drainage systems has shown that both increased pasture utilisation and extra pasture growth from drainage can be very competitive with other feed sources.
However to take advantage of this extra growth, more cows, and farm improvements including a feed pad may be necessary. However, subsurface drainage provides many intangible benefits such as improvement in soil health due to the increased aeration of the soil, increased responses to fertiliser use, reduced mineral imbalances in the soil and sometimes in the plants as well, are long term sustainability benefits. Salinity can be prevented or greatly reduced if in its early stages.
Therefore the resultant fodder has the potential to be of much higher quality producing improved animal production.
Drainage, by reducing pugging and creating favourable soil environments, results in the retention of the improved pasture species versus the influx of plants suited to waterlogged conditions such as rushes, fog grass, glyceria species, water couch, etc.
Most importantly, reduced stress in managing stock and pastures during the wetter months of winter or spring has very large benefits for the farm operator. Subsurface pipes can be used to drain heavy clay poorly drained soils successfully but their spacing would need to be so close together that they are uneconomic in extensive farmland systems.
Subsurface pipe drainage was referred to as 'tile drainage' in the past due to the use of short clay pipes. Clay was expensive and difficult to lay and has now been replaced by slotted PVC or flexible corrugated plastic pipes of variable diameters. Specifically designed drainage trenchers Figure 1 , usually fitted with laser guidance equipment, dig the trench, lay the slotted pipe and place permeable backfill into the trench on top of the laid pipe.
Benefits of a subsurface drainage system
This backfill is delivered by trucks or trailers fitted with conveyor belts which feed the backfill into the hopper. The forward speed, hopper channel opening size, material size, etc. In very permeable soils, very little backfill is needed but in less permeable soils, or where moles are to be pulled through above a pipe, the backfill depth reaches to near the ground surface.
Mole drainage can be classified as mole drains, mole drains over a collector pipe system or gravel mole drains. The action of the mole plough forms a mole channel in the area of the soil profile with a specific clay content. The plough also cracks the soil profile immediately above the mole channel allowing water to flow into it.
Mole drains are used in heavy soils where clay subsoil near moling depth to cm prevents downward movement of ground water. The success and longevity of mole drains is dependent on soils having a high clay content so that once a mole channel is formed, it will maintain the channel for many years.
Mole drains are not suited to soils with clay types which have dispersive or slaking characteristics.
Subsurface Drainage Design and Installation
A mole plough Figure 2 is used to form mole drains. Simply, a mole plough contains a leg or blade to which a torpedo or foot is attached to its bottom. Sometimes a plug or expander and having a slightly larger diameter, is attached to the rear of the torpedo, and ensures the mole channel is left with the correct shape. This system is used in soils where it is not possible to form moles that reach the outfall.
This includes the presence of stones, sandy pockets, uneven surfaces or excessive distances to the outfall. In heavy soils where mole drains would need to be very long over 80 m before they reach an outfall, installing subsurface pipes at approximately 60 to m, over which mole drains are pulled, can be very successful.
Washed sand or small diameter gravel is backfilled into the pipe trench to near the ground surface at installation. Mole drains are then installed at or close to a right angle to the direction of the pipes.
What is Subsurface Drainage?
Excess ground water flows into and along the mole drains, then drains into the porous backfill above the pipes, and then is quickly removed to outfalls via the subsurface collector pipes.
Gravel mole drains are best suited to soils and situations where subsurface pipes are unsuitable, where mole drains have a very short life span, or in slaking soils so the mole channel will maintain its shape at or soon after moling.
Unfortunately there are very few gravel mole drainage machines Figure 5 available in Australia. Also gravel mole drains are expensive due to the amount of backfill and the close spacing required.
However, they do offer an alternative in some "difficult to drain" situations. They may be useful in slaking and dispersive soil types but expert opinion should be sought if considering their use in these situations.
These drains are installed at the base of slopes at the change of gradient, usually where a steeper slope meets the flats to intercept the downhill flow of subsurface water Figure 6.
Often the soil type on the slope is more permeable than those of the flats and this forces the water to come to the surface, usually at the change of slope. Interceptor drains can also be installed below springs and spring lines to intercept spring water.
Grazing animals severely pug the areas surrounding springs and damage is usually more concentrated down slope. This affected area increases over time as the 'soak' area spreads outward and down slope. Drainage reduces stock damage, or pugging as the soil maintains its strength and so, structure. These remove water from aquifers so that the water table will fall or be maintained at a suitable level below the ground surface.
The act of pumping causes a drawdown of ground water leading out from the pump's location with its effect being much less at depth. The extent of effect will depend on aquifer depth, soil type, height of water table, etc. The cost, benefits, disadvantages, and most importantly, whether they can be used or not and the need for a permit must be discussed with the Regional Rural Water authorities. Ground water pumping will not be discussed further in this Wet Soils Management series.
To decide which drainage system to install, the soil's characteristics, its permeability speed at which water can move through the soil , and suitability for mole drainage clay type and content must first be determined.
Farmers and drainage contractors can often decide which drainage option to use based on some simple on-farm tests. For a detailed explanation of these tests, see Agriculture note: AG Managing wet soils: determining which subsurface drainage system to use. The Water Act provides guidance for the management of waterways and swamps. Before considering draining a wet area you should contact your local Catchment Management Authority and Regional Water authority for advice, as a permit may be required.
AG Managing wet soils: pipe drainage AG Managing wet soils: mole drainage AG Managing wet soils: determining which subsurface drainage system to use.
The previous version of this Agnote was developed by David Hopkins and was published in October This publication is copyright. No part may be reproduced by any process except in accordance with the provisions of the Copyright Act The advice provided in this publication is intended as a source of information only.
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Types of subsurface drainage systems
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Types of subsurface drainage systems Note Number: AG Published: October Updated: October This Agnote explains the options available for draining excess subsurface water to a specific depth from the soil profile.
Introduction Many soils in southern Victoria used for dairy farming have had their drainage characteristics assessed.
Benefits of a subsurface drainage system Costing of drainage systems has shown that both increased pasture utilisation and extra pasture growth from drainage can be very competitive with other feed sources. Types of subsurface drainage systems There are four main types of subsurface drainage systems.
Drainage trencher installing subsurface pipe drains and depositing permeable backfill on top. Fig 2. Mole plough.
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Fig 3. Mole drain over collector pipe System. Fig 4. Gravel mole drain slot filled with gravel. Fig 5. Gravel mole drain machine. Fig 6 Interceptor drain. Back to top.
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