LO 6.0 - Explain the role and principles of drainage

Question 1

Question

What are the causes of poor drainage?

Name 4

Drainage Causes

Answer 1

There are 4 main causes:
* Fine textured soils
* High water table
* Perched water table
* Springs/seepage

In addition, any soil that has been compacted will drain less well because the largepore spaces have been destroyed.

Drainage Causes

Question 2

Question

How does Fine-textured soil cause poor drainage?

Drainage Causes

Answer 2

Fine textured soils:

• Soils with high clay content tend to have poor drainage due to their small particle size and tendency to compact, which restricts water flow. They are also prone to waterlogging, leading to poor root development and plant growth.

Drainage Causes

Question 3

Question

How does a High water table soil cause poor drainage?

Drainage Causes

Answer 3

High water table:

• The water table is the level in the soil profile below which soil is saturated. It moves up and down with the seasons; higher in winter and lower in summer.

Drainage Causes

Question 4

Question

How does a Perched water table soil cause poor drainage?

Drainage Causes

Answer 4

Perched water table:
A perched water table occurs when a layer of soil with low permeability overlies a layer of more permeable soil, creating a barrier that traps water.

• A layer of finer material like clay.
• A Plough pan.
• A layer of rock.

Drainage Causes

Question 5

Question

How does a Spring or Seepage soil cause poor drainage?

Drainage Causes

Answer 5

Spring/Seepage:

• In sloping ground, where an impervious layer intersects the land surface, water can flow out at the surface. This leads to persistently or intermittently wet areas downslope.

An aquifer is a body of permeable rock that contains or transmits water.

Drainage Causes

Question 6

Question

What are some signs and indications of poor drainage?

Indications of poor drainage

Answer 6

Signs that a piece of land is poorly drained include:

• Indicator plants (rushes, sedges, creeping buttercup, flag irises), particularly in grassland
• Poor or patchy crop growth
• Shallow root systems ie.. not saturated, air available
• Standing water flooding, puddles or ponds.
• ‘Poaching’ the breakdown of soil structure caused by the passage of animals over an area of wet soil.

Poaching

Indications of poor drainage

Question 7

Question

What are the effects of poor drainage on roots?

Indications of poor drainage

Answer 7

Effects of poor drainage on roots

• Roots require oxygen for respiration and in waterlogged conditions roots will die.
• Early in the growing season, roots may remain shallow to find air. Later shallow roots may leave the plants at risk of drought stress.

Indications of poor drainage

Question 8

Question

What are the effects of poor drainage on nutrients?

Indications of poor drainage

Answer 8

Effects of poor drainage on nutrients

• Breakdown of OM is slower in waterlogged soils. Bacteria and fungi need oxygen.
• Nitrogen is easily lost to the air, and nitrogen fixing by bacteria are impaired in waterlogged soil.
• Roots that are compromised by waterlogging are less effective at absorbing nutrients.

Indications of poor drainage

Question 9

Question

What are the effects of poor drainage on soil structure?

Indications of poor drainage

Answer 9

Effects of poor drainage on soil structure

• Wet soils lose strength and are more prone to compaction. Compaction adds to the problems of poor aeration.
• Wet soils are also less effective at providing anchorage for plants this can cause trees to fail.

Pond and fallen tree.

Indications of poor drainage

Question 10

Question

What are the effects of poor drainage on soil biota?

Indications of poor drainage

Answer 10

Effects of poor drainage on soil biota

• Microorganisms are reduced in waterlogged soils. Plants depend on some of these.
• Earthworms are compromised in very wet soils.

Indications of poor drainage

Question 11

Question

What are some other effects of poor drainage?

Indications of poor drainage

Answer 11

Some other effects of poor drainage

• Fungal rot is more prevalent.
• Wet soils are slow to warm up. ‘window of workability’
• Harvesting is more difficult.

Indications of poor drainage

Question 12

Question

Why would you drain soil?

Purpose and Benefit

Answer 12

• The purpose of draining soil is to achieve greater depth between the surface and the water table. This depth is available for root growth.

Purpose and Benefit

Question 13

Question

What are benefits of draining soil?

Purpose and Benefit

Answer 13

Benefits of drained soil

• Improved soil workability and trafficability.
• Greater rooting depth and improved growth/productivity.
• Reduction in fungal diseases, seedlings/young plants.
• More rapid soil warming in spring.

Purpose and Benefit

Question 14

Question

What are the Negatives of draining soil?

Negatives

Answer 14

Negatives of drained soil

• Habitat destruction - Wetlands and bogs.
• Carbon loss - OM breakdown incresases.
• Nutrient loss - Washed away to local rivers and streams
• Contamination of watercourses -
• Risk of flooding elsewhere - Alter hydrology, flooding downstream

Negatives

Question 15

Question

What are methods of investigating drainage problems?

Investigating drainage

Answer 15

Investigating drainage problems.

• Desk study - Online sources about topography and water courses. Climatic data, local knowledge.
• Test pits - Dig pits in well drained and poorly drained soils, at top middle and bottom of hills.
• Safety and test pits.
• Examination of the soil - Take samples and assess texture, colour, rusty is intermittent and blu soil is poorly drained gley. Check root depths.
• Levels - Check for hills and slopes

Investigating drainage

Question 16

Question

What are the types of drainage systems?

Drainage Systems

Answer 16

Types of drainage systems

• Groundwater drainage systems
• Shallow drainage systems

Drainage Systems

Question 17

Question

Describe ground water drainage systems?

Groundwater drainage systems

Answer 17

Ground water drainage systems

• A network of pipes in a free-draining layer.
• Leads to larger drain, then to water course.

Groundwater drainage systems

Question 18

Question

Why is there often a permeable layer at depth?

Groundwater drainage systems

Answer 18

Many of our soils were formed from glacial deposits. The heavier, coarser particles were dropped first, the lighter material (silts and clays) on top. That’s why there is often a permeable layer below a poorly drained one.

Groundwater drainage systems

Question 19

Question

What are some characteristics of a Groundwater drainage system regarding depth?

Groundwater drainage systems

Answer 19

Drain depth

• Determined by the depth of the permeable layer.
• Could be anywhere from 0.8m to 2.5m
• A minimum depth of about 0.6 m is required to allow for machinery traffic.

Groundwater drainage systems

Question 20

Question

What are some characteristics of a Groundwater drainage system regarding spacing?

Groundwater drainage systems

Answer 20

Drain spacing

• Hydraulic conductivity of the soil
• Depth of permeable layers
• Typical rainfall amounts
• Depth of water table you are trying to achieve
• Pipes closer together in heavy soils (5 to 15 m). Lighter soils, 10m-30m
• The cost of drainage increases as spacing decreases.

Groundwater drainage systems

Question 21

Question

What do you know about drainage pipes?

Groundwater drainage systems

Answer 21

Drain pipes

• Corrugated plastic pipes, perforated, typically 6 - 12 cm diameter.
• Smooth plastic pipes, perforated, typically 5 - 9 cm diameter.
• Older tile drains: porous clay/ceramic pipes laid end to end, typically 7.5 cm diameter.

Groundwater drainage systems

Question 22

Question

What do you know about drainage layouts?

Groundwater drainage systems

Answer 22

Drain layouts

• Parallel network drains for flat or evenly graded land. Field drains run across the slope and the main drain runs down the slope.
• Where falls are uneven, a herringbone system allows the field drains to feed into main drains at an angle.
• Interceptor drains can be placed at the base of a slope to ‘intercept’ water moving into the land from higher ground.
• ‘Natural’ drain layouts can be used to deal with springs and seepage that affect isolated areas.

Groundwater drainage systems

Question 23

Question

What do you know about drainage stones?

Groundwater drainage systems

Answer 23

Drain stones

• Pipes are surrounded by a layer of drainage stone, typically 10 - 40 mm.
• The stone helps to speed water movement to the pipes, it filters out sediment that would otherwise silt up the pipe and it helps to support and protect the pipe.

Groundwater drainage systems

Question 24

Question

What are Shallow drainage systems?

Shallow drainage systems

Answer 24

Shallow drainage systems
* Where soil permeability is low in heavy soils.
* Moling and subsoiling are used together with piped collector drains.

Shallow drainage systems

Question 25

Question

What is Land forming?

Shallow drainage systems

Answer 25

Land Forming

• Aims to eliminate unevenness in the land creating an even fall across the whole field to an open drain.
• If a lot of soil needs to be moved, strip off the topsoil, regrade or level the subsoil and then replace the topsoil.

Shallow drainage systems

Question 26

Question

What is Mole Drainage?

Shallow drainage systems

Answer 26

Mole Drainage

• A mole plough is a bullet shaped implement (the mole), 7 - 8 cm diameter, attached to a narrow leg. A cylindrical expander, slightly larger than the mole, is attached to the back of the mole.
• Mole ploughs create channels in the soil, 1.5 to 2.0 m apart and up to 60 cm below the surface. These channels (or mole drains) carry water to piped drains or open drains at right angles to the mole drains.
• Mole drains can last from 2 - 6 years after which they need to be re formed.
• Work well in clay with high clay content.

Shallow drainage systems

Question 27

Question

What is Gravel Mole Drainage?

Shallow drainage systems

Answer 27

Gravel Mole Drainage

• In soils with a lower clay content, and peat soils, the moles will collapse. Gravel can be fed into the mole channels from a hopper as the moles are created.
• Gravel moles last much longer than moles without gravel and are more expensive to install.

Shallow drainage systems

Question 28

Question

What is Subsoiling?

Shallow drainage systems

Answer 28

Subsoiling

• Consists of loosening of the soil and is useful with very hard or compacted soils.
• An implement similar to a plough, but deeper, is used to shatter compacted layers.

Shallow drainage systems

Question 29

Question

What Drain maintenance should be carried out?

Drain maintenance

Answer 29

Drain maintenance
* Regular inspection and maintenance.
* Fine soil particles can block pipes.
* Plants/roots can be a problem in open drains and at pipe outlets.
* Blockages can occur anywhere but most likely at pipe outlets and junctions.

Drain maintenance

Question 30

Question

How do you design to reduce blockages?

Drain maintenance

Answer 30

Design to reduce blockages

• Keep network as simple as possible.
• Avoid junctions if possible.
• Install access points at junctions (mapped).
• Protect outlet points at installation

Drain maintenance

Question 31

Question

How do you clear out pipes?

Drain maintenance

Answer 31

Clearing pipes

• Pipes leading into open drains should be cleaned regularly – drain jetting.
• 2-3 years after installation and then 3-5 yearly.
• Hose is fed into pipe to wash sediment out.
• Purpose-built equipment – approx. €80 per hour.
• Rodding can clear blockages near outlet.

Drain maintenance