An Introduction to Agricultural Tile Drainage

John Panuska PhD, PE

Natural Resources Extension Specialist Biological Systems Engineering Department

UW Madison

SWCS Meeting September 21, 2012

Tile System Design Objectives

Maintain water table at proper level for healthiest plant growth.

Keep soil voids free of excess water, which

permits air flow and allows important biological processes to take place in soil.

Minimize inefficient equipment operation caused by wet areas.

Improved Root Development

Tile System Design Objectives

Benefits of Subsurface Drainage

Increase crop yields and field trafficability.

Greater soil water storage capacity. Conserve topsoil by reducing runoff.

Raises soil temperature

Dry soil is warmer than wet soil. It takes 5 times as much heat to raise wet soil 1 as it does an equal volume of dry

soil.

Environmental Risks of Tiles Increased export of nutrients (NO3 and P),

pesticides (Atrazine) and pathogens.

Surface inlets act direct conduits to receiving waters.

Macro-pores (roots and earth worm holes) are natural direct conduits.

Drainage of wetlands is illegal !

Environmental Risks of Tiles - Macropores -

Preferential flow

– Earthworm burrows

– Root holes

– Shrinkage cracks

– Structural porosity

Dyed Burrows

Dyed Burrow

0 -

1 -

2 -

3 -

Drainable Water

Natural or Random

Follow natural depressions. Used frequently

in “pot hole” landscapes to drain isolated depressions.

90

92

90

Herringbone

Advantageous for heavier soil often found in narrow depressions.

Double drainage

around main. More junctions –

Added cost

90 92

Gridiron

Drainage of level areas, uniform slopes

and soils w/ wide-spread wet areas.

One main or sub-

main serves as many laterals as possible

Outlet

Cutoff or Intercept

Sloping water table creates wetted area.

Used to intercept shallow lateral flow.

Impervious Layer

Tile Drain

Water Table

Clay pan or tight subsoil

Seep Area Tile Drain

Tile System Components

1. Laterals are the initial collectors.

2. Sub-main or collector-main

collect from laterals.

3. Mains collect from sub-mains and collector-mains. Main or Sub-main

Air vent line and riser

Main or Sub-main

Air vent line and riser

Main or Sub-main

Air vent line and riser

Main or Sub-main

Air vent line and riser

Laterals

Main or Sub-main

Air vent line and riser

Laterals

Main or Sub-main

Air vent line and riser

Laterals

Main or Sub-main

Air vent line and riser

Laterals

Outlet

Tile System Layout Always start with contour map of the field

Put mains and sub-mains on steepest grades to decrease pipe size.

Field laterals places on contours to maintain a uniform depth and improved drainage uniformity.

Surface Inlets

Sediment entry into tile lines

Adverse water quality impacts.

Equipment maneuverability

Grass

Hickenbottom Inlet

Blind Inlet (French Drain) Can be used when water quantity to remove is small or

sediment load is high

Do not function satisfactorily for more than a few year

They are expensive to construct, but do not interfere with farming operations

Field Locating Tile

As-Built Plans Recorded during or shortly after installation. Not often available

There is no hard and fast method !!

Field Locating Tile Remote Sensing

Color Infrared (CIR) photos can indicate tiles via plant stress or soil moisture. Dry soils appear light, while wet soils are dark. Photo taken in the spring. Photo by Ashok Verma

Field Locating Tile

Electronic soil survey map from Outagamie Co., WI

Pattern tile laterals at ~ 60 ft spacing.

Air Photos

Drain spacing, water table depth and crop response

Field Locating Tile

Field Locating Tile

Ground Penetrating RADAR

Ohio study was able to locate 72% of the total amount of drainage pipe present at 13 test plots. Does not work well in WI calcareous soils.

Drain Tile Installation Equipment

Tractor Backhoe Tile Plow

Chain Trencher Wheel Trencher

Drain Tile Materials

Clay Tile (organic soils) Concrete Tile (mineral soils)

Drain Pipe Materials - Polyethylene Plastic -

Single wall slotted Dual wall (not slotted)

Gravity Outlet

MUST have sufficient grade for gravity flow !

< set preliminary grade> - If not, a pump station will be necessary.

Receiving water must have adequate capacity.

Provide guards to keep animals out.

Daylight outlet pipe 1 ft above base flow in receiving channel

Drainage Pump Stations

When you don’t have the fall to use gravity ….

Hydraulic Design Considerations

Drain System Capacity Drainage Coefficient (Dc) = Depth rate (in/day) Area x (Dc) = [ac] • [in / day] / 23.8 = Flow rate (ft3/sec)

Profile View

Area 1

Plan View

Area 2

1 2

L

S S

Also applies at the whole field scale

Drainage Coefficient

Drainage coefficient (Dc ) is a desired water removal rate.

The Dc equals the volume (depth (in) x area (ac)) of water to be removed from a field in 24 hours.

Drainage area can be computed from the length and spacing of the drains.

Recommended Drainage Coefficients (in/day) for Pipe Drains in Humid Areas

Crops and Degree of Surface Drainage

Mineral Soil (clay and silt)

Organic Soil

Field Crops Normal With Blind Inlets With Surface Inlets High Value Crops Normal With Blind Inlets With Surface Inlets

3/8 – 1/2 1/2 – 3/4 1/2 – 1.0

1/2 – 3/4 3/4 – 1.0 1.0 – 1.5

1/2 – 3/4 3/4 – 1.0 1.0 – 1.5

3/4 – 1.5 1.5 – 2.0 2.0 – 4.0

This is essentially a simple risk management framework

Lateral Depth and Spacing

A relationship exists between depth and spacing of drains. For soils of uniform permeability, the deeper the drains, the wider the spacing (within limits). Need to provide adequate root depth above the saturated zone.

S d

Lateral Depth and Spacing

Varies with soil permeability, crop and soil, kind of management practices crop, extent of surface drainage. Typical drain depth range = 3 to 6 ft. Typical spacing = 30 to 100 ft. Depth / spacing balance to minimize cost. Minimum cover greater than 2.5 ft.

2

21

2

2 )4()8(L

hKL

hdKDC ∗∗+

∗∗∗=

Hooghoudt Equation, 1940

Drain Depth / Spacing - Equation

Image from Gary Sands – U of MN

Dd

m

h

Drain Spacing, L

confining layer

equivalent confining layer

soil surface

Dd water table

tile drain

K1 (in/day)

K2 (in/day) Flows horizontally toward drains

DRZ

Photo from Gary Sands – U of MN

Flow Though Porous Media

P. Gradient

Gravity

Drain Depth / Spacing - Table

Varies with soil permeability, crop and soil management practices, kind of crop, extent of surface drainage.

Soil Texture

Spacing (ft)

Depth (ft)

Clay Clay Loam

Average Loam

Fine Sandy Loam Sandy Loam

Peat and Muck Irrigated Soils

30 – 50 39 – 69 59 – 98

98 – 120 98 – 197 98 – 295 148 - 590

3.0 – 3.6 3.0 – 3.6 3.6 – 4.0

4.0 – 4.6 4.0 – 5.0 4.0 – 5.0 4.0 – 9.8

Manning’s equation for gravity pipe flow

n = .009 smooth interior pipe .015 3” to 8” sizes .017 9” to 12” .020 > 12”

Dc (in/day) x Area (ac) = Flow rate (ac • in/day)

(ac • in/day) / 23.8 = Flow rate (ft3/sec)

From Gary Sands U of MN

Pipe Hydraulic Capacity

Pipe capacity (cfs) = 0.4631 x D 2.667 x S 1/2

n D = pipe diameter (ft) and S = pipe slope (ft/ft)

Pipe Size and Grades

Desirable minimum working grade is 0.2 %

Typical minimum lateral size is 3 - 4” in humid regions and 5”- 6” for organic soils.

Minimum grade sufficient to maintain 0.5

ft/sec or 1.4 ft/sec with sand and silt in flow.

Tile Drain Design Tools - Web Based -

www.extension.umn.edu/AgDrainage/online-calculator.html

Engineering Design Aids - Slide calculators -

Tile System Maintenance

Rodent Guards Keep rodents from moving into tiles lines. Guards can sometimes plug so they need to be checked.

Photo from Eric Cooley, UW Discovery Farms

Tile System Design & Maintenance - Pipe Flow Velocity -

Very high velocities can cause “sink holes” when soil is actually pulled into the tile line.

“Blowouts” can occur when lines become pressurized.

Soil Texture

Max. Velocity

ft/sec Sand & sandy loam 3.5

Silt & silt Loam 5.0

Silty clay loam 6.0

Clay & Clay loam 7.0

Course sand or gravel 9.0

Watch out for steep-to-flat grade changes and overloading mains …. Blowouts !

Tile Line Blowouts Time

During storm event After storm event

Photos from: Eric Cooley, UW Discovery Farms

Tile System Maintenance

Iron Ochre A filamentous bacterial slime composed of organic masses and iron oxides. It is bacterial growth supported by soluble (ferrous) iron in the groundwater along with organic matter. Photo from: VanGluck and Novy, (2009)

Tile System Maintenance

Iron Ochre There is no long-term economical control method. On-going maintenance is the only option. Water jet cleaning is the best option. Source: VanGluck and Novy, (2009)

UWEX Information Resources

UW Extension Tile Drainage Web Site http://fyi.uwex.edu/drainage/

UWEX Information Resources - Publications -

Tile Drainage in Wisconsin: Maintaining Tile Drainage Systems

Tile Drainage in Wisconsin: Understanding and Locating Tile Drainage Systems

learningstore.uwex.edu/

Drainage System Cost - Approximate ! -

Drainage system installation costs can vary significantly based on terrain, soils,

outlet availability, etc.

Rough Range ~ $1,000 – 1500 / ac

QUESTIONS ??