Catchment Sensitive Farming: Practical Tips

Practical Tip: Swales

What is a Swale?
What are the benefits of using swales?
What makes a good swale?
Where should the swale be located?
How do I design a swale?
How do I build a swale?

What is a Swale?

Swales are shallow grass-lined channels designed to collect water and move it gradually away downslope. They can encourage infiltration along their route and the grass can provide filtration of suspended sediments as well as taking up nutrients. Commonly, their storage capacity is increased and the rate of water flow along their length is decreased by the construction of check dams.

Unlike ditches, they are normally dry without wet weather and their sides and base are grassed. The grass needs to be kept to a height of approximately 100mm you can do this by mowing, strimming or light grazing.

What are the benefits of using swales?

  • effective at improving water quality
  • cheaper to construct than piped systems
  • able to be incorporated into the landscape
  • a low-maintenance option
  • visible in operation

What makes a good swale?

  • The longer the swale the better. A longer swale gives more time for the entrapment or settlement of suspended solids. Swales of more than 70m length are optimal.
  • Swales should have no sharp bends they should curve gently.
  • Swales should have shallow gradients of no more than 50. Steeper ground than this can have a swale built on it but the swale should curve to and from across the slope with a shallow gradient. Take care that, as above, the curves are gentle.
  • Swales should have gently sloped sides with a gradient no steeper than 1:3. This can make the whole swale rather wide though. If you need to, make the bottom of the slopes of the swale shallow and steepen them as you go upwards. Keep the slopes safe though.
  • An established grass sward is beneficial and so standing water should be avoided.

Where should the swale be located?

This will depend on the layout of the site and, in particular in the case of a retrofit, on the available space. Normally though the swale would run either parallel with or at right angles to the run of the houses with hard-standing areas draining towards the swale.

Note that, to comply with Standard Farming Installation Rules 2.3.3.3 (pigs) and 2.3.3.4 (poultry), outflow points from hard-standing to swale should be able to be blocked off to contain heavily contaminated run-off such as may occur during wash downs. At these times, run-off should be collected and diverted to waste tanks for subsequent land spreading.

On large sites, subsidiary swales between houses may link with a main swale off to one side. Roof water may be led to swales via clean surface drains or small swales.

How do I design a swale?

The key to designing a swale is to know the size of the area being drained. This is the area of lightly contaminated hard-standing and roof that is going to be contributing run-off to the swale. To size the swale, this area is multiplied by 12mm in order that the first 12mm of rainfall in any one rainfall event can be held in the swale any additional run-off will overflow downhill. This volume of run-off (area x 12mm) is known as the Treatment Volume, Vt. 12mm is chosen because that amount of rain will generally lift any light contamination from the surface and further rain after that is likely to run clean. The gradient of the slope is then used to calculate how many check dams are required and then their spacing and sizes are calculated.

Here is a worked example

Step Worked Example Data
1 Measure the roof and hard standing areas 5450 m
2 Work out the swale length available on site 120 m
3 Measure the gradient along the line of the swale (see Swale Note 1 for how to do this, or use a inclinometer) 2%
4 Calculate the Treatment Volume, Vt 5450 x 0.012 = 65.4 m
5 Calculate the number of check dams . . .

a from Swale Table A, look up the distance between check dams for the gradient determined in Step 3

b divide the Swale Length from Step 2 by the distance between check dams from Step 5a to work out how many are needed

 

25 m

120 / 25 = 4.8 ? 5

(round your answer up)

6 Calculate the volume of the check dams. .

a from Swale Table B, look up the volume held by each check dam. Initially assume a swale floor width of 1 metre.

b multiply the volume per check dam (Step 6a) by the number of check dams (Step 5b)

10m

 

10 x 5 = 50m

7 Ensure that the volume of all the check dams will contain the Treatment Volume, Vt Volume held = 50m

but Vt = 65.4m3 - volume insufficient

8 If the volume is insufficient, repeat Step 6 and Step 7 using a greater floor width Floor width = 1.5m

Single check dam = 13.1m Volume held = (5 x 13.1) = 65.5m this is OK

Swale Table A Distance between check dams

Swale gradient % 1 2 3 4 5
Distance between check dams m 50 25 16.5 12.5 10

Swale Table B Volume per check dam

  Swale gradient %
1 2 3 4 5
Floor width m 1.0 20.0 10.0 6.7 5.0 4.0
1.5 26.3 13.1 8.8 6.6 5.3
2.0 32.5 16.3 10.8 8.1 6.5

Swale Note 1 - Measuring gradients without needing a survey

You will need:

  • Two sturdy wooden laths, one 2m long and one 1m long
  • A spirit level

How do I build a swale?

First excavate the line of the swale

  • Excavate the swale to a depth of 750mm. Store the topsoil separately
  • Make the width of the swale floor equal to the width calculated. Where possible make the side slopes no steeper than a 1:3 slope - use a shallower slope if space permits.
  • Excavate the floor of the swale a further 150-250mm and fill the resulting trench with some of the retained topsoil

Now build the check dams

  • Excavate a trench across the swale 200mm deep and 3.3m long
  • At the lower end of each trench, build up the check dam from the 75-150mm grade broken stone to a height of 500mm above the floor of the swale. The sides of the check dam should have a 1:2 slope.

To finish the swale
  • Spread the remaining topsoil over the sides of the swale and apply fertiliser and/or lime as appropriate to bring the nutrient content and pH to a standard to enable the grass to establish quickly. BS 3992:1994 "General Purpose Topsoil" specifies good topsoil parameters.
  • Grass the swale with a mixture that requires little maintenance and provide a dense, well-knit sward. For instance, a mixture of 70% creeping red fescue, 20% smooth-stalked meadow grass and 10% creeping bent. Apply the grass mixture at about 24 grams per m or 240kg per hectare.

Source: Guidance for treating Lightly Contaminated Surface Run-off from Pig and Poultry Units, Environment and Heritage Service, Northern Ireland


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