Catchment Sensitive Farming: Practical Tips

Practical Tip: Constructed wetlands

What is a Constructed Wetland?
What are the benefits of using Constructed Wetlands?
What makes a good Constructed Wetland?
Where should the Constructed Wetland be located?
How do I design a Constructed Wetland?
How do I build a Constructed Wetland?
When might I want a pond and a constructed wetland?

What is a Constructed Wetland?

Constructed Wetlands are designed to mimic natural pond and wetland systems. They can take a wide variety of forms but they will all have at least an area of relatively deep water at their inlet end which does three things:

  • The deep area allows sediments to settle out 
  • The deep area can help to contain water coming from a sudden storm event and so provide some storage capacity 
  • Delaying the movement of the water through the system by giving it a deep area to pass through first allows time-based water treatments to start and encourages, for instance, pathogens to die off

Constructed Wetlands will also have a "marsh zone" with varying depths. The deeper marsh comes first and the water depth will gradually decrease until, at the outlet end, the depth is as a little as 100mm or less. This marsh zone provides for a variety of vegetation types, and habitats too. The different vegetation gives different biological treatments for nitrate removal and nutrient uptake as well as physical treatments such as sediment entrapment.

What are the benefits of using Constructed Wetlands?

  • good at slowing down flow through a system and containing a large volume of water for time-based treatment
  • good at encouraging sediment settlement and entrapment
  • excellent at providing habitats and landscape benefits
  • when properly designed, provide a low-maintenance and effective treatment option

What makes a good Constructed Wetland?

  • A good Constructed Wetland has a deep pond at the inlet end.
  • A planted shallow bund immediately after the deep pond provides a physical buffer to sudden inrushes of water following a storm. It encourages the water to spread out and the planting provides a further sediment trap.
  • The marsh zone should have a gradually decreasing water depth from bund to outlet and should be planted to vegetation typical of the area (it may be allowed to partially colonise naturally).
  • A planted shallow bund or sub-surface barrier separating the deeper first part of the pond from the shallower second part is an important feature. The planted bund filters sediment, slows down flow and spreads it out across the surface. See the section on “How do I build a Constructed Wetland?” for details of bunds.
  • Although seasonal variations in pond depth can be tolerated, and indeed can give a more natural appearance and better biodiversity, the pond must have enough of a water supply in the summer that it doesn’t dry out.
  • Some tall waterside planting can help give a variety of habitats and a more natural appearance, but the pond must not be unduly shaded. Try to have a transition zone of rough grass and shrubs between the pond and surrounding grassed areas. This will benefit wildlife as well as creating a slightly more natural appearance.
  • A "calm" pond is best able to settle out sediment and is less likely to re-suspend sediment once it has settled. Having the inflow below the water surface helps to reduce splash and turbulence.
  • A long pond rather than a round pond means fewer deadspots in which water could become stagnant. This will reduce any tendency for the water to become stale and to start to smell.
  • Heavy livestock should be kept out of the pond. Light grazing of surrounding grass areas by, for instance, sheep can be beneficial.

Where should the Constructed Wetland be located?

A constructed wetland should not be formed on the site of en existing wetland of high biodiversity or ecological importance. You should check this before you begin to plan. In a system to treat lightly contaminated site run-off from pig and poultry units a Constructed Wetland should come "after" one or more swales. If the anticipated sediment load is particularly high it should be immediately preceded by a stand-alone pond but the pond cannot substitute for the constructed wetland.

Much of the advice for the location of ponds also applies to Constructed Wetlands. As with ponds, Constructed Wetlands are designed to hold water and lining them is an expensive and problem-prone option. Ideally therefore they should be on an area of impermeable sub-soil, preferably clay. A clay content of at least 20% should provide the necessary watertight base.

Because of the deep water, it is necessary to provide a safety margin around the deep part of the Constructed Wetland to minimise the chances of anyone falling in accidentally. Back-slopes above the pond should be as gently sloped as possible, ideally no more than 1:4. Siting a pond on a piece of ground with a significant slope will therefore take up a large amount of space if there is to be a safe surround as well as a deep pond section. Keep the pond to gently sloping areas of ground where possible.

Don’t be tempted to incorporate an existing natural wetland. It is not acceptable to use natural water features for treatment of even lightly contaminated water.

Be very cautious about starting to build a Constructed Wetland in a very boggy piece of ground and certainly avoid areas with peaty soils.

How do I design a Constructed Wetland?

As with all other designs presented in this guidance, the first step is to determine the area of roof and hard-standing that will be contributing run-off. You then need to decide whether you plan to build several small Constructed Wetlands or one larger one. The sizes of the pond, bund, and the varying depth marsh zones can then be calculated, including an estimation of the number of reed seedlings needed to plant up the bund.

Step   Worked Example Data
1 Measure the roof and hard standing areas 8400 m²
2 How many similarly sized wetlands do you wish? 3
3 Divide the contributing area by the number of wetlands 8400 / 3 = 2800 m²
4 For each wetland, calculate the volume of rainfall resulting from a 12mm rainfall event. This is the Treatment Volume, Vt 2800 * 12 / 1000 = 33.6m³
5 CIRIA publication C521 "Sustainable Urban Drainage Systems - design manual for Scotland and Northern Ireland" recommends that the volume of water in a treatment wetland is three times the Treatment Volume, Vt 33.6 * 3 = 100.8m³
6 The average depth of the wetland including the deep pond will be 0.6m, therefore calculate the wetland area. 100.8 / 0.6 = 168m²
7 Add 30% to the wetland area to accommodate a marginal shelf. 168 * 1.3 = 218m²
8 The length:width ratio of the wetland is important. A wetland that is long and narrow rather than short and wide is best. A length:width ration of at least 2:1 is recommended. Enter your desired length:width ratio. 2:1
9 Using the length:width ratio, calculate the length and width of the wetland. Length = 21m

Width = 11 m

10 The pond will comprise the first third of the length of the wetland. Calculate the position of the bund. The bund’s top will be 100 to 150mm below the water surface. The bund’s top surface will be 1m wide. 21 / 3 = 7m from the inlet
11 The three marsh zones will each take up one third of the remainder of the length of the Constructed Wetland. So, each zone will be :- (21 – 7) / 3 = 4.7 m long

How do I build a Constructed Wetland?

  • Mark out the overall length and width of the proposed Constructed Wetland(s) using the calculated dimensions. The finished wetland need not be perfectly squared off – a much more natural appearance could be created by curving the corners and following the landscape. However, the overall shape should have the length:width ratio used in the calculations. Avoid :
    • narrow waists
    • “dead” spots
    • tight bends
  • Remember safety at all times. Ensure that slopes are as gentle as possible and certainly no steeper than 1:3 above deep water or below water. Ensure that the site is always left safe during construction.
  • There should be at least 500mm of freeboard above the normal water level.
  • The water inlet pipe should be from 200-300mm below the finished water level to avoid splashing. This helps to avoid sediment being re-suspended during high flows.
  • Install a larger diameter pipe at the outlet than at the inlet. This will help to avoid flooding should the outlet pipe become partially blocked. Surrounding the outlet pipe with a mesh screen, and checking it regularly, will also help reduce flood risk. If the outlet pipe is fitted vertically on a 90o bend, the top height and hence the wetland’s water level can be altered by rotating the joint slightly.
  • The three varying-depth marsh could be three separate zones with a stepped join between them, or they could be a continuous gradually-shallowing slope from the bottom of the bund up to the outlet end.
  • The regulatory body may require you to have a water sampling point at the inlet or the outlet end of the Constructed Wetland. If they do, ensure that safe access and a secure footing is available for personnel taking water samples.
  • Depending on the size of the Constructed Wetland, it may be help to even out flow and avoid dead spots to have more than one outlet across the “outlet" end of the shallow marsh. Consider more than one outlet if the width of the Constructed Wetland is greater than 5m. An exit trench, with a perforated pipe across the "shallow" end of the marsh zone, may be preferable.
  • If possible, create an overflow channel by excavating a shallow depression in the banking. Face it with stone to avoid erosion and ensure that it will overflow to a “safe" area.
  • When planting up, do not introduce any exotic plant species. Wherever possible, use plants that occur naturally in the locality. If there is a waterbody close by, natural colonisation will occur. Also, help invertebrates to colonise by scooping a pail of sediment and water out of a nearby watercourse and adding that to your finished Constructed Wetland.

When might I want a pond and a constructed wetland?

In situations where run-off is relatively heavily contaminated, the benefits of having a stand-alone pond as well as a constructed wetland are considerable. Although, in these cases, the normal route for water could be swale-pond-wetland-discharge, with a parallel pond the flow could be switched in the event of a contaminating spillage. The initial contaminated run-off could be caught in the pond and the pond then could be isolated by switching the flow to the route swale-wetland-discharge. This would give the opportunity to pump out the pond and remove the contaminant from the system. Normal routing could then be restored. Or, when sediment was being removed from the constructed wetland’s deep pond, the stand-alone pond could be used to collect run-off until the de-sludging was complete.

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

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