Safety

Before tackling any work by hand, consider using a machine to do the heavy work. Clearing and digging out muck and vegetation is potentially hazardous because of the combination of mud, water, slippery slopes and hidden debris. Follow safety advice, and take care to avoid back injury. Note the symptoms of Weils Disease and Hepatitis A, and take measures to prevent infection. Avoid working close to blue green algae, and wash off any that comes in contact with the skin.

Timing the  work

Periodic digging out or dredging may be needed on small ponds and sluggish ditches and streams. In general, clearance is best done in autumn, when any adverse effects on wildlife associated with the pond or waterway will be minimised. Water levels should be near their lowest, making work easier, and conditions for working are still pleasant. Emergent vegetation will be dying down, and excess growth can be pulled or dug out.

Ponds

Village ponds or those in fields or on commons with eutrophic water sources may need some clearance every few years. Manage areas in rotation, clearing no more than half the pond in one season, and then leaving it for at least a year before the other half is touched. Other ponds may be regularly managed each year, clearing sections in rotation to maintain a range of conditions from bare mud to thick emergent vegetation.

Drainage ditches

Artificial drainage ditches need dredging periodically to prevent them silting up. Long or short rotations can be used, dredging from different sides alternately, or clearing sections in rotation. The aim is to create a varied structure with the full range of conditions from newly cleared to silted. Another regime leaves random strips to create a series of small pools. For further details see Kirby (1992).

Clearing ditches whilst maintaining varied structure

Watercourses

Slow flowing streams and rivers may need periodic clearance, depending on various factors including amount of rainfall, incidence of storms, aquatic and bankside vegetation, and changes upstream which may alter the flow. ‘In-channel’ works can include removal of recently deposited silt, and also the removal or redistribution of gravel. For more details see RSPB, NRA and RSNC (1994).

Feasibility

Consider these questions:

  1. How big is the area to be dug out or cleared?
    An area just 9 x 9m (30 x 30’) and 600mm (2’) deep in silt provides about 60 tonnes of spoil, enough fo fill 2,500 buckets, or two tipper lorries.
  2. How much water may have to be drained before bottom clearance can begin?
  3. How much time must be allowed for preliminary work such as construction of coffer dams or initial drainage?
  4. How deep is the bottom muck and how difficult will it be to work in? Test the work area and follow the safety precautions.
  5. Is there adequate access and support for any mechanical equipment needed?
  6. If a pump is used, where will the water or slurry be discharged? Is there any danger of runoff into adjacent property?
  7. Can spoil be disposed of effectively and without damage to other areas?

Minimising harmful side effects

  1. Never clear an entire pond in one season. At the most work on half, and allow that to recover before touching the remainder. For managed ponds and waterways, limited annual maintenance is better than a major clean out every few years.
  2. Unless the pond or waterway is completely lifeless from pollution, the bottom mud will contain pond organisms. Leave some undisturbed. The sediment in dried out ponds will contain dormant seeds, so if you are relining the pond, set aside some of the sediment for use in the new pond.
  3. Undrained, stagnant or eutrophic waterways should be disturbed as little as possible in hot weather. Bottom sediments contain locked-in organic matter which, if suddenly stirred into the water, may promote an algal bloom or directly lower the water ’s oxygen content. If clearance is necessary at this time, try to confine activities to only a quarter or so of the area in any day. Where waterways are seriously polluted with toxic metals or other material, complete and thorough clearance of the bottom muck should be take by the Environment Agency or equivalent. Volunteers should not work where toxic materials are present.
  4. Take great care not to damage a puddled clay bottom by digging into it or letting it dry out.
  5. Minimise the effects of trampling around the waterway by limiting access for working to a few defined routes. Provide scaffold boards or other temporary walkways as necessary.
  6. Avoid smothering the bank with sediment. As work proceeds, pile the mud, rhizomes and vegetation into a few locations along the edge of the bank. Leave for a while to let the water drain back into the pond, and to allow some of the mobile pond organisms to escape back into the pond. Then fork or shovel the spoil clear of the bank for removal (see below).
  7. Choose the driest bank from which to work, if possible. This is not only easier, but a dry bank is less sensitive to disturbance, and often less ecologically valuable, than the wetter margins.
  8. Marshy areas at the edge of a pond are a valuable part of the ecosystem. Only clear out what is necessary to prevent the pond silting up. If silting is a problem, consider installing a silt trap at the inflow point.
  9. Don’t drain a pond except as a last resort.

Disposal of spoil

  1. To minimise handling, put spoil into the ultimate disposal carrier as near as possible to where it is dug up.
  2. If spoil is to be left on site, place it where it can’t be washed back into the waterway or pond by rain. Don’t change the nature or profile of the bank by piling up the spoil, unless this is part of the management scheme.
  3. Some spoil can be ‘lost’ by using it as the basis for islands or causeways. Within the site, spoil can be used to create banks or other features, or to repair areas eroded of soil.
  4. Make sure you have permission from the landowner or relevant authority before tipping onto another property. Roots of marginal plants contained in spoil will regenerate if the spoil is spread on damp ground, which may be undesirable. Silt containing vegetation matter  spread  onto  cultivated,  free-draining  land should not cause any problems. See also requirements of the Environmental Protection Act.
  5. Where the spoil is contaminated, or contains a lot of rubbish and debris, it will need to be tipped into skips or trailers for disposal. Check with the local authority about arrangements for disposal.
  6. Where clearance involves flushing sediments down- stream, the Environment Agency should be consulted because of possible siltation downstream. The use of ‘Sedimats’ is being developed to trap waterborne sediments.

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