Repair of dams, weirs or sluices takes extremely careful planning and should not be attempted unless it is certain that each of the following steps can be carried out successfully:
- Inspect the site carefully for signs of erosion, seepage or leaks on the downstream face of the dam, weir or sluice. Look for cracks, animal holes or tree roots in earth banks. Check the condition of brickwork, concrete and timbers. Can you be certain that it is the barrier itself which is leaking? Consider other causes such as seepage through permeable soils, a drop in the water table or damage to the pond lining.
- If you suspect leakage at a particular spot but cannot confirm this by simple visual inspection, use fluorescein dye to trace the route of the leak.
- Analyse the cause of the problem. The reasons why dams fail have been listed earlier in this chapter. Very often the same reasons contribute to leakage through the barrier even it if doesn’t collapse.
- Decide on an appropriate repair method and analyse the order of work required.
This is most important. You must establish all control points and other peculiarities of the pond’s water supply and decide how and in what order to cut them off or divert them to isolate the barrier for repair. - Provide the proper repair materials. Poor quality or inappropriate materials will guarantee failure.
- Create a safe, dry working area around the installation before attempting repairs, by draining the site or by providing a coffer dam.
- Get below or beyond the leak. Repairs must be keyed into impermeable foundations or banks unless the need is simply to patch small holes in a sound structure. In the case of earth dams especially it is often necessary to dig a trench along the centre line of the dam right down to the foundations and then to fill the trench with impermeable material. Be sure that trenching can be carried out safely. Provide shuttering if necessary.
- Make repairs to the highest standards possible. Faulty workmanship only causes problems later.
- If the dam has no proper overflow or outlet it is best to install one when carrying out repairs.
- After finishing the repairs, remove the coffer dam and restore the water supply. Keep a close check on the structure particularly during and after heavy rains to make sure it is safe and functions properly. Remember that seepage can seldom be eliminated. You have succeeded if you have cut water loss to an acceptable level.
Repair Methods
Leaks in earth banks
If the problem is one of leakage through a puddled clay or other lining, carry out repairs.
If the problem is one of leakage around the edge of an installation, the most likely causes are animal burrows, tree roots or seepage due to improper keying in. In either case it is best to dig out the affected area and plug the excavation with puddled clay. Remove trees which are growing too close to the installation.
Leaks between dam or sluice boards
First inspect the boards to see if they have rotted and need replacement. Water pressure is greatest at the bottom so trouble is most likely at the base. If the problem seems to be one of small cracks or ‘finger holes’ between the boards, it is easiest either to pack clay between them or shovel in dark earth or peat on the upstream side to be carried by the current into the cracks. This treatment may need to be repeated frequently. Strips of foam rubber pushed into the cracks make a longer-lasting seal. Badly warped boards should be replaced.
Erosion of channels
Unless erosion is due to water being where it shouldn’t, for example where a dam is overtopped, the solution is to protect the parts of the installation exposed to the current or to rebuild the installation so that the water flows more gently over or through it. The designs suggested earlier in this chapter should prevent serious erosion from occurring.
Seepage or ‘piping”
The installation must be torn down to below the seepage level and the impermeable barrier, ruined due to internal erosion, must be re-established. Larger earth dams need only be excavated along the centre line and converted into diaphragm dams by packing clay or by laying concrete into the trench. These materials should be compacted by tamping in 150mm (6″) layers.
In most cases sheet metal, plywood, polythene, PVC or butyl inserts do not work, at least not for long. However they are worth trying on small dams where the cost of replacing them is not prohibitive. Sheets of butyl rubber, factory-joined to the correct size, are likely to work best and longest. If galvanised iron sheeting or polythene- wrapped wood is used, make sure that any joints in the metal or wood are well plugged with clay or are otherwise sealed and that sections of polythene are joined with mastic and pressure-sensitive tape. Consider using these materials along the wet face of the dam to convert it to a blanket-type dam rather than digging out the centre to make a diaphragm dam. This is easier and makes the task of replacement simpler should the material work poorly.
Decaying masonry and concrete
The problem is to properly key the repairs into the old material. This may not be possible if the structure is rotten and any repairs of this sort will be temporary. Scrape out crumbling mortar from between bricks before repointing and scrub down the area to be repaired. Chip away the surface of old concrete to expose the coarse aggregate. Use sulphate-resistant cement for repairs and leave the oiled shuttering in place for at least a week to allow the repairs to bond. Sometimes puddled clay can be used more easily than concrete to plug small leaks, but try this only if the existing installation is still structurally sound.
