Dam construction

 

Although on level ground a pond is most easily made by excavation, on sloping ground, a dam may be the best answer. The most common fault when building a dam is haste! Water can be very powerful, and if care is not taken, a winter flood or even a few weeks' seepage may swiftly undo many hours of hard work. The creation of a marshy corner or a primitive, albeit productive, pool, can often be achieved by putting a few sheets of corrugated iron across a sluggish-running ditch. But where there is any appreciable flow of water, a proper dam must be planned with care.

The following points should be noted:

(a) The height of the dam and the gradient of the land upstream determine the length of the pond.

(b) The width of the valley at dam height determines the width of the pond.

A simple method of establishing the first point is to drive two sticks into the ground 6 ft. apart at the position of the dam: one stick at the pond face and the other at the downstream face. Then fasten a straight cross-piece between the two and line up with a spirit level. By sighting upstream along the cross-piece, a rough indication of the extent of the flooding can be obtained. The cross-piece may be moved up or down until the correct amount to be flooded has been estimated. A rifle with a telescopic sight can be levelled and used in the same way. This will give the approximate height of the dam required.

When the site of the dam has been chosen, the construction is usually fairly simple, if certain rules are followed:

(i) The width of the top of an earth dam must be at least equal to its height;

(ii) The width of the base must be five times the height (e.g. a 4-ft.) high dam must be 4 ft wide at the top and 20 ft. wide at the base);

(iii) A concrete or brick spillway must be constructed of sufficient capacity to cope with the maximum amount of flood water;

(iv) The discharge from the spillway must be well away from the base of the dam to avoid any possibility of erosion. The water should fall onto a concrete `apron'. The importance of the spillway cannot be emphasised too strongly. We have seen many old ponds and lakes which are dry, due to the failure of the dam. Almost invariably this has been caused either by incorrect design of the spillway or failure to maintain it.

Where very heavy floodwater can be expected, it is advisable to have a second bypass spillway. The alternative is to have a deep main spillway with hatches to hold up the summer level. These can be expensive and must be well maintained.

During construction the dam must be carefully consolidated by running the machinery back and forth, gradually building up the height in layers not more than 1ft. thick. If the construction has to stop for any length of time, the top layer must be scarified before continuing, otherwise water will seep between it and the new layers. Made-up ground is not as strong as natural ground which has had years to settle.

The vegetation should be cleared from the wings of the dam before starting to build as the layer of topsoil and decaying organic matter can create a line of weakness at each end of the dam. Where possible, make the dam in an arc facing into the lake so that the weight of water against the dam face presses the end of the dam tight against the bank sides.

The middle portion of the dam should be built higher than the ends to allow for subsidence where the filling is deepest.

Vegetation should be established as quickly as possible to avoid erosion of the dam faces by rain and waves. One method of preventing wave erosion on the pond face is to make a baffle of floating logs. Grasses, reeds and rushes are suitable for planting on dams, but never plant trees as their roots allow the water to permeate through the dam and cause washouts.

The best machine to use for dam-building is a heavy bulldozer or tractor bucket - the heavier the better.

If the soil is light and quickly permeable, such as sand or peat, a core trench should first be dug and filled with clay or similar material. This trench should project into the bank on either side and be deep enough to reach into impermeable subsoil. On some sites it may be necessary to build a concrete curtain wall along the centre of the dam, projecting into the bank at both ends, and down into the subsoil.

The chief attraction of a dam is that quite large areas of water can often be impounded by a very short length of dam. With modern earth-moving equipment, however, it is advisable to weigh up the pros and cons of making a dam compared with digging out an area below the existing water level. This produces a maintenance-free pond. Many old dams were constructed before the days when powerful machinery was available. Apart from the cost of a dam, which can be considerable, there is a another disadvantage and that is the possibility of silting. A lake with a stream running into it has the same effect as a settling tank. The slowing up of the water-flow allows material in suspension to sink to the bottom where it accumulates over the years, gradually reducing the area of water as the deposits rise to the surface level. In shallow lakes with a large influx of storm water, this action can be fairly rapid and is usually followed by invasive vegetation.

There are two ways to avoid this problem. The first is to construct a deep settling tank where the stream enters with a low dam dividing it from the lake. This will trap much of the silt before it enters the lake. The tank can be cleared with an excavator every few years. The second method is to construct a new channel for the stream, around the side of the lake, holding the water level to dam height, with a hatch at the lower end.

In flat country digging is nearly always the best method; among hills a dam is usually the easiest. In undulating country the decision as to which is suitable is often marginal and here we would advise digging for safety's sake. If the lake is also to be used for fishing, then a dam has a considerable advantage because facilities for draining the lake can be included in the construction. This is very valuable for fishery management.

Before constructing a dam it is always wise to consult your legal adviser and it is necessary to inform the local district engineer of your Regional Authority who will doubtless give much good advice, and who must sanction the construction of the dam before work commences.

Although we have given some general guidance on dam construction it is really essential to get professional advice from a chartered civil engineer before undertaking any work. For instance, without proper examination of the site, a dam might inadvertently be constructed over layers of peat, or where impermeable soil was not present. In both instances the dam would probably be unstable and possibly dangerous. Explosives Marshy areas are sometimes so soft they cannot support pond-digging machinery of any kind. Until now these places have been largely unproductive, except for an occasional snipe drive.

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