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Book:	Waterways & Wetlands
Chapter:	13 Dams, weirs and sluices
Section:	Factors to consider
Metadata:	Details	Buy this book

Environmental effects

How will the installation affect existing habitats? Dams frequently flood wetlands upstream while at the same time drying out wetlands below. Even minor impermeable banks across marshes, bogs or wet heaths may subtly alter drainage patterns, often to the detriment of existing communities of plants and animals. This may be balanced to some extent by the development of new seepage marshes at the foot of the dam.
Sluices allow water levels to be controlled and are often incorporated into dykes in order to manage wetland water tables. The likely effects on plants and animals adapted to the existing water regime should be studied first so that changes can be timed to be least destructive to ground nesting birds.
On-stream dams and high weirs may block the movement of migratory fish and must be equipped with suitable fish passes, approved by the relevant water authority, if on a trout or salmon river.

Hydrological factors

The following factors must be considered in the design of any water barrier.

Water pressure

Pressure is a function of the 'head' of water. Dams which fail due to pressure always do so at the base, where the pressure is greatest. Therefore the bottom of the dam must be its strongest part. The 'top water' of the upper foot or so seldom poses a threat unless current rather than pressure is the dominant factor.
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Note that water pressure is independent of current. The higher a dam the greater the pressure behind it and the stronger it must be whether the water is flowing or stagnant.

Upthrust pressure

Water also creates an upthrust pressure on non-porous installations which are not keyed into impervious strata. The upthrust is proportional to the head and, if greater than the weight of the installation, may cause it to float, shift or crack. The rule of thumb is to make concrete or brickwork at least half as thick as the head, i.e. if the head is 'h' the weir or sluice must be 0.5h thick. The maximum head over non-porous weirs and sluices of the sort which volunteers can easily construct is about 4'-5' (1.2m-l.5m). Even at this head, concrete or brickwork must be 2'-2'6" (610-760mm) thick.

Sheet piling or a concrete cut-off wall can be embedded at the upstream end of the installation to force seepage water to come up farther downstream, lessening the upthrust on the installation and allowing the use of thinner slabs. However, placing the footings requires either a pile driver or the use of pumps to keep the cut-off trench dry while pouring and curing concrete, and is usually beyond volunteer capabilities.

Current

Current exerts a force upon objects in water just as wind does upon objects exposed to air. For most dams this force is insignificant compared to that exerted by water pressure. Structures in swift watercourses, however, may fail due to current alone. This is especially likely in the case of very small dams, weirs and sluices made of weak, often improvised materials the strength of which cannot be adequately assessed relative to the force of the current.

Current can erode earth or sandbag dams and weirs and concrete barriers which have not had time to set fully. For this reason it is important to divert the stream before building an on-stream dam and to protect the finished embankment. Tidal barriers must usually be protected over the range of the tides since they are subject to continuous scour.

Sometimes the failure of an on-stream dam in a flood appears to be due to the current itself but is actually caused by a combination of increased water pressure over an inadequate spillway and erosion aided by flood-borne debris.

Preventing erosion

An earth dam which is overtopped usually fails because the dry face of the dam is rapidly eroded by falling water. This may happen to sea walls overtopped by exceptional tides. Weirs and sluices may erode where a change in slope causes the current to accelerate, or where turbulence occurs. Proper sluice design is largely a matter of preventing turbulence during normal flow as well as occasional floods.

Erosion is also a problem just below a weir, which can quickly be undermined and collapse. Small weirs in slow-flowing streams where the velocity is less than about 200-250mm (8-10") per second may be protected with a simple breakwater. This may consist of a stone slab laid at a 45° slope against the downstream face, a number of chunks of stone or concrete which project above the surface of the water below the weir or several upright stakes of varying lengths to scatter the overspill, driven into the bottom just below the weir.

Where the velocity is likely to be greater than about 200-250mm (8-10") per second, erosion can be reduced by designing the weir to the general shape shown below:

It is necessary also to harden the channel at the downstream toe of the weir, as shown. The simplest method uses a double layer of stones too big to be shifted by the current, packed together from the toe to a distance about half again as far as the estimated area of scour, e.g. 600-900mm (2-3') for weirs having a head over the crest of a foot or two. The stones form a flexible mat which sags in front as the channel bottom starts to scour and in this way prevents erosion from progressing farther upstream towards the weir itself.

Seepage

Water seeks its own level. When unable to overtop a barrier it attempts to seep under, through or around the ends. Seepage through porous soils or wet peat causes them to become unstable and slump. This, rather than simple 'leakage', is why barriers must be made of relatively impermeable material unless they are stone, brushwood or gabion weirs which are deliberately designed to allow the flow-through of water. Internal erosion of a dam, or 'piping', is most likely to occur where two materials meet, as where pipes run through the dam or where it joins the bank or substratum. Piping is treacherous because at first only the finest particles are carried away but by the time it becomes noticeable it may be capable of rapidly eroding the remaining material.

While it is almost impossible to prevent seepage it can be minimised by forcing seepage water to travel the longest possible distance. This is done by 'keying' the barrier well into an impermeable base and bank and by providing seepage collars or other obstacles to the flow of water along the outside of pipes. Even the smallest barrier should be carried at least 150mm (6") into relatively impervious material on either side. Where the barrier is keyed into earth there may be a problem with voles and rats tunnelling around the end and causing a leak. Adding a hard corner to the barrier serves to protect the bank at this point. A corner also helps prevent seepage where the bank is so hard that is cannot easily be cut away for a keyed joint. Small gaps between the barrier and the bank can be sealed with clay or even packed with earth.

Wind and waves

Wind can cause erosive wave action at the water line unless a barrier is well consolidated or protected by vegetation or stone pitching. A dam must be designed with enough freeboard to avoid being overtopped by wind-whipped waves.

Impurities and acids

Mortar and concrete remain weak and may not bond properly if they are mixed with water containing much organic material or silt. If mortar or concrete is likely to come in contact with acids in water or soil it is best to use sulphate-resistant cement.

Siltation

Any barrier to the flow of water causes it to drop its load of suspended solids. These build up and may drastically shorten the barrier's useful life. Improperly designed inlet channels which suffer erosion may contribute to siltation and turbidity.

Safety and practicality

Any water barrier is in some degree hazardous. Even quite a small installation may cause a minor disaster if it fails. For this reason, volunteers should attempt to design and construct only those dams, weirs and sluices which fall well within the limits suggested below.

Under the Reservoirs Act of 1975 and more recent legislation, any dam which impounds more than 25,000 cubic metres of water must be constructed and operated under the supervision of an engineer approved by the Department of the Environment. This would include for example a lake of 2.5 hectares (6 acres) of one metre depth. Annual inspections and monthly logs of water levels are also required.
On-stream dams must be designed with suitable overflows to handle runoff both from normal rainfall and occasional storms. The size of the catchment area needs to be taken into account. Take advice from the Environment Agency as necessary.
The overall size of concrete installations is limited by the need to provide sealed joints and reinforcing mesh or bars in slabs larger than about 3m (10') in any dimension (less for upright or structural slabs and where the concrete is under tension). All reinforcements must be covered by at least 50mm (2") of concrete.
Size and design of concrete barriers is further limited by difficulties in erecting formwork or shuttering for large or irregular slabs. Shuttering is difficult to place in cramped conditions and even more difficult to retrieve despite being well oiled beforehand. Try to use the cheapest wood you can given the required strength if you are uncertain about being able to get it out afterwards. Calculate the weight of the concrete before deciding on how strong the shuttering must be. Where the design calls for curved surfaces or bevelled corners, sheet metal can be used for flexible shuttering, held in place by wooden struts and staked to the ground. Leave the shuttering in place for at least a week to allow the concrete to strengthen.
It is wise to have an expert check the plan and siting of any earth dam before beginning work since this type may fail if built without the necessary skill and experience.

For further advice see The Game Conservancy (1993).