Weirs are barriers, usually in flowing water, which hold back water and are overtopped by it in normal conditions. They may be part of the overflow systems of dams or they may be placed on their own where only a small head of water is to be ponded back, for example to create fish pools. Small weirs on feeder streams act as silt traps to reduce siltation of on-stream ponds.
Banks with spillboards
Simple weirs can be made by inserting a spillboard between the existing banks of a stream or ditch or by partly cutting off a water course with an earth bank containing a spillboard. The advantage of this over a plain earth bank is that the spillboard, arranged to be lower than the bank and made of an erosion-resistant material, takes the overflow rather than the easily-eroded bank.
Artificial banks should be built up of compacted layers of impermeable soil just like a small earth dam. Key the spillboard into a groove in each bank. Wooden spillboards swell slightly but if seepage continues plug the gaps with clay.
Any material may be used for a spillboard as long as it is durable, resists decay in water and is strong enough for the current:
- Wooden planks make cheap spillboards. Old discarded pressure-treated timber such as that used in board dams is particularly good. Don’t cut up valuable elm or oak boards, though, unless you are sure they can no longer serve their original purpose.
- Sleepers make very solid spillboards. Like planks, they can be placed one on top of another to raise the effective level. If they are not badly warped, seepage between the cracks is usually tolerable. Think twice before cutting good sleepers into short lengths. You can use old part-rotted sleepers which cannot be trusted for bridges if you don’t mind them becoming progressively more leaky over the next few years.
- Sheet metal, which may be available as scrap, works well but is ugly and becomes dangerous as it rusts. Sheet metal for spillboards should be at least 3mm (1/8″) thick or, if thinner, should have welded angle reinforcements.
- Concrete spillboards are lasting and can be cast to fit the situation. They can easily incorporate a notch to measure stream flow.
Porous weirs
Porous weirs slow the flow of water rather than stop it. The greater the flow the higher the water raised by the weir. In times of low flow the water simply goes through the gaps, which is why porous weirs are ineffective in still water and should not be used where seepage must be minimised. Materials for porous weirs include:
- Piled stones or gabions
- Brushwood or logs laid across the channel
Whatever the design, the bed below must be protected from scour and the material making up the weir must be heavy enough or firmly enough bedded to resist shifting in flood conditions. Stones for small weirs should weigh 50kg (1cwt) or more. Where these are not readily available, gabions can be filled with smaller stones to the required weight. Gabions may be purchased or made from chainlink fencing.
The following designs for boulder weirs can be used on curved and straight channels respectively.
Temporary slab weir
The weir shown below is designed to maintain summer water levels in trout streams and to be inactivated when levels rise naturally in winter.
In autumn the steel stakes should be removed and stored and the concrete slabs laid flat on the stream bed, ready to be replaced the next spring when water levels begin to drop.
Wooden weirs
Temporary board weir
This simple design may be used to raise summer stream levels by a few inches to form fish pools, and should be removed before winter. Drop pools are unlikely to form in this time, even without downstream protection, and if they do the weir may be easily re-sited elsewhere.
Crib weirs
Crib weirs of rough tree trunks, short dam boards and stones, are economical where felling is being done and a simple but strong structure is needed. The logs, 100-150mm (4-6″) in diameter, are placed 600-900mm (2-3′) apart and spiked to others placed across them at right angles. The spaces between are packed with stones or clay and the upstream face covered with boards and sealed with clay to prevent seepage. The downstream face may also be boarded over.
Another arrangement is to step the downstream face to give it a gentler slope and break the force of the water, as shown below. The steps are protected with boards.
In either case the channel downstream must be protected by stones to prevent erosion. Where the weir does not rest on rock, it may be made more stable and watertight by driving pre-cut dam boards along the base both up and down stream as shown.
Concrete and masonry weirs
Concrete weirs incorporated into earth dams should have the general shape of the one shown below. If the faces are no steeper than 1 vertical to 3 horizontal they may be cast without the placing of complex formwork. Concrete side walls must also be provided to protect the channel where it cuts through the earth dam.
Free-standing designs
The design below provides an efficient shape for a concrete weir, and should have a thickness at the base at least 50% greater than the height. The apron is designed to turn the flow slightly upwards to dissipate the energy of the water and help prevent erosion downstream.
This design requires complex formwork using sheet metal bent to the required shape, and where this is impractical the design shown here should be adopted.
Weirs may be built of rough stones bound by concrete, with the base the same dimension as the height or greater.
In all cases the weir must be built down to a solid and permanent footing to prevent leakage and shifting and the downstream channel must be protected with stones.
