Sluices are channels for conducting water through regulating valves or gates. They may be self-contained or part of larger dams or weirs where the aim is to regulate as well as impound water.
Small ponds frequently have sluices at their outlets which function either as single overflows or as the primary overflows on dual systems. When a pond’s level is high due to rain or heavy runoff from feeder streams, boards can be removed or the gate lowered to allow more water out. In times of drought the outflow can be blocked to keep the pond at the desired level. Ideally, the sluice also functions as a low-level outlet which allows the pond to be drained by the removal of all the sluice boards or by lifting the gate so that water flows out at the bottom.
Sluices installed at control points in ditch or dyke systems aid in the management of a number of nature reserves, such as Woodwalton Fen National Nature Reserve in Cambridgeshire where a complete grid of dykes has been supplied with sluices which allow control of levels within a tolerance of a few inches. By maintaining slightly different water regimes in various sectors within the fen a highly diverse flora is protected and promoted. Sluices are also used for flood and tidal control, for example on the Ouse Washes, at Minsmere in Suffolk and at Arne RSPB reserve in Dorset. Tidal sluices allow the salinity of impounded water to be controlled or, where one-way flaps are installed, prevent salt water from flowing into freshwater lagoons.
Open sluices
When used as part of the overflow system of an on-stream pond, the dimensions shown below should be checked against the critical storm flow from the catchment to ensure that the sluice is big enough. Take advice as necessary.
The concrete base ensures that water falling over the sluice boards does not erode the installation. The outflow channel should be protected with stone pitching as necessary.
Where the maximum flow through the sluice is likely to cause erosion around the ends, the design may be modified as shown below. It is important, where the sluice outflow is also a spillway, that the downstream apron is extended beyond the base of the dam as shown. A stilling basin may be necessary to handle fast flows.
Notice that natural scour areas occur around the downstream ends of the sluice where the flow is swift. These should be allowed for in the design of the dam or embankment and may be cut out in advance and protected from further erosion. Protection may be by stone pitching or by gabion ‘mattresses’ made of 900m-1. 2m (3-4′) wide chain-link fencing laid on the bank, covered with stones, folded back over itself to hold the stones in place and pinned to the bank to keep from slipping. Jute matting or other biodegradable geotextiles may be used where the flow is slow enough that a dense cover of grass will suffice once it establishes itself and the matting rots. Stronger protection in the form of galvanised wire/polypropylene geotextile may be necessary where flows are stronger.
Closed sluices
The standard closed sluice is the ‘monk’, so called because it is supposed to have been developed for use in the old monastic fish ponds. It is still widely used in Scotland, where it is claimed that for combining water level control with the penning of fish it has never been bettered. The basic design, shown below, should be provided with a removable top cover and a galvanised metal fish screen across the front where fish are to be kept in the pond.
The size of pipe depends on the required outflow and should be at least 225mm (9″) diameter. For on-stream dams the capacity must be planned according to storm flow requirements. A single section of uPVC pipe should be used to avoid joints.
Where vandalism is likely, the pipe may be extended out into the water so that the end containing the control boards is out of reach. Or the head may be set into the dam and the upstream edge made flush with the bank for unobtrusiveness and ease of cleaning.
Sluice gates
Plank gate
A standard plank gate set in a metal or pre-cast concrete channel is a simple and effective design. Elm or oak boards are most durable. They can range from 25 x 150mm (1 x 6″) size up to sleepers for very large gates.
Metal channels should be rolled steel. They should be fixed to wooden structures with galvanised steel screws and to concrete with galvanised rag bolts. Channels fixed to a wooden dam are shown here. U shaped channels range from 75 x 38mm (3 x 1.5″) upwards.
Board gates have several problems:
- They leak unless constructed as double sets of boards with earth packing.
- They may be very hard to remove under great pressure or a strong current. This problem occurs only on large sluices.
- They can be easily vandalised and the boards taken, unless set away from the bank.
- They do not allow fine adjustments of the water level.
- They do not control the direction of flow. On tidal sites and occasionally elsewhere, directional control may be most important.
Among the many alternatives to the plank gate, those discussed overleaf are perhaps the simplest and most useful.
Lifting gates
Solid gates can be provided with heavy-duty hardware to deter vandals and with ratchets, turnscrews or other mechanisms to make lifting easier. Single gates are designed to drain the pond or channel by providing a low-level opening, as shown below.
For fine control of water levels a double gate is best, provided with an independent lift mechanism for each gate. In small sluices both gates can be placed in the same channel as shown.
Box gate
This is desired for use in tidal situations or on fully regulated dyke systems where first one side of the sluice and then the other may have higher water. The box allows flow only in one direction because it is fitted with a hinged lid which swings shut when the current reverses. This gate can be easily incorporated into the standard board dam.
Tidal sluices can also be controlled by means of swing valves (see below).
Pipe sluices
Pipe sluices are simple constructions using one or more adjustable plastic pipes, which pass through earth or concrete dams to allow inflow and outflow levels to be controlled. In recent years they have been widely used on RSPB and other reserves, and have proved cheap to construct, simple to operate and durable. Two basic types have been constructed. These are small bore rigid uPVC pipes (e.g. ‘Osma’ pipes) with right angle bends, or larger bore flexible uPVC pipes. Small bore pipes are widely available from builders’ merchants, and larger bore pipes from agricultural suppliers.
Rigid pipes
To control water levels in a ditch, the following simple design can be used. uPVC pipes of diameter 110mm or 160mm are suitable, supplied in 3m lengths. Lay the pipe along the bottom of the ditch. A 90˚ bend is then fitted to the upstream end of the pipe, and earth is banked over the pipe and compacted. By rotating the bend, water level can be varied by about 200mm (8″). Further lengths can be inserted into the bend to control greater depths of water. Being simple to operate, these type of sluices are easily tampered with by vandals. Where the location makes a sluice vulnerable, it may be necessary to position the upper end of the bend out of reach of shore, and only operated using a suitable long pole, or from a boat.
The size of the pipe or pipes will depend on the required flow of water. Several pipes can be fitted, and then one or more opened as necessary. RSPB have used 110, 160 and 200mm diameter uPVC pipes at several reserves, including Nene Washes, Minsmere and Titchwell Marsh. Where large amounts of water need to be drained rapidly from lagoons, sluices with a low-level large diameter on-off pipe are constructed, with adjustable pipes above. For further details see Burgess, Neil and Hirons, Graham JM (1990).
The same principle can be used to control ditch flow into a drain, by fitting the adjustable bend into a concrete blockwork chamber. The chamber also serves as a silt trap and a point from which drains can be rodded. For further details see Coleshaw, Tim (1995) in Enact (Vol 3 No 1).
An alternative use is to control water level in field drains, as shown.
Flexible plastic pipe sluices
Flexible plastic pipes of diameter 350 or 500mm, normally in 6m lengths, can be used to rapidly move large volumes of water through sluices. The corrugations, which give flexibility to the pipe, also help slow seepage along the outside of the pipe. Flexible plastic pipes are light and easy to handle, can be cut with a hacksaw, and joined with push-fit connectors. The pipe should be set in the dam as it is constructed, about one-third up from the base of the dam. This allows the sluice to give maximum flow when needed, and prevents surface weed getting sucked into the pipe. To close the sluice, the end of the pipe is lifted up and tensioned by a rope and attached to a stake or similar on the dam. Use a chain and padlock or similar where vandalism may occur.
Pipework
Pipes passing through embankments, whether inlet pipes, primary overflow pipes, bottom outlet pipes or piped-in sluices, must be:
- Flexible and strong enough to withstand the high loads caused by uneven settling
- Fitted with watertight joints
- Fitted with anti-seepage collars if the pipe passes below the top water level of an earth embankment.
Otherwise the soil inside the bank may erode due to seepage along the outside of the pipe. Collars are usually made of concrete at least 150mm (6″) thick. To be effective the collars should have a diameter five times that of the pipe and should be spaced ten pipe diameters apart along the line of the pipe. For larger pipes the number of collars can be increased in order to decrease their overall size.
The following types of pipe may be used:
- Vitrified clay pipes. These are widely available, and are easy to join with push-fit polypropylene flexible couplings.
- uPVC pipes. These are widely available, with fittings as required. They are light and easy to handle, and are very resistant to deterioration.
- Cast and spun iron pipes. These are rigid but can be provided with flexible joints. They are very strong and resistant to deterioration but are costly. For this reason they are usually used only where the earth loading is high, e.g. the bottom outlet to an on-stream dam.
- Spun concrete pipes. These are rigid and so should be connected with flexible joints. They are strong and can be used anywhere except in water or soil high in sulphates.
Where diversion pipes are taken off at an angle, their openings should be made flush to the bank or concrete slab by means of a galvanised metal collar or dish.
Valves and plugs
All plugs and, when possible, control valves on pipes passing through embankments should be at the upstream ends to reduce the pressure within the pipes.
The advantage of a valve is that it allows the outflow to be controlled, whereas once a plug is opened it cannot be easily replaced until the pond has drained to a level within working distance of the pipe.
One simple valve for use in tidal control situations is the swing valve shown below. A closed-cell foam gasket may be used instead of the ‘O’ ring but it tends to deteriorate more rapidly.
Plugs are appropriate for very small dams and off-stream embankments where the bottom outlet pipe is unplugged only when draining and cleaning the pond. Whatever the design, two points should be observed:
- The plug should be greased, otherwise it may be very hard to pull loose when needed.
- The plug should be marked and preferably attached to a chain for retrieval. Its position can be indicated by a line and float. The free end of the chain should be fastened to the dam so that it can be more easily found when needed.
Large builders’ merchants carry sewer pipes and stoppers. You can make your own plug out of an appropriate diameter circular wood block, sealed in the pipe with mastic and sealing compound. A more permanent plug can be made by machining a plastic block.
