Canal Features & Structures

A navigation canal consists of several elements, each of which exists in considerable variety.

Prism:

The most common element is the canal itself, often referred to as the "prism". The cross-section of this prism is usually trapezoidal in shape. The dimensions of the prism vary from canal to canal. Some early American canals had cross sections 40 feet wide at the water line, 26 feet wide at the bottom, and four feet deep. Later canals were wider and deeper. As needed, the prism was dug into or built up from the surrounding land. If the earth that the prism was built through was not porous, the canal could be just dug. But, in porous soils, the canal had to be lined with a clay puddle. In more recent times, linings can be fabric or concrete.

Towpaths

In the era before engines, canal boats were towed by animals walking along a towpath along one side of the canal. These towpath canals were a great improvement in efficiency over the rough roads of the times as the same animals and teamsters could move several times more freight in a canal boat verses a wagon. On towpath canals, the towpath is usually on the river or downhill side allowing the rougher features of the landscape and ports to be on the opposite (berm) side.

Culverts:

When a canal crosses a local drainage, it is necessary to deal with water flowing down that drainage, across the canal line. One method used where elevations allow is to dump the drainage into the canal and use it as a feeder. While this adds water to the canal, it can also add too much water during heavy runoff and bring silt into the canal, which then must be removed to maintain the channel.

Often, a better approach is to direct the side drainage under the canal by installing a culvert through the canal fill. Culverts have been built of wood, masonry, concrete, and steel. Many masonry culverts have wooden floors and foundations. With wood, it is important to keep the wood submerged at all times to prevent it rotting.

Culverts can be very large and impressive when viewed from the side of the canal. In the general use of the term, culverts are not visible when looking along the line of the canal and do not result in a narrowing of the prism.

Aqueducts:

Like culverts, aqueducts carry streams under the line of the canal. But, they are usually larger than culverts and are very noticeable from the canal. Often, an aqueduct is narrower than the canal on both sides. Often, only one way traffic can occur at any time. Aqueducts have been built of wood, iron, masonry, steel, and concrete. Often a combination has been used such as a wood trunk on masonry piers and abutments on a submerged wood foundation. Some aqueducts like five by John Roebling in Pennsylvania and New York had a wooden trunk supported by iron cables and masonry piers and abutments.

In some instances, aqueducts span roads or railroads. A rare few span both streams, roads and railroads.

Basins:

Natural irregularities in the landscape and the need for loading areas result in wide places along the canal. Basins allow for loading docks, for turning boats around, and act as small reservoirs above locks.  Above locks, the larger surface area of a basin allows the lock to be filled while producing a minimal drop in the level of the canal above.

Spillways and overflows.

For a canal to operate properly, it is necessary to maintain the water at a constant level. But rainfall, either into the canal or into areas draining into the canal can result in too much water. To control this, spillways are included to allow this excess water to leave the canal without overflowing the banks and causing damage.

Locks:

The lock is probably the most complicated and noticeable element of a transportation canal. Locks allow boats to move from one level of a canal to another. Basically, the lock is a long box with a floor, two parallel walls, and gates at each end. The floor serves as the bottom of the box and as the foundation of the walls and gates. Early locks had wooden floors, which were supported on piles. There may also be transverse walls under the floor to stop water flowing downstream under the lock. Wood in the floor and under the floor will not rot as long as it is kept submerged. More recent locks have concrete floors and foundations. Often the spaces between timbers under wooden floors were filled with clay as further waterproofing.

Above the floors are two parallel walls. The space between the walls is the lock’s width. Walls may be made of wood cribs, which are cheap, but don’t have a long life, masonry, steel, or concrete. Many rough masonry locks have wooden liners to protect the boats. These are known as "composite" locks. Lock walls have to enclose the water in the lock and hold back the earth on each side. They also have pockets at each end to receive the lock gates in the open position and may have channels for water. The walls usually extend a little upstream and downstream of the lock gates.

At the upstream end of the lock is the sill. The sill is the point that the floor of the canal changes elevation and its height above the floor is equal to the lift of the lock. Sills may be located just inside of the upper gates, in which case the upper gates are shorter than lower gates or just above the upper gate pocket, in which case all gates are of the same height.

Gates:

At either end of the lock are the gates. Many locks have miter gates at each end. These gates pivot around the end next to the lock wall and meet with a mate at the center of the lock forming a "V" pointed upstream when closed. In the closed position, the gates also seal against a miter sill on the lock floor. The "V" formed by the pair of gates causes water pressure to wedge them closed and transfer forces through the gate to the hinge forcing it into the lock wall.

Some locks have full height intermediate gates allowing short boats to use less of the lock and therefore less water. Other locks have extra sets of gates at one or both ends as backups. Some locks have gates facing in opposite directions in cases where the height of the water at one end may change relative to the other such as due to tides or floods.

Lock gates come in other forms. On narrow locks, a single leaf may be used, swinging all the way across the chamber from one side. This is very common at the upper end of these locks.

Deep locks often use guillotine gates at their lower end that open vertically. Some lock gates slide horizontally from a pocket in the lock wall or rise vertically from the lock floor. A commonly used gate at the upper end of a lock is the drop gate. This gate pivots at the bottom and falls in the upstream direction to the floor of the canal to open.

Another type of gate is the sector gate. Sector gates pivot at the lock wall end like a miter gate, but rather than having a flat surface to the water, they are shaped like a piece of pie with the surface being an arc. Sector gates can open and close under control against a head or flow. They are often used as additional, emergency gates at the upper ends of locks or lifts as they can be closed safely after an accident to another gate.

Gates can be moved by balance beams attached to the gate or by operating rods, chains or hydraulic cylinders.

With the above elements in place, a means is also needed to let water into the lock from above and let it out to the level below. This can be accomplished by valves in the gate, either vertically sliding as in England or butterfly as commonly used in the US.. However, gate valves in an upper gate that is less than full height have a danger to a boat in the lock. On a full height gate, water will flow into the lock below the boat. But on a shorter gate, the valve is higher, raising the possibility that the flow through the valve will land in the boat, sinking it. This can be prevented either by deflector plates or by building a tunnel around the gate hinge and admitting the water into the lock at right angles below the lower water level. The inflow will then be dampened by the water in the lock.. With these "ground paddles" on a lock that is much wider than the boat, it is usually best to open the valve on the same side as the boat is secured so that the incoming water will impact the far wall and return to pin the boat against the near wall rather than the reverse which will throw the boat across the chamber.

More modern, larger locks have tunnels behind each wall that are connected to the lock chamber by side tunnels. The side tunnels connect either through the lower wall or through the floor. Filling and emptying of the lock is then controlled by valves between the canal above the lock and the tunnel and between the tunnel and the canal below the lock. This arrangement allows for a faster, but smooth flow into and out of the lock chamber..

Further elements of lock hardware are bollards or snubbing posts on the lock walls. These are needed to stop an unpowered boat entering the lock and to hold boats in place while filling or emptying the chamber. Modern locks may have floating bollards that move with the water level. Snubbing post were usually made of wood. But granite, iron, and reinforced concrete have also been used. Many locks now have wires along the lock walls for securing small craft. Some locks also have winches to pull unpowered craft into and out of the lock. Locks may also have safety ladders to allow escape from the chamber for anyone who falls into the water.

Less obvious items at locks are a basin above to give a larger reservoir for the water to operate the lock and a bypass flume and spillway to allow water to flow down the canal regardless of whether the lock is being operated. Bypasses may be open channels on the off side of the lock or a culvert. Where locks are paired, the bypass is often in a culvert through the island between the chambers.

Where large changes in elevation occur, locks are often arranged in flights. Sometimes each step of the flight will have a small basin between. In other cases, the locks will be arranged together with the upper gate of one lock serving as the lower gate of the next. Such flights have been built with as many as five or six locks. One limitation on the number of locks so grouped is that all traffic must be cleared from the flight before the direction of travel is changed. In some cases, such as Foxton in England, fights of locks are subdivided with a passing basin between flights.

Where water conservation is important, locks are often equipped with side ponds. This allows part of a lock of water to be saved in the side pond and used to partially fill the lock on the next filling.

Flights of locks are time consuming and use much water. To speed up the process and reduce water usage, incline planes with dry or wet carriages, vertical lifts, and water slopes have been used.

Guard Locks:

Where a canal crosses a river at the same elevation or joins a river at a dam, there is a need for protection of the canal from flooding of the river. This is often accomplished with a guard lock to allow movement between the canal and the river even if the river rises while controlling the flow into the canal. Often when the river is not in flood, the gates of the guard lock are left open.  Guard locks with two guillotine gates are on both sides of the modern Erie Canal crossing of the Genesee River.

Other sites with guard locks are Clinton, OH on the Ohio & Erie Canal where one lock protects the canal south of a crossing of the Tuscarawas River. (The canal north of the crossing locks up at Lock 3.) A second Ohio site is at Lockbourne, OH, where the Columbus Feeder crosses Big Walnut Creek with guard locks on both sides.  These Ohio guard locks have masonry structures at each gate pocket, but turf walls in between.

Tunnels:

When a canal must pass a ridge, particularly if water is scarce on the summit, the hill is often tunneled through. In the towpath era, the earliest canal tunnels in England did not have towpaths. Rather the boats were manually "legged" through. Later tunnels were built wider to allow for two way traffic and included towpaths. While canal tunnels are common in England, they were used less in the US. The first US canal tunnel was on the Schuylkill Canal in Pennsylvania. This tunnel was later daylighted. The oldest surviving US canal tunnel is just west of Lebanon, PA on the Union Canal. This tunnel has been recently restored and is open for boat tours. Another well known, but not navigable canal tunnel is on the Chesapeake & Ohio Canal at Paw Paw, MD. Tunnels have also been built to carry roads and railroads under canals.   List of US & Canadian Canal Tunnels

Dry-docks:

Regularly, it is necessary to perform maintenance on canal craft. For this purpose, dry-docks are provided along the canal. These are chambers with a gate to the canal where a boat can be floated in and then be isolated from the canal. Once the chamber is closed off, a valve is opened draining the dry-dock into a lower level of the canal or into a nearby stream. Often, a dry-dock is larger enough to hold several craft at once.

Weigh locks:

On early American canals, boats were charged a toll based on the type and weight of the cargo. To establish the cargo’s weight, weigh locks were provided at strategic points. A weigh lock had a chamber like a lock with gates at one or both ends. After a boat was admitted, the gates were closed and the water allowed to run off. The boat then settled on to a very large scale that could then determine its loaded weight. At the start of the season, all boats were weighed empty. With the empty weight known, it could be subtracted from the loaded weight to determine the net weight of the cargo.

Mileposts:

On early canals, tolls were charged based on weight and the distance traveled. To keep everyone honest, the canal companies were required to erect mileposts along the line. These were numbered from a common point. Mileposts were usually made of wood, but granite and cast iron were also used.

Reservoirs:

The heart of any canal system is water. This is especially true of summit canals, which must have an adequate supply to the summit level to allow locking in both directions. To supply water during the dryer months, canals usually include reservoir systems in the neighboring territory. These systems can be quite elaborate and extensive.

In areas where canals have been abandoned, the reservoirs are often converted to other uses such as recreation lakes. Buckeye Lake and Lake St. Marys in Ohio are examples. Some canal reservoirs have become infamous when they were poorly maintained after canal abandonment. The most infamous of these was Lake Conemaugh near Johnstown, PA. After the abandonment of the Pennsylvania Main Line Canal, this lake was converted to recreational use. But, the dam was poorly maintained. During a period of heavy rain over the region, this dam was overtopped and failed, releasing a wall of water known as the Johnstown Flood.

Dams:

Dams are used to create reservoirs. They are also used in river navigations to create the river pools used between locks. Some canals paralleling rivers have dams and pools in the river to supply the adjacent canal. Early dams were wooden cribs (boxes) filled with stone and covered with a wooden sheath.  Some of these early dams are still in service, with a concrete cap.  Later, in some cases, a masonry dam was built just down river from the original dam, submerging the original dam in the pool above. Others were built with masonry. More recently, dams are built with concrete, or concrete with steel gates.

Feeders:

Feeders are used to bring water from a reservoir or river pool to the canal. Often, the feeders are made navigable and serve as a branch to the main canal.

Bridges:

Canals are a water barrier running through the countryside and dividing the land on each side. To maintain communication across the canal, it is necessary to construct bridges over the canal. These bridges are of many types. Some of these are draw bridges, either lifting or pivot. Others are fixed bridges at a height sufficient to allow boats and towpath users to pass underneath. Frequently, the canal narrows at the bridge to reduce the span needed. Bridges are often quite low, defining the maximum height of craft above the water. The lowest bridges are often railroad bridges as these railroads most need to minimize the grade on each side. On towpath canals, the towpath as well as the canal must pass under the bridges with no piers on the edge of the canal.

Stop gates:

Where canals have a long level between locks, it is often desirable to be able to close off sections of the level for maintenance or in the event of a bank failure. This is accomplished by a stop gate, which may pivot from the bottom of the canal or the sides or can be a guillotine overhead structure. Another approach frequently used where the canal narrows to pass under a bridge is a stop groove where timbers can be manually inserted. Stop gates are also provided where canal diverge from or cross rivers so as to be able to isolate the canal from the river in case of flood or for winter drainage.

Docks:

Canals need docks to allow boats to come close to the edge for loading and unloading. On towpath canals, docks are usually on the berm side (opposite to the towpath) keeping the towpath clear for towing. Docks are often equipped with warehouses for the storage on goods in transit. Some of these warehouses are on the very edge of the water and have hoists to allow goods to be directly moved between boats and upper floors. In other cases, cranes are provided for freight handling. On the coal canals, chutes were used at the loading points to rapidly fill canal boats.

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Tunnel page revised 10/18/07