2.7 STONE FORD
Fords have been traditionally used on all access routes, from roads to hill tracks and stalkers paths, where wheeled vehicles or stalkers’ ponies require access. On upland paths this common feature has a stone pitched or cobbled surface, with variations in style from place to place.
The ford provides a solid, hard-wearing stone surface through rivers, large streams or burns, where bridges or stone culverts are not appropriate or feasible. It also serves to dissipate the waterflow by increasing the width of the watercourse, and possibly reducing the depth. A regular surface enables ponies and wheeled traffic to cross easily, and walkers to have a safer crossing, though probably a wet one. The whole structure should withstand fast flowing water, and also help prevent erosion of the river bank where it meets the path.
Bill of Quantities (example)
Using local weathered stone construct a stone ford, with a pitched surface lining the full length and width. Extend at least 300mm wider than the path on each edge, and to the top of each bank. Re-grade the path surface at least 2 metres either side of the ford.
POSITIONING OF THE FORD
This will obviously be where the path crosses the watercourse, which may limit the option of choosing the best place. The ideal position for a ford is on a wide, level stretch of slower flowing water, where its force is reduced. This may mean re-aligning the path, or the burn. The ford should not be placed where walkers will by-pass it on dryer days.
The pitched surface comprises the number of courses of stone required to run from the path on each bank down to the centre of the stream. The courses normally run parallel with the bank. These form the ford:
- Channel - the lowest, centre course of the pitched surface; largest stones form a firm base for the upper courses of the ford, and a shallow channel where the water flow is fastest
- Edges - where it joins with the stream’s natural bed; large stone used for the end stones in each course forms strong edges; the largest on the uphill edge which takes the force of the waterflow
- Ends - at the bank edge; large stones also required for the end courses to solidly join the ford with the path surface
Dimensions of the ford vary according to the width of the watercourse and the width of the path.
- the width should be at least 300mm wider than the path width at each edge
- the length should extend to at least one course of stones past the top of the stream bank, or to the width of the waterflow when in spate, whichever is the greater
- the edge or end stones, of the centre courses, should provide a surface level with the stream bed at the upper end or edge, so as not to impede water flow, and no more than 150mm above the bed at the lower end
- all lower edge stones should be pitched in to at least 2/3 of their depth, for stability of the structure
- the surface of the bank edge courses should be flush with the path surface
- all other stones should form as level a surface as possible for the ford, sloping gradually down to the base of the stream, providing a good surface for path users, and water to flow over
Local block stone should be found within the area that the path passes through, and from the stream bed where this does not have an adverse impact on the flow of water.
The size and shape of all stone should be such that the depth can be pitched into the bed of the stream with a level tread surface on the top side. Other faces should be as even as possible, to form a tight match with adjacent stones.
The 'key' stones of the ford, edges and ends should have a minimum depth of 300mm, dug into the stream bed.
Method of Construction
Having assessed the dimensions required for the ford during high waterflow, the construction should preferably be undertaken during the period of lowest flow.
Prepare the stream bed.
- divert or block the waterflow taking care that damage is not caused to the adjoining path
- where the stream is wide, half or part of the bed may be blocked for work to take place
- the force of the water may be reduced by placing large boulders upstream
- remove any stone or material from the stream bed that may cause obstruction, over the area to be constructed, saving suitable stone for construction.
Pitch the ford centre channel.
To provide a firm base, and achieve the required levels, start with the centre course of stones, at the lowest point of the stream bed. If the drop from each bank is minimal and the stream narrow, construction may start at the path edge.
- work upstream from the lower edge, and parallel with the stream bank
- set the depth of the stones into the stream bed to achieve the required ford surface level
- fit adjoining stones together tightly, with top surfaces flush with the adjacent stone
- tightly wedge and pack all gaps with smaller stones and gravel
Pitch the remaining courses.
Starting each adjoining course at the lower ford edge, pitch parallel courses, until the ford ends are reached, at the bank
- make sure top surfaces are flush with the adjoining course to achieve a gradual slope down to the centre channel
- fit all adjoining stones, and courses together tightly
- overlap all joins with the previous course of stone
- tightly wedge and pack all gaps with smaller stones and gravel, to form an immovable structure that won't be undermined by the force of water
Re-grade and compact the path surface, over approximately 2m length, down to the level of the ford end stones on the stream bank, in order to achieve a draining fall towards the stream.
Restore any damaged areas, particularly the path and stream bank edges above and below the ford, to ensure there is no risk of water flowing around the ford causing damage to the path.
Key points to watch out for:
- make sure that all adjacent stones are tightly butted, with overlapping joins, and gaps wedged or filled to allow no movement of the stone
- ensure that no stones are protruding above others
- make sure that the ford is adequate for the waterflow of the stream at its fullest: wide, long and deep enough, to prevent water washing out behind the ford onto the path
Where no wheeled traffic is expected the design can be varied by incorporating a defined step, at each end forming an edge to the path where it joins the ford. This would use large block stones butted tightly together. The ford becomes like a very large cross drain, with a wide stone liner - with large edge stones and water flowing below walking level.
This, or other designs, may also incorporate stepping stones to give walkers good footholds when the waterflow is particularly high. These may be the edge stones on a wide river, or a single large stone in the centre of a narrower stream.
If the ford is constructed where there is a steeper fall, such as where the path is traversing a steep cross-slope, it may be preferable to construct courses of stone running parallel to the path. The course on the down stream edge will require the largest stones, as the base of the structure.
Fords should require minimal maintenance. They are self cleansing, and if constructed properly and soundly, should only require minor repairs. Maintenance should take place during dry weather.
- re-pack stonework where there is movement or any visible gaps
- re-pack path surfacing above the ford end stones, where it may have been washed out or settled
- re-build path or stream banks where water is flowing around the ford ends and damaging the path - minor extension to the ford width and length may be required
- build a ford that will blend in with the surrounding landscape, using stone from the stream bed or surrounding ground
- check that changes in waterflow will not affect the ecology down stream
HEALTH AND SAFETY HAZARDS
- do not work on the ford when the waterflow is strong
- be aware of slippery surfaces on boulders and stone underfoot
- build a ford that is large and solid enough to prevent washout with the highest and fastest flows of water; in particular after sudden snow melt and long periods of rain.