Fords can be graded natural river beds or consist of a concrete pad in the river bed to assist with vehicle traction and to reduce sedimentation from vehicle passes. Fords can be used to cross small, low-flow rivers but are more generally used to cross broad, shallow or intermittent rivers, where the river bed is stable. For example, shingle or rock substrate. This topography makes alternative types of river crossings difficult or impractical to construct. Ideally, fords are used where traffic volume is light, or where traffic use is only for a short duration. For example, in a small woodlot harvest with a low productivity contractor. Fords are also often used as a secondary crossing point, where bridges cannot provide access for heavy and large forestry machinery, like haulers or construction machinery, due to weight or width restrictions. The use of fords can create significantly more sedimentation than other forms of river crossings, because sediment is generated from the approaches through water wash from the moving vehicle, storm water not being effectively diverted and treated before entering the river, and from mud off tyres.
The advantages of fords are that they can be used where other crossings would not work. Also, their design is not sensitive to flow. Fords can accommodate very large flows and associated debris that cannot be not reasonably passed through culverts or short bridges. Also, they are typically the lowest construction cost of all river crossings.
Fords have both traffic management and environmental disadvantages to other crossings. There will be periodic safety and traffic delays during high flow conditions, where traffic cannot cross the river or where traffic tries to cross when it should not. Also, heavily used fords generate considerable instream sediment. Concrete fords are the least favoured of fish passage structures because they become a fish barrier, especially in low flow. If possible, consider converting a ford to a drift deck, battery culvert or bridge crossing when traffic volumes increase at harvest. This will create a better structure and help reduce sedimentation.
The NES-PF has specific rules around the fording of rivers. It is essential that the regulations are clearly understood at the planning stage. The NES-PF Regulation 97 means that both a discharge permit and a resource consent for disturbance of the bed of a river will be needed, where:
- There will be more than 20 axle crossings per day
- If certain fish species are present, and
- Where there is a conspicuous change in colour or clarity beyond a 100 m mixing zone for more than 30 minutes after use.
Note that the NES-PF Regulation 97 requires a resource consent for any new concrete pad in the bed of a river, where that river is listed in a regional plan or water conservation order as a habitat for threatened indigenous fish, or a fish spawning area.
8.3.1 Ford design
One of the first considerations is to decide whether to use the natural river bed or to construct a concrete pad crossing. The factors to consider include river bed substrate, volume of traffic movement, acceptable risk of road closure, downstream impact of sediment generation and impact on fish. Another important consideration is identifying a good crossing point. It is essential that the best site is chosen that meets constraints related to road location, grade and alignment.
Characteristics of a good crossing site include:
- Locations on a straight section of river. Approaches that are perpendicular to the river help reduce scour of the approaches and stream bed
- Locations that do not alter the natural course and gradient of the river, or create erosion of the banks and bed
- Concrete pad fords that are located on stable and low gradient sites
- River beds with shallow water depth, and substrate that is hard and stable. For example, natural crossings on rock, as opposed to muddy substrate, help to reduce sedimentation and improve vehicle access
- Crossing approaches that have suitable gradient and transitions, so that vehicles are not grounded, especially low loader transporters
- Road approaches that are not steep, as these are ongoing sources of sediment
- Concrete pad fords that are designed to withstand flood flows. Rivers are prone to bed shifting. If water gets under the pad it can be undermined and displaced.
Like other river crossings, it may be prudent to consult with a forest engineer, hydrologist or other specialist to help with design and construction if the skills in-house are limited.
8.3.2 Ford construction
The key to good construction is doing the construction at the right time, keeping the approach gradients as low as possible, and making sure that the sediment generated from the crossing is reduced by diverting water from the approaches to sediment control structures. The keys to controlling erosion and sediment delivery from roads to rivers include providing adequate surface cover, that is gravel, and reducing runoff volume and velocity with frequently spaced water control structures. The following are good practice guidelines for ford construction:
- Construct in suitable weather, and with low base water flows
- Check for any fish spawning timing constraints under the NES-PF
- Limit earthwork disturbance to the immediate construction site
- Minimise the need for machinery to operate in flowing water
- Limit sedimentation entering the ford from the approaches
- Divert road surface water off the approaches
- Ensure fish passage is not impeded
- Construct stormwater and sediment control measures as close as practicable to the crossing, and ideally within 10 m. For example, use berms, cut-outs, ditches and culverts, flumes and sediment traps. Build these above the annual flood flow level
- Provide surface cover on the approaches. Use clean gravel where the existing road surface would create a sedimentation problem
- Check regularly during and on completion of construction. If the work does not meet the design plan and standards, then initiate corrective actions.
An additional factor for natural river bed crossings is to use clean rock fill where the carriageway requires strengthening of the river bed. It is important to use graded rock that is large enough to resist displacement by the flow of water. Fill the gaps or voids between this material with clean, small rocks or gravel to provide a better driving surface.
Additional factors for concrete pad crossings include ensuring that the river is diverted to assist in constructing the foundation, to reduce the risk of contaminants entering the water, and to minimise discharge of sediment. Also construct the concrete pad so it extends well beyond the extent of the river channel when at medium flow. This will help reduce entry and exit erosion of the gravel at the transition zone from the concrete to gravel, and further reduce sedimentation from the wet area generated by vehicle wheels displacing water when exiting the crossing. An important construction requirement is to armour the leading edges of concrete pad fords with aprons. This will reduce the river undermining the structure.