River crossings serve two principle purposes. The first is to access a harvest site, where the crossing and possibly the road, will be decommissioned after harvest. The second is in harvesting operations, where log extraction requires the need to temporarily cross a river. Temporary road prefabricated single span concrete bridges will be discussed in the single span bridge section of this chapter (9.10.3 Single span bridges). These structures can be designed for both temporary and permanent placement.
Temporary river crossings can be in place for up to two months, whereas temporary single span bridges can be in place for up to two years. So be careful to meet the regulatory requirements around ‘temporary’. Also, the NES-PF does not consider a ford to be a temporary crossing; it is a crossing irrespective of how long it is in use. Constructing, using, maintaining or removing a temporary river crossing or temporary single-span bridge is a permitted activity if regulations within the NES-PF are met, otherwise a resource consent will be required. Many of the construction requirements have been incorporated into this section, however, it is essential to refer to the regulations to understand the relevant obligations.
Temporary river crossings, like permanent ones, must be carefully planned, constructed and maintained otherwise they could cause a significant safety or environmental risk. Plan for all temporary crossings at the infrastructure and harvest planning phase. Key factors to consider in determining the type and size of the structure is the catchment size, the river’s banks, width and substrate, and downstream infrastructure. Ensure the crossing locations are clearly marked out for the operator.
Temporary road river crossings need to meet design standards for transport-related loadings. Therefore, the structure and its components need to be engineered. These can include a removable culvert and log structure sitting in the bed of the flow path, or removable concrete or steel bridge spans and abutments. The design varies with the river and approach of the extraction track. No crossings are needed for machines crossing dry gullies, however this is obviously weather-dependent. It may be prudent to have a temporary crossing plan ready for when these gullies become wet.
Flood plain shingle river beds can be a dilemma for all crossings. The best option may be for these to be temporarily crossed without a structure, because built-up access approaches and the volume of logs or metal required to cover the culvert(s) may compound potential risk during floods.
The following image is an example of a low-cost, portable river crossing alternative to a ford, drift deck or battery culvert that is suited to a smaller, short-term operation. It is an option to consider for a river not subject to frequent flooding or gradients steeper than about 3%, with high bedload movement. Otherwise, it will have high maintenance due to blocking up with gravel and structure displacement, leading to it needing to be pulled out and reset. It was made of untreated rough sawn Douglas-fir and larch, braced by steel cross ties, and cost about $3500 to build.
The following is recommended practice when constructing temporary river crossings:
- Minimise the disturbance of the natural shape of the river. Excavations of banks and bed must not exceed 200m2, otherwise a resource consent will be required
- Minimise soil entering the river during construction. Pull back material to a position where it will not enter the waterway
- Reduce potential sediment entering the water body from the crossing approaches. This is an important consideration for their location, design and construction. Where practicable:
- Flatten the steepness of the crossing approaches
- Divert water from the approaches. Methods include outsloping, installing cut-outs and sediment traps
- Consider factors like the catchment size, the waterway’s banks, width and substrate, and local climate
- Use the right equipment. An excavator is essential for most crossing construction
- For crossing designs that restrict the flow of water, install a culvert of at least 375 mm at the base of the crossing to allow water movement. Do not use only logs at crossings. Water does not easily drain though, which leads to upstream ponding.
Harvest extraction temporary crossings
- Plan for temporary harvest crossings. Avoid letting the contractor determine the number and location of them
- Consider the harvest crew equipment in the design of the structure
- Log bridges are effective at spanning small waterways, but need to be carefully constructed. It is recommended to limit bridges to less than 1 m above the ground, otherwise the structure needs to be designed by an engineer and meet legal requirements. Bridges must be constructed to:
- Carry the logging equipment across it
- Pass the flood flow from a 5% AEP event under the bridge soffit
- Enable the passage of bed material
- Locate as few crossings as needed to safely and productively harvest
- Design approaches so that logs do not sweep off the crossing into the waterway when extracted. For example, logs can be driven vertically at corners and crossing entrances to keep trees aligned to the crossing
- Ensure the contractor knows the crossing construction requirements. Provide instructions or other supporting paperwork (job prescription) and on-site guidance to the contractor if needed. Sign-off the job with them
- Ensure the crossing locations are clearly marked out for the operator
- In addition to the recommended practice for all crossings, consider the following to reduce potential sediment entering the waterway from the approach tracks:
- Use corduroy or slash to cover the approaches within 20 m of the river, unless approaches are of low erosion material like rock or river gravel
- Wherever practicable, maintain the track grade over the crossing to reduce potential sediment entering the water body from the approach tracks
- Remove all crossing material from the banks and river bed within a week of constructing the crossing and finishing the job.