This chapter discusses options for river crossings. It covers the different types of crossings and their advantages and disadvantages, and gives an overview of installation methods. It does not cover drainage culverts that are used to direct water from the ditches under a road. These are described in Chapter 7: Erosion, sediment and slash control.
The term river, to be consistent with the definition in the NES-PF, includes all continually and intermittently flowing bodies of fresh water; it includes what foresters and forest engineers commonly refer to as streams and other waterways. The NES-PF definition is different from a dictionary definition, where a river specifically refers to a larger stream.
Crossing rivers is an integral part of road construction. Roads regularly need to cross rivers to access forest operational areas. River crossings must be correctly designed and constructed to meet safety, environmental, operational and cost requirements. Crossing waterways should be kept to a minimum. They are expensive, and increase the risk of adverse environmental effects.
Crossings must comply with relevant Acts and regulations, including the Building Act 2004, the NES-PF, and the Freshwater Fisheries Regulations 1983 (FFR83). The NES-PF has specific conditions for various types of river crossings, and clear permitted activity rules. The general rules are for notice, effects on other structures and users, fish passage, erosion and sediment, location, flow calculations, contaminant discharge and organic matter. This Manual has tried to strike a balance, so that the specifics of the rules are not repeated verbatim, but are instead paraphrased and incorporated into the design and construction requirements for river crossings.
The common types of river crossings used on forest roads in New Zealand are:
- Fords, either unsurfaced fords or concrete pad fords
- Temporary crossings, including corduroy log bridges
- Single culverts – including round, arch (half pipe), box, or oval culverts
- Battery culvert crossings
- Drift decks
- Single-span bridges, either permanent structures or temporary portable structures.
Ensure that higher risk, or more significant, river crossings are located and designed by an experienced person. If in doubt, seek expert advice; the consequences of failure can be significant. Not only are river crossings expensive to rebuild, there can also be significant operational and environmental implications. Bridge construction requires structural design input, and will usually involve a chartered professional engineer (CPEng) for certification, so that building consents can be obtained.