Take time to decide on the river crossing site, and the type of crossing. River crossings are expensive and should be designed as long-lasting structures, unless they are designed for temporary use and removal. Potential crossing locations and their conditions need to be assessed and understood. A good river survey is essential, and substantial or sensitive crossings will need more planning. Surveys should include measuring channel profiles and cross sections to better understand flow conditions, and assessing the river bed and bank condition along with any indications of normal flow and past flood levels. Knowing the shape of the river at the proposed structure location improves the data accuracy for hydraulic modelling, which helps to optimise design solutions, and assists in developing construction plans and details.
The road approaches to the river crossing are important to get right in terms of grade and alignment, not only for the intended traffic but also to mimimise the potential for sediment entering the river at the crossing.
For bridges and culverts, the optimum crossing sites are likely to be where the waterway is narrow and straight with stable banks, the crossing length is short, and the roadway can be arranged to cross perpendicular to the river. For battery culverts, drift decks and ford crossings, the preferred sites will be where the waterway is wide and shallow, with low banks that allow river crossing approaches to enter the river with gentle gradients, while also being perpendicular to the river. Note that ford crossings are only a permitted activity where the number of axle movements per day is less than 20.
The following factors influence what type of waterway crossing may be most appropriate for a site. Consider how these factors will affect the design, construction and ongoing use of the crossing:
- Catchment size, shape, and topography
- Rainfall timing, frequency, and intensity
- Rainfall runoff characteristics, as influenced by soil type, geology and vegetative cover
- Normal water flow level and design flood flow level
- Stability of the river bed and banks
- Risk of woody debris transport during storm events
- Channel geometry. For example, wide and flat versus narrow and incised
- Size of forest area served by the road and crossing
- Forest access requirements. For example, long-term versus temporary access; all-weather versus fair-weather access
- Traffic characteristics. For example, logging traffic versus light vehicles only
- Water quality requirements
- Ecological importance of the particular river. For example, aquatic habitat and fish passage
- Availability of construction materials and machines
- Permitted Activity conditions for River Crossings in the NES-PF (Regulations 37-46).
A profile of the river at the proposed crossing location