Battery culverts consist of several pipe or box culverts installed beside each other in a river bed. The road is formed over the pipes to create a low-profile crossing. The normal river flow passes through the culverts, but during flood events water flows over the top of the crossing. This allows the dry passage of vehicles in normal conditions, but may result in the road occasionally being closed to vehicle access for short periods during large storm events. Battery culverts need careful planning and installation to prevent failure.
Consider using a battery culvert when a single culvert is impractical to meet full design flood flow, or a bridge is too expensive or has other design challenges. It is recommended to use a battery culvert instead of a ford when there will be more than 20 axle crossings per day, and the ford would generate fine sediment with vehicle passes. A good alternative to consider is a drift deck. These are discussed in the section 8.7. They may be cheaper, and have the option of being able to be used as a temporary river crossing.
Battery culverts are not recommended in high-gradient, high-energy river reaches because these are susceptible to high rates of bed load movement. This can result in gravel and boulders lodging in and blocking culvert pipes, and increased maintenance. During flood flows, the structure can be outflanked, which can scour out the approaches. It is a regulatory requirement to ensure fish passage is maintained, except where approved by a relevant fisheries manager.
Always refer to the NES-PF at the planning stage. It has specific rules for battery culverts, and they should be clearly understood. Resource consents will be required where the battery culvert crossing does not meet the permitted conditions within the NES-PF. For example, for a battery culvert to be a permitted activity, the contributing catchment area must be less than 500 ha.
8.6.1 Battery culvert design
How the battery culvert is designed is critical to ensuring that it does its job, has reduced maintenance, and provides for fish passage. It is important to get the location right. Choose a site, if possible, on a straight section of river where it will not alter the natural course and gradient of the river channel. Avoid long steep road approaches, as these are ongoing sources of sediment. Finding a crossing which meets these criteria will reduce scour of the approaches, and erosion of the banks and river bed. It is also best to make sure there is enough room to design approaches that are perpendicular to the river, so that water does not get directed to either end of the structure. Again, always refer to the NES-PF rules at the planning stage.
Make sure the carriageway is high enough, so that the crossing is safe to travel on during moderate flow conditions. This will reduce crossing closures that would otherwise be highly disruptive to traffic. Also, it is important that the approaches have acceptable gradient and transitions so that vehicles, especially low loader transporters. are not grounded. Note that where the height of the structure is greater than 1.5 m above the river bed, a building consent will be required.
Determine the correct type and size of culvert pipes for the structure. Reinforced concrete pipes have very good hydraulic characteristics because they are smooth, and have a high load bearing capacity. They may create a barrier to fish passage, however. Calculate the flood design, and use engineering formulae to determine the required culvert size. This includes considering the contributing catchment area and average annual flow. It is essential to design the structure to resist hydraulic pressure and erosion effects during flood flow conditions or debris flows. Major damage of the battery crossing will occur if they are not built to be robust. This may require reinforced aprons or deeply set rip rap on the outflow of the crossing. Also, it is critical to design to resist damage or blockage from woody debris. This may require the design of flared or chamfered culvert inlets or slash deflectors.
It is always good to look at existing structures, where present, to see what design features worked well, or not so well. For example, to assess the culvert pipe size against that derived from the flow calculations.
Given the costs and risks involved with battery culverts, it is recommended that the crossing design plans and calculations are peer reviewed. It is also suggested that in higher risk situations, or where a building consent is required, it may be necessary to consult with a forest engineer, hydrologist or other specialist to help with both design and construction. For additional information on how to calculate culvert size, refer to 8.9 Prediction of flood flows and sizing culverts, and Schedule 2 of the NES-PF for flood design flow calculators.
8.6.2 Battery culvert construction
As with all crossings, it is essential that construction occurs in suitable weather, and when the river has low-base water flow. Check for any fish spawning timing constraints. Restrict earthwork disturbance to the immediate construction site, which will include an area upstream and downstream of the crossing. Minimise the need for machinery to operate in flowing water. During construction, check regularly that the installation specifications and procedures are being followed, and signoff the work on completion of construction. If the work does not meet the design plan and standards, then initiate corrective actions.
The following are additional general construction good practice measures:
- Divert water flow around the construction site to assist in the foundation work, reduce the risk of contaminants entering the water, and minimise discharge of sediment
- Excavate the crossing bed, as required, to the correct depth and grade
- Wet or curing concrete must not be in contact with flowing water. Cement is a contaminant, and is toxic to invertebrates and fish. When pouring concrete, the water channel will need to be temporarily diverted
- Bed culverts so that they lie flat, and are supported on a firm or concrete base
- Elevated sediment discharge levels will inevitably occur during construction. The NES-PF sets a permitted activity limit of no more than eight consecutive hours of elevated sediment. Any longer will require a discharge permit
- Reduce the risk of contaminants entering the water, for example, from the foundation work. Minimise discharge of sediment
- Ensure culvert pipes lie at or below the natural river gradient, to avoid creating plunge pool erosion in the bed of the watercourse at the outfall of the culverts. Also, one of the culverts must be at least 100 mm below river bed level, and located to carry low or base flow. This will allow for fish passage
- Take care not to damage the culverts during installation. Concrete pipes are heavy, hard to place into position, and need heavy equipment to transport, load, unload, and position them
- Protect the inlet and outlet of the structure. Armour outlets with concrete aprons, rip rap, reno mattresses or other energy dissipating structures. Inlets are best protected by having deflectors that force most woody debris up and over the structure
- Stabilise the banks upstream of the structure inlet, if necessary, to prevent bank erosion
- Use clean gravel on approaches where the existing road surface could create a sedimentation problem.
It is important to divert road water off the approaches to minimise sediment entering the crossing. It is best to 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.