Single culverts are still the most common structure used to cross small to medium-sized rivers on forestry roads. They are best suited where there is a low level of river bed load (gravel) movement that could infill the culvert, and a low risk of debris entering the river. Compared with most other waterway crossing structures, culverts are easy to install and relatively low cost. Careful planning and installation are required to prevent failure and ensure fish passage. Resource consents will be required where the culvert does not meet the permitted activity conditions within the NES-PF. With regard to location, a resource consent is needed when the river crossing is within 500 m of a dwelling that is within 15 m of a river bed greater than 3 m wide, or downstream of a dwelling with a ground floor level that is less than 1 m above the highest part of the river crossing.
8.5.1 Types of culverts – pipe, box and arch
Many types of pipes are suitable for waterway crossings, but in terms of design, the pipe and box culverts are generally used. However, arch type culverts are becoming more common, especially for larger rivers, as they retain the natural river bed. Culvert pipes can be smooth or corrugated, and can be made from a wide range of materials – plastic, concrete, galvanised steel or aluminium. The service life, ease of transport and handling during construction, the relative cost between types, the importance of fish passage, and soil and ground conditions will determine which type of pipe is best for the site. Plastic pipes are now most common as they are lighter, but more importantly, can flex a little to accommodate differential settlement of the road over time. Other pipe sections, for example second hand steel drill casings, are relatively low cost and make functional culverts, especially for temporary waterway crossings.
Corrugated pipes are commonly used for small to medium applications. They are made of polyethylene, galvanised steel or aluminium. Polyethylene is typically used for the smaller culverts, and steel, or aluminium in some instances, is often used for the medium-sized structures. Transportation and handling costs are lower than for concrete, due to their lightweight design. Polyethylene is an extruded product, and steel and aluminium come from roll-formed pipe sections; they all come in long lengths (6 m), and require less pipe joining. They can easily be assembled on the job; for example, polyethylene can be cut to length on-site with a saw. Small pipes may need only one or two people to lay them into position. Some pipes require the use of the manufacturers’ proprietary joining systems.
Corrugated pipe culverts have a lower water discharge capacity than the same sized, smooth-walled pipes. The disadvantages of corrugated pipes are that they are reliant on the soil envelope to provide strength, are easily damaged during installation, and are less durable than concrete because they are subject to abrasion. Galvanised steel also corrodes. They are also easily damaged by floating debris, so they need to be installed where there is a low risk of debris damaging the structure, or where debris traps are installed upstream. With pipe culverts, it is difficult to establish and maintain fish passage, especially if they are not constructed below the bed level of the river. Polyethylene pipes are resistant to corrosion, but are more sensitive to improper installation due to their greater susceptibility to collapse under a non-uniform load. Corrugated aluminium pipes are used in situations where the soil or water is likely to corrode galvanized steel pipes. Due to their durable characteristics, they have a longer life potential in harsh environments.
Corrugated multi-plate pipes are larger corrugated, galvanised steel culverts constructed using a ‘multi-plate’ system. The pipes are assembled on-site from a series of curved plates bolted together to form a large circular or arched pipe culvert barrel or pipe. These enable the installation of large pipe culverts in locations where transport of a large concrete or roll-formed steel pipe to the site is impractical.
Reinforced concrete pipes have very good hydraulic characteristics, and are excellent for situations where a high load-bearing capacity is required. They are less dependent on the soil envelope, less susceptible to construction damage and can be re-used. Concrete culverts will last for long periods without corroding or losing their structural strength, and are resistant to bedload abrasion. They do have several major disadvantages. Due to their hydraulics, it is difficult to establish and maintain fish passage, which may necessitate the fitting of fish passage baffles within the base of the culvert. Also, supply and installation are relatively expensive. They are heavy, so machinery is required to load, transport, unload and place them in the river. For example, a 2.4 m (diameter) concrete pipe segment with a length of 2.5 m weighs nearly 6 tonnes. As such, it is important to understand the lifting capacity of available machinery. For example, a 20-tonne machine can lift around 5 tonnes, while a 30-tonne machine can lift around 8 tonnes. The rubber ring joining system makes laying more difficult. Also, the pipes are in shorter lengths, necessitating more pipe joins and, if the fill moves, socket connections can separate.
Arch culverts are generally open-bottomed structures, sometimes referred to as half-pipes, or they can have a flattened steel base. Arch culverts can either be a steel pipe arch or steel multi-plate. They can span up to approximately 8 m and can cope with relatively high flow rates. The advantages of arch culverts over pipe culverts are that they maintain a wide natural river bed, improve hydraulic capacity during low flows, and cause less heading up of the river during flood flows. These factors will also improve fish passage. Arch culverts require foundations that can resist the arch compression loads. They may be an attractive alternative to a bridge where foundation soil bearing capacity is good on each side of the river. They need to be designed by a suitably experienced person, preferably an engineer.
Box culverts typically consist of rectangular concrete box segments tied together with longitudinal steel bars to form the required culvert length. They are available in a range of sizes up to several metres wide and high. Other configurations include U-shaped troughs with lids, and an inverted U system founded on a concrete base. Using box culverts has several advantages. They can cope with large flows where headroom is limited, and for an equivalent waterway area, they can accommodate significantly larger flows than corrugated pipe alternatives. They have a large and robust mass, so they are able to withstand overtopping, and are resistant to heavy bedload movement. Another advantage is the minimal excavation and backfilling required. Off-the-shelf precast units are loading certified and as such, box culverts can be designed to carry heavy wheel loadings with little or no fill material placed over the culvert to distribute the load. Also, baffles can be easily installed to aid fish passage. An obvious disadvantage is that precast concrete units are heavy, and may require a crane to put them in place. This makes supply and installation relatively expensive. For example, a 4 x 2 m concrete box culvert can weigh nearly 11 tonnes. Also, like other culverts, they can create a permanent control point in river beds, and require ongoing bed and channel maintenance.
8.5.2 Culvert design
The necessary design criteria include a determination of the flow rate (volume per unit time, in m3/s) and depth of water associated with a 1-in-20-year storm event for a given river location. The culvert should be sized to pass the design storm flow without heading up. Refer to the section for calculating storm flow. Other important criteria consider environmental and social effects of the structure. For example, fish passage or downstream river users. This requires using a mix of engineering formulae and judgement to determine the required culvert size. The flow capacity of the selected culvert is a function of its cross-sectional area, surface roughness and length. Use existing structures, where present, as a tool to gauge the culvert pipe size against that derived from the flow calculations. Also consider that smaller pipe diameters may be more prone to plugging, and require more frequent maintenance to keep them open.
Culverts must be designed to cope with the expected flood flows from the catchment. The NES-PF’s permitted activity rules are specific around culverts. For example, the NES-PF permitted activity rules require that a single-pipe culvert must pass a 20-year storm event of no more than 5.5 m3/s without heading up. Also, if the invert gradient is greater than 6% on river channels greater than 3 m wide, a resource consent will be required. The NES-PF also specifies a minimum diameter for a single pipe culvert to be 450 mm.
There may be some circumstances where design to a lesser flood event is appropriate. Examples include temporary harvesting access roads where the culvert will be removed within two months, culverts in locations where an alternative flood overflow path can be provided, and/or culverts on access roads where it is acceptable that they are inaccessible to vehicles on occasions during periods of high rainfall. Where culverts are designed to carry a lesser flood, they should be installed so that they will not contribute to any adverse environmental or downstream effects on landowners if the culvert overtops.
Given the costs and risks involved for large culverts, or culverts higher than 3.5 m (measured up from the bed of the river at the inlet – including the pipe and fill), it is recommended that flood design calculations are peer reviewed. It is also recommended that in higher risk situations 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 section 8.9 Prediction of flood flows and sizing culverts, and Schedule 2 of the NES-PF for flood design flow calculators. The regional council can request a copy of your flow calculations, so keep a record on file.
When designing a culvert, consider the following to determine the type and size of the culvert:
- The catchment size and its geology, soil type and topography
- The expected flood flows, and the probability of these occurring during the period the culvert is in place
- The traffic usage and the importance of the road; for example, a short-term spur road versus an arterial route
- The potential to provide overflow paths, to avoid damage should the culvert capacity be exceeded in a major storm event
- The risk of adverse upstream and downstream effects; for example, to the environment, infrastructure and dwellings
- Upstream and downstream passage of fish, if present or likely to be present
- The amount and type of woody debris that might reach the crossing in a storm event.
- To determine the specific location of the culvert:
- Locate the crossing on a straight section of river to reduce scour of the approaches, if possible
- Try to avoid locations that alter the natural course and gradient of the river, or create erosion of the banks and bed of the river.
Also, ensure that the culvert is the right length. If the culvert is too short, the batter slopes are over-steepened. This can lead to the fill slope slumping, and the discharge of sediment into the river. Consider designing armoured spillways where culverts may be at risk of overtopping.
A culvert pipe of adequate length
8.5.3 Culvert installation
Trench size recommendations for culvert installationCulverts must be installed correctly. Most culvert manufacturers provide specifications for the installation of their products. These may vary between types of culvert pipe and between manufacturers. Also, it is critical to meet the requirements of the NES-PF for culvert installation. Therefore, it is very important to refer to both the NES-PF regulations and the supplier’s specifications. The following are general points for culvert installation. Some points are specific to pipe culverts; however, most are applicable to any type of culvert. Also, some of these points are linked to permitted activity requirements within the NES-PF.
- Construct in suitable weather, and with low river flow
- Check for any fish spawning timing constraints under the NES-PF
- Limit earthwork disturbance to the immediate work site, which will include an area upstream and downstream of the crossing site
- Minimise the need for machinery to operate in flowing water
- Divert the river around the culvert trench temporarily to make sure the culvert foundation is properly prepared. This also reduces the risk of contaminants entering the water, and minimises discharge of sediment. Culvert pipes should never be installed directly into a flowing river bed, because proper installation cannot occur
- Take care not to damage the culvert during installation
- Construct the culvert trench or bed at the correct depth and grade, so that when constructed, both the inlet and outlet are 20% below river bed level. This will allow for fish passage
- Bed the culvert in, so that it lies flat and is supported on the firm base of the trench. Ensure the pipe will bed into the surface underneath it, preventing water from flowing underneath it. A uniformly sloping bed is required
- Make the trench wide enough for proper compaction at the sides during backfilling
- Backfill, using clean fill for example, with no organic matter. Compact around the pipe, to eliminate water bypassing the culvert, and scouring it out. Compaction should be carried out in layers, using vibrating plate or oscillating plate compactors. This allows a significant proportion of the applied vertical load to be transferred to the surrounding side fill material
- 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 must be temporarily diverted
- Armour the inlet headwall and outlet, to help protect the structure from erosion, where necessary. Use rip rap, reno mattresses, durable logs, gabions, wing walls or energy dissipating structures. Always armour the culvert mouth and exit to above the full pipe diameter. Do not use tyres, untreated wood or logs to construct the headwalls of the structure
- Divert road surface water away from the river crossing and culvert fill, within 10 m where practicable
- Use storm water and sediment control measures to limit sediment entry into the river; for example, berms, cut-outs, ditches, flumes and sediment traps
- Check regularly during, and on completion, of construction. If the work does not meet the design plan and standards, then initiate corrective actions.
If an earth channel and bunding is used to divert the river, consider using a geosynthetic cloth to reduce the release of sediment into the river. Also, before redirecting the river through the installed culvert pipe, pump out any sediment-laden water that may have accumulated in the pipe or at its ends. Discharge well clear of the river into a soakage hole, or where it can filter through vegetation or soil before re-entering the river.
When constructing the bedding, do not use large rocks that would apply uneven point loads on the pipe or box. This could cause excessive stress in the pipe wall, leading to failure. In situations where high settlement of the underlying ground is expected, it is advisable to crown the bedding slightly upward near the centre of the culvert, so that after settlement, the pipe will be near the desired grade, rather than having a sag in it.
Flexible culverts, which are manufactured from polyethylene, steel or aluminium, are susceptible to deformation and possible collapse if the backfill around the pipe is not uniformly compacted around the full diameter of the pipe. Rigid culverts, such as concrete pipes or concrete box culverts, have their walls reinforced to carry the load. Rigid pipes still require adequate compaction of backfill to ensure there is no leakage along the outside of the pipe, and to avoid future settlement of the pipe backfill. Large concrete pipes will deform under load if the backfill is not well compacted. The diagram above gives recommendations for the size of the trench required for culvert installation.
Ensure the fill batter slopes are not too steep. It is important that the culvert pipes are long enough to ensure the batter slopes are not steeper than normal earthworks fill slopes, except where specific culvert headwalls or retaining walls are used. Debris barriers should be considered. There are many useful designs in guides such as the US Federal Highway Administration hydraulic engineering guide, Debris Control Structures: Evaluation and Countermeasures.
8.5.4 Minimising culvert failures
Culverts are susceptible to blockage and overtopping, especially in recently harvested catchments where there may be a high proportion of debris entering rivers during floods. This is the predominant cause of culvert failure. For this reason, special care in the design and installation of culverts is necessary to minimise the potential adverse environmental effects of a culvert blockage or blowout. Good design, location and culvert construction methods, along with good harvest practices, will reduce the risk of culvert failure. Avoid locating culverts in bends in the river channel immediately upstream of the culvert entry. Also avoid double barreled culverts, where two smaller culverts are used instead of one larger one.
The damage caused by a blocked culvert can be significantly reduced by providing a low overflow point offset from the culvert fill location. In the event of blockage, water will flow over more stable ground rather than erode out the culvert fill, discharging large amounts of sediment into the river. For example, on higher risk sites where exceptional flood flows could carry debris that may block a culvert. Consider designing culverts to allow flood flows to pass over the culvert via a spillway. Design the structure to have a low point at one end of the crossing on undisturbed ground adjacent to the structure. Building an armoured spillway is a controlled activity in the NES-PF; controlled means that a resource consent is required to divert the river or otherwise modify the bed of a river.