2 Culvert Guidelines
2.1 Resource consent
If you wish to construct a culvert in a stream you are likely to need a resource consent. Consent requirements for culvert construction vary throughout the country. Contact your local regional council to discuss your proposal to find out if a consent is required and what information you need to provide.
2.2 Guideline criteria
These guidelines are not suitable when the following situations apply:
- overtopping of the culvert could cause flooding to nearby houses or buildings
- the crossing point is within 1 km of a residential area where the backup of flow behind the culvert could cause flood problems
- high debris loads are likely, eg, significant gravel bedload, flood debris such as trees or logs
- locations where the embankment above the culvert is greater than 1.5 m above the soffit and/or overtopping could cause embankment failure with significant consequences
- steep hill catchments
- catchments larger than 500 ha.
2.3 Culvert size
To determine what size of culvert is required, first look near your proposed crossing to see if there are any other culverts on the same stream which work well in storms (ie, do not flood upstream in smaller floods and do not regularly overtop). This may give you a guide as to an appropriate pipe size, although in many cases culverts in place on the stream may not be appropriately sized.
To size a culvert using this guide:
- Check the criteria to confirm that these guidelines are suitable for your location.
- Determine the catchment area above the crossing point. If using NZMS 1:50,000 maps note that one square on the map equals 100 ha.
- Locate the catchment on the rainfall maps and identify the rainfall band. Note: if the catchment lies across two rainfall bands, use the higher band.
- Refer to the culvert sizing tables for the relevant rainfall band and choose the catchment area closest to your catchment. This will give you a recommended culvert diameter. Note: a single large pipe can be replaced by two or more smaller pipes. See the table below for equivalent culvert capacities using smaller pipe.
If your catchment is larger than those in the culvert tables you should contact your local regional council or an agricultural/rivers engineer for advice. You may require a bridge rather than a culvert.
The minimum recommended culvert size is 300 mm in all situations. This is because smaller culverts are easily blocked by only a small amount of debris.
In some cases a large single culvert may not be the most practical option. Table 2.1 gives equivalent multiple barrel culvert installations, which will provide the required culvert capacity.
Table 2.1: Equivalent capacities for multiple barrel culverts
| Pipe diameter | Equivalent to | ||
|---|---|---|---|
| 300 mm | |||
| 375 mm | 2 x 300 mm | ||
| 450 mm | 2 x 375 mm | 3 x 300 mm | |
| 525 mm | 2 x 450 mm | 3 x 375 mm | 4 x 300 mm |
| 600 mm | 2 x 450 mm | 3 x 375 mm | 4 x 375 mm |
| 675 mm | 2 x 525 mm | 3 x 450 mm | 4 x 375 mm |
| 750 mm | 2 x 600 mm | 3 x 450 mm | 4 x 450 mm |
| 825 mm | 2 x 675 mm | 3 x 525 mm | 4 x 450 mm |
| 900 mm | 2 x 675 mm | 3 x 600 mm | 4 x 525 mm |
| 975 mm | 2 x 750 mm | 3 x 600 mm | 4 x 525 mm |
| 1050 mm | 2 x 825 mm | 3 x 675 mm | 4 x 600 mm |
| 1200 mm | 2 x 900 mm | 3 x 750 mm | 4 x 675 mm |
| 1350 mm | 2 x 1050 mm | 3 x 825 mm | 4 x 750 mm |
| 1600 mm | 2 x 1200 mm | 3 x 975 mm | 4 x 900 mm |
| 1800 mm | 2 x 1350 mm | 3 x 1200 mm | 4 x 975 mm |
| 1950 mm | 2 x 1600 mm | 3 x 1200 mm | 4 x 1050 mm |
| 2100 mm | 2 x 1600 mm | 3 x 1350 mm | 4 x 1200 mm |
| 2550 mm | 2 x 1950 mm | 3 x 1600 mm | |
The culvert sizes provided in this guide will in most cases pass storm flows equating to about the 1 in 5 year storm. Therefore from time to time they can be expected to overtop and in very large storms may scour out. If you wish to have a higher level of storm protection, or gain a more site-specific understanding of your risk, you will need to take advice from your regional council or a suitably experienced consulting engineer.
The maps and culvert tables on the following pages show the Rainfall Bands for different locations in New Zealand together with the recommended culvert size depending on the size of the relevant catchment.
- Upper North Island rainfall bands
- Lower North Island rainfall bands
- Upper South Island rainfall bands
- Lower South Island rainfall bands
- Table 2.2: Culvert tables
2.4 Culvert materials
There are a number of possibilities for culvert materials such as concrete, polyethylene/plastic, and corrugated steel. When choosing a material, consider the site conditions and the expected traffic loads over the culvert.
- In difficult to access locations a lighter material may make construction simpler whereas if the traffic loading is high the pipe material will need to be able to cope with this.
- In areas where agricultural and fertiliser runoff occur, steel culverts can suffer accelerated corrosion.
- In soft soils a single length flexible pipe will cope better than multiple section rigid pipes. In very soft soils, settlement of the pipe can affect the culvert performance - in these cases an engineer should be consulted.
- If high traffic loads are anticipated eg, logging trucks, contact an engineer to provide a specific design.
Contact your local rural supplier for prices and availability.
2.5 Culvert construction
You will need to consider crossing location. This may be determined by track locations but there may be more suitable alternative crossing points. Ideally a crossing point should be at the narrowest point of the stream with flat approaches on either side.
The culvert sizes in this guide allow for low flood flows. An allowance needs to be made for larger flood flows to prevent damage to the culvert and track. The best way to achieve this is to create a lowered spillway (approx 0.5 m deep) to the side of the culvert to allow controlled overtopping of the culvert. The spillway should be wide and level across the track and away from the culvert fill material. The outlet side of the spillway should be gently sloping back to the stream and grassed or rock armoured to prevent erosion.
If there is no suitable location for a spillway adjacent to the culvert, the spillway can be over the culvert. In this case special care must be taken in the construction of the culvert headwalls to protect the fill material from being washed out.
Allowance must be made in construction for fish passage through the culvert. The simplest way of achieving this is to make sure the base of the culvert is slightly below the streambed and that the culvert is laid on the same grade as the natural stream. As a rough guide culverts less than 600 mm should be buried 75 mm into the streambed; culverts less than 1000 mm should be buried 100 mm; and culverts larger than 1000 mm should be buried in the stream by 150 mm.
Headwalls should be provided at the culvert entry and exit. Headwalls retain earthfill and improve the hydraulic capacity for culverts as well as protecting against turbulence, which can erode stream banks and undermine culverts and tracks leading to expensive maintenance. A variety of headwall constructions may be suitable including precast concrete, posts with timber lagging, gabions, concrete filled bags, insitu concrete and rock.
Using rock armouring in the streambed at the culvert outlet will help prevent undermining of the culvert and improve conditions for fish to pass through the culvert. Rocks 200-300 mm in diameter will be suitable for most situations, preferably with a geotextile layer below to prevent fine material washing away and gradually eroding the area.
Runoff from tracks leading to and from the culvert should be diverted away from the stream using earth cutoff drains.
Fill over the culvert should be no higher than 1.5 m. High fills allow for greater depths of water backing up and will increase the flow velocity and erosive potential at the outlet and immediately downstream of the culvert, increasing the likelihood of damage. The fill depth should be checked against the pipe manufacturer's recommendations to ensure the pipe is capable of supporting the load. In most cases the culvert should have at least 0.5 metres of compacted material above. This protects the culvert by using the soil structure to spread loads across the pipe and preventing high point loads.
Backfill around the culvert should be compacted during construction and it is important that the pipe bedding (the material the pipe is directly sitting in) is firm and supports the pipe barrel. Fill should be clean and free of debris, vegetation and topsoil. Vegetation and topsoil must be cleared from the crossing point before the culvert is installed.
2.6 Culvert maintenance
It is important that culvert entries and outlets are kept clear of debris as this can seriously reduce the flow capacity of the culvert and result in additional flooding and damage. Ensure culverts are clear by checking regularly particularly before and immediately after heavy rain. A small amount of time can save a large amount of dollars!
2.7 Culvert costs
Culvert costs vary with the location, pipe material and the pipe size. Typical costs would be in the range of $2,500 to $25,000 depending on size and location.
