If your crossing point is not within the criteria for culverts referred to in Section 2.2 or the geography of the crossing point make a culvert impractical you are likely to require a bridge. If this is the case you will need to contact an engineer or bridging contractor to develop a design suitable for your location.
As is the case with most culverts, resource consent will be required for all bridge structures. Contact your local regional council to discuss your proposal and find out what information you need to provide. A building consent will also be required from your local district council.
As a general rule the bridge deck should, where possible, be 0.5 m higher than the surrounding ground/floodplain. This allows debris to pass under the bridge during flood events and prevents the bridge becoming a major obstruction in high flows, as water can pass around the bridge.
A number of companies are able to produce prefabricated concrete bridge decks for spans up to 12 m. These are often quick to construct and cheaper to install. The costs to construct a bridge vary depending on a number of factors eg, bridge location, access to the site, the bridge span required, the expected bridge loading, abutment piling, and erosion protection. For a typical situation the costs generally range from $2,500 to $4,000 per metre span of the bridge.
As for culverts, regular inspections of the bridge can save money and reduce flood risks. Monitoring the abutments for signs of erosion is also important as is checking for damage to the bridge deck.