Site Analysis

Understanding all the features of a site, using and protecting the best, and minimising the impact of the worst.

Flood risk

Floods can be devastating. Flooding in Edgecumbe in 2017, for example, destroyed over a dozen houses and left a dozen more permanently uninhabitable.

Flood risk will determine whether and how a site can be built on.

Flood risk occurs in low lying areas of land such as:

  • flood plains
  • close to rivers, watercourses, lakes and the sea
  • below a hill
  • close to a valley mouth
  • close to drainage channels
  • natural depressions or swamps that have been drained.

If a site is prone to flooding, decisions must be made about:

  • whether the site is safe to build on
  • where to locate a building on the site
  • the finished floor level – the council may have specific requirements under section 72 of the Building Act 2004
  • landscaping and drainage.

Check council records for the property, such as the project information memorandum (PIM) or land information memorandum (LIM) reports, and look for information about past flooding in the area. Consider the impact of flooding on stormwater and sewer systems and erosion.

Some local authorities have defined and mapped flood management areas. The council may have a requirement for new buildings in these areas to have a floor level higher than other areas. If you are planning to build or extend a house in the Christchurch flood management area, for example, your minimum floor level may need to be high enough to protect from a 1-in-200-year flood event.

Areas at risk of flooding are also changing as a result of rising sea levels.

Smaller but more frequent flooding may actually cause more problems for the built environment than larger but less common floods. A research team found in their calculations that when sea levels rose by 0.3 m, the more common smaller floods actually made up more than 80% of the average annual building replacement costs in half of the areas they studied.

Research by NIWA scientists published in early 2020 found that the risks of flooding that is not the result of a storm or heavy rain are increasing, especially in the first 6 months of the year. There is a natural annual cycle where the sea warms up and expands, leading to higher sea levels over these months. When this rise is added to forecast sea level rises from climate change – 300–400 mm in the next 30–40 years – and there is also a spring or king tide, “sunny day flooding” affecting coastal buildings and roads will become increasingly common.

Many local authorities are mapping coastal areas at risk. For example, in 2021 Northland Regional Council updated its maps of areas that may experience coastal flooding and erosion within 50 and 100 years. Once these sorts of maps are finalised, they typically go into local district plans and councils are required to use them when managing coastal hazard risks. The information usually becomes part of property information records such as LIMs and PIMs.

Another example of this is a 2021 report for the Christchurch City Council prepared by Tonkin+Taylor. This found around 25,000 properties in coastal and low-lying areas of Christchurch and Banks Peninsula are at risk of flooding or erosion from rising sea levels. Around 6500 of this number were assessed as high or medium risk. Christchurch has developed an online interactive tool that allows residents to see how coastal flooding and erosion could affect their properties over coming decades.

Making enquiries with insurance companies should also be part of the assessment of a potential building site that has flood risks. Some insurance companies are tailoring their premiums to the risks faced by individual properties, so a house owner may have to pay higher premiums. In some cases the insurance excess applied by an insurer for flooding claims may also be high. While an excess of $2500 to $5000 is not unusual for flooding, the excess could be up to $10,000.

Minimising the risk

If construction of a new building goes ahead on a site prone to flooding, minimise the risk by:

  • ensuring the building is located on the highest section of the site
  • building away from natural drainage paths or channels
  • making the finished floor level of the lowest floor well above (600 mm minimum) the maximum flood level
  • installing additional land drainage for low-lying areas (there must be somewhere for water to drain to).

 

Updated: 11 October 2021