River flooding modelling

River flood modelling is used to provide detailed information on the flood hazard of a river or stream. It can be used for many purposes, including:

• To inform flood hazard areas, or areas that may be unsuitable for development or require mitigation.
• To provide flood levels at specific locations to inform:
  • Design requirements for flood protection infrastructure such as stopbanks
  • Minimum floor levels of buildings – note that these are set by District/City councils, not WRC, but they are often informed by flood levels from our models.
• To inform the location and/or design of key infrastructure

To construct a river flood model, you need the following key inputs:

1. Topographical information for the river channel and floodplain
   • From LiDAR and/or river channel surveys
2. Information on rainfall
   • From telemetry such as rain gauges and tools such as HIRDS (provided by NIWA)
3. Information on the catchment and the level where the river flows into, i.e. the sea, another river (known as the boundary condition)

All these inputs can be very time consuming and/or costly to develop at the level of detail that is required for an accurate model, as is the modelling itself. Because of this we don’t have detailed models for all the rivers and streams in the Waikato region. We focus on the rivers and streams with the greatest flood risk and where we have flood protection schemes.

There are two main types of flood modelling, hydrological and hydraulic. Hydrological modelling is used to assess how hydrological processes within a catchment culminate in runoff to allow flow predictions within a watercourse. These flow predictions can be used in hydraulic models which computationally analyse the flow through physical features such as rivers and infrastructure.    

River flood models are only as accurate as the data that is put into them. With developments such as LiDAR, the accuracy of flood modelling is improving; however, a model can never fully represent reality. Some common limitations of river flood modelling are:

• Older models are likely to be less accurate than more recent models due to improvements in technology, data and climate change projections, as well as changes that may have occurred to the catchment (see below).
• Models are based on the condition of the catchment at the time of modelling (a snapshot in time). Changes such as land use changes, intensification in development and recontouring of the land (e.g. for a subdivision) may affect the flood hazard.
• Models are based on the river channel and floodplain topography; however, this does not include obstructions such as fences, trees and buildings, which may locally affect the flood extent, depth and/or speed.
• Flood models do not consider the potential for large debris blockages (such as a landslide) or debris flows.
• River flood models are generally only relevant to identified waterways and WRC drains. Other sources of flooding, such as localised ponding, stormwater infrastructure, the sea and other waterways are often not included.

When viewing our river flood models on the Waikato Regional Hazards Portal, clicking on the model data (in the map) will allow you to see in which year the model was developed.

A datum is a fixed vertical elevation or level used as a reference point. Flood levels from river flood models are provided relative to a specific datum, e.g. RL 14.6 m (MVD-53). RL stands for reduced level and means the level is in reference to the specified datum; MVD-53 stands for Moturiki Vertical Datum 1953, which is a commonly used datum in the Waikato region. TVD-52, or Tararu Vertical Datum 1952 is another commonly used datum in our region.

When viewing our river flood models on the Waikato Regional Hazards Portal, clicking on the model data (in the map) will allow you to see which datum the model is relative to.

The New Zealand Vertical Datum 2016 (NZVD2016) is the official vertical datum for New Zealand and its offshore islands. WRC and other councils are working towards using this datum for all flood models (and other applications) to achieve better consistency. However, due to legacy data, models and other information this process will take time.

Climate change is projected to increase the intensity of heavy rainfall events and cause sea levels to rise, both of which increase the risk of river flooding. Because of this, when developing river flood models, the effects of climate change must be considered. The Ministry for the Environment has produced guidance on how to do this, and HIRDS provides a tool with which to achieve it. 

As our understanding of climate change science improves, the guidance on how to incorporate its effects continues to be developed. This means that older flood models may use a different climate change scenario(s) to newer models, i.e. different increases in flow and sea level. When viewing our river flood models on the Waikato Regional Hazards Portal, clicking on the model data (in the map) will allow you to see which climate change scenario was used for that model.