Floodplain management planning
A floodplain management plan (FMP) emphasises the need to ‘keep people away from the river’ rather than ‘the river away from people’.
The Floodplain Management Planning Process
The process for developing the floodplain management plan is shown in Figure 1, and involves three steps:
- Phase 1: Establish the context,
- Phase 2: Understand the risk and treatment options,
- Phase 3: Achieve sustainable solutions
to a level of detail appropriate to the location and circumstances of the community.
Figure 1: The Floodplain Management Planning Process (adapted from NZS 9401:2008)
Community involvement in floodplain management
To be effective, the floodplain management planning process requires the active involvement of the people who live and work on the floodplain. This is the same whether developing a floodplain management plan for urban areas (such as the Hutt Valley or the Kapiti Coast), or for rural areas (such as the Wairarapa).
Each plan reflects the values, aims and responsibilities of those who would be affected by flood risk in a particular catchment. The community is then involved in making decisions on the level of risk they are prepared to live with, identifying ways to reduce the effects of flooding or erosion and deciding how to manage the residual risk.
Phase 1: Establish the Context
To develop a floodplain management plan, Greater Wellington must first research information about the river and its floodplain. This is done by:
The densely populated Hutt Valley floodplain
- Studying rainfall and runoff, and identifying catchment conditions that may lead to increased run-off and possible flooding;
- Assessing the flood and erosion hazard (describing the hazard);
- Reviewing existing flood and erosion hazard management;
- Considering present and future uses of the floodplain, such as housing, recreational, commercial, or agricultural development;
- Considering the impact on environmental factors - such as the habitats and ecosystems that exist on the floodplain and in the corresponding river systems;
- Identifying cultural, landscape, recreational and community values;
- Identifying potential damages on the floodplain.
This information is put together in a 'flood and erosion hazard assessment' which becomes a summary document and completes Phase 1 of the floodplain management planning process.
Phase 2: Understand the Risk and Treatment Options
Identify and assess risks
In terms of floodplain management planning, flood risk management measures fall into two broad categories, non-structural and structural, as shown in Figure 2 below.
Each flood risk management measure needs to consider economic, environmental, cultural and social impacts.
The combination of structural and non-structural measures for managing flood risk is influenced by:
- Existing Risk. This is a measure of how developed the floodplain is. For example, if there is an existing community living there; and
- Whether the river’s natural pathway has already been confined by development.
Figure 2: Flood Risk Management Measures
Non-structural measures
Non-structural measures address flood risk by firstly helping communities to avoid it. Rather than building structures to keep the river away from people they focus on keeping people away from floodwaters and helping the community cope when flooding occurs.
Non-structural measures are used to influence the way:
- Waterways and land are used;
- How buildings are constructed; and
- They also seek to improve the community’s resilience to flooding.
These measures are implemented through encouraging voluntary actions and by enforcing planning policies and rules.
Non-structural measures are considered the most cost-effective flood risk management approach because they can prevent considerable flood damages at little cost to the wider community. They are vital when a flood occurs that is larger than that with which the flood risk management measures have been designed for (this is called an over-design event). An over-design event may expose formerly protected communities to the full forces of a flood.
Planning Options
Planning options are regulatory measures which raise community awareness of the risks and realities of living on a floodplain. These alert people to areas subject to flood risk and encourage appropriate land-use in flood and erosion prone areas. Planning options may include zoning and cover activities such as construction and earthworks. Examples include:
- Set-backs from watercourses
- Requiring floor-level controls
- Restricting subdivision and development in floodable or erosion hazard areas
- Encouraging sustainable development that will not increase flood water to watercourses e.g. stormwater run-off controls
Catchment Options
Catchment management options include measures such as reforestation to reduce rain run-off and wetland restoration to absorb floodwaters. Additionally, swale drains, rain gardens, and detention storage in public parks or sport fields may be used to reduce the amount of water that may reach waterways in a flood. These measures work by holding water in an area, stopping it getting into the stream or river as quickly. They are of great benefit in smaller events and they will also help to reduce long term damage from larger events.
Flood-proofing Options
Flood-proofing options include house-raising and flood-proofing of at-risk properties. These are an option for people who own flood affected properties. They look at addressing the impact of a flood by reducing the potential for damage such as lifting electrical circuits, installing tiles instead of carpets in floodable rooms, or replacing under floor vents with ones that can be closed.
Civil Defence and Emergency Management
Civil Defence and Emergency Management is about encouraging emergency preparedness in the community, and providing flood warnings. These options are most often used to cover the ‘what if?’ situations, but are equally useful to have ready for those who live in a floodable property. They are about knowing what people should do when a flood happens, and having a plan ready to put in action. A plan may include making sure people can move precious items upstairs easily to prevent them getting damaged, to making sure they have an escape plan if things are getting dangerous.
Non-structural measures cannot prevent flooding, but they can reduce the risk associated with flood events and help limit people from becoming affected by flooding. Such options are adaptable, and implemented through district plan processes and voluntary measures.
Structural measures
Structural measures are the physical flood risk management structures, designed to help keep flooding away from existing communities and development.
Structural Works
Flood defences such as stopbanks, floodwalls and pumping stations address the consequence of flooding up to their design period event. Sometimes a larger than design flood may happen, or climate change may occur faster than allowed for meaning that they become difficult to upgrade to greater design period event standards.
Channel Management
Active channel management measures include bed and beach re-contouring and gravel extraction. They are used to reduce the opportunity for the river to erode its banks, reducing potential erosion hazard and may also prevent damage to structural works.
Upgrading bridges or other structural constraints for the watercourse will also improve flood capacity and avoid the risk of debris dams forming (logs and other obstructions catching against the structures).
Bank Edge Protection Works
Rock linings, vegetation buffers and groynes are bank-edge works and river berms, which are used to reduce erosion hazard, maintain the position of the river channel.
Recent research (Waikanae Floodplain Management Plan Review 2010) shows in many cases construction of flood defences such as stopbanks has not significantly reduced potential flood damage because additional development has continued behind the stopbank, and this development remains vulnerable to residual risk if the design period event of the stopbank is exceeded.
Phase 3: Achieve Sustainable Solutions
Once a number of options have been identified, combinations of non-structural and structural measures for managing the flood risk are usually possible. These should look at managing the risk over the full flood risk spectrum i.e. from an annual event to an extreme event. Greater Wellington, works with the community to select a combination of options for more detailed study. A preferred option is then chosen after looking at non-structural options, the cost of any construction work, the level of flood risk that results, and the social and environmental effects. This forms the basis of the final floodplain management plan.
Operative plans and those in progress
Greater Wellington has developed floodplain management plans for the Hutt, Waikanae and Otaki River floodplains. Each of these planning processes took several years and involved many groups and organisations in each community.
There currently Floodplain Management Plans underway for the Waiwhetu Stream, Mangaroa River, Waiohine River and Pinehaven Stream.
In the Wairarapa, river management schemes (which are similar to Phase 1 of the FMP) are in place for the Waiohine, Mangatarere, Upper Ruamahanga and Waipoua rivers.
Eventually, Greater Wellington will work through floodplain management plans for all of the region's catchments where flood risk management is an issue.