Flooding is the most common natural hazard we experience in Tasman. Floods can cause injury and loss of life, damage to property and infrastructure, contamination of water and land, and other environmental impacts. There are two types of flooding: river (fluvial) floods or overland flow floods.
River, or fluvial, flooding happens when a river, lake, or stream overflows its banks due to excessive rainfall and inflows from their respective tributaries. The resulting damage can be extensive as the overflow floods low-lying land of the adjacent flood plain. Additional impacts can arise should flood flows lead to dam failures or breaches in stopbanks.
The severity of a river flood depends on the amount and duration of rainfall in the river's catchment area, how saturated the soil is from previous rainfall events and the catchment’s shape and steepness. In flatter areas, floodwaters tend to spread out and rise more gradually and are shallower. In hilly or mountainous areas, floods can develop rapidly after heavy rainfall and drain quickly with high flow velocities, potentially causing further damage should debris flows occur. Given Tasman’s hilly topography, floods in the District can, and do, develop rapidly.
Large floods can inundate large areas across their flood plain. For example, in 2010 extreme rainfall resulted in a large flood in the Aorere valley with floodwaters extending right across the entire flood plain in its lower reaches.
Flooding in the Aorere Valley in 2010 (looking upstream towards the Ferntown Bridge).
Overland flow flooding occurs when intense rainfall accumulates on the ground and flows downhill leading to flooding without the involvement of an overflowing river, lake, or stream. Many people mistakenly believe that flooding only poses a risk if they live near a water body, but overland flow floods can happen anywhere — urban or rural — even in places far away from any water body.
In urban areas overland flow floods often result in urban stormwater drainage system becoming overwhelmed, causing water to spill onto streets and surrounding properties. The water level typically remains shallow (less than 1 metre in depth), however greater depths can occur where it is constrained and concentrated by such things as buildings, fences and landscaping. Overland flow flooding can cause considerable property damage, and the cost of recovery can be significant for homeowners and communities.
In 2013, an extreme rainfall event (approximately 100mm in 1 hour) caused extensive overland flood flows and surface water ponding across parts of Richmond. More recently, in August 2022 Richmond experienced another, but less severe, rainfall event.
Flooding in Richmond from the August 2022 severe rainfall event.
Floods can occur very quickly with rapidly rising flood levels accompanied by a rapid, powerful surge of water as a result of heavy rainfall over a short period of time, often in or near elevated terrain where gradients are steeper. Such rapid flooding can also result from the sudden release of water from the failure of an upstream dam. These floods are extremely dangerous and destructive, not only due to the speed of the flow and the sheer force of the water, but also because of the debris it picks up and carries along in its flow. Much of the larger debris is deposited when the gradient flattens out with floodwaters laden with the remaining debris continuing down onto the lower fan. Debris flows can be very destructive, typically arriving in abrupt pulses often described as “walls” of water and debris.
In 2011, prolonged and extreme rainfall caused flooding in the catchments draining into Ligar Bay and Pohara. Associated landslides triggered debris flows (rapidly flowing slurry of soil, sand, rocks, boulders and vegetation) which continued down the steeper catchment channels and on to the apex of the outwash fan.
Debris flow in Ligar Bay (December 2011)
Flooding is projected to become more common and severe in the future, as climate change brings more frequent and intense storms.
Given the District’s hilly topography and weather patterns, large parts of the district are susceptible to overland flow flooding. Thunderstorms can produce very heavy rainfalls locally, resulting in overland flow flooding where they occur. Tasman can also experience long duration, district-wide, high rainfall events impacting the larger catchments and bigger rivers. Such circumstances occurred in 1983 when many of the district’s catchments, the Tākaka and Motueka in particular, experienced significant flooding.
There are 13 river catchments in our District with varying degrees of management along their lengths. Some rivers, when in flood, have the potential to impact several of our towns to varying degrees. Council manages various stopbanks along the Waimea, Lower Motueka and Riuwaka rivers and Brooklyn Stream for flood protection purposes. However, even with stopbanks in place there is still a residual flood hazard. Stopbanks can overtop or fail during an event, and no form of protection is able to prevent damage in every flooding situation. Read more about river management in Tasman.
Tākaka has no formal flood protection measures. Some limited flood protection to the town is provided by a private stopbank, however much of the town remains exposed to varying degrees of flood hazard.
Stopbanks on the Lower Motueka River were designed and constructed in the late 1950s to provide protection to adjacent farmland from a 2% AEP flood event (a flood with a return period of 50 years on average). Parts of Motueka town are susceptible to flooding from a breach of the Motueka River stopbanks during a large flood. The Riwaka and Brooklyn areas are susceptible to flooding from multiple sources which include the Riuwaka River, the Brooklyn and Little Sydney Streams, and from a breach of the northern Motueka River stopbanks.
Parts of both Brightwater and Wakefield and surrounding areas are susceptible to flooding from the Wairoa and Wai-iti Rivers, the Mount Heslington and Pitfure Streams and a number of smaller tributaries. Parts of Richmond are susceptible to surface water flooding from the hill catchments above the town.
Council holds information on flooding including historic flood maps, images of past flood events and computer simulations (flood modelling). Council uses this data in our decision-making processes and when providing advice to public.
We’ve started a project to bring together our natural hazards spatial data into a single, online, easy to use, natural hazards map viewer. While this work is under development, you can view some of our flooding data (and other natural hazards) on our Nelson Tasman Future Development Strategy Map(external link) (under layers/constraints and protected areas).
My property is shown within an area susceptible to flooding, what does this mean?
The mapped areas are where land may flood during an extreme rainfall event based on flood hazard data held by Council. The degree to which an individual property would be affected by such flooding will depend on a number of factors including the location of the property in relation to the river or overland flow path, the depth and velocity of the flood water, and duration of ponding. Building characteristics such as the floor height and construction materials, as well as the nature of landscaping and fencing around a building can also influence the extent of flood damage that may occur.
Can I subdivide or build if my property is shown as being susceptible to flooding?
Subdivision requires a resource consent. All natural hazards, including flooding, are already taken into account when considering an application to subdivide land under the requirements of the Resource Management Act 1991.
When you apply for a Building Consent, flooding and the property’s exposure to other natural hazards are taken into consideration through the Building Act requirements.
How will this mapped information affect my property value?
Implications of the mapped information will vary from site to site and we’re not able to determine whether there will be any effect on property values.
How will this mapped information affect my property insurance?
In the first instance you can contact your insurance provider to discuss your specific policy.
The Council cannot advise property owners about the implications of this mapped information on your ability to obtain insurance or on insurance premiums.
We do know that the insurance industry uses local government-produced natural hazards data, along with their own assessments, to quantify natural hazard risk to help determine insurance coverage and/or premiums.
The Natural Hazards Commission Toka Tū Ake (NHC) has information on insurance and the claims process and making your home safer from natural hazards. They also have a Natural Hazards Portal Claim Map, illustrating settled NHC claims at a property level from 1997 to present.
Deep South National Science Challenge has developed an easy-to-read infosheet for homeowners, with important frequently asked questions about how insurance companies might respond to increasing climate hazards. Read more about house insurance and climate change.
What if I want to sell my property?
When you’re selling a property, you and your agent are legally obliged to share all relevant information about it to buyers. It’s also important that potential buyers do their own due diligence on any property they wish to buy.
Under the Local Government Official Information and Meetings Act 1987, councils have an obligation to make natural hazards information available.
District and unitary councils also have specific obligations under the Building Act 2004. This allows people to access information held by council about their property or any property they are interested in.
The data used to prepare the maps is applied to Land Information Memorandum (LIMs) and Project Information Memorandum (PIMs).
How is the Council using this coastal flooding and sea level rise information?
Consideration of flooding has been embedded in the Council’s work programmes for a number of years. We use this information in:
Next Steps
The next steps in reviewing the TRMP natural hazards plan change will be to engage with the community on the ‘issues and options’ for managing our natural hazards, including flooding. Engagement is proposed to begin early to mid 2025 and will seek the community’s views on the options for updating the TRMP provisions that relate to natural hazards.
Landowners, iwi, stakeholders and the wider community will have opportunities to input into the natural hazards TRMP plan change as we progress through the plan drafting stages.
Where do I find information about what to do following a natural hazard event?
Information, advice on getting prepared and what to do during a natural hazard event can be found on the Nelson Tasman Emergency Management Group website.
The Natural Hazards Commission Toka Tū Ake also has information and advice for home owners, tenants and home buyers to ensure your home is safer for natural hazards.
