Flood mapping

A flood depression is a place that has the potential to ‘fill up’ with water in heavy rain if the stormwater pipe system, drainage networks or culverts are blocked, if the system is reaching capacity, or if the natural contours of the ground create a ‘bowl’. Some of these areas are naturally present around Hamilton, and others can be created if care isn’t taken around planning, construction, and development.

We manage stormwater through a network of pits, pipes, culverts, open drains, and streams. This network carries water away from roads and properties but it doesn’t have unlimited capacity. Once exceeded, stormwater may flow on roads or within properties, and will pond in some locations.

Water will flow downhill. Our roads will generally form a path to take floodwaters away via culverts and pipes, and there’s also flowpaths across land (including private property).  If our pipes become blocked, flooding can get worse and water can pond in ‘depression areas’ which rely on these pipes to take the water away.

Pipes and culverts can be blocked by debris carried by floodwaters. Smaller debris may also accumulate around partial blockages. Debris can include rubbish, leaves and branches, or anything that can be carried by flowing water. In very heavy rain events this can include things like mattresses, outdoor furniture, caravans and even cars. Some of these may end up in our streams or streets and block our pipes and culverts.

Culverts and pipes take water away and are needed to drain Hamilton. The flood extents assume the pipes and culverts are functioning and conveying water away, whereas the depression areas represent the ponding area in a flood event if pipes and culverts are blocked. When blocked, they can’t take water away and water can be trapped behind man-made features such as development areas or roads.

This information shows areas around Hamilton where water might fill up if the stormwater system reaches, or exceeds, capacity. This information should be considered when developing property or applying for a building consent. The information also allows better readiness and understanding of extreme events, such as those the Auckland and Hawkes Bay regions experienced in 2023.

The more information we have, and can share with our community, the better prepared we all are to plan ahead and protect our properties and families.

There is a lot of uncertainty when it comes to blockages. We don’t know if and when they may occur, but we know they can happen and they were a significant contributor to the 2023 Auckland Anniversary Weekend flooding events. When they do happen, the flooding can be severe. They can also occur fast, leaving little time to evacuate as water level rises.

Floodviewer shows a lot of data at an individual property level. Modelled flooding is shown by mRL (Reduced Level, in metres). Reduced Level means the height from a specific data point which provides consistency for flood mapping. Contour is the height of the ground from that same data point. Ground levels can change either through natural movement or man-made earthworks and can be quite different across sloping sections or hilly properties. To work out the relative flooding in a particular event, just subtract the height of the flooding from the height of the ground. For example, a property with a mRL of 34.33 and a Contour of 34 will have .33m (33cm) of floodwater in that modelled event. If the neighbouring property has the same flooding but a contour of 33, (that part of the section may slope toward a gully) it will have 1.33m of flooding at that point in the same event. If you want more detail on your individual property, contact city.development@hcc.govt.nz

The flood information will not directly affect your rates. Any works that Council undertakes to address flooding would normally be determined through the 10-Year Plan process and funded from either rates or a combination of rates and development contributions.

Floodviewer shows whether properties within modelled areas are affected by flooding in a 100-year flood including Over Land Flow Paths (OLFP) and flood depression areas.

Flood information shows people whether their property is likely to flood during a 100-year flood. These floods have a 1% probability of occurring in any given year.

The information shows whether this is low, medium or high flood hazard and enables you to prepare an emergency plan. The flood information can also inform what you do on your property. For example, owners of properties with a flood hazard showing should consider where flood water will rise and flow on the property particularly if intending to develop. People must avoid making flooding worse on any other properties by blocking or diverting flood water. For example, this could happen by constructing a raised garden, garden shed or playhouse in a flood area. They should also take care when altering the level of their property that they do not inadvertently direct water where it is not wanted, such as towards the door of a building.

Flood maps can look like they show flooding on top of the buildings. In reality, the water may go under, through or around the building. Whether flood water gets into a building depends on how high off the ground the floors are compared with the depth of the surrounding flood water. Council is unable to provide floor levels of existing buildings but can supply detailed maps showing maximum flood depth and velocity and design flood level for individual properties in a 100-year flood for flood-modelled areas. These maps may be requested by:

• Phoning (during normal business hours) 07 838 6699, or
• Emailing city.development@hcc.govt.nz.

You may wish to understand the flood risk to your property and modify your property to reduce the risk. Flood hazard rules in the District Plan apply to new subdivisions, land use and development. The rules also guide the development of new buildings and extensions to existing buildings, but they don’t require you to modify your home.

If any part of your property is shown in a flood hazard area in the District Plan, then the floors of any new buildings and extensions to existing buildings are required to be a minimum height above the floodwater level.

If Floodviewer updates the flood hazard area on your property from that shown in the District Plan, or if your property is in a flood hazard area not identified in the District Plan, then contact Council’s customer services on 07 838 6699. Staff will direct you to someone who can advise you of any relevant requirements for new buildings and extensions to existing buildings.

Floodviewer shows best available flood information, but this does not mean there is no risk of flooding for the following reasons:

• Some areas are yet to be modelled, so we don't have data available for them; these are greyed out in Floodviewer.
• Because modelling relies on data inputs and assumptions, and these may change, Floodviewer should be considered a tool to understand the potential for flood risk and management rather than a guarantee of what would happen.
• Flood hazard mapping is for a 100-year event. A larger flood event could flood a larger area and/or to a greater depth than shown on the flood hazard maps.

The earthworks layer shows areas that have been affected by significant, consented earthworks. Within this area, the flood hazard mapping is no longer reliable because land contours and, as a result, flooding, may have changed. The earthworks layer will be updated at least annually.

The District Plan identifies flood hazard areas and has objectives, policies and rules specific to each area. These provisions set out the requirements to be met, whether resource consent is required, and the criteria to be used to assess whether to grant or decline resource consent. When making decisions on resource consents, Council will also consider the flood information displayed on Floodviewer if the District Plan provides the discretion for this to happen. New subdivision, land use and development should avoid areas subject to flood hazards, if the level of risk is unacceptable. Refer to Chapter 22 and the features maps of the District Plan for details.

The flood hazard information displayed on Floodviewer is the best available information. Whether a resource consent is required for any proposed activity on such a property depends on the requirements of the District Plan and whether the District Plan provides discretion for consideration of Floodviewer information with respect to the proposed activity. Best available information, regardless of whether it is shown in the District Plan, should be considered whenever planning development. For example, it is expected to be used as part of the Building Consent process.

Council may consider the flood information displayed on Floodviewer when making decisions on building consents. The Building Act requires any building consent application for a site that has a flood hazard to demonstrate that the proposed buildings and site work will protect the outfalls of drainage systems and safeguard people from injury or illness, and other property from damage, caused by surface water. This includes demonstrating, amongst other things, that:

• The disposal of surface water collected or concentrated by buildings or sitework will avoid damage or nuisance to other property in a 10-year event (an event having a 10% probability of occurring any year); and
• Surface water resulting from a 50-year event (an event with a 2% probability of occurring any year) will not enter any housing, communal residential or communal non-residential buildings.
• Adequate provision has been or will be made to protect the land, building work or other property from the likely flood hazard (considered as a 100-year event, i.e. an event with a 1% probability of occurring any year).

Council cannot advise on how flood information affects insurance cover. If you have questions we recommend discussing this with an insurance provider. Insurance policies often require policy holders to advise the insurer of any new information about natural hazards on an insured property. The public, including insurance companies, can access Floodviewer.

Council is required to produce the best available flood information and make it available to the public on request, including anybody considering buying a property. Property valuations reflect sales evidence up to the valuation date. Over time, property sales evidence will reveal any effect the flood hazard information has on property values. If you are concerned about the potential impact of flood hazard information on property value, then you could seek advice from a registered valuer.

Yes. A LIM outlines all information a council holds on a property, including permitted land use, consents, stormwater and sewer drains, rating information and potential natural hazards including erosion and flood hazards.

Where the property that is the subject of the LIM lies within a flood hazard area identified in the District Plan, the LIM will simply state what types of flood hazard are present. That is, whether it is high, medium or low. Where the property lies in a flood area identified on Floodviewer, additional details of the flooding will be included in the LIM.

Specialised computer flood modelling is used to produce the flood information shown in Floodviewer. The modelling and mapping is prepared and peer-reviewed by experts.

Flooding shown in Floodviewer is the best available information. Models will be created or updated from time to time. Updates may include changes to modelling methods, the inclusion of new infrastructure (e.g. new pipes), new or updated inputs and assumptions (e.g. how much allowance is made for climate change), and changes to landforms (e.g. from earthworks that may change the way water flows).

This means flooding shown on properties may change.

Council will include in its District Plan only flood maps that are unlikely to be changed by development anticipated during the life of the plan. Such maps are included in the District Plan by way of a plan change or plan review, which is a lengthy and expensive process. Until this happens, new flood hazard maps must remain outside the District Plan.

As flood hazard modelling is useful to people planning or managing development, it is appropriate to make it available to the public by means other than the District Plan, such as Floodviewer.

Write to Council stating what it is that you disagree with, and why, and Council will consider your feedback. Such feedback should be sent to: city.development@hcc.govt.nz

Following our flood mapping and Floodviewer work that’s well underway, the next step is to consider further changes to planning rules to help create more climate resilient communities and better manage flooding hazards.

The work is known as Plan Change 14 and will look to remove out-dated flood hazard maps from the District Plan and introduce improved rules to manage flood hazards based on the most up-to-date information available on Floodviewer. Formal notification of any rule changes will happen in 2024.

Because of this, we are proposing to take the flood hazard maps out of the District Plan. This will be apart of the 2024 Plan Change 14.

No.

Whenever stormwater runoff exceeds the drainage capacity of the stormwater network, ponding and/or overland flow is expected. It would be extremely expensive to provide a drainage network that avoids all ponding and overland flow. Council first set design levels of service for the stormwater network in Hamilton in the 1980s but has since amended them.

As at 2018, different design levels of service have applied to existing and new development. For existing development, Council plans to maintain the following levels of service for the stormwater system: 2-year event for residential, 5-year event for commercial, and 10-year event for industrial (page 123 of the 2021-2051 Infrastructure Strategy).

For new development, the Regional Infrastructure Technical Specifications (the RITS) requires new primary stormwater systems for residential, industrial and commercial areas to be designed for a 10-year rainfall event.

These stormwater systems may include (but are not limited to) wetlands, ponds, lakes, rain gardens, swales, filters, pipelines, inlet/outlet structures and soakage areas (RITS, updated May 2018, page 15).

Council will consider flood information when managing its assets and planning future stormwater network improvements. These improvements could include increasing culvert or stormwater pipeline sizes or constructing stormwater detention facilities. Council may also require developers to install on-lot stormwater devices in new developments. Decisions on the priority and timing of stormwater upgrades are made via Council's Annual Plan and 10-Year Plan processes.

We encourage you to seek professional advice (e.g. from a stormwater engineer) about ways you can reduce the risk of flood damage to your property. Reducing the risk of flooding on your property must not make the flooding of any other property worse.

Freeboard is the vertical distance between the modelled top level of flood water and the underside of a concrete floor slab, or the underside of a floor joist.

Minimum freeboard provides a safety factor to reduce the risk a building will be flooded in a 100-year event.

The freeboard allows for:

• the predicted flood level
• some uncertainty in the energy of flowing water (think of how water “breaks” or “climbs” up an obstacle placed in flowing water)
• waves created as water passes around or over obstacles
• the wake of people or vehicles moving through flood water.

The freeboard may also provide a margin of safety should the flood be greater than the 100-year event.

Where any part of a property is within a flood hazard area, then all new buildings and extensions to existing buildings must comply with the minimum freeboard.