Across the Region - River Management Edition

Welcome to our latest Across the Region. This issue we created a special edition that shares with you a bit about what goes on behind the scenes in terms of managing our Region’s river systems.

River management is one of council’s core business areas and plays a key role in our goal of keeping communities safe from flooding. From what river management and drainage schemes are, to which staff work in your area, this edition provides an insight into how Horizons operates. The team is led by group manager Ramon Strong, a chartered professional engineer with over 20 years experience, including two years at Horizons prior to working in the South Island and then returning to us.

When it comes to deciding what work Horizons undertakes for river management, we have defined areas known as schemes that people within the area contribute towards. Changes to, or creation of, schemes take place through our Annual or Long-term Plan process in consultation with the community.

We have recently been through an Annual Plan round, which following community submissions and deliberations with the rest of Council, we have determined our key priorities for the coming year. In addition to changes to transport activities, managing pests and predators, and reviewing and improving policies and programmes, funding allocation has been assigned to developing/updating numerical models to provide better and more robust solutions for river management services, as well as improvements to the way we manage dams.

Rates have been set at an average rate increase of 4.32 per cent for 2017-18. This increase is slightly less than the 4.49 per cent increase for Year 3 of the LTP. Council strives to strike the delicate balance between keeping rate rises to a minimum while managing the Region’s natural resources prudently. If you have any suggestions as to how we could improve our service or do things differently I invite you to bring these to Council when we open submissions for the Long-term Plan around March 2018.

Bruce Gordon, Chairman

HORIZONS' ROLE IN RIVER MANAGEMENT

The Horizons Region is carved by three main river systems – the Whanganui, Rangitikei and Manawatu. These river systems then break down into catchments, which are land areas within the Region defined by water sources.

One of our key business areas is river management within these catchments. This is paramount to our role in keeping communities safe in our Region. Managing rivers through engineering works allows us to help prevent floods and provide adequate land drainage where necessary. With many urban communities located on flood plains, it is important to have an appropriate standard of flood risk mitigation for the safety of the community. The protection of the flood plains is also important for protecting productive land, which is key to the economic goals of the immediately affected areas, as well as the wider Region.

TERMINOLOGY EXPLAINED - WHAT'S A 100 YEAR FLOOD?

We've all heard the phrase "a 100 year flood" but what does that actually mean? The answer can be a bit complicated so we've put together a video to help explain the term.

WIN A HELICOPTER FLIGHT!

For the months of June and July, we’ll be highlighting our role in river management via the Horizons’ Facebook page. As part of this campaign we are running a competition using the historical photo below. Email us at communications@horizons.govt.nz and tell us what part of the Region you think this photo was taken in and go in the draw to win a helicopter flight for you and a friend over your local river scheme with one of our engineers. Entries close Friday 21 July 2017.

WHAT DOES THE HORIZONS' RIVER MANAGEMENT TEAM DO?

In the Horizons Region we manage 34 river and drainage schemes, maintaining over 490km of stopbanks, 1,090km of drains, 23 pumping stations and 53 dams. That sounds like a lot but in fact it only covers approximately 40 per cent of the river catchment areas in the Region. Within a scheme we work to reduce the risk and impact of flooding and erosion or provide land drainage using funding provided for by scheme rates.

This is achieved by more that just stopbanks. Although important, they are just one part of our overall flood defence picture. Assets such as spillways, dams, pump stations, floodgates and drainage networks all help keep communities safe and dry.

Finally, it’s also important to note that we are in the business of river management – not river control. Our river systems are a force of nature, and as a result managing them can be challenging at times. However, our schemes play a key role in making our Region productive and prosperous.

HOW SCHEMES WORK

A scheme is determined by identifying an area of properties and land that have a reasonable flooding or erosion risk. Once a scheme area is identified, properties within the area are assessed and rates are set to reflect the amount of benefit they receive from the works.

This generally breaks down into three sections:

Direct benefit – those properties that receive direct protection of their land, for example from a stopbank or river bank erosion protection

Indirect benefit – those properties who receive general benefit from flood protection and river bank erosion protection, for example protection of community assets like roads, bridges and network services that result from reduced flooding and erosion

Contributor benefit – those properties upstream that are considered to contribute to flooding or erosion problems downstream

How much each section pays is either based on capital value, land area or a set rate. All schemes also receive a 20 per cent contribution from the general rate, recognising the benefit the entire Region receives from less flood damage.

Outside of a scheme some limited financial assistance is available for river works. Unfortunately, Horizons only has a small amount of funding available for these types of works each year. This means that additional funding has to be sought from other parties, such as the district council or landowners, who will receive direct benefit from the work.

The other option is to leave the river to run its natural course, which most of the time does not put too much land at risk. There is also the option of creating new schemes or extending the size of an existing scheme, however this can only be done through an Annual or Long-term Plan process and in consultation with the community that will have to pay for the scheme.

RIVER MANAGEMENT IN THE HOROWHENUA DISTRICT

For the Horowhenua area the biggest scheme is the Lower Manawatu Scheme which starts upstream of Palmerston North and extends right down to the coast. This scheme includes stopbanks which are designed to contain a flood that has a 1 per cent chance in any given year of occurring, commonly called a 1 in 100 year flood, for the rural area.

The Lower Manawatu Scheme is quite sophisticated and does not solely rely on stopbanks. There are a number of spillways that operate in large events, temporarily storing water in large areas of low lying land until the river is low enough for the water to drain out. The Taonui Basin is the Region’s largest floodwater ponding area, and this can be seen from the Rangiotu township in a flood. A key asset in this area is the Moutoa floodgates which divert river flows down a purpose made spillway 10km long and 600m wide, joining the Manawatū River at Foxton. This diverts floodwaters from the lower reaches of the Manawatū River where the channel is windy and flat.The Moutoa floodgates took three years to construct in 1962 and when finished were the biggest river

Above: Erosion control with willows, Ohau

control structure undertaken in New Zealand at the time.

In addition to the Lower Manawatu Scheme, there are seven drainage schemes in Horowhenua. In total these schemes include approximately $30 million worth of assets, 19 pump stations, 139 floodgates and 385km drains. Drainage is particularly important for this area as it provides an outlet for properties to clear floodwaters.

The Makerua Scheme is the most complicated of these schemes and is responsible for protecting productive land within the Opiki Basin. The scheme includes floodgates, pump stations and stopbanks. Direct benefit ratepayers have invested a lot of money in this scheme as they recognise the importance of clearing floodwaters from their property as quickly as possible during a flood event.

RIVER MANAGEMENT IN THE MANAWATU DISTRICT

The Manawatu floodplain has always been a very productive district, but flooding has made realising its potential a major challenge. From as early as the 1890s, several proposals for protecting the entire Manawatu floodplain from flooding were put forward, however it wasn’t until the great flood of 1953 that a comprehensive Lower Manawatu Scheme was designed, and finally completed in 1962 when the Moutoa floodgates were finished.

Today, 300km of stopbanks, 200 floodgates, 600,000 tonnes of rock and 250 km of trees provide flood and erosion protection to 320km2 of rural and urban land stretching from Ashhurst to the sea. If constructed today, we would have to find over $200 million to replace these assets.

The stopbanks that provide the majority of our flood protection are designed to contain a flood that has a 1 per cent chance in any given year of occurring, commonly called a 1 in 100 year flood in the rural area. For the city of Palmerston North, they are designed to contain a 1 in 500 year flood, which has a 0.2 per cent chance of occurring in any given year.

Above: Horizons staff checking the 'Gas Works' drain, part of the Lower Manawatu scheme

The Lower Manawatu Scheme is quite sophisticated and does not solely rely on stopbanks. There are a number of spillways that operate in large events, temporarily storing water in large areas of low lying land until the river is low enough for the water to drain out. The Taonui Basin is the Region’s largest floodwater ponding area, and this can be seen from the Rangiotu township in a flood. A key asset in this area is the Moutoa floodgates which divert river flows down a purpose made spillway 10km long and 600m wide, joining the Manawatū River at Foxton. On a smaller scale, but using the same principles, Feilding is also protected by a floodgate structure that diverts flows from the Makino down a floodway to the Kiwitea Stream. In times of flood, all these structures and systems are crucial to keep people and property safe.

RIVER MANAGEMENT IN PALMERSTON NORTH

The Manawatu floodplain has always been a very productive area, but flooding has made realising its potential a major challenge. From as early as the 1890s, several proposals for protecting the entire Manawatu floodplain from flooding were put forward. However, it wasn’t until the great flood of 1953 that a comprehensive scheme was designed. It was finally completed in 1962 when the Moutoa floodgates were finished. These gates took three years to construct and when finished were the biggest river control structure undertaken in New Zealand at the time.

Today, 300km of stopbanks, 200 floodgates, 600,000 tonnes of rock and 250km of trees provide flood and erosion protection to 320km2 of rural and urban land stretching from Ashhurst to the sea. If constructed today, we would need to find over $200 million to replace these assets.

The stopbanks that provide the majority of flood protection in the Palmerston North district are simple earth banks, typically with a 3m wide top width and a 2:1 batter slope. Their height is set using a sophisticated computer model which predicts flood heights based on rainfall events. The stopbanks in the rural area of the Manawatu floodplain are designed to contain a flood that has a 1 per cent chance in any one year of occurring, commonly called a 1 in 100 year flood, and a flood that has a 0.2 per cent chance of occurring in any one year, 1 in 500 year flood, for Palmerston North.

The Manawatū River originates north of Norsewood in Tararua District and drains 6000km2 of land as it makes its way through the Te Apiti – Manawatu Gorge and to the sea at Foxton. On reaching the Manawatu floodplain, the river adjusts to a shallower gradient and meanders over the last 48km of its journey, with a fall of just 18cm per km. The Manawatu floodplain was formed from silt carried downstream by the river. This silt is still being deposited today, between the confines of our stopbanks, raising the height of our berms, and slowly decreasing the flood carrying capacity of the channel.

Above: Anzac cliffs, April 2016

Our flood protection is quite sophisticated and does not solely rely on stopbanks. There are a number of spillways that operate in large events, temporarily storing water in large areas of low lying land until the river is low enough for the water to drain out. The Taonui Basin is our largest floodwater ponding area, and this can be seen from the Rangiotu township in a flood. We also have the Moutoa floodgates which divert river flows down a purpose made spillway 10km long and 600m wide, joining the Manawatū River at Foxton. The floodway, and then the river, are crossed when using SH1 just below Foxton.

Lower Manawatu City Reach

The City Reach project, which is part of the Lower Manawatu Scheme and began in 2007, provides Palmerston North with flood defences designed to withstand a 1 in 500 year return period flood. Without the protection between 12 and 20 per cent of the City would flood. Upon its completion in 2017-18 the City will be completely protected down the right bank of the Manawatū River. Stopbank upgrade works from Albert Street to Fitzherbert Ave, and rock lining at Dittmer Drive are just some of the works you can see when walking the Manawatū River shared pathway.

Anzac Cliffs

A major project to realign the Manawatū River, protecting the integrity of the City’s stopbanks opposite Anzac Cliffs has recently been completed. An overtight bend in the river was directing flow into the base of the cliff, undermining the easily eroded material and posing a risk to public safety. The river realignment component of the works, including the establishment of an approximate 28,800 tonne heavy armour rock lining along the new bank alignment, cost $3.7 million and was funded as part of the City Reach Project.

RIVER MANAGEMENT IN THE RANGITIKEI DISTRICT

The Rangitikei District is part of the northern river management area. It comprises of 14 separate schemes that are spread wide from above Taumarunui in the north, Tangimoana in the south, east of Feilding and Ashhurst to the Ruahine Ranges in the west. There are five river and drainage schemes in the area; the Turakina, Makirikiri, Rangitikei, Powera and Tutaenui. Collectively these schemes protect rural and urban properties from flooding and erosion in the Turakina, Rata, Hunterville, Bulls and Marton areas.

Although there are a number of schemes in the Region that address larger events, Rangitikei schemes are designed to deal with smaller, more frequent events. This means that highly productive land and community infrastructure will not be impacted as frequently as it would without the schemes. For example the Porewa Scheme,

Above: Erosion bar repair on the Rangitikei River at Sanson Metals

protects the townships of Rata and Hunterville, together with approximately 800 hectares of rural land in the valley floor, from flood events with return periods of up to 25 years. The scheme assets (27 detention dams) also reduce the effects of flood events with a greater return period than 25 years, which has a 4 per cent chance of happening in any given year.

The Tutaenui Scheme is another example, which is designed to manage the flows during a 20 year rainfall event and protects Marton and Bulls and surrounding rural land. This is done by capturing rainfall in 18 detention dams.

RIVER MANAGEMENT IN THE RUAPEHU DISTRICT

The Ruapehu District is part of the northern river management area. It comprises of 14 separate schemes that are spread wide from above Taumarunui in the north, Tangimoana in the south, east of Feilding and Ashhurst to the Ruahine Ranges in the west.

Three of these schemes are located in Ruapehu and provide their own unique level of service to the community as described below:

Taringamotu Scheme

This is our newest scheme and provides channel clearance work between the Ongarue River confluence to 32km upstream. At the request of the community the initial seven year proposed work programme was modified into a two stage approach.

The first of these was to clear log blockages from the channel to allow uninterrupted river flows. The second was to clear vegetation back from the banks to avoid trees falling into the waterway. At the end of the 2016-2017 year, and at the completion of the stage one work, a hold was placed on further work. This is to allow the community to decide how, or if, the scheme should continue into the next Long-term Plan period.

Above: Work being carried out at Mangaeturoa, Raetihi

Upper Whanganui Scheme

This scheme provides landowners adjacent to, and in the floodplain of, the Whanganui River through Manunui and Taumarunui with a degree of protection against flooding and riverbank erosion. The control of flooding is largely limited to the urban area of Taumarunui downstream of the SH4 Bridge. The surrounding community receive benefit from the works carried out to directly protect Taumarunui.

Pakihi Scheme

The primary objective of this scheme is to protect rural land from flooding through the management of two detention dams and minor channel clearance work. It has been in existence since at least 1962 and continues to be in demand for flood and erosion protection. Horizons has demonstrated cost-effective, purposeful, sustainable and environmentally sound management of the dams and possesses the engineering expertise and corporate knowledge to own and manage these for the foreseeable future.

RIVER MANAGEMENT IN THE TARARUA DISTRICT

There are seven schemes in the Tararua District. The smaller ones include the Akitio River Scheme, Eastern Manawatu Scheme, Ihuraua Scheme at Alfredton and the Tawataia-Mangaone Scheme. The annual works budgets for these schemes vary between $2,000 and $26,000 and involve clearing willow tree congestion to improve flood carrying capacity, to spraying and cleaning of drains and streams. The Tawataia Stream has a detention dam that provides flood protection for that community.

The other three larger schemes, which were formed between 1943 and 1954, include the Upper Manawatu-Lower Mangahao Scheme, the Mangatainoka and South Eastern Ruahine (SER) Schemes. These focus on river bank erosion works and some flood protection in certain areas. The schemes have important assets that require regular maintenance including various types of bank protection, eg. tied tree work, willow tree plantings, rock linings, groynes, timber retaining walls and in channel assets such as grade controls, weirs, stock gates, along with guide banks, stop banks and drains. The total value of these assets for each of the schemes are; $8.9 million for the Upper Manawatu-Lower Mangahao, $15.2 million for the Mangatainoka Scheme and $14.2 million for the SER Scheme.

The Upper Manawatu-Lower Mangahao Scheme includes management of 50km of river channel and 19km of drainage channel and collects $315,500 of rates from 11,500 properties.

The Mangatainoka Scheme manages three main rivers including 52km of the Mangatainoka River, 20km of the Makakahi River and 16km of the

Above: Confluence of the Upper Manawatu and Tiraumea Rivers at Ngawapurua rail and road bridges just south of Woodville. A re-alignment of the Tiraumea river was completed recently, in conjunction with the underpinning of the rail bridge.

Mangaramarama Stream along with 50km of drains. The management of these rivers and drains attracts approximately $650,000 of rates from 6,560 properties.

Finally, the SER Scheme which covers an area of 786km2, consists of managing approximately 18 streams between Woodville and Norsewood that flow from the Ruahine Ranges into the upper Manawatū River. One of the main features about this scheme is the management of gravel movement in many SER streams. To address this issue, the scheme has a combination of weirs and “gravel reserves” in the upper reaches of many of the streams to control the bed levels so that flooding and bank erosion are mitigated. The rates collected for this scheme total about $488,000 and come from approximately 9,000 properties.

Apart from managing the schemes within the Tararua District, the River Management team does deal with a significant number of general enquiries about a range of non-scheme issues to do with drainage, stream bank erosion and channel congestion leading to flooding. Some of these enquiries, if they meet certain guidelines, can be approved for an Environmental Grant of up 30 per cent of the total cost of the repair work.

RIVER MANAGEMENT IN THE WHANGANUI DISTRICT

The Whanganui District is part of the northern river management area. It comprises of 14 separate schemes that are spread wide from above Taumarunui in the north, Tangimoana in the south, east of Feilding and Ashhurst to the Ruahine Ranges in the west.

Five of these schemes are located in Whanganui and provide their own unique level of service to the community. Learn a bit more about some of these schemes below:

Lower Whanganui Scheme

The main objective of this scheme is to provide flood protection to certain Whanganui urban areas. It also provides bank protection to erosion prone areas through the use of vegetation management in the urban river reaches. The scheme area extends from Upokongaro in the north, to 19km downstream at the river mouth at Castlecliff. This reach of the river is very flat graded and has a very large tidal exchange capacity.

Assets include the Anzac Parade/Kowhai Park and Balgownie stopbanks. Earlier this year Horizons approached the Whanganui community to see if there was an appetite to increase the level of service for this scheme. Those impacted by the 2015 floods were supportive but the rest of the community was less receptive to the idea. It has now been proposed that community flood resilience be increased through potentially moving or raising properties in the Anzac Parade/Kowhai Park area. Detailed information about this option will be put to the community for consultation as part of the 2018-28 Long-term Plan.

Matarawa Scheme

This scheme is located to the east of Whanganui City in the Matarawa and Mangamoku Stream catchments. Established in 1981 to control flooding of the low-lying land in the Matarawa, Kaukatea, Mangaone and Mangamoku Stream valleys, the scheme includes five detention dams in the upper catchment.

Above: Balgownie stopbank, Wanganui

In addition to the construction of the five detention dams, extensive works were carried out in the stream channels to clear heavy vegetation, and stopbanks were constructed along selected parts of these channels to improve their flood-carrying capacity. To date, maintenance of the channel has fallen to landowners. However, the June 2015 flood event highlighted that this was to varying degrees and channel congestion enhanced flooding. Council resolved to commence the channel clearing work in 2017-18 but to fund that work initially from reserves, consulting next year on spreading the cost more widely across the community as part of updating our Long-term Plan.

Haunui Scheme

The Haunui Scheme was established to provide a degree of flood protection to 10 rural properties that are situated in, or beside, the old course of the Whangaehu River.

The Scheme is located in the Haunui Road area of the lower Whangaehu River Valley, upstream of SH3. While it protects properties from river flooding, opposed to other schemes that include river works, this scheme is focused on drainage. It consists of four separate drains, totalling 9km of open channels, that drain the left bank of the Whangaehu River and adjoining hill country.

WHAT RIVER MANAGEMENT STAFF DO DURING A FLOOD EVENT

Before a flood event, we often receive a forecast for heavy rain. The team looks at this, and in combination with data collected via our rain and river gauges, assesses what state the rivers in the area are in and what impact the rain might have. River Management staff will check assets such as flood gates to ensure they are ready to respond, touch base with staff and contractors that may help with a response, and identify what points in the network may need extra attention.

During an event River Management staff are based in the Emergency Coordination Centre (ECC) and out in the field. In the field they are monitoring the flood and inspecting assets to ensure they are performing as they should be, reacting to any emerging situations (such as if a stopbank was going to be over topped or a culvert was blocked), and deploying pumps. The current state on the ground is reported back to the ECC via the River Management representative there. Staff can also be involved in the operation of flood gates and flood barriers, as directed by the ECC.

After a flood event the team will visit assets to assess any potential damage and prioritse any damage repairs. They will also evaluate what went well during the event and what they may need to keep in mind for next time.

WHERE TO FIND MORE INFORMATION

To learn more about Horizons’ role in river management please visit our website www.horizons.govt.nz and visit the Flood Protection page.

COUNCIL MEETINGS

August 2017

Environment Committee, 8 August, 9am
Catchment Operations Committee, 8 August, following Environment Committee
Strategy and Policy Committee, 9 August, 10am
Regional Council, 29 August, 10am
Audit, Risk and Investment Committee, 29 August, following Regional Council