Te Kāuru Upper Ruamāhanga Floodplain Management

What are they and why are they important?

A diverse mix of flows and depths is important in a river system to help create the variety of habitats for fish and invertebrate life. In meandering gravel bedded rivers this diversity is largely provided by the occurrence of Pool – Run – Riffle sequences.

Figure 1: Plan view of a pool-run-riffle sequence

· A pool is an area of slow flowing, deep water, often on the outside bend of a stream or river

· A riffle is an area of fast flowing, shallow water where the surface of the water is broken from flowing over stones

· A run is a smooth, unbroken flow of water that connects pools and riffles, acting as a transition between the two.

Figure 2: Section through a pool-riffle sequence

Good for nature

Each of these habitats and their specific characteristics (depth, velocity, substrate etc.) cater to particular fish and invertebrate species and help to promote and maintain diverse instream river ecology. The information below shows the preferences some fish have¹;

· Rapid/riffle – torrentfish, bluegill bullies, kōaro, alpine galaxias, and upland longjaw galaxias

· Run – juvenile eels, trout, and some galaxiid and bully species

· Pool – adult eels, lamprey, various juvenile galaxiid species, and adult kōkopu.

¹ – Based on survey worked carried out by NIWA 2014.

Good for recreation

Pools can have a significant recreational value to people in terms of providing ‘swimming holes’, particularly over the summer months. There are a number of locations in the Wairarapa where a particular pool is virtually a permanent feature and has been popular for swimming for generations. These locations tend to be where the river meets a hard rocky bluff, on an outside bend, and the river bed is eroded down creating a deep pool.

Another popular pastime is fishing, in particular for trout, for which pools – riffles – runs play an important part in providing good feeding grounds and protection.

Changing environments

The location and frequency of Pools – Runs – Riffles can change over time due to natural events and processes; such as floods passing through the river system, which rework gravels by the process of erosion and deposition.

Human intervention also has the potential to alter and affect the nature and frequency of these habitats. Here, in the Wairarapa, river management schemes were set up many years ago with a mandate, from the riverside community, to manage and mitigate flood and erosion hazard. Some of the activities carried out in these rivers, using mechanical equipment to move gravel around the river bed, can create situations where established pools, in particular, are destroyed, or severely diminished to mitigate the effects of bank erosion. Sometimes the effects will be short-lived and other times it may take a longer period of time to re-establish the pool-riffle-run sequence. This is general down to the flows in the river and how long it is before ‘freshes’ or floods come along to re-establish a natural order.

Traditionally this work was carried out on the basis that financial/economic values outweighed other instream, ecological values, however, as local/national values and economic drivers have changed this has led to a shifting emphasis on how these values should be weighed up against one and other. This changing balance of values has been reflected in scheme activities in more recent years but further work is being carried out to promote improvement.

This does not necessarily mean that mechanical intervention in the river should not take place, but it should be planned and managed in such a way that all the values are weighed up and appropriately considered to help deliver good outcomes for the wider river community (in-water and out-of-water).

Continued research into the effects of these types of activities, education and good practices around when to, and how best to, intervene are important in driving improvements.

Tools to achieve good river management outcomes

The Floodplain Management Plan is looking to develop a range of tools to help deliver sustainable, long-term management of flood risk, based on its established vision and aims.

One tool to potentially help offset instream channel work could be to ensure that the sequence if pool-run-riffle is recreated, to some extent, after works have been completed.

Another tool could be to identify a number of pools, runs, riffles that would be indicative of a healthy diverse river system and to try and maintain this number (or more likely number envelope).

Further work needs to be carried out to identify what the most appropriate set of tools should be and how they could be applied.

We’d like to hear you thoughts though so get in touch and let us know what you think.

GWRC provides a flood warning service across the whole region. People access this service in different ways:

1. By checking river levels on our website, http://graphs.gw.govt.nz/

2. By being signed up to a telephone warning list for one of our rivers or streams

3. Other means, such as calling our Hydroline voice mailboxes to check river flows

A wide range of people use our flood warning service, including:

· Farmers and other landowners with flood-prone land

· Civil Defence (and through them, local councils and emergency services)

· Contractors working in the rivers

Metservice provides severe weather warnings and weather forecasts, and GWRC provides information about current and predicted river behavior using our network of rain gauges and river flow gauges. These sites are connected to our network and website using radio and cellphone transmitters. GWRC staff will notify a flood warning when the river gauges reach predetermined alarm levels. These levels vary, but are generally around two thirds of an “annual” flood, ie. A flood expected to occur once a year on average. There would typically be a handful of alarms for each river in a given year.

Times and technology are changing, and a flood warning review is currently underway. GWRC will be getting in touch with selected stakeholders for feedback on how the current system works and discuss what sort of service we should be providing as well as how we can improve. The review will also take into account feedback received through the TKURFMP process. Some suggestions so far from the FMP Options Development are:

· Additional/upgraded rainfall and river flow sites (especially on the Waipoua River through Masterton, where a temporary site has already been set up)

· Improved flood forecasting

· Automated voice calling for the flood warnings instead of the manual telephone distribution list

· Use of cellphone txt messages

· Monitoring flows for the presence of landslide dams forming after earthquakes or heavy rain

Buffers are an envelope of land beyond the river channel which is allocated to provide for erosion control and protection. This is often in the form of vegetation.

The use of planted willow buffers for river and erosion management has been a practice in development for more than 30 years. It is a proven technique to provide certainty to landowners about erosion extents from frequent flooding, and is an accepted practice. The willows operate to increase resistance to erosion along a bank edge without preventing erosion occurring altogether. In effect it slows the erosion process, meaning less area of land will be eroded if planted with willows that when compared to bare, unplanted land. The willow roots serve to bind the river bank material together.

The use of willows is reliant on them and the land they are planted on being available to erode, meaning that this land is sacrificial for the purposes of limiting erosion damage, and a buffer zone planted with willows may ‘vanish’ at any time, eroded by a flood event. This is their purpose and what they have been designed for. At times these buffers naturally refill with gravel and are replanted as the river meanders transition downstream, and at other times these buffers are artificially reconstructed by machine work and replanted.

Buffers are used as a tool which supports the River Edge Envelopes and River Bed Envelope tools.

Planted buffers are not suitable in some areas, for example they are not appropriate to use at the top of high banks where the roots cannot protect the base of the slope from erosion. They are also not appropriate in places where the erosive forces of the river are too great. In these situations, other tools may be used instead of or in addition to planted buffers.

Planted buffers in most instances currently serve only a single purpose of making land available for erosion control and protection. There have been some alternate land uses trialed to recognise potential alternate revenue streams from these parcels of land that are not available for the adjacent rural land use (usually cropping, dairy or sheep and beef). These additional revenue streams include beekeeping, and growth of willows as a fodder crop.

Nothing stays the same forever and this applies to the bed of a river.

The bed level of a river can rise (aggrade) and lower (degrade) over a period of time and over a longer period of time can fluctuate between these two states. This happens due to natural events but can also be significantly affected by human activities.

This is particularly evident within a gravel bedded river, where rising and lowering bed levels can be observed within relatively short timeframes (years and decades)

In the Wairarapa the river beds are generally lowering in the Waingawa and Waipoua River, and the upper reaches of the Ruamahanga, and rising in the lower reaches of the Ruamahanga. This is an over simplification but represents the overall observed trend.

Why do bed levels go up and down?

Bed levels will change based on the natural balance between the amount of gravel available to be transported, downstream, and the physical ability of the river to transport this material.

River beds tend to lower when the supply of gravel is limited upstream, through a combination of natural and human processes, and the capacity for the river to move it is high.

On the flip side river bed levels will tend to rise when the supply of available gravel increases and the ability of the river to transport it is lower.

This balancing act involves a number of complex natural factors including;

· Climate

· Geology

· Topography

· Vegetation

· Soil type

And human influences, such as river management (eg. stable resilient river banks and gravel extraction)

What’s the issue?

Bed levels play an important role in flood risk and river management and significantly influence wider the natural environment.

If bed levels increase too much in a particular area it may increase flood risk to people, buildings or land.

However, lowering of the bed levels can lead to bank instability particularly where high, steep banks have developed. The continued lowering increases the risk of the banks falling into the river and can threaten structures in or near to the channel, such as; bridge foundations or stopbanks.

Shallow groundwater supply that has connectivity to the river can also be affected by bed level changes in a river. Lowering river bed leads to lower water levels in the river which then has the potential to provide the groundwater with a path of lower resistance back into the river. This can have real implications of people who rely on this source of water for personal and stock use, particularly during periods on low rainfall and low river flows.

In general intervening in natural variability is likely to be counterproductive in the medium to long term, but there are instances when immediate intervention may be required to protect important infrastructure.

What Influence does GWRC have?

GWRC have an extensive network of cross-sections on all the main rivers in the region and these have been surveyed over the past 20 plus years – though in the lifespan of a river this is no time at all. This information helps us build up a better understanding of river processes taking place and can also help to determine what management actions might be required, if any.

Bed levels can and are be managed, to some extent, using mechanical means (diggers and dozers) of moving and reworking river gravels, or by loosening up (ripping) hard packed gravel (to make it more mobile). Extraction involves getting a Contractor to remove gravel from a particular gravel beach. This material is usually used for construction purposes such as roading or building.

What are river bed envelopes?

A river bed envelope helps to define the natural variability of a river over a period of time. It relies on monitoring the river, at specific points, over an extended period of time to build a picture of the trends occurring in the river.

If the upper or lower limits of the envelope are being approached this is the time consider why this might be happening, in the context of observed trends, and what the implications might be!

From this it may be appropriate, or necessary to define a management activity to remedy or mitigate the situation.

We are looking to develop a range of tools to better monitor and manage the rivers in the Wairarapa for which GWRC have responsibility. Defining bed envelopes could be one such tool. We would love to hear your thoughts.

Greater Wellington is developing a Floodplain Management Plan that is the culmination of many years of work and is used to understand the flood risk across the Upper Ruamahanga Valley.The development of this plan has distinct stages. Phase 1 has been completed and we are currently finalising Phase 2. You can read an overview of the first two phases below.

Phase 1 - Gathering and reviewing local information

Between 2012 and 2014 investigations into flood risk across the Upper Ruamahanga Valley were completed, this included identifying flooding and erosion problems, and collecting information about the environment and how people use and value the rivers. This culminated in a community openday to share and verify this information.

A new flood risk map was produced which used 20 years of data and well established mapping techniques. The techniques were peer reviewed by industry experts, and finally the information was released on the 22nd August 2014.

• Click here to read some Frequently asked questions and answers about flood risk
• To find out about specific flood risk to your property email us: info@gw.govt.nz
• For Masterton District Council Land Information Memorandums (LIMS), and building and resource consents, call Masterton District Council on 06 370 6300
• For Carterton District enquiries, call Carterton District Council on 06 379 4030.