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.