Banks are unstable and may need reshaping.
 

Bank has been reshaped to prevent erosion and aid planting. Note that banks are yet to be grassed and fenced.

If your stream has steep-sided, unstable banks, it can be useful to reshape them before you do any fencing or planting. Steep-sided, unstable banks are often
encountered in drains that are regularly cleared with mechanical excavators.

Improving the slope of the banks will mean:

• Bank erosion is less likely
• Plantings are less likely to reduce the flood carrying capacity of the stream.

However, the process of reshaping the banks, called rebattering, may release sediment into the stream and can cause environmental damage. To make sure that best practice is followed, Environment Canterbury requires you obtain a resource consent, which guides you through the steps you can take to minimise damage.

Tips for bank reshaping

• Ideally, banks of drains should be less than 1 m vertical to 2 m horizontal  - see diagram:
   
• Other waterways, such as streams, may have sections that are steep and eroding and need battering to this ratio before it is possible to plant them.
• 'V' shaped drains help water to flow faster, reducing sedimentation and weed growth. However, many species of fish like undercut banks, so consider leaving some of this habitat if you have fish in your stream.
• Keep the riffles: riffles are areas of shallow, fast-flowing water in which the surface is rough or broken by rocks. They are the preferred habitat of many species of fish and insects. Mark these and other features that are not to be disturbed.
• Timing: to minimise the impact on aquatic life, work in non-tidal zones from November to April, and tidal zones from November to February.
• The site will be vulnerable to erosion following battering. To minimise damage, avoid rebattering long stretches at a time, sow grass seed to re-vegetate disturbed areas and exclude stock from the banks until it is stabilised.

Be very careful when removing old gorse hedges as the soil underneath is particularly crumbly and erosion-prone.

Checklist

• Keep riffles
• 'V' shape for drains
• Time work to minimise environmental damage
• Shape small stretches at a time
• Plant banks soon after shaping
• Exclude stock from banks.