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Erosion along riverbed Erosion along riverbed

Erosion control

Many properties in the region are steep and hilly. While our soils are versatile and provide a good basis for farm production, they are vulnerable to erosion. Erosion can reduce the potential productivity of the land and contributes large amounts of sediment to waterways and estuaries, ultimately lowering water and soil quality. 

Soil erosion

There are three main categories of soil erosion types in the region:

  • Surface eroison 
  • Mass movement
  • Fluvial erosion

These are classified into 13 erosion types and each has a range of management options. The best management option will need to be tailored to meet specific site needs. Recognising the potential for erosion to occur will enable you to manage the land according to your level of acceptable operational risk.

Sheet eroison

Sheet erosion (also known as sheet wash), removes surface soil over large areas, such as cultivated paddocks, tracks, areas of heavy stock concentrations and bare ground.

It is caused by raindrops dislodging soil particles that are then transported away by overland flow.  

Factors that increase the risk of sheet erosion include soil parent material, slope angle, slope aspect, drought conditions and overgrazing. It commonly occurs on dry hill country, cultivated slopes, and steep land in pasture.  

Soil slip

Soil slips are shallow, rapid slides or flows of soil and softer weathered rock. The failure surface is parallel to the ground surface and less than one metre deep. The failure occurs when the surface material becomes saturated with water and ‘slides’ off the relatively impermeable layer it sits on. It can be triggered by prolonged rainfall, earthquakes, and undercutting of slopes by streams or wave action. Slips may also be induced by human activities, such as slope modification or roads, tracks and buildings. Slips generally occur on slopes of greater than 20 degrees and are more common under a pasture land cover.  

Earth flow

Earth flow erosion is a slow mass movement of soil and weathered rock (up to 25mm per year). Earthflows may be shallow (<1-2m) to deep-seated (tens of metres, but normally 3-5m). Deep earthflows occur on slopes between 10 and 20 degrees and shallow on slopes >20 degrees. The original vegetation often remains in place and looks humped with tension cracks. Earthflows are most active in wetter months and on sites where the toe is undercut by streams or roads or where gullies have developed.  

Slumps

Slumps are deep-seated failures of rock and subsoil. They involve rotational slide movements along failure planes resulting in a raised lower toe relative to the upper slope. Sometimes this results in formation of a pond at the head of the slump. Slumps often occur in those areas prone to earth flow erosion.  

Streambank erosion

Streambank erosion is the removal of material from the bank usually during high flows. Bed and bank scouring removes support and leads to the toppling in of the bank. This normally occurs when flood waters are receding, and the bank material is saturated, heavy and loses the support of the river. Water velocity along the streambank can also shear off large blocks.  

Rill erosion

Rills are closely spaced channels resulting from the uneven removal of surface soil by running water. Rills are <60cm deep and <30cm wide and commonly occur on cultivated slopes where they normally erode down to the base of the cultivated layer. They are occasionally observed on slopes with established pasture cover. In general, the potential for rill erosion increases with slope angle. 

Gully erosion

Gullies are formed by the removal of soil, subsoil or rock by water. They are large, permanent features >60cm deep and >30cm wide that initially form when water is channelled and the sides and head of the channel begin to erode. The channel normally only carries water during rainstorms.  

Tunnel gully erosion

Tunnel gully erosion is initiated by the subsurface concentration and flow of water resulting in scouring and the formation of narrow conduits, tunnels or pipes underground. As softer material is eroded away the tunnel collapses and can be visible as either holes in the ground or continuous open channel.  

River erosion

Intense rainfall can quickly flood rivers and bring down trees, causing erosion and other damage. 

When this occurs, unless the river forms part of a ratepayer funded flood scheme (see below), it’s likely the landowner will need to remedy any problems themselves. This includes fallen trees if they are blocking the natural flow of a waterway (the owner of the land where the tree fell from is responsible). 

Rivers and drainage schemes 

The Regional Council is the owner, manager and operator of five rivers and drainage schemes, which are which are funded by targeted rates.   

Map of Bay of Plenty rivers and drainage schemes

If a stream on your property has been impacted by flooding and/or erosion and the property is located within one of our schemes, please get in touch with the Rivers and Drainage Operations Team on 0800 884 880. 

Find out more about our rivers and drainage schemes

Erosion prevention 

The best way to manage erosion is prevention. Below are some things you can do on your property to prevent erosion.  

Illustration of farmer standing by fence

Fencing 

Farm fences contain stock safely on your property and helps prevent stock damaging steep, erosion prone land. They also make moving stock easy with little labour, and create laneways, yards and races for handling stock.  

Things to consider: 

  • Choose the best line possible for stability, ease of construction and maintenance. Fences close to streams are vulnerable to damage and more stable lines will be found along adjacent terraces or ridges. Consider using electric fencing in floodways to allow for flood flows and easier repair.  
    • A temporary electric tape fence can help confirm your thoughts on fence placement. You will be able to observe stock reaction, grazing patterns, and make accurate paddock size measurements to cost the permanent fence 
  • Minimise any line preparation earthworks that may create erosion problems.  
  • Building fences is a skilled job and the use of an experienced fencer is strongly recommended.  
    • Fences can be costly so it’s important to install a fence that is fit for purpose and suits your stock type.  
  • Gates are not necessary in a protection fence, however, to muster stray stock out or gain machine access, installing a gate opening at a convenient point is recommended.  
Illustration of person putting plants in the soil

Pasture cover and riparian planting 

Good vegetation cover (pasture and trees) is the first line of defence against soil erosion.  

Things to consider: 

  • Autumn is the optimum time for pasture establishment/ renovation due to warmer soil temperatures and reliable rainfall.  
  • When deciding on pasture species for steep land, think about dual purpose (forage/ground cover) species.  
    • Consider introducing mat-forming species such as brown top into the mix, as well as species that grow quickly after grazing.  
  • Maintain a mix of grasses with occasional deep rooted woody vegetation to help reinforce the riparian margin and steep, erosion prone land.  
    • Avoid planting flax on steep stream banks as they will block the stream if they are undercut and fall in.  
  • Soil conservation trees can help stabilise steep slopes.  
    • Deciduous species, such as willow or poplar species, can also supply fodder and shade for stock. Pruning these trees will allow more light to the base for even grass growth throughout the year.  
    • Avoid planting large trees close to the water’s edge because they may topple in as they mature.  
  • A nutrient budget is helpful to ensure just enough fertiliser is added to keep steep land productive. Use regular soil tests to determine requirements.  
  • Avoid competition from existing pasture species when over sowing by pre-spraying with herbicide.  
  • Avoid cultivating slopes steeper than 12 degrees. For lesser slopes, cultivate along the contour or at a slight angle to it. Divert runoff from drains, roads and tracks away from arable paddocks.  
Illustration of hill with trees

Land retirement 

Pasture on steep hill country is vulnerable to erosion and can often cause financial loss through farm damage, rehabilitation costs and loss of production. Retiring steep land is one of the most effective ways to prevent this.  

Things to consider: 

  • If you are thinking of changing the land use of difficult-to-manage pastoral units, consider woodlots, forestry, space planted conservation trees or retire the land to native bush.  
  • Establishing woody vegetation (native or exotic) will bind the soil and help with stability. 
    • Woody vegetation can also reduce peak flood flows by slowing down overland flow. 
  • Establishing woodlots for timber production can provide alternative income.  
  • Retired land has no fertiliser requirements and therefore allows you to concentrate your fertiliser budget on the most productive areas.  
  • Timber harvesting requires good access for machinery and trucks. Steep land can be difficult to access so make sure access is available early in the planning phase.  
    • A farm plan will help identify a farm’s resources and how to make best use of them.  
  • Weed management on an annual basis is essential to preventing pest species from establishing.  
Illustration of cow by fence

Land use, grazing and stock management 

How you use your land can have an impact on preventing or causing erosion.  

Things to consider: 

  • Sensible grazing regimes will protect grass species and improve recovery. Even the most persistent species can be severely damaged by inappropriate grazing regimes and lack of maintenance.  
    • Avoid overgrazing in summer and heavy or prolonged rotations in winter, which open up bare ground and expose it to soil erosion and weed infestation.  
    • Avoid stock camping areas by planting more trees in paddocks and locating troughs to encourage stock to distribute more evenly throughout the paddock. 
    • Avoid placing water troughs in waterlogged or floodable areas where increased stock traffic may damage soil structure or water supply is washed away.  
  • Excluding livestock from waterways prevents direct contamination from effluent and reduces stream bank erosion. It also keeps stock secure and safe.   
  • Avoid using sacrificial paddocks near waterways.  
  • Avoid cultivating soils that are too wet or too dry as this destroys soil structure and increases the erosion risk.