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RIVERBANK STABILISATION USING VEGETATION AND BIOENGINEERING

Riverbank erosion is a naturally occurring process where water wears away the banks of the river. However, increased human and animal activity with poor management of riverbanks have increased the rate of erosion. With no protective shield (vegetation or fencing), erosion can lead to riverbank failure. There are many ways by which riverbank erosion mitigation works can be carried out:

  • Physical bank stabilisation using bamboo or soil bioengineering (e.g. soft or vegetative gabions).
  • Riparian revegetation and bush regeneration including native species of grass or trees.
  • Stock management such as fencing or cattle gates to reduce animal activity

To control riverbank erosion, it is best to use a combination of techniques.

Bank stabilisation can be undertaken in a variety of ways:  

  • Bamboo plantation along the banks: Bamboo is widely used for soil protection because it grows   rapidly Image removed.and has a strong root system. It can be used in a variety of   soil profiles and types. Two types of Bamboo varieties - Common   Bamboo (Bambusa vulgaris) and Pacific Bamboo (Schizostachym   glaucifolium) – are commonly used in the Pacific.

 

 

 

  • Soil bioengineering structures for riverbank erosion:  Soil bioengineering methods integrate civil Image removed. engineering techniques with natural materials to obtain fast,   effective and economic methods of protecting, restoring and   maintaining riverbanks. Two types of soil bioengineering structures   used for riverbank erosion control are: retaining walls and spurs.   Retaining walls and spurs can be gabion (civil engineering structure   filled with earth and stones), stone or combination of vegetation   and gabion/stone.
     

 

 

 

  • Native vegetation (e.g. vetiver grass growing along a river or stream) helps in retaining sediment.
  • Bamboo plantations can make otherwise wasted land productive as it requires minimal nutrients from the soil and provide shade and foliage back to the topsoil thus providing better conditions for other species to thrive.
  • Bamboo is widely used as a raw material in industry. For example, in handicrafts, its fibres are used to weave clothes and make paper, and its shoots and sprouts are used for food.
  • Bamboo cultivation provides many indirect benefits like local energy production, fodder for livestock and poultry, a source of useful material therefore income for local communities.
  • The vegetation or material required for bioengineering can often be locally sourced.
  • Revegetation of a riverbank prevents sedimentation which helps in improving health of seagrass and coral reef near the river mouth.
  • Bamboo plantations can create a carbon sink (i.e. absorbs carbon dioxide) and helps in mitigating climate change.
  • Due to the economic value of bamboo, it can be harvested unsustainably by the community for economic benefit.
  • As bamboo plants can grow vigorously, choosing the wrong variety of Bamboo can make it invasive.
Indicative cost

Riverbank stabilization using vegetation and bioengineering implementation is low cost. Whilst providing cost estimates is difficult given the broad range of influencing factors across the Pacific, the below table outlines the key components that should be taken into account when estimating an bioengineering riverbank stabilisation project budget for a specific location.

Components

Unit
(A)

Number of units (B)

Unit Cost
(C)

Budget
(B x C)

Establishment Cost

        
  • Land (if not already held)

e.g. km2

      
  • Planning and design including community outreach

e.g hours

      
  • Materials (e.g. seeds, plants, stones, gabions, biotextiles)

e.g. number

      
  • Labour

e.g. hours

      
  • Reporting to donor (if required)

e.g. hours

      

Ongoing Cost

        
  • Maintenance Cost (materials and labour)

e.g. number and hours 

      
  • Reporting and Monitoring

e.g. hours

      

Riverbank stabilisation using vegetation and bioengineering is compared to hard infrastructure like channel concreting. Comparisons between these approaches include:

  • Bamboo plantation or soft gabions are often cheaper
  • Bamboo plantation reduces sediment flowing in the river and prevents erosion.
  • Bamboo complex root system acts as a water filtering system thus maintaining the river water quality.
On-ground impacts that the EbA option addresses
Increasing riverbank erosion
Increasing river sediment loads
Increased occurrence of flooding
Increasing prevalence of invasive species

Case study

Riverbank erosion control for Waimanu River

Fiji

Waimanu River is one of the three rivers draining large amounts of freshwater into the Rewa River. It is also one of the drinking water sources for the Fijian central island of Viti-Levu. The riverbank has been eroded which affects the river system as well as the livelihood of communities. The community have stopped using the eroded riverbank for any recreational activity and houses near the bank are prone to landslide during the rainy season. During heavy rainfall events, the high turbidity of the river affects the availability of freshwater fish, eel and mussels.

As part of the Fiji Ridge-to-Reef project, Vertiver grass was introduced to prevent soil erosion and support water retention along the riverbank. Vertiver grass grows fast, is strong and grows large root networks which holds the soil together.

The grass planted on the riverbank has successfully stabilised the bank, reduced channel widening, reduced excessive sedimentation and helps ensure freshwater fish availability.

Reference: Riverbank erosion control for Fiji’s Waimanu River, https://www.pacific-r2r.org/news/riverbank-erosion-control-fijis-waimanu-river