The benefits of using groynes for river training and river bank erosion protection

Very commonly erosion of river banks and beds in the Western Cape occur when:-

  • Un-naturally high flow velocities are experienced: This occurs when soil and rocks in rivers have been bulldozed to the sides making the river channel narrower and deeper. This also happens when thick stands of woody alien vegetation has been allowed to get established on the inside of river bends, and these effectively block part of the river channel leaving the rest deeper and narrower than usual. Water flowing in narrow and deep channels flows with a higher flow velocity, and this higher flow velocity increases the water’s ability to pick up and carry away sediment in the river.
Black wattles have colonized the inside of a bend (right) and diverted the flow of water into the river bank (left)
  • The resistance to erosion by soil plus natural vegetation has diminished: Many indigenous wetland plant species protect the soil from erosion by having root masses that bind the soil, and plant upper structures that lay flat and cover the soil surface during floods. When a river has been bulldozed, this protection is removed leaving the soil susceptible to erosion. When invasive alien vegetation such as Black Wattle and Port Jackson Willow displace indigenous wetland vegetation, because they are woody, they do not lie flat during floods. They then trap floating debris during floods and create obstructions in the river channel that diverts the flow of the river into the river banks, promoting erosion.
A bulldozed river bed with no vegetation to hold the sediment during a flood
  • There is un-natural soil erosion in the channel upstream: Soil erosion in a river channel due to un-natural causes increases the sediment load in the river and further de-stabilizes the river. When the water flowing in a channel carries an abnormally high volume of sediment, it deposits some wherever a lower flow velocity zone occurs, such as at the inside of a bend in the river. This deposition of sediment builds up the inside of the bend and forces the flow outwards, this results in an acceleration of the meandering process and often results in the fresh erosion of the bank on the outside of the bend.
Large blocks of peat washed out from the river bed upstream, obstructing teh flow of the river and causing fresh erosion downstream. This is an extreme case, but not uncommon.

When groyne structures are used to stabilize a watercourse, if done correctly the design addresses all of the above causes of erosion. In particular the following are important:-

  • The river channel must be widened and made less deep. The groyne structures (if low) assist directing the flow of water around the bend without high erosion-causing flow velocities being encountered at the river bank. The groynes should be designed to be low enough so as to be overtopped at a low flood level and enabling a wide flow area, but must be high enough to redirect the flow of the water.
  • The spaces between the groynes should be arranged so that an environment is created where indigenous wetland vegetation may be established. Apart from contributing to the environment’s biodiversity, this vegetation plays the important role of assisting the groynes to manage erosion during high flow situations. At the project design stage, if the intention is there to establish a healthy stand of indigenous wetland vegetation between the groyne structures, it is reasonable to motivate a wider groyne spacing. The saving on the cost of building structures can be used to propagate and plant the vegetation.
  • Ideally when erosion at a site is being encouraged by un-natural erosion upstream, the river should be rehabilitated all the way up to the source of instability. If this is not possible, a field of groyne structures still has the advantage that it can trap a lot of the sediment arising from the upstream erosion and have a beneficial impact of the river stability.
Long low groyne structures, designed to overtop during relatively small floods, but capable of safely supporting a sustainable wide slow flowing flow channel

Note that with longitudinal bank erosion protection methods (such as retaining walls or rip-rap armouring of the banks), while having their own particular advantages, do not provide the benefits that groyne structures listed above do.

Apart from creating a more natural river cross section and facilitating the use of natural vegetation to stabilize rivers, additional benefits of using groynes are that:-

  • The construction of groynes uses a lot of manual labour and so these projects feed readily into local job-creation schemes.
  • Groynes trap surplus river sediment and help protect the downstream riparian community from sediment driven river instability.
  • Especially when the landscape permits long and low groynes, groynes provide a cheaper solution than longitudinal protection (such as retaining walls and rip-rap).
A field of groynes protecting a river bank from erosion downstream of a bridge