Riverbank training and protection

Black wattles have colonised the inside of a bend (right) and diverted the flow of water into the riverbank (left)

A bulldozed riverbed with no vegetation to hold the sediment during a flood. This has created unnaturally high flow velocities Large blocks of peat washed out from the riverbed upstream, obstructing the flow of the river and causing fresh erosion downstream. This is an extreme case, but not uncommon

Unnaturally high flow velocities, progressive loss of vegetation and increased sediment loads are key contributing factors that cause riverbed and riverbank erosion. The downstream effects include flooding and siltation build-up in dams and weirs. The installation of groynes serves as an effective countermeasure.

By Hans King, Pr Eng*

Many indigenous wetland plant species naturally minimise erosion by having root masses that bind the soil, and plant upper structures that lie flat and cover the soil surface during floods. However, when invasive alien vegetation, such as black wattle and Port Jackson willow, displace indigenous wetland flora, there’s ensuing havoc. That’s because they’re woody, so they don’t lie flat during floods. As a result, they then trap floating debris and create obstructions in the river channel, which diverts the flow into the riverbanks, promoting erosion.

This scenario is compounded where unnaturally high flow velocities are experienced. This may occur where soil and rocks in rivers have been

Long and low groyne structures, designed to overtop during relatively small floods, but capable of safely supporting a sustainable, wide, slow-flowing channel

bulldozed to the sides to optimise surrounding land use, making the river channel narrower and deeper. This also happens when thick stands of woody alien vegetation become established on the inside of river bends. They effectively block part of the river channel, leaving the rest deeper and narrower than usual, and significantly increasing the flow velocity. That in turn increases the water’s ability to pick up and carry away sediment in 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 on the inside of the bend and forces the flow outwards. That results in an acceleration of the meandering process and often causes fresh erosion of the bank on the outside of the bend.

Groyne structures

Faced with these escalating environmental threats, one of the best ways to counter them is to install groyne structures to stabilise the watercourse. If designed and installed correctly, groynes are highly effective. In this respect, there are some important points to note: • The river channel needs to be widened and its depth reduced. The low-lying groyne structures installed will then assist in directing the flow of water around the bend without high, erosion-causing flow velocities being encountered at the riverbank. The groyne design should ensure that the installation is low enough to be overtopped at a low flood level, enabling a wide flow area, but high enough to redirect the flow of water. • The spaces between the groynes should be arranged so that an environment is created where indigenous wetland vegetation can 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. If the intention is to establish a healthy stand of indigenous wetland vegetation between the groyne structures, it is reasonable to motivate for a wider groyne spacing at the project design stage. • Ideally when erosion at a site is being encouraged by unnatural 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 on the river’s stability. Groynes trap surplus sediment and help protect the downstream riparian community from sediment-driven river instability.

Groyne installations protecting a river bend

Purpose-designed and labour-intensive

It’s important to note that while longitudinal bank erosion protection methods (such as retaining walls or rip-rap armouring of the banks) have their advantages in terms of soil retention, groyne structures are purpose-designed for watercourse stabilisation.

Apart from creating a more natural river cross section and facilitating the use of indigenous vegetation to stabilise rivers, groynes also provide a host of additional benefits. The key one is that they lend themselves to labourintensive construction and Expanded Public Works Programmes. Where the landscape permits long and low structures, groynes also provide a cheaper solution than longitudinal protection (such as retaining walls and rip-rap).

*Hans King, Pr Eng, is the founder of HansKingSRS. Email: hans@hanskingsrs.com