Rehabilitating channels using a novel stabilisation approach

When five new and very active gullies formed on the Vergelegen farm in Somerset West, Western Cape, an urgent remediation response was required. A key challenge was that the watercourses had to be reshaped since the gulley sides were too steep to plant vegetation as part of a bioengineered response.

By Hans King, Pr Eng*

Cascades of small weirs were recommended to reduce channel slopes, as well as flow velocities during floods. The design objective was to keep the number of weirs to a minimum, both from a cost perspective and in terms of minimising the presence of intrusive engineered structures within the natural environment.

Erosion protection was required along the total fall of the gullies, with the drop height per weir, plus the slope of the channel between the weirs, determining how many were required. In terms of design options, configuring a steeper slope between the weirs would have reduced their number, in turn reducing construction costs. However, the downside is that a steeper slope between the weirs translates into higher flow velocities.

When planning a cascade of weirs, it is therefore important to find the optimum balance to minimise the risk of future erosion in the channels, especially during floods.

The dilemma when stabilising streams with vegetation

In the Western Cape, perennial wetlands and streams that are well vegetated with indigenous vegetation are quite resistant to soil erosion. But when a watercourse (such as those at Vergelegen) has been severely disturbed, there is insufficient time within one growing season, including construction, to establish enough new vegetation to prevent the disturbed soil from washing away in the following rainy season.

An innovative approach

The novel solution to this problem at Vergelegen was to cover the base of the landscaped channel with a 150 mm thick flexible geocell mat. The initial intention was to fill this honeycomb structure with topsoil and densely vegetate it with plants, including palmiet reed. The latter tends to lie flat during floods and protects a lot of the soil adjacent to the plant from erosion.

Small-scale trials of this planted geocell technique were carried out on Vergelegen prior to agreement on the final design approach.

Severe gully erosion at Site 5 at Vergelegen farm

The result

Construction commenced on 1 September 2020 and the weir cascades for gully sites 1 and 5 were completed in the late summer of 2021. Indigenous wetland vegetation was planted immediately but did not have time to mature before the onset of winter.

The observed containment of erosion during the first rains was initially successful, especially where the plant density was high; however, in areas where plants were sparse, the topsoil started washing away.

In affected zones, once the honeycomb cells had been partially emptied of soil by erosion, piping underneath the mat became a problem and whole areas of the mat became devoid of soil fill. This was a problem particularly where the rehabilitated channel had to blend into the existing watercourse with short lengths of steep slope.

Late in the winter of 2021, some remedial work was done to the areas suffering from erosion. The then deformed mat was levelled and filled with a gravel and rock mixture. In addition, a shallow trench was dug at the mat joints (every 5 m down the length of the channel) and filled with gravel wrapped in a geotextile to cut off extended piping flow paths underneath the mat. Indigenous wetland plants were immediately planted in the gravel.

In addition to the general planting of vegetation, palmiet ‘logs’ (palmiet rhizomes

laid head to tail with a small overlap) were placed across the entire width of the channel every 2 m and staked into position. The key benefit of these logs is that they create surface roughness, which reduces the flow velocity in the channel. They also help to form a sequence of small pools that trap sediment and seeds, and encourage the establishment of new wetland vegetation.

Conclusion

The use of geocell mats at Vergelegen has proven to be very promising for future interventions. Based on the lessons learnt, and with hindsight, the upfront design would have included small cut-off structures under the mat, the filling of the mat with gravel, and the use of palmiet logs.

For future reference on other projects, it would also be valuable to research and quantify the allowable design channel flow velocities for a given fill material. This would enable the optimisation of the channel slope with the intention of minimising the number of weirs needed for the cascade.

*Managing director, HansKingSRS

INDIGENOUS WETLAND PLANT SPECIES EMPLOYED

The following plant species (which occur naturally on the farm) were used: In the channel wet zone: - Prionium serratum (palmiet reed) - Carpha glomerata - Ischyrolepis subverticillata (besembos) – transplanted clumps take a long time to establish - Wachendorfia thyrsiflora - Juncus effusus - Juncus lomatophyllus - Isolepis prolifera – this usually appears naturally and does not have a strong root system; however, it facilitates other plants in getting established, and creates a dense surface cover quite quickly On the banks of the channel: - Ischyrolepis subverticillata (besembos) - Pennisetum macrourum (African feathergrass) Representation of the impact that the number of weirs in a cascade has on the slope between the weirs

Lower end of Site 1: for a short distance, the slope of the channel is 10-15% to tie in with the existing downstream channel. This caused an increase in flow velocity and erosion of the fill, requiring an alternative design approach

A weir cascade installed on one of the remediated channel slopes Filling the geocell mat with gravel

Attaching palmiet ‘logs’ to pegs

A cascade weir section with vegetation well established