Regenerating Fynbos, Riverine and Dune Systems

4 November 2024

The Lower Silvermine River Flood Alleviation Scheme

The locations of the Silvermine River in Clovelly, and the Lourens River in Somerset West, the subject of this article, are indicated in Figure 1 below.

The need to construct a railway bridge over the Silvermine River in the late 1880’s, resulted in the floodplain of the river’s lower reaches being reduced by 80% to a relatively narrow 200m wide corridor. Subsequent residential development resulted in the river being restricted to a narrow earth channel running through residential gardens resulting in flooding of the residences when the river was in high flow or flood.

The flood alleviation scheme allowed for the ‘Low Flow’ to continue in the earth channel through the residential properties, but a ‘choke’, an in-stream pipe, installed upstream of these properties, allowed the higher volumes of water to back-up and spill over into a series of wide detention ponds. In this way the detained water moved slowly towards the restricting railway bridges before entering False Bay (Figures 2 and 3).

The site is exposed to strong southeasterly, salt-laden winds in summer. The soil, which is typical of sandy costal dunes, was partly covered by invasive alien vegetation.  To successfully restore the local vegetation, typical habitats needed to be re-created, requiring the seed-bearing sand and indigenous plant material to be used. The bare soils had to be stabilised and the revegetation carried out in the optimum season, which in the area is late autumn or early winter.

The landscape design involved the creation of various habitats in and around the detention ponds, including water bodies, adjacent wetbank zones, seasonal wetlands and dunes (Figure 4).

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Prior to the creation of the detention ponds, indigenous plant material was harvested from the remaining pockets on site, stored, propagated and maintained until ready to be planted. The invasive plant material was also cut for brushwood and chipped for mulch for use in the stabilisation of the exposed sands (Figure 5).

The topsoil from the project was stockpiled for return to the site prior to revegetation. The stockpiling was specified to be less than two metres high to prevent compaction, free-draining to prevent leaching, and covered with shadecloth to prevent wind erosion. The optimum stockpile period was considered to be less than 12 months for seeds to still be productive.

Once the earthworks were complete, fine grading allowed for habitat creation, with topsoil being spread and stabilised using non-invasive wheat straw, rotovated into the topsoil, while brushwood windrows and shadecloth fences were installed at 90 degrees to the wind direction (Figure 5).

The various areas were planted with species specific to each habitat, whereafter the whole area was ‘hydroseeded’ with seed collected from the area. This was programmed to be done in late autumn and early winter at the onset of the rainy season (Figure 6).

While the priority of the project was flood alleviation, other low-key interventions were provided for passive recreation, such as a gravel footpath around the 1.75 km circumference of the ponds. Bridges and boardwalks across the detention ponds provide pedestrian access on either side of the river and ponds, as well as some seating (Figure 7).

The area was maintained for 12 months after practical completion of each phase to ensure establishment of the vegetation. Maintenance included weeding and replacing dead material, fixing minor erosion and watering the area in the initial summer months. Well established habitats 5 years after re-vegetation are indicated in Figure 8 below.

Since the completion of construction some 20 years ago, excess silt from wildfire erosion and stormwater runoff, sludge from sewage spills and invasion of reeds associated with siltation, has decreased the capacity of the wetland ponds. Silt traps were constructed but have either not been cleared of silt regularly or have not been able to cope with silt quantities.

The wetland ponds are currently being dredged, i.e. the accumulated silt deposits including invasive reeds, litter and solid waste, are being mechanically removed. This has resulted in the indigenous vegetation and habitats also being destroyed.

While funding was made available for the construction of this nature based scheme, the funding for ongoing maintenance was left to another City Department whose priorities are merely clearing of the water channels. Even though a community based organisation is actively involved in the management of this system, they are reliant on the City for the larger expenses, such as clearing of alien vegetation and silt. Unfortunately the dredging was left to inexperienced and uninformed machine operators which resulted in the local community based volunteers scurrying around capturing tadpoles of the Endangered Western Leopard Toad and other wildlife which inhabit the wetlands before the dredging.

New open water bodies are being created where seasonal wetlands once existed, which together with the remnant wetlands will provide habitats for the endangered Western Leopard Toad, as well as bird and fish species.

The Lourens River Flood Alleviation Scheme

In Somerset West, nestled in the Helderberg Basin, the Lourens River flows from the Hottentot Holland Mountains through farmlands and urban development to the sea. Flooding of the adjacent urban areas occurs as a result of development encroaching onto the floodplain, as well as increased runoff from surface hardening of surrounding areas.

The Lourens River Flood Alleviation Project is a two phased scheme, which commenced in the late 1990’s and continues to the current day (Figure 9). Phase 1 is a predominantly soft engineering phase through the residential areas to the False Bay coast, approximately 6,5 km as the crow flies. Implementation has taken place through a number of sub-phases still underway. This soft engineering phase is only able to deal with floods of less than 1:2 storm events

Phase 2, will consist of a diversion canal that will allow a certain amount of water through the natural channel, but carry greater than 1:2 year flood waters along a separate path, linking it back to the river further downstream where more flood space is available.

The low-flow channel area, which form part of the soft engineering works, include the creation of lower adjacent terraces, which become inundated after substantial rainfall. Space is not always available for terracing resulting in the use of earth berms or mounds and walls, such as ‘Loffelstein’ block walls (Figures 10 and 11).

The vegetation of the river corridor contains both the remnants of historical rural development, as well as the original indigenous riverine vegetation. While many exotic and invasive species were removed as part of the widening of the river, some non-invasive species were retained for their cultural value.

The requirement was to re-vegetate using endemic species from the local riverine area, with a dedicated nursery being established for propagation. A range of plant types suitable for river edge, wet-bank and dry-bank situations were propagated and replanted on completion of the civil works (Figures 12 and 13).

The vegetation became well established during the contractual maintenance period. As the phases unfolded, some lessons were learned, the plant material being restricted to low growing ground covers and shrubs together with a tree canopy, which allowed for surveillance of the footpaths (Figure 14).

Residential areas adjacent to the completed flood alleviation work areas have recorded reduced annual flooding, while areas adjacent phases under construction still experience flooding, which should reduce after the flood alleviation works are completed.

Once the contractual maintenance period has ended, ongoing maintenance is carried out by the City. Because maintenance has been irregular, the river has become overgrown with garden weeds from upstream. In addition, sedimentation from stormwater runoff and sludge from sewage spills have also built up in the river reducing flood alleviation capacity. The City have recently cleared most vegetation, including weeds, indigenous riverine plants, silt and sludge from certain sections of the river.

Conclusion

In the 15 to 20 year period post construction of both projects, the re-vegetation has been successful, stabilising the riverine corridor, creating new habitats, providing passive recreation facilities for residents, while at the same time alleviating annual flooding.

Lack of regular maintenance work has led to the buildup of plant material and silt affecting flood water capacity. Where large machinery has been brought in to clear the areas, a significant change in the previously created habitats has occurred.

An ongoing concern is that a dedicated ‘Custodian Department’ within the City needs to take the lead in managing operational maintenance of areas such as these. To this end, Operation Management Plans and Manuals have been produced but seemingly shelved. Such a Custodian Department would need input from stormwater engineers,environmental/biodiversity managers and the Parks Department to manage the operational phase of these schemes. The Water and Sanitation Department’s maintenance crews have not had a thorough briefing on the systems used in these areas, resulting in much of the previously done ‘soft engineering’ works being damaged.

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