Stabilisation of river catchments

Over the past thirty years, large tracts of natural vegetation have been cleared for cultivation in the Chitsa, Chizvirizvi, Fair Range and Chipimbi resettlement areas. Agricultural fields have been positioned across drainage networks, resulting in large volumes of erosive runoff. Soil erosion has reached alarming levels in the communal lands, and flooding into Malilangwe is having negative impacts on habitats within the reserve. For example, the dambo grasslands of the Chiloveka, Benzi and Mahande Rivers, which are important sources of grazing for white rhinoceros, are being desiccated by rills and gullies, and the thousands of tons of sediment being deposited annually into the Malilangwe reservoir may be responsible for prolonged blooms of toxic algae that threaten the health of both people and wildlife. Remedial action needs to be taken urgently before the damage becomes irreparable.

Hedges of vetiver grass (Chrysopogon zizanioides) have been used globally in tropical areas to successfully stabilise runoff from agricultural fields. Vetiver provides a “softer” option than earth banks and other engineering works because its hedges are porous and serve to slow down the rate of flow as opposed to blocking it completely. Interventions that block flow often fail after heavy rain because the volumes of water are too great to hold back.

The vetiver system for controlling erosion consists of a simple vegetative barrier (a hedge) comprising upright, rigid, dense, and deeply rooted tillers of clump grass that slows runoff and holds sediment on site. Over time, sediment accumulates on the upslope side of the hedge, eventually forming a natural terrace. Vetiver’s deep and massive root system can reach down to two to three metres in the first year after planting. The measured maximum resistance of vetiver roots in soils is equivalent to one-sixth that of mild steel, and the root system has been shown to increase soil shear strength by as much as 39%. Hedges are propagated and established vegetatively, and recommended cultivars are sterile and non-invasive.

 To control runoff and stabilise soil erosion, it is recommended that vetiver hedges be planted along contours located at 1 metre height intervals. Following these recommendations, 50km of hedge have been planted in the Chitsa Communal Land as a first step towards the stabilisation of the Chiloveka River catchment. The intention is to systematically stabilise the agricultural fields in the river catchments, starting in the south and then working anti-clockwise around the boundary of the reserve.