Erosion Control on Unconsolidated Volcanic Ash Deposits at Rabaul
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Scientific registration n° : 2146 Symposium n° : 31 Presentation : poster Erosion control on unconsolidated volcanic ash deposits at Rabaul, Papua New Guinea Contrôle de l'érosion sur des dépôts de cendres volcaniques non consolidées à Rabaul, Papouasie, Nouvelle Guinée HUMPHREY Bill (1), IGUA Passinghan (1) (1) Lowlands Agricultural Experiment Station, PO Keravat, ENBP, Papua New Guinea The volcanic ash deposited in and around Rabaul, Papua New Guinea (PNG) in the 1994 eruptions is very erodible. The soil surface is heavily rilled and gullies as deep as 10 m have developed in the fresh ash. In some areas, the disrupted drainage patterns have resulted in erosion of the developed soil under the ash. Civil engineers have determined that the ash bed slopes must be stabilised with vegetation before work to restore Rabaul town can begin. This has prompted a program of plantings on the ash beds aimed at building up soil organic matter and decreasing runoff and ash flow during heavy rainfall. On-going farming systems research into the management of sloping lands, conducted jointly by the PNG Department of Agriculture and Livestock, the IBSRAM PACIFICLAND network, and the EU Pacific Regional Agriculture Programme is providing a variety of strategies to stabilise these ash beds. These strategies include planted hedgerow barriers and living check-dam structures, and cultivated broadacre plantings of appropriate species. Preliminary plantings have shown leguminous ground covers such as Macroptilium atropurpureum and Vigna luteola can be established in the ash. Leguminous trees such as Gliricidia sepium and Acacia auriculiformis have also been successful. However, the formation of a dense sward and the buildup of organic matter require grasses and durable mulches that are high in tannins. A program to test combinations of legumes and non-legumes, eg. Vetiveria zizanioides and tropical pasture grasses, has been developed. Flemingia macrophylla is included as a legume the produces a durable mulch. The project is also evaluating the impact of fertiliser additions in the establishment of the above planting combinations. The project provides a unique challenge as the ash is unconsolidated, sterile, and completely devoid of nitrogen and organic matter. However, local soil developed from previous eruptions is very fertile and our project is merely accelerating the natural processes that transformed the previous parent material into living soil. Keywords : Papua New Guinea, volcanic ash, ash bed, revegetation, stabilisation Mots clés : Papouasie Nouvelle Guinée, cendre volcanique, lit de cendre, revégétalisation, stabilisation du sol.