Erosion on Dog Island Airstrip, Foveaux Strait
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EROSION ON DOG ISLAND AIRSTRIP FOVEAUX STRAIT J.G. BRUCE and W.H. RISK (M.A.F.) REPORT GG6 Soil Bureau District Office D.S.I.R. Private Bag GORE SOIL BUREAU DISTRICT OFFICE REPORT GG6 DEPARTMENT OF SCIENTIFIC AND INDUSTRIAL RESEARCH, NEW ZEALAND, 1983 Information in this report may not be quoted or cited in publications without the permission of the Director, Soil Bureau, Lower Hutt. -3- EROSION ON DOG ISLAND AIRSTRIP FOVEAUX STRAIT At the request of Mr W.J. Mcintosh, District Administration Officer, Ministry of Transport, Invercargill, the Dog Island airstrip was inspected on Wednesday 27 May 1981. Those involved were W.H. Risk (M.A.F.), J.G. Bruce (.D.S.I~R.), W.J. Mcintosh and Mr Turner (S.A.C. Pilot). The group left Invercargill airport by Cherokee aircraft at 2.35 pm returning at 4.05 pm. Time on ground at Dog Island, where we were accompanied by the lighthouse keeper Mr w. Russell, was approximately 3/4 hour. Dog Island is a small island in Foveaux Strait off the entrance to Bluff Harbour, and lying about 5.5 km south-east of Stirling Point (Fig 1). The island is approximately 1 km long (north-west to south-east) and ranges in width from about 150 m to 300 m. It has an almost continuous rocky shore line. Elevation above sea-level is mainly between 6 m and 7 m with that part, where the lighthouse and attendant buildings stand, rising to about 14 m. The airstrip extends for some 600 m along the length of the island and is sited approximately E-W. The strip is required for servicing and supply for Dog Island lighthouse which marks the approach to Bluff Harbour. There has been considerable trouble with the western end of the airstrip through lifting of the pasture, and deflation of soil. Approximately 100 m has been eroded and deflated. Two barriers, consisting of bales of straw,. have been secured across the strip at the western end about 50 m apart, in order to reduce erosion. A row of flax bushes marks the end of the strip at the extreme west of the island. Prevailing winds are from a westerly quarter and reach gale force frequently. Soils on the island are peats (Tentative name: Dog Island peat) or are peaty a~d are about 75 cm to 80 cm deep, resting on a rock platform. The rock platform, which is composed of norite of Bluff Complex (Watters, et al., 1968), has a discontinuous layer of well weathered and rounded pebbles (mainly quartz) and ground rock on its surface. , . ( Rly. Sin. '·./ "I Awarlia Plluns 5 ke,tch map of' DOG l5LAND FOVE'AU)( STRAl7 ' ' LOCAL 11'( l\il~P / I Eroded parf of Airs/rip ///// S ""'ple Sites: • c, _re) ··-.. soo :JJrQtA.-A /ro1n cter1nl p1ie;IC!jrc-PhS J.C81?u € 6011., But<t:t:Ju :J>.s.1. fh.N 6N6415 /..// l'-1./2. ' I -5- The eastern end of the island has a broad sand dune lying approximately north-south and extending across the island for about 150 m. A smaller dune occurs on the south-east extremity. The soil on the major dune is very similar to Toetaes sandy loam (N.Z. Soil Bureau, 1968) which occurs on the south-east coast of Southland. Material from this dune has been excavated and used to topdress the deflated parts of the airstrip. The strip was laid down in the 1960s and has been covered with a coarse hemp open weave matting. This has kept the pasture down and has proved successful in maintaining grass cover. Repairs to blow-outs on the strip have been carried out using this matting and in general have been successful. Where there have been gaps in the matting, however, small blow-outs have occurred and reestablishment of grass cover has been poor. The pasture mixture appears to be dominantly browntop, but in many places there were patches of Yorkshire fog. At the western end of the airstrip a blow-out has occurred east of the latest bale barrier. It appears to be due largely to turbulence and the eddying effect of wind across the barrier. A strip 4 m to 6 m wide, immediately adjacent to the barrier, has not been affected by deflation. In places, between 15 cm and 25 cm of infilled sand has been eroded down to the surface of the underlying peat. This area had previously been stripped of pasture through lifting. On the windward side of the barrier well dried and comminuted peat, very similar in appearance to fine lignite chips, has accumulated against the barrier. Matting was used on the deflated area to the west of this barrier (Fig 2) but has been completely lifted and peaty top eroded. There are a few patches of pasture, but most vegetation has gone. The deflated surface is slightly hummocky (amplitude 15-20 cm) and within the hummocks there are remnants of roots from the original sedge like vegetation. On the westernmost part of the original airstrip (i.e. between the flax and first bale barrier) the completely stripped surface now has a ground cover of Cotula and similar species which are very low to the ground. Scattered over the surface are a considerable number of small bleached quartz pebbles. The peat here is about 60-70 cm deep firm, fine grained, and very waxy. MaU.~q v sed .~ .f.-f.\(s E C\ r II o &c.>:"' '3 w ~ec Co Or•g•n.a.I e.. ... os;o"" Bale ba..-r1u Sole ba..-n<l(' I 64-l'"ipput ~..,. e..ct. 'I H""-.,""' 0C1k.~ Sur~ce. I Prevct'1(,·..,lj ::Dea.d roo4-s c. ... "'""''"""'cok d..-v t ff) °"I W1Md C. Cl- W4)(y. bo -~ S" p e.a.-~ 1 \\\ 1, Z, a.nd b. sa"""p\a. si \-a.s. Fig 2. Diagramatic section of western end of airstrip showing eroded area. -7- In this area there may be a certain amount of moisture stress, as well the effect of salt spray. It is possible that if sand could be scarified into the top of the peat there would be sufficient fine material for the establishment of pasture. Some form of firmly pegged matting cover would be needed to avoid further deflation. It is probable too that a regular maintenance topdressing of the entire airstrip would be of assistance in bringing the soils to a reasonable level of fertility. Soil samples for analysis were taken on the airstrip at~u~ sites as follows: (sites as on map (Fig 1) and section (Fig 2). Site 1: Western end of airstrip where Cotula and other species have established on the surface of the deflated peat. Sampling depth: 0-12 cm and includes vegetation (Field No AG355). Total depth of peat at site 65 cm, very waxy at depth, moist, and resting on rock. Site 2: Beyond easternmost bale barrier on sand that had been added as infill and pasture e~tablished on it. Some mixing of very fine peat chips. Sampling depth: 0-15 cm (Field No AG356). Underlying is further sand overlying eroded peat surface comparable to site 1. Site 3: North side of airstrip in well established pasture. Site is opposite lighthouse and attendant buildings. Sample depth 0-12 cm. Site 4: East end of airstrip near sand dune. Well compacted and well established pasture. Sampling depths: 0-5 cm (Field No AG358), 5-15 cm (Field No AG359). Description of Samples Fie~d No AG355 0-12 cm black (5YR 2~5/1) peat; firm; blocky fracture; moderately developed medium and fine nut structure; with crumbs and and granules; few small bleached pebbles; many roots. Top 3 cm somewhat friable, with fine granular and crumb structure. -8- Field No AG356 0-15 cm black to dark reddish brown (SYR 2.5/1-2) sand; friable to loose; some mixing of very fine dry peat chips; weakly coherent; weakly developed nut structure; many fine roots. (Transported material from east end of island to fill in eroded area). Field No AG357 0-12 cm black (5YR 2~5/1) peat; ftiable to firm; moderately developed fine nut structure with granules; few to many fine roots; few small bleached pebbles. (Somewhat tu~y in top 3-4 cm - which may relate to development of airstrip - also added fertiliser). Field No AG358 0-5 cm dark reddish brown (5YR 2-~5/2) peaty (turfy) sand; firm; weakly developed granular structure; many roots; strongly matted throughout; distinct boundary (Boundary may relate to development of airstrip and added fertiliser). Field No AG359 5-15 cm dark reddish brown (5YR 3/2) sand; compacted; structureless; coherent; many fine roots. Comment Sites 3 and 4 are on well established pasture on the airstrip but both appear to be very turfy on top 5 cm. This may relate to establishment of the airstrip pasture and application of fertiliser. These samples are from areas that have been covered with matting. It is also likely that site 4 has had more sand added from time to time as it is close to the excavation for infilling. Samples from site 4 have been split at the turfy break. Sample at site 3 has riot been split but it may be possible to do so. It is estimated that pH in all samples will be low wich would affect the availability of what nutrients may be present. It is also likely that at site 1 and possibly at site 2 salt content may be fairly high. The turf iness of other samples suggest that most of the available nutrients (through added fertiliser) are near the surface.