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
NEW ZEALAND, 1983 DEPARTMENT OF SCIENTIFIC AND INDUSTRIAL RESEARCH, 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
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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.~qv sed .~ .f.-f.\(s E w C\ r ~ecCo II o &c.>:"''3 Or•g•n.a.I e..... os;o""
Bale ba..-r1u Sole ba..-n 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. -~S" e.a.-~W4)(y. bo Cl- p 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. TABL& 1. Chemical Analyses .... so1L TYPE: --·---~.o.-~-.::. __Ls...k_a,._'.:):::i, __ ~.0 .. 1.1..S____ [~f-~.~~-a.v_~--.S.:r_it.B..1.IL .. ---··-·~- LAB. Nos. _____ .t:::\.X.._0.~.•. 9'..::i:._ _(.~"-"-f.k~-}-n;.&--SDc.;<;-J;..l.~, A . 0.:.L"<.t.0.?1--- Field Nos. -~~---3...S.~'L:.~.S...1_ .ClASS!FICATION-Common: __t' e.o..\:.;,---S.o..i.il":I _____ ,____ '-_ · · Pit/Comp. ·----·---- Technical: Altitude:. __ Rainfall:_, ___ . PARENT MATERIAL:'------,...,-----~--=------'---'--o-----· Slope and Aspect:, ______VEGETATION:------'~------' Grid Ref. --'------ Topdrcmng H1Story:. Couniy, ------ . lo.ea lity: ·- SURVEY: ------· Soil District:. Collected by d .. S'., ..'if>.B.._\J..LJ: Depth pHH,o(l:l-5) ' I C N PJmg')'.) I P Rein. C.CO, Stone• ~ • lab. No. HoriL I C/N (>2mm) Moislur• I _ln. Moirl I Ory . % .% T ru..<>~ I I I ,., '!. % Fa Anelyst (initials}: r.....i.,.,.;., A\.?ro'l(:it: . 0 Depth\ Cation ExChange {me. / 0 ) s (mg%) I ;.\\;~., "Toh..\ I Lab. No. Ii Horii. Sol..J.\._, in: CEC jrEBJliln) %BS .• c. Mg K Na Tola! IAds!x! I I ~~t.>1 I I I I I I 5'1\>C-i I _l:'.1.~.-~i.. ~-: ...... Q.:J.i..\\..!..t,,J.:.o. --·--·--·-- -~1:.. 1 ...\ .... ~-3...... 1.5.:.~--- ...L'L.L ...l:l.i .. LLL:.5. ·-~----· ·-···---·· ..O.:.Jl.L. ..•O..:.:UI...... _-.. --·-----l-~1- -··--·----·-·b·'...... 0.:..!5\·····S:!i:l ...... - ...... l.~.. :~ ...... ;!) .. L...... 3.:.. 'iL ..... A.:.'.:l ...... O.:J.. 6. __ l:.'.':l.11: ··-·--· ···---··-·--· .. 0"..10 ...... 0..:.0.~ .. ..;,----·-·--··· ··--··-- O..:~.~-- ....O..:..L9.. _...:__.!______··-· - '-·-······--··c.·'·-········· 0.: ..!~..1.:5.Cl: .. '.l .: ...... _. __ .....!t-J.:.!o...... ~.'t_ ... .l.3.:.'.\I ...... 1.:5.:.~ ...... 0..:.9.0 ...... 5.:..0.. ··-·-···--··· --·· ... I ·. , ----·-····--····d·\:--··--· o.~\-~-~!t: ..:!· ...... '.ll.::o. .. ··--·~--It...... ?! ..:sr .. ·-~:~~.---~~--J .... Q.~"~· ···---·· ---·-·· ...0 ..: . .1.L . ...Q.:.0 .. 1*. ·------· ---·--· --~ ·-- 0 5 ...0 ..:..\.1!: .. .. o... ~o~ J ... _...... L.•..... J_____ ...... el ...... s.. ..1s~ ...... Q ...... ·····---····-·.·····-~- .. .:s. ·-··-----~--r·· ·- ·~r.. o...... ~r~~-i~:~-~·~1---··~ .. -·---r--~----~ r ...... - . . . I . - I...... -10- Analysis for the five samples are given in Table 1. Composite core samples for quick test analysis were collected throughout the length of the airstrip along the medium line (site 5) and from the eroded part of the western end of the airstrip (site 6). Analysis for these samples are gi~en in Table 2. Table 2. Quick Test Analysis for Samples from Dog Island Airstrip Sample Site & Location De pt pH Ca K p Mg 5 Airstrip medium line 0-7.5 cm 4.7 7 3 110 70 6 Eroded western end 0-7.5 cm 4.1 4 3 80 70 -11- RECOMMENDATIONS FOR MAINTENANCE AND RE-ESTABLISHMENT 1. General Maintenance for whole of strip (a) Fertiliser Analyses indicate that phosphorus and potassium levels are satisfactory at present. To encourage and maintain optimum growth it is suggested an annual topdressing of 15% potassic superphosphate at a rate of 300 kg/ha be applied in the spring. (b) Lime Requirement The pH (soil acidity) measurement along the median line of the airstrip averaged 4.7 which is low for satisfactory grass growth. To raise the pH to a satisfactory level (i.e. pH 5.6+) it is suggested that lime be applied at a rate of 7.5 tonnes/ hectare when practical. The range of pH values is somewhat variable over the airstrip depending on the amount of peat or sand in the surface layers. (c) Grass Seed Mix for Bare Areas The most suitable species for this situation are fine leaved hard wearing grasses such as browntop (Agrostis tenuis) and Chewings fescue (Festuca rub£~). A mixture consisting of equal weight of each grass sown at a rate of 60-70 kg/ha, which is similar to a lawn mix, should give satisfactory results. (d) Grass Grub and Perina Control During the inspection a few porina grubs were noted to be present on the more sandy eastern end of the strip. Grass grubs, although not seen, are likely to be present. These pests if present in high numbers can cause the death of pasture grasses. Next autumn an inspection to determine their number should be carried out, and if present in pasture damaging numbers (more than 10-20 per square metre) they should be treated for. A number of commercial products are available with details supplied if required. -12- 2. Re-establishment of Wind Eroded Areas (a) The proposed method of filling in holes and leveling the area with sand from the eastern end of the Island would seem the most satisfactory. If practical some peat material could be mixed in with the sand as this would increase soil stability during the establishment of pasture. The final surf ace should be formed when fertiliser and the above seed mix can be broadcast and lightly raked in. It is recommended that the black plastic mesh (as we were shown on Dog Island) be laid on top of the seeded area and firmly pegged with wire staples. It is also recommended that the mesh on the windward edge (i.e. to the west) be taken over in a bull nose and buried in order to minimise any effects of wind lifting. It is also suggested that only small stages be attempted at any one time - say strips 10 m wide. (b) Fertiliser Newly grassed areas should be topdressed with reverted super at 500 kg per hectare plus nitrogen, in the form of 125 kg/ha of ammonium sulphate. The nitrogen should be reapplied about six weeks later at the same rate as it is readily leached from the soil. The nitrogen application is to encourage rapid grass establishment. The fertiliser, both super and nitrogen, should be applied prior to seeding and worked into the ground during preparation. Subsequent fertiliser application should be as discussed in section 1a above. (c) Wind Break Barriers We feel the first windbreak barrier should be removed to decrease the considerable eddying effect as this appears to have aggravated erosion. In its place a windbreak of 1 m high plastic mesh (same as ground covering material) should be erected immediately to the west or windward of the first -13- newly established grass area. This type of windbreak reduces wind velocity but does not cause turbulence and has a sheltering effect for about 10 m to leeward. If the airstrip is to be extended to the west the windbreak should be progressively moved to protect each new area. The windbreak can be constructed by putting in a strainer post and stay on each side of the strip.~ Heavy wire (No. 9 fencing wire) can be strained between these posts corresponding to the top and bottom of the plastic mesh. The plastic mesh can then be attached to these wires by doubling it around the wire and either sowing with string or using plastic clips. Additional support wires, not attached, can be strung across the mid point of the mesh to reduce billowing and excess wind pressure. (d) Eroded Areas on Western Extremity of Airstrip On this area (west of bale barriers) surface soil has been eroded away exposing the underlying peat. To prevent further erosion of this peat, Cotula which has already colonised a small part of this region could be transplanted to the rest of the area. This could be done by digging up small plugs 2.5 cm in diameter and transplanting them over the bare surface. If such nucleus areas are established as close together as available plant material permits the area would be protected from further erosion. This is a stabilising alternative which could be carried out prior to rebuilding and regrassing this portion of the airstrip as this may take several years. -14- References New Zealand Soil Bureau, 1968: General Survey of the Soils of South Island, New Zealand. N.Z. Soil Bureau Bulletin 27. 404 p. Watters, W.A.; Speden, I.G.; Wood, B.L. 1968: Sheet 26 Stewart Island (1st Ed.) "Geological Map of New Zealand 1:250,000". Department of Scientific and Industrial Research Wellington, New Zealand. SOIL BUREAU DISTRICT OFFICE D.S.I.R ... GORE APRIL 1983