Southland Coastal Hazard Assessment
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Southland Coastal Hazard Assessment Prepared by Dallas Bradley, September 2009 Southland Coastal Hazard Assessment 1. Introduction The Southland Coastal Hazard Assessment has been prepared to assist with resource management and engineering lifelines processes. The principle purpose of the assessment is to flag the existence of particular hazards for further investigation, as and when necessary. The assessment is more in the nature of a “once over lightly” examination, rather than an in-depth study of the natural hazards that affect the coast. 2. Scope The assessment covers the mainland Southland open coast from the western end of Te Waewae Bay to the Otago/Southland boundary at Waiparau Head. The assessment does not cover Stewart Island and only touches lightly on harbours and estuaries, the reason being that within the developed parts of Stewart Island (parts of Paterson Inlet, Halfmoon and Horseshoe Bays) and in mainland estuaries and harbours, there are numerous areas subject to coastal hazards, principally coastal erosion. As such, it is recommended that any development adjacent to the shores of any estuary or harbour is subject to a site specific natural hazard investigation. In many cases, the hazard will have already been addressed to a degree, usually by physical works, some dating back many years. 3. Hazards Investigated The hazards specifically investigated are erosion/accretion, marine inundation and tsunami. The assessment also records the Land Resource Inventory Classification of the erosion potential of the land adjoining the coast. Sea level rise has not been treated as a hazard in its own right, but as an exacerbator of erosion and marine inundation hazards. The assessment is principally based on what has happened in the past. Predicted climate change and, more particularly, sea level rise are likely to accelerate or instigate more coastal erosion in the future. Although estuaries and harbours were not rigorously assessed, coastal erosion is generally occurring in these areas as extensively as it is on the open coast. The assessment tends to be dominated by the erosion hazard, mainly because it is extensive, long lasting, persistent and relative to other hazards, obvious. Hazards such as marine inundation and tsunami are episodic and generally less frequent. Coastal erosion becoming an increasingly significant issue in some places, especially Colac Bay, and it is an issue to some degree almost everywhere. As far as the tsunami hazard is concerned, the “Review of Tsunami Hazard and Risk in New Zealand” undertaken by Geological and Nuclear Sciences in January 2006 is the most authoritative source. Taking both local and distant sources into account the 100, 500 and 2500 return period wave heights above mean sea level are 0-2, 2-4 and 4-8 metres respectively for the area covered. Significant local variations could occur. The significant effects of historic tsunami have also been noted. Two further reports, “Understanding local source tsunami: 1820’s Southland Tsunami” jointly published in November 2005 by GNS and NIWA and “Otago region hazards management investigation: tsunami modelling study” published in September 2007 by NIWA contain some additional information on the modelled behaviour in particular locations of tsunami generated from different sources. The former also includes the outcome of a search of historic documents that reported tsunamis in the 1800’s. The marine inundation hazard as created by storm surge events is dependent on the land height and the wave and sea level regime within or along any coastal reach. Land heights are not generally well known but as a general rule one should be wary of land below 3.5 and 6.0 metres above mean sea level adjoining harbours/estuaries and the open coast respectively. 4. Format of the Assessment The main part of the assessment consists of an excel spreadsheet that describes various sections of coastline and the hazards contained within. That is followed by a collection of photographs and plans that illustrate the hazards referred to. 5. Use of the Assessment The assessment should be used to identify areas or infra-structure that may be vulnerable to the occurrence of the natural hazards discussed, then implement an appropriate management regime that is underpinned by concepts of sustainability and resilience. A far as Local Authorities are concerned, such decisions should be guided by guided by Coastal Hazards and Climate Change – A Guidance Manual for Local Government in New Zealand published by the Ministry for the Environment in July 2008. Coastline Upper Land Notable Features Engineering Hazards Comments References Reach Beach/ Behind Lifelines Coast type Coastal Coastal *Hinterland Marine Tsunami Erosion Accretion Instability Inundation Te Waewae Sandy Flat forested Cribs on flat land Logging access Yes, western west of Slight Slipping/ probable Probable, Net westward longshore 1. Kirk R.M. & Shulmeister Dr Bay (Track bench above MHWMS at road along land two thirds Waikoau Slumping especially at sediment transport.1 J. (1994): "Geomorphic Burn to initially then Waikoau Mouth immediately (1950 -1981) mouth river "the longshore drift processes and Coastal Waikoau rising. above beach. decreasing mouths2 - direction varies with wave Change in the Lagoon River) Relatively new towards west see attached regime, although in the System, Lower Waiau River, Swing Bridge wave height/ long term may be more Southland." Report to the (walking) at probability generally to the south Waiau River Working Party Waikoau Mouth - maps - east."16 convened by the Southland previous bridge extract from Significant change around Regional Council, May 1994." (road) washed out p96-98 of Waikoau Mouth (accretion 2. Berryman K (2005) by riverine GNS report on west side, erosion on "Review of Tsunami Hazard flooding east - see plot of past and Risk in NZ" Institute of coastline positions) See geological and Nuclear Fig.1. Sciences Ltd. 4. URS (2008) Manapouri Tailrace Amended Discharge Project (MTAD): Geomorphology Assessment. Te Waewae Boulders Western half Hump Track walk Road on bench Active substantial Negligible definitely, Ditto Net westward longshore 2. Berryman K (2005) Bay ( transitionin steep, begins about half above beach was erosion post accretion erosion debris on sediment transport.1 "Review of Tsunami Hazard Waikoau g from west eastern half way along this eroded in 2007 1981 See 1947 -1981 road from "the longshore drift and Risk in NZ" Institute of River to to gravel narrow flat reach. (See Figs 2a & Fig 2 See Fig 2 time to time direction varies with wave geological and Nuclear Rowallan then sand bench with 2b)to the point regime, although in the Sciences Ltd. Burn) steep where it was long term may be more land/terrrace replaced in 2009 generally to the south above by a new inland east."16 road. Te Waewae Sandy from Cliffs or steep from the Rowallan Burn to Grove Burn, Active Generally Slight Wind probable but Ditto There used to be a road at 3. NZAM Aerial Photograph Bay Rowallan to wide bench backed by cliffs from the Grove Burn to erosion post accretion Erosion at present the top of the beach along Runs 1247 - 1253 (1947), (Rowallan Groveburn, Waiau Mouth. Degrading gullies in and 1981. See 1947 -1981. tends to be the western part but it has Environment Southland Burn - Waiau gravelly behind - land has an uplifted appearance. Fig 3 See Fig 3 confined to been completely eroded Aerial Photo Runs A0423 & Mouth) from southern away. A2182. Groveburn margin of Net westward longshore 15. Halliday Consulting, to Waiau bench. sediment transport.1 August 2004. Stability of "the longshore drift Proposed Building Sites at direction varies with wave Crooymans Subdivision, regime, although in the Papatotara; long term may be more Coastline Upper Land Notable Features Engineering Hazards Comments References Reach Beach/ Behind Lifelines Coast type Coastal Coastal *Hinterland Marine Tsunami Erosion Accretion Instability Inundation generally to the south east."16 Te Waewae Gravelly Cliffs above Cribs above Former Bluecliffs Active No Slight Wind Yes Ditto A very dynamic area 5. Turnbull I.M. & Allibone Bay (Waiau the western western cliffs (See Beach Road has erosion, Erosion subject to mouth closures A.H. (Compilers) "Geology of Mouth and and eastern Fig 4b), a few, been eroded away particularly and mouth movements the Murihiku Area (2003)" barrier end of the (but reducing between Waimotu at the over 4km of the 6km Institute of Geological & beach) Lagoon number) of cribs and Cameron western end. barrier length. Coastal Nuclear Sciences. created by on floodplain to Creeks processes dominate over the barrier east of Waiau riverine processes.1 beach. River Mouth appears to be Cliffs at west moving westward but that end of may change. Bench lagoon and below western cribs is west of the currently eroding and cribs are barrier beach appears to slipping. be moving inland over western most 2km. The lagoon itself has extended westwards by about 300 metres since 1981. See Figs 4 & 4a. Te Waewae Gravelly Cliffs or McCracken's SH99 in close Active No Slight Slumping No Ditto It appears that the Te Waewae Bay coast east of the Waiau Bay (Waiau steep slopes Rest, proximity to top of erosion and Wind is under supplied with sediments and is consequentially Mouth - above a cliffs or steep along full Erosion erosional.1 "the Waimeamea narrow slopes length (See longshore drift direction varies with wave regime, although Stream) bench. Figs 6 & 7) in the long term may be more generally to the south