A Generic Coastal Erosion Hazard Zoning for Tasmania. Chris Sharples, Hannah Walford & Luke Roberts April 2014

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1 A Generic Coastal Erosion Hazard Zoning for Tasmania Chris Sharples, Hannah Walford & Luke Roberts April 2014

2 Why do we need comprehensive coastal erosion hazard zones for Tasmania? Roches Beach 2011

3 Why do we need comprehensive coastal erosion hazard zones for Tasmania? The need for a coastal erosion hazard zoning arises because: 1. Sea-level is rising (causes erosion) 2. More people are building more infrastructure on the coast (exposes more assets to erosion) Hence the coast is becoming more at risk of erosion at the same time as more people are wanting to live there. Coastal erosion hazard banding is one of a series of hazard zoning projects for Tasmania being undertaken by Department of Premier & Cabinet (landslides, coastal flooding hazards, etc).

4 Options for assessing coastal erosion hazards for developments & planning 1. No hazard assessments required, hope for the best (the old way, no longer acceptable given current pressures on coast). 2. Detailed site-specific hazard assessment for all coastal development proposals as they arise; using site specific data and modelling Sbeach, Xbeach, Bruun, Shoreface Translation Model, etc (expensive, reliability questionable for many sites, may be onerous for proponents, no strategic overview of coastal hazards for planners). 3. Provide a conservative First Pass hazard zoning for whole coast based on generic setback allowances: developers can either comply, or fund detailed site-specific assessment themselves (the approach taken for DPAC: provides strategic overview of coastal hazards spatial variation, etc)

Generic erosion hazard bands Aim of the zoning scheme is to zone the coast into four simple erosion hazard bands: 1. Acceptable: negligible hazard expected before 2100 2.

5 Generic erosion hazard bands Aim of the zoning scheme is to zone the coast into four simple erosion hazard bands: 1. Acceptable: negligible hazard expected before Low: minor and infrequent damage may occur before 2100 only basic measures needed to limit impact to tolerable levels 3. Medium: significant damage likely to occur before 2100 substantial measures must be applied to manage impacts 4. High: repeated significant damage expected before 2100 development should generally be prohibited

Coastal substrate types in Tasmania However

substrates, which erode in different ways at

erosion susceptibility zones that easily

For Tasmania, we grouped coastal landform

different erosional responses to coastal

6 Coastal substrate types in Tasmania However there are a wide range of erodible coastal substrates, which erode in different ways at different rates; cant define a single set of erosion susceptibility zones that easily applies to all these types. For Tasmania, we grouped coastal landform substrates into four types with notably different erosional responses to coastal processes: Soft sediment shores (sand, mud, etc); Soft rock shores (semi-lithified sediments or deeply weathered bedrock); Hard rock shores (sloping rocky coasts, hard sea-cliffs). Artificial

7 Procedure Define high-med-low-acceptable hazard zones for each coastal substrate group (each with sub-types) using methods appropriate to substrate type. This produces a significant number of differentlydefined coastal erosion hazard zones (total 27 zones considering sub-types) These zones are then compared in a pairwise fashion to derive a final grouping of all 27 hazard zones into four final hazard bands

sub-types: behave differently; Sand is highly mobile, eroded shores may recover (accrete); But

8 Soft sediment coasts Old erosion scarp Loose sediment, easily eroded and deposited; Mainly sandy beaches, but also some muddy estuarine shores and cobble beaches; Open coast and sheltered sub-types: behave differently; Sand is highly mobile, eroded shores may recover (accrete); But increasing prevalence of progressive recession (nonrecovering erosion) expected with ongoing sea-level rise;

9 Natural recession limits Relevant to softsediment shores only Recognises that soft sediment shores will only erode landwards to the point at which underlying bedrock rises above sea-level May reduce hazard area in many cases where soft sediment shores have narrow recession limits

10 Soft sediment coasts available mapping Mapping datasets used: Line map of coastal landform types (Smartline); complete for whole Tasmanian coast (Updated version of original 2006 Tasmanian coastal landform map) Soft sediment polygon mapping (complete for whole Tas coast, reliability varies) (Based on Geological Survey mapping (MRT) plus additional mapping and inferences by C. Sharples & H. Walford: Geological Survey maps omit some sediments to show bedrock beneath); Inferred natural recession limits mapping (complete for whole Tas coast, reliability varies) (prepared by Chris Sharples, Paul Donaldson, Hannah Walford)

Generic open coast sandy shore erosion hazard setbacks (zones) Standard coastal erosion hazard modelling techniques (as used for Clarence, widely

al. 2012) Calculated components for: S1: Short term storm bite (using Sbeach, Xbeach) S3: Allowance for recession due to future sea-level rise (Bruun

11 Generic open coast sandy shore erosion hazard setbacks (zones) Standard coastal erosion hazard modelling techniques (as used for Clarence, widely used in NSW & Queensland) Generic hazard zones for Australian coastal regions developed by Water Research Laboratory (UNSW) for ACE-CRC (Mariani et al. 2012) Calculated components for: S1: Short term storm bite (using Sbeach, Xbeach) S3: Allowance for recession due to future sea-level rise (Bruun Rule: yielding Bruun Factor ) S5: Allowance for dune scarp slumping (Nielsen method) S2 underlying recession & S4 Beach rotation ignored for generic zones

12 Open coast (swell-exposed) sandy beaches - erosion susceptibility zones WRL defined four different hydraulic zones around Tas coast: modelled differing generic setbacks for each based on a generic beach wave climate, beach slope and sand characteristics for each hydraulic zone; Then we used natural recession limits mapping used to truncate generic hazard zones to limits of actual potentially erodible areas: Near term erosion susceptibility zone: Modelled 2 x 1:100 year storm bites for generic beach plus dune instability allowance Recession to 2050 susceptibility zone: Bruun Rule recession modelled to 2050 for generic beach Recession to 2100 susceptibility zone: Bruun Rule recession modelled to 2100 for generic beach Acceptable zone: Landwards of 2100 zone

13 Open coast (swell-exposed) sandy beaches - erosion hazard zones: Mapped as zones landwards of HWM using soft sediment polygon mapping; truncated to landwards using natural recession limits mapping.

14 Sandy beach hazard zones - example

15 Soft rock coasts Margate Taroona Cohesive materials, but not hard bedrock, usually clayey; Erosion slower than for soft sediments, but no recovery:- soft rock shores recede persistently; In Tasmania, mainly Tertiary-age sedimentary rocks Taroona

Soft rock coasts available mapping Mapping datasets used: Polygon mapping based on Geological Survey mapping (MRT); plus: Soft rock under thin superficial sediments

16 Soft rock coasts available mapping Mapping datasets used: Polygon mapping based on Geological Survey mapping (MRT); plus: Soft rock under thin superficial sediments interpreted by C. Sharples & Hannah Walford based on known geological structures and gravity data provided by MRT; Interpreted mapping provided as single standalone soft rock GIS layer.

(limited data available from historic to recent air photos): 0.35m/yr maximum observed recession rate (Barilla Bay 1946-2010 0.

17 Deriving generic erosion hazard zones (setbacks) for soft rock coasts No existing generic setbacks available; We used: Barilla Bay shoreline recession (scarp positions) from air photos Active erosion Rokeby Beach Observed soft rock shore recession data (limited data available from historic to recent air photos): 0.35m/yr maximum observed recession rate (Barilla Bay m/yr minimum observed recession rate (Rokeby Beach ) Plus x 2 allowance for acceleration with sea-level rise (based on modelling by Trenhaile 2011)

18 Soft rock coasts erosion hazard zones Mapped as zones landwards of HWM using soft rock polygon mapping. Near term erosion susceptibility zone: Maximum observed (historic) recession rates extrapolated to 2030 x 2 conservative allowance for acceleration with sea-level (storm bite not appropriate short-term hazard measure for soft rock) Recession to 2050 susceptibility zone: Maximum observed (historic) recession rates extrapolated to 2050 x 2 conservative allowance for acceleration with sea-level Recession to 2100 susceptibility zone: Maximum observed (historic) recession rates extrapolated to 2100 x 2 conservative allowance for acceleration with sea-level

However hard rock cliffs are more prone to instability

19 Hard rock coasts Generally the most resilient coastal type erosion rates mainly negligible on human time scales; However hard rock cliffs are more prone to instability (that s why they are cliffs) may show slumping etc on human time scales

20 Hard rock coasts available mapping Mapping datasets used: Line maps used landwards extent of substrate not relevant; Mapping used: Smartline coastal geomorphic line map includes sloping and cliffed hard rock shores based on geological maps, air photo interpretation and significant groundtruthing by C. Sharples State-wide coverage

21 Hard rock coasts mapped sub-types Three hard rock shore types recognised for susceptibility assessment: - Gently to moderately sloping hard rock shores and backshores - Soft sediment shores backed by bedrock above sea-level with some soft sediment over bedrock - Steeply sloping and cliffed hard rock shores

Hard rock erosion hazard zones Gently to

22 Hard rock erosion hazard zones Gently to moderately sloping hard rock shores All considered acceptable (no significant hazard) Hard rock cliffs Generic 50m hazard setback applied in lieu of modelling (future work requires full coastal LIDAR) Coastal landslide regression modelling method developed by Colin Mazengarb (Mineral Resources Tasmania)

well-designed and robustly-constructed coastal erosion protection

23 Artificially protected or modified coasts Many are constructed as quick and dirty solutions and fail quickly in storms; However well-designed and robustly-constructed coastal erosion protection (walls, boulder revetments) may work well; Nearly always some ongoing maintenance costs

24 Artificially protected or modified coasts Mapping datasets used: Line maps used (generally narrow linear features) Mapping used: Smartline coastal geomorphic line map includes artificial shores from air photo interp. & some ground mapping; Incomplete but no better state-wide dataset

25 Artificially protected or modified coasts: erosion susceptibility (hazard) zones If artificial shores considered resilient: Considered Low hazard close to shore (failure possible but unlikely); acceptable to landwards If artificial shores not considered resilient: Zoned as per natural substrate type without protection

26 Pairwise assessment of all (27) defined erosion hazard zones Each pair scored according to Which of these is more susceptible to erosion more immediately?

27 Pairwise score ranking yields final hazard band definition

28 Final hazard bands combine different shoreline types according to magnitude (physical scale) and likelihood (time frame) of potential hazard; e.g., High Hazard bands all = soft sediment storm bite (near term likelihood) erosion zones; Medium Hazard bands = longer term recession zones for soft sediment + near to medium term soft rock recession zones

29 Caveats Hazard banding subject to errors or incomplete data in underlying mapping; we identified about 10% of hazard banding as having known deficiencies in underlying data, may be more (ongoing data maintenance is the answer) Regardless of data errors, definition of hazard zones & thus hazard bands remains very generic and First Pass To allow for these uncertainties we have tried to be fairly conservative: hence use of system is intended to be: Either comply with zoning (cheap conservative option); or Undertake detailed hazard assessment at proponents cost if wishing to challenge hazard banding

30 Thank you

A Detailed First Pass coastal hazard assessment for a long complex coast: Kingborough LGA, Tasmania

A Detailed First Pass coastal hazard assessment for a long complex coast: Kingborough LGA, Tasmania Chris Sharples & Paul Donaldson University of Tasmania September 2012 Kingborough LGA, south-eastern