Identifying attributable physical effects of contemporary climate change-driven sea-level rise on soft coastal landforms

Changing climate has always been a fundamental driver of sea-level change. Changes in climate over the nineteenth and twentieth centuries have produced increased global sea-level rise which is expected to lead to increased erosion and progressive recession of many soft erodible coasts. However most swell-exposed sandy beaches have not yet shown such a response because other confounding processes such as beach erosion and recovery cycles are still commonly of larger scale and prevent the emergence of a detectable sea-level rise signal in beach behaviour. This thesis tests the hypothesis that there may nonetheless be some susceptible coastal landform types that are already responding to contemporary climate change-driven sea-level rise with an observable change in behaviour, for example a switch from shoreline stability to progressive recession, or an increase of previous shoreline retreat. This study analyzed air photo and beach profile records for 35 coastal sites around Tasmania (Australia) over an approximately 70-year period to compile shoreline behaviour histories. Sites were selected from a range of geomorphic types hypothesised to be susceptible to early responses to sealevel rise. Twelve distinctive sites from four different coastal environments were analysed in some detail, with these results informing analysis of 23 other sites. For all 35 sites, a shoreline behaviour history was compiled using all suitable air photos to map shoreline position changes over time using the seaward vegetation line as a shoreline proxy. The sites include some that were known from previous work to have changed their behaviour during the air-photo period, and others whose histories were unknown at the outset. Sites influenced by local anthropogenic influences were mostly avoided, but in a few cases were used if the extent of artificial interference was clearly defined. Photogrammetric error margins were quantified for arguably the most important source of uncertainty, albeit more sophisticated uncertainty analyses are possible. Some confounding factors which might prevent an early sea-level rise signal being detectable in shoreline behaviour are minimal on the Tasmanian coast, including vertical land movement and interannual sea-level variability associated with the El Nino Southern Oscillation. Present day relative sealevel rise around Tasmania is commensurate with the global average, implying that contemporary climate change-induced sea-level rise is the dominant component of local relative sea level rise. Variability in swell wave direction is another potentially confounding factor that is also minimal on Tasmania’s western and southern coasts. Study sites included 18 swell-exposed sandy beaches such as Ocean, Hope and Roches Beaches. However in order to explore shoreline responses in the absence of the swell-driven sand transport and shoreline recovery processes that may prevent detectable early sea-level rise signals, study sites were also selected at 11 sandy and sandy-saltmarsh shorelines in swell-sheltered tidal re-entrants such as Duck Bay and Cloudy Lagoon, and at 6 sites on soft-rock (cohesive clay) shorelines at Rokeby Beach and the Barilla shore in Pittwater estuary. All the sites are readily erodible and are expected to eventually recede in response to climate change-induced sea-level rise. Of the 35 study sites, 10 exhibited a change of long-term shoreline behaviour over the air photo period. Sites showing a change from stability to long-term progressive recession include swellexposed Ocean and Roches Beaches, with Nebraska and Prion Beaches showing similar but more recently emerging changes. A significant long-term increase of prior recession was found at several swell-sheltered sites at West Duck Bay and a swell-sheltered soft-rock shoreline at Barilla (Pittwater). The geomorphic and oceanographic conditions at each site were identified to frame multiple hypotheses allowing assessment of whether an early response to rising sea-levels provides the best explanation for the observed changes, or whether other plausible explanations were available. Sea-level rise and increasing onshore wind speeds emerged as the only identified drivers able to account for the observed changes at 8 of the 10 sites showing significant change. Both drivers are expected to result in storm waves more frequently running further landwards over deeper water and impacting higher on the shore profile, leading to increased shoreline erosion and recession. Both processes could drive such change at six of these sites, whereas sea-level rise is the only plausible driver identified at two sites (Roches Beach and South Barilla). At the other two sites (Stephens Bay South and Gordon), variability in sand supply and artificial interference are more likely to have driven the observed changes. The 8 study sites exhibiting long term shoreline behaviour changes consistent with sea-level rise and/or increases in onshore wind speeds were in all cases characterised by (1) the presence of an active sediment sink capable of permanently sequestering increasing quantities of eroded sediment as shoreline erosion increases in response to more frequent higher wave attack, and (2) by a persistent (commonly unidirectional) sediment transport pathway capable of efficiently delivering eroded sediment to the sink with little or no return to the eroded shore. These two inter-related conditions exist for some but not all sandy swell-exposed beaches but are common (via differing mechanisms) for soft-rock coasts and for sandy shores within tidal swell-sheltered coastal re-entrants such as estuaries and lagoons. For swell-exposed sandy beaches where these critical conditions are present, the rapid permanent loss of eroded and mobile sand minimizes the effect of cross-shore and alongshore sediment exchanges that might otherwise prevent an early sea-level rise signal in shoreline behaviour, and instead allow a “tipping-point” style of switch from stability to recession to occur. A range of other site conditions may also contribute to changing shoreline behaviour in certain process environments but are not always associated with long-term changes. However, a relatively high degree of local wind-wave exposure and fetch were found to also be critical conditions for changing shoreline behaviour within swell-sheltered tidal re-entrants such as Duck Bay, Cloudy Lagoon and Pittwater. Of the 25 (out of 35) studied shorelines which have not yet shown any detectable long-term change of behaviour over the study period, 21 do not exhibit the critical conditions of having persistent sediment transport to sufficiently large active sediment sinks (e.g., Hope Beach, Cloudy Bay Beaches East and West), or in the case of swell-sheltered re-entrants these sites have only relatively limited wind-wave exposure and fetch. However, another four swell-exposed sandy beach sites did not exhibit any longterm behaviour change despite having the critical conditions of significant active sand sinks and persistent active sand transport pathways from the study shore to active sand sinks. In each of these cases, an actively gaining sand supply equal to or larger than the amount being permanently lost from increased erosion is inferred to be the key factor preventing an early change of behaviour by compensating for increasing sand losses attributable to climate change-induced drivers of shoreline change (e.g., at Mulcahy Bay Beach and Adventure Bay South Beach). The range of differing site conditions and differing historic shoreline behaviours investigated in this study support the expectation that some coastal landform types will respond earlier than others to climate change-induced drivers including contemporary sea-level rise and increasing onshore winds. This study identifies some critical conditions differentiating early from late responders to coastal climate change-driven processes. This will improve capacity to plan adaptation to coastal climate change impacts by identifying shores susceptible to earlier changes under a continuing warming climate