Modelling of vegetation change in response to sea level rise using SLAMM was deemed unsuccessful in this study. Model performance improvements were
attempted as suggested by the technical documentation, using high quality elevation data and including as many site-specific parameters as possible (Clough et al. 2012), and as suggested by the literature, such as aggregating wetland vegetation categories (Wu et al. 2015), in this study mixed and mangrove vegetation combined into the same SLAMM vegetation category (Table 6.6). While landward movement of vegetation was predicted by the model as expected, the model did not reflect recent observations of vegetation change, such as the invasion of saltmarsh by mangroves at the mangrove-saltmarsh boundary in the northern area of the Badu Wetlands. The primary factor determining vegetation change within SLAMM is elevation relative to the tidal frame, vegetation category changed by the model when the elevation of a cell falls outside the elevation range of the input vegetation category assigned by the user, the vegetation category the cell is converted then determined by SLAMM. (Clough et al. 2012). For example, when irregularly flooded marsh is inundated it is converted by SLAMM to regularly flooded marsh (Clough et al. 2012). However, at Homebush Bay there is significant elevation overlap between vegetation categories (Table 6.3), exaggerated within SLAMM by the lowering of saltmarsh elevation
ranges in the Waterbird Refuge due to the artificial tide regime determined by SOPA, tidal regime variable across the landscape, but not modelled as such in SLAMM. Expected vegetation change was likely not observed as expected in SLAMM as elevation did not change relative to sea level enough that the elevation fell outside the elevation range of the input vegetation category. While the Waterbird Refuge could have been excluded from modelling by clipping northern area including the Waterbird Refuge and open water from the input raster files, doing so would also have clipped some of the northern areas of the Badu Wetlands.
SLAMM also assumes static wetland surfaces, surface elevation change for each wetland habitat remaining constant through time, when this is unlikely to be the case under future sea-level rise. In the model, wetland surfaces are assumed to be static, with no morphological change in conjunction with sea-level rise (Akumu et al. 2011; Clough et al. 2012; Traill et al. 2011). In reality, inundation by sea-level rise would
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likely increase sediment accretion by stimulating biomass production and allowing more sediments to be trapped by vegetation (Morris et al. 2002).
On a more regional level, it has also been noted that SLAMM lacks mechanisms to account for some site and regionally specific factors. For example, the model has no mechanism to account for damage to vegetation caused by disturbances such as storms (Glick et al. 2013). Wetlands modelled in Louisiana were regularly subject to hurricanes, which are expected to increase in intensity over time due to climate change, the effects of large hurricanes on the wetland present a ‘wildcard’ factor with the potential to affect future wetland vegetation configuration (Karl et al. 2009). While tropical hurricanes are not a concern at the Badu Wetlands, this example
demonstrates the difficulty of modelling wetlands worldwide which are subject to spatially variable influences on vegetation.
However, SLAMM will likely become suitable for use at Homebush Bay in the future as the model is improved, with a greater range of user defined parameters, and further refinement of the processes modelled. For example, the implementation of modification of elevation ranges and other parameters on a sub-site basis, spatial variation on a sub-site scale could be more completely accounted for in the model. These model improvements would likely allow for SLAMM to be used at sites such as Homebush Bay, where elevation ranges are far from discrete.
Though SLAMM was unsuccessful in modelling the likely impacts of sea level rise on vegetation at Homebush Bay, future sea level rise and resulting impacts are likely to occur, and may require management attention. Even though surface elevation currently exceeds localised sea level rise for mangrove and mixed vegetation at Homebush Bay (Table 6.5), global rates of sea level rise are predicted to far exceed current rates of surface elevation change (IPCC 2015), the wetlands unlikely to increase their surface elevation rates to match future rates of sea level rise and maintain their current extent and distribution. Recent estimates of mangrove
inundation based on tidal range, SET-MH measurements and sediment availability, including measurements from Homebush Bay, placed the inundation of
Southeastern Australian mangrove forests assuming that landward migration is not possible, as likely to occur sometime between 2070 and 2075, based on a sea level
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rise of 1.4 m by 2100, and sometime after 2100, based on the IPCC RCP6 prediction of 0.48 m by 2100 (Lovelock et al. 2007). Though this study estimates that
mangroves will not be inundated for over 50 years, and as saltmarsh is located high in the tidal frame (Table 6.3) and therefore of low vulnerability while accretion and surface elevation rates and local sea levels remain stable, impacts on wetland vegetation are likely to emerge far earlier. For example, mangrove invasion of saltmarsh is already well documented in Southeastern Australia (Saintilan & Rogers 2013), at the Badu Wetland addressed by a mangrove seedling removal program (Pacific Wetlands 2014), and will become more susceptible to colonisation by mangroves in areas where mangroves were previously excluded by low inundation and high salinity (Oliver et al. 2012).
If sea level rise and vegetation change were not responded to by SOPA vegetation would attempt landward migration, but would succumb to coastal squeeze due to the surrounding roads and Bennelong Pond, which would prevent migration of
vegetation. Even if roads and paths were removed to allow vegetation migration within Bicentennial Park, wetland vegetation migration might be prevented by steep slopes to the south and east of the Badu Wetlands (Figure 6.8), and the original amenity of grassy parklands invaded by wetland vegetation lost. Combined with the economic cost of infrastructure removal and relocation, allowing the vegetation to migrate is not likely to be the most advantageous solution.
Instead, if SOPA is to maintain the current elevation of the Badu Wetland relative to sea level, and therefore the current distribution and extent of vegetation, surface elevation change rates must be equal to or greater than future rates of sea level rise, and the rise of the tidal frame. The hydrology of the Badu Wetlands, including the tidal frame, may be artificially controlled through the use of a SlipGate, as has been done successfully at the Waterbird Refuge (Paul 2009). Adjustment of the wetlands through sediment accretion could also be assisted through sediment nourishment, has been done internationally at larger scales (Khalil et al. 2010). Sediment
previously extracted from channels could be used for this nourishment project, the on-site location likely substantially reducing the cost of acquisition of suitable sediment (Khalil & Finkl 2009; Khalil et al. 2010). However, the increased sediment load would likely increase the sedimentation rates of the channels in the Badu
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Wetlands, sedimentation of excavated channels previously identified as a management issue (Rodgers et al. 2013)
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