4.1.2 Ice Thickness and Headwall Retreat
IT values in 2016 and 2017, and the mean IT across all three years, produced weak and variable correlations with HWR rates. However, IT in 2018 produced significant positive correlations with HWR across all timespans, with an overall correlation coefficient of +0.47 between the 2018 IT and the 2016 to 2018 HWR rates (p < 0.05). However, no statistically significant differences were noted on the influence of the IT categories in 2018 (no ice, 0-2 m, 2-4 m) on the mean HWR rates (Figure 5a).
In the western section of the headwall the MI was partially covered in 2016 (Figure 6a), but exposure between 2016 and 2017 resulted HWR rates faster than the site average. In combination with the significant relationship between the presence of exposed headwall ice in 2018 and the average HWR rate, this suggests that the presence of MI inland of the headwall, regardless of its absolute thickness, is an important factor controlling HWR rates.
4.1.3 Overburden Thickness and Headwall Retreat
The thickness of the exposed overburden in all years produced statistically significant negative correlations with HWR rates, the strongest being between the average OT across all three years and the average HWR from 2016 to 2018 (-0.52). Examining the mean OT across all three years, the influence over HWR rates can be split into three broad categories; below 2 m, 2 to 4 m and above 4 m. An average OT of 0-2 m produces an average HWR of 27.3 m a-1, 2-4 m OT averages 14.8 m a-1, while all higher categories average between 7.7 and 9.0 m a-1 (Figure 5b and Figure 6b). This suggests maintaining an OT of below 4 m tends to produce substantially faster HWR rates than otherwise.
4.1.4 Proportion of Ice and Headwall Retreat
As with the IT, the PI in 2016 had no significant influence over the subsequent HWR rates. Statistically significant correlations were present between the PI in 2017 and HWR rates, but the strongest correlations occurred with the 2018 PI values. The correlation between the 2018 PI and the 2016 to 2018 HWR rate is +0.58 (p < 0.05) and with the 2017 to 2018 HWR rate is +0.59. However, as with the IT metric, the correlation is non-linear, influenced heavily by the low HWR associated with the 0% PI category, while any PI above 0%, regardless of their values, are associated with faster rates of HWR. This further supports the idea that the presence of ice inland of the current headwall position, rather than its thickness or headwall proportion, is associated with enhanced rates of HWR (Figure 5c and Figure 6c).