Sea Level Rise, Erosion & Sand Supply

Coastal spits, and island types such as sand cays, are examples of supply supported coast. These often have a level of retentive capacity, which may be developed by the sedimentary landform structure, or an underlying geological framework. Where supply exceeds this retentive capacity, supply supported landforms develop. They are often dynamic, reflecting fluctuations in supply, such as spit sequences.

Under conditions of sea level rise, both the retentive capacity and the sediment supply can reduce. Behaviour depends on which reduces more rapidly:

1. If retentive capacity reduces faster than supply (e.g. SLR reducing reef sheltering), then supply supported landforms will become increasingly dynamic. In the longer-term, if supply reduces, they may begin to assume similar behaviour to previous conditions, likely with smaller coastal structures.

2. If retentive capacity declines slower than supply, then behaviour is likely to appear as gradual retreat. However, when supply declines below the retentive capacity, there is a rapid change in coastal behaviour, as it switches to being a supply restricted coast. Initial development will typically involve downdrift erosion and often involves switching from near linear (or convex) coastal segments to arcuate coastal forms.

Conceptually, this requires a change in thinking, where bypassing of headland structures is a function of the retained beach volume, rather than a direct function of wave forcing.