The pattern is more complex for wave-orbital velocity. SLR reduces orbital velocity on narrow reef flats but increases the velocity once the reef flat width exceeds around 500 m (Fig. 4). The depth of the reef flat has a non-linear impact on orbital velocity such that velocities typically increase to a maximum at a depth of around 1–1.5 m, and then decline as depth increases. However, under the highest SLR scenario considered (SLR = 1 m), orbital velocity follows a monotonic decline as reef flat depth increases. The influence of SLR is significant, leading to increases in wave height of order 10–15% for SLR = 0.25 m and to increases of order 50–75% for SLR at the upper bound of current projections for 2100 (Fig. 3). These changes in wave height are similar to those found by Storlazzi et al. (2011) and Grady et al. (2013) for fringing reefs. Changes in the orbital velocity are likewise similar or even greater. This is discussed further in the next sub-section.
SLR also appears to provide the potential for significant shifts in wave period (not shown) which also influence the changes in orbital velocities and have a significant effect on wave forces on corals, as discussed later. The wave period plays an important role in the generation of wave forces on corals and on sediment transport on reef flats and at the shore. The wave period is relatively constant on the fore-reef under SLR, but increases at nearly all other locations for nearly all reef bathymetries under both average and cyclonic conditions. The greatest increases in wave period occur on the shallower top reefs.
Increased lagoon width clearly increases nearshore wave height because of a greater fetch length, and again significant changes occur with SLR due to the increased water depth over the reef (Fig. 5), consistent with previous modeling (e.g. Sheppard et al., 2005). Changes in lagoon width also induce subtle variations in wave period over and above those induced directly by SLR (not shown). SLR enables longer period waves to propagate over the reef; however, the peak wave period may then decreases as the lagoon fetch increases and new shorter wind waves are generated. Underlying this change is a gradual increase in the period of locally generated waves in the lagoon as lagoon width increases. The impact of these changes in wave conditions for the stability of reef island beaches will be considered in a later paper.