Sea Turtle Nest Inundation in Ras Baridi

Abstract

Sea turtles, with six of the seven species listed as endangered by the IUCN, face significant threats. Their eggs exhibit sensitivity to fluctuations in temperature, salinity, and moisture during their 6-8-week incubation period. Inundation of these eggs can substantially decrease their viability and influence sex ratios. This thesis centers on mitigating the wave runup-driven flooding of sea turtle nesting beaches. Mitigating nest flooding can involve relocating at-risk nests to higher elevations. This thesis aims to assist in responsible nesting relocation by providing flood risk information as a function of beach elevation. The study site, Ras Baridi, is a significant nesting site for green turtles (\textit{Chelonia mydas}) in the Red Sea. The research focuses on the development of a methodology for assessing flood risk in environments with limited data.

The risk of sea turtle nests to flooding at Ras Baridi was assessed by employing two different metamodels, BEWARE 2.0 and HyCReWW, that were recently developed to assess wave runup in coral reef environments at low computational costs. These models employ different strategies to model wave runup. BEWARE 2.0 uses real bathymetry, whereas HyCReWW schematizes the input bathymetry. 40-year hindcast datasets of waves and water levels from 1978-2018 were used in combination with 10 m horizontal resolution bathymetry data from the Allen Coral Atlas as model inputs. HyCReWW exhibited maximum runup at an 80 m reef width (0.95 m for 1 year return period (RP), 1.95 m for 40 year RP) BEWARE 2.0’s highest runup was at an 80 m reef width (1.35 m for 1 year RP, 2.7 m for 40 year RP). Median inundation durations from BEWARE 2.0 and HyCReWW were 4 and 7 hours, respectively. Beach elevations associated with 5-year return periods were identified as suitable minimum nesting elevations to mitigate inundation risk.

BEWARE 2.0 and HyCReWW results for Ras Baridi lacked direct real-world validation, prompting a methodological validation approach. Six validated XBeach 1D NH models in fringing reef environments globally were utilized for comparison against BEWARE 2.0 and HyCReWW. The comparison revealed increased median scatter indices and root-mean-square (RMS) errors for both BEWARE 2.0 (0.07 and 0.1 m increases) and HyCReWW (0.1 and 0.15 m increases) with low-resolution bathymetry (10 m horizontal). BEWARE 2.0 had lower median RMS errors and scatter indices for high-resolution (2 m horizontal) and low-resolution bathymetry than HyCReWW, as well as smaller spreads. Consequently, utilizing BEWARE 2.0 results is recommended for Ras Baridi and similar data-scarce coastal environments over HyCReWW.

In summary, this thesis addresses the pressing issue of sea turtle nest flooding, providing a methodology to assist in responsible nest relocation in low data environments. The 5-year return period runup elevation of the BEWARE 2.0 results (1.25-1.75 m along the beach) has been identified as a suitable minimum nesting elevation in Ras Baridi, and the likelihood of flooding as a function of beach elevation has been provided. These results can aid coastal managers in making informed decisions for the protection of these endangered species.