Modelling Hydrodynamic and Groundwater Processes on a Sea Turtle Nesting Beach

Abstract

Galveston Island is crucial for sea turtle nesting but faces erosion and increasingwater levels, limiting nesting opportunities and posing dangers to nests in flood-prone areas. A numerical model was developed to estimate groundwater levels and hydrodynamics at specific locations on two onedimensional transects, located at the Tipsy Turtle and the Sea Wall. The model was used to assess the risk of turtle nest inundation at six fictitious nest locations and compared to a previous method used in a study by Ware et al. (2019), which used the 𝑅2% formulation developed by Stockdon et al. (2006).
The model was validated using data from a two-month field campaign in Galveston, Texas. Two five-day periods were simulated to represent different tidal and wind conditions. The model was also simulated for five different hydraulic conductivities.
In the nearshore, hydraulic conductivity did not influence the results. Although mean surface
elevations were captured well, the infragravity and incident wave heights were exaggerated in most cases, possibly due to wave spreading not being accounted for in 1D models and, therefore, concentrating energy transfer of incident waves to the infragravity waves.
The model demonstrated that small hydraulic conductivities performed better in estimating
groundwater levels in calmer conditions when the mean sea level was considerably lower than
the groundwater level, as in the second period. The RMSE close to the dune foot was 0.075 m for 𝐾 = 0.0001 m/s and 0.4 m for 𝐾 = 0.001 m/s in period 2 close to the dune foot at the Sea Wall. In contrast, the intermediate and higher hydraulic conductivities showed minor errors in more dynamic conditions close to the shoreline in period 1. The RMSE at TT4 (approximately halfway at the Tipsy Turtle) during period 1 was 0.164 m for 𝐾 = 0.001 m/s and 0.239 m for 𝐾 = 0.0001 m/s. In all cases, the best hydraulic conductivity decreased in landward direction.
Not a single hydraulic conductivity performed well in all conditions. Therefore, the results in a
test case assessing turtle nest inundation also varied. In most cases, the model overestimated turtle nest inundation, while the method by Ware et al. (2019) underestimated in all but one case. The run-up estimations of Stockdon et al. (2006) as used by Ware et al. (2019) were sensitive to changes in bed slope, with a steeper slope of 0.05 aligning better with modelled run-up during high tide in period 1 and a flatter slope of 0.01 in period 2. A slope of 0.0316 aligned mainly with the modelled run-up in intermediate conditions in period 1.
This report demonstrated that not one of the cases simulated is reliable enough for assessing turtle nest flooding accurately in all hydrodynamic conditions and is too conservative in most, but not all, cases. Therefore, the current model cannot be used as an alternative yet. Moreover, it concluded that the method using the empirical run-up formulation as in Ware et al. (2019) should also be used cautiously due to consistent underestimations in this experiment.