Estimating Natural Boundary Change on a Large Lake

Abstract

The natural boundary is intuitively understood to be the line dividing the bed of a water body from the adjacent land. It is used in many jurisdictions to define the boundary of ownership between a land parcel and a state-owned watercourse. It is typically defined on the ground through observations by a professional surveyor. Predicting changes to the natural boundary has obvious utility (e.g., with climate change); however, its’ observational nature does not lend itself to such predictions. This paper uses a case study of a large lake to explore the relationship between shoreline morphology and hydrology, and the elevation of the natural boundary. Significant correlation was found between the natural boundary elevation and average wave power, but not with variables beach slope, sediment type, or presence of vegetation. Below 0.4 W/m of average wave power, the correlation with natural boundary elevation is very weak, suggesting a lower limit of influence. Two process-based methods for estimation of natural boundary change are proposed, one based on the observed statistical relationship between the average wave power and the natural boundary elevation, and another method based on detailed analytical modelling of bottom stress from breaking waves using the Xbeach software (v1.23). Both methods were used to estimate natural boundary change under a hypothetical change in water level regime due to proposed weir upgrades. While observational data are not available to support evaluation, both approaches predict similar changes to the natural boundary despite their significantly different basis. While the modelling approach requires significant analyst and computational effort, the parametric approach is comparatively efficient, making it practical to apply at high resolution over very large shorelines.