Which comes first in the U.S. Arctic – the tidal datum or the shoreline position?

Abstract:

The U.S. arctic coast extends more than 50,000 km (tidally-influenced, 1:63,360 scale) along the Arctic Ocean (1 active tide station), Beaufort, Bearing and Chukchi Seas (3 stations), and includes the Aleutian Island Chain (5 stations). The best available vector that defines this regulatory, ecological and navigational boundary is a compilation of Mean High Water (MHW) features that have been visually interpreted from satellite or aerial imagery. Despite documented rates of rapid shoreline change in the arctic, the vast linear extent, remoteness, and limited ice-free season create unique challenges in maintaining an updated shoreline vector for the Alaska coast; this is compounded by a lack of high resolution digital elevation models (DEMs) and topography of the sea surface grids to conduct datum-derived shoreline mapping using elevation-intercept techniques widely employed elsewhere. Best-available shoreline positions in arctic Alaska span a wide temporal range with 32% dating from before statehood (1959), 33% from 1960-2010, 16% of unknown age, and only 19% of the total extent has been mapped since 2010. We present a hybrid approach to update shoreline vectors that uses co-registered orthoimagery and DEMs to obtain a local MHW tidal datum approximation by sampling the average elevation along a manually-digitized High Water Line (HWL) segment and applying appropriate corrections for beach slope and local wave climate. This elevation is used to conduct an automated, elevation-based shoreline extraction in the immediate vicinity of the sample and the process is iteratively repeated in segments along the coast. Preliminary results suggest that shoreline vectors derived in this manner are comparable to existing, contemporary MHW vectors (< 4 m horizontal offset in low-grade coastal environments); this technique can accelerate map updates and also produces a more repeatable shoreline position in data-sparse regions that are undergoing rapid change. Furthermore, by exploiting known relationships between HWL and the MHW datum (proxy-datum bias corrections), we can provide tidal to geodetic datum conversion approximations to areas without water level records for use in preliminary inundation modeling and coastal flood decision support.