PHOTOGRAPHIC MEASUREMENT OF SAND MOVEMENT IN GIGAPIXEL PANORAMAS REVEALS SHORT TERM VARIABILITY IN GEOMORPHOLOGICALLY SIMILAR LAKE MICHIGAN DUNES

We measured sand movement in open-sand environments exposed on the interiors of parabolic dunes at two sites along the eastern shoreline of Lake Michigan, at Saugatuck Harbor Natural Area (SHNA) in Saugatuck, Michigan and the Kitchel Lindquist Hartger Dunes (KLH) in Grand Haven, Michigan. Both sites are isolated from the Lake Michigan shoreline. The SHNA site is a blowout at the nose of a stabilized, dissected, 11.5 m high parabolic dune located behind an established dune ridge, 360 m from the beach. The KLH site is the northern of two adjacent, 14 m high parabolic dunes 400 m from the beach, with an intervening row of houses and roadway. The SHNA dune axis has a bearing of 75°, while the dune axis at KLH has a bearing of 70°.

We used web-hosted gigapixel panoramas of digital images to record and measure sand surface elevations relative to dune pins designed for photographic measurement (photopins). Photopins were installed at SHNA in July 2012 and were in place at both sites from June 2013 through June 2014. Error analysis based on geometric principles constrains the maximum error attributable to parallax when measuring on-screen images of our photopins in a panorama. Parallax errors can be non-trivial when measuring photopins photographed at close range, but maximum error drops to less than 2 percent for photos taken at 3 or more meters, regardless of the angle of the pin relative to the line of sight or its elevation above or below the camera.

SHNA saw an average sand loss of 6.76 cm over the two year period, but sand movement was variable, with an average net loss of 13.46 cm in 2012-13 partially offset by a net gain of 6.83 cm in 2013-14, recorded at 103 photopins. In contrast, from June 2013 to June 2014 the KLH site saw a net sand loss of 18.80 cm as measured at 39 photopins. The sites are 40 km apart so they see similar large-scale storm patterns but are affected by different smaller-scale fluctuations in wind direction and strength. Even within a single site, sand movement is patchy, with accumulation and erosion occurring within a few meters. The contemporaneous observation of net sand accumulation at SHNA but sand erosion at KLH during 2013-14 shows that sand movement at similar dune sites likely responds to highly localized events and conditions, the time-averaging of which produces broad regional similarities in dune morphology.