Using oblique imagery to measure hypsometric changes in sandbar volume following controlled floods in the Grand Canyon

Measuring changes in the elevation distribution of sub-aerial fine (< 2 mm ) sediment and estimating sandbar volume multiple times per year can improve sediment budget calculations in fluvial systems. In the Grand Canyon of the Colorado River, effects of dam operations on sandbar size and distribution is of long-term management interest. Bar-building controlled floods have been implemented in 1996, 2004, 2008, 2012, 2013, 2014, 2016, and 2018 to mitigate sandbar erosion. Annual topographic surveys provide a single measurement of sandbar change caused by the integrated effects of all flows in one year (both controlled floods and normal dam releases), but do not measure erosion and deposition caused by specific operations or individual floods. On one sandbar monitoring site in Grand Canyon, we demonstrate that imagery from autonomous digital cameras can be used to provide quantitative measures of sandbar hypsometry several times per year without costly and labor-intensive surveys. We describe methods for measuring changes in the storage of fine sediment at monthly or seasonal timescales by constructing hypsometric (area-elevation relation) curves. These curves are created and updated with sandbar area measurements from georectified images taken multiple times each day. As the water surface elevation fluctuates with daily, seasonal, and monthly discharge patterns, sandbar area and volume can be estimated using known stage-discharge relationships. We present parameters extracted from image-derived hypsometries to estimate sandbar volume and elevation relief ratio, which provides a new way to quantitatively measure monthly or seasonal changes in fine sediment storage.