Abstract. The accurate quantification of sediment mass redistribution is central to the study of surface processes, yet it remains a challenging task. Here we
test a new combination of terrestrial gravity and drone photogrammetry methods to quantify sediment mass redistribution over a 1 km2
area. Gravity and photogrammetry are complementary methods. Indeed, gravity changes are sensitive to mass changes and to their location. Thus, by
using photogrammetry data to constrain this location, the sediment mass can be properly estimated from the gravity data. We carried out three joint
gravimetry–photogrammetry surveys, once a year in 2015, 2016 and 2017, over a 1 km2 area in southern Taiwan, featuring both a wide meander of
the Laonong River and a slow landslide. We first removed the gravity changes from non-sediment effects, such as tides, groundwater, surface
displacements and air pressure variations. Then, we inverted the density of the sediment with an attempt to distinguish the density of the
landslide from the density of the river sediments. We eventually estimate an average loss of 3.7 ± 0.4 × 109 kg of
sediment from 2015 to 2017 mostly due to the slow landslide. Although the gravity devices used in this study are expensive and need week-long
surveys, new instrumentation currently being developed will enable dense and continuous measurements at lower cost, making the method that has been
developed and tested in this study well-suited for the estimation of erosion, sediment transfer and deposition in landscapes.
Quantifying sediment mass redistribution from joint time-lapse gravimetry and photogrammetry surveys
