Wave-induced horizontal erosion at the edges of salt marshes constitutes a significant mechanism contributing to the rapid reduction in coastal salt marsh areas. This mechanism is notably characterized by the alternation between continuous minor erosions (such as the detachment of small soil clumps or the loss of sediment particles) and occasional mass failures (erosion at the base of steep edges leading to cantilevered failures). Traditionally, the understanding of this complex mechanism has been limited due to the absence of techniques capable of high-frequency, continuous observations. To overcome this challenge, we have developed the Acoustic Marsh Edge Monitor (AMEM), an instrument that utilizes acoustic echoes to measure the distance from the probe to the marsh edge, significantly minimizing external environmental interference. It is capable of automatically conducting observations during low tide periods, with an effective measurement range up to 4 meters and a resolution reaching the millimeter scale. Moreover, the instrument adopts a modular design philosophy, allowing for flexible adjustments in observation height and sensor configuration in response to actual changes at the marsh edge. In the Jiangsu coast, China, we conducted several rounds of observational experiments, each lasting 15 to 60 days, respectively, with synchronous observations of wave activity. The accuracy of the instrument was validated through comparison with unmanned aerial vehicle (UAV) photogrammetry and traditional manual measurement methods. The observational results revealed: (1) Mass failures exceeding 1 meter occurred at the marsh edge during the observation period, with significant differences in the phenomena of mass failures between the two observations; (2) A certain temporal lag exists between mass failures and strong wave activities; (3) Multiple minor erosion events had occurred prior to the occurrence of mass failures. These findings provide us with a deeper understanding of the horizontal erosion mechanism in coastal salt marshes.

marsh edge cliff, erosion, Acoustic Marsh Edge Monitor