Utilizing airborne LiDAR data to quantify marsh edge morphology and the role of oyster reefs in mitigating marsh erosion

Abstract

Marsh habitats, experiencing accelerated change, require accurate monitoring techniques. We developed methods to quantify marsh edge morphology using airborne LiDAR data. We then applied these methods within the context of oyster reef restoration within the shallow coastal bays of Virginia, USA, by comparing retreat and morphology quantified at paired reef-lined and control marsh edges at 10 different marsh sites. Retreat metrics were analyzed between 2002 and 2015, utilizing a LiDAR derived edge for the year 2015 from points of maximum slope and aerial imagery pre-2015. Retreat was also compared before and after oyster reef restoration to determine if reefs slow erosion. We found that slope statistics from airborne LiDAR elevation data can accurately capture marsh edge morphology. Retreat rate, measured at edges typically found near the vegetation line, was not significantly different between reef-lined and control marshes and ranged from 0.14 to 0.79 m yr-1. Both retreat rate (ρ = -0.90) and net movement (ρ = -0.88) were strongly correlated to marsh edge elevation. Exposed control marshes had significantly greater mean and maximum slope values compared to reef-lined marshes. The mean edge slope was 11.4° for exposed marshes and 6.0° for reef-lined marshes. We hypothesize that oyster reefs are causing an elongation of the marsh edge by reducing retreat at lower elevations of the marsh edge. Therefore, changes in marsh edge morphology may be a precursor to changes in marsh retreat rates over longer timescales and emphasizes the need for repeated LiDAR measurements to capture processes driving marsh edge dynamics.

Seasonality of Retreat Rate of a Wave‐Exposed Marsh Edge

“…Another technique for edge delineation when DSMs are available is identification of the maximum slope, as used in Hogan et al. (2021) and Farris et al. (2019).…”

“…In a system with ramplike morphology and no clear elevation distinction between marsh and mudflat, this histogram-based technique is likely to fail to identify a geomorphically relevant boundary elevation value. Another technique for edge delineation when DSMs are available is identification of the maximum slope, as used in Hogan et al (2021) and Farris et al (2019). We found this technique challenging to implement in our heterogeneous system where some portions of shoreline were notably steeper than others, some portions of the marsh had multiple steep scarps between terraces, and small clump features could be difficult to differentiate from the main scarp edge in their slope.…”