Effect of Hurricane Wrack Deposition on Coastal Marsh Surface Elevation Change

Science Center Objects

Storm surge waves and tides of hurricanes have the propensity to wash up marsh detritus of dead reeds and leaf debris along with plastic trash and lumber, commonly referred to as wrack, from overwashed beaches, marshes, forests, streets, and lawns.

Polyurethane tarps covered a subset of SETs
Polyurethane tarps covered a subset of Surface Evaluation Tables (SETs)

The Science Issue and Relevance: Storm surge waves and tides of hurricanes have the propensity to wash up marsh detritus of dead reeds and leaf debris along with plastic trash and lumber, commonly referred to as wrack, from overwashed beaches, marshes, forests, streets, and lawns. As storm tides recede, wrack debris settles on high ground and marsh settings smothering vegetation and ground surface. These residual piles of wrack are generally distributed patches of varying area, shape, composition and thickness. Little is known of wrack persistence and breakdown over time after storm and their effects on surface elevation change and subsequent vegetation recovery and succession. It is postulated that interior marsh ponds may have their origins tied to wrack deposition and marsh dieback of previous storm events. To test this hypothesis, a manipulative field study was conducted with the benefit of a legacy experimental project of block design including multiple surface elevation tables (SET) in a coastal saltmarsh at McFaddin National Wildlife Refuge in southeast Texas. 

Methodology for Addressing the Issue: To mimic Hurricane Sandy wrack piles of Northeast marsh settings, an experimental field study was conducted to cover a subset of SETs and live marsh settings with polyurethane tarps to flatten live stems and block sunlight. Resurveys of surface elevations of a dozen SET stations were accomplished prior to treatment at two seasonal intervals, summer and fall 2013. Real-time kinematic GPS surveys combined with complimentary ground leveling was accomplished to create a digital elevation model of the area land surface and to rectify referenced datum comparison of SET measurements. Polyurethane tarps covered a subset of SETs and the ground surface resurveyed in fall 2015, allowing 2 years of surface adjustment and biotic response. Treated sites showed degrees of vegetation loss and species change. Surface elevations of treated SET stations generally decreased relative to untreated stations with some variance from root ingrowth of newly colonizing plants and species more as an adaptation response of plant physiology than physical surface amendment or deposition.

Real-time kinematic GPS surveys combined with complimentary ground leveling
Real-time kinematic GPS surveys combined with complimentary ground leveling

Future Steps: An understanding of the interaction of wrack deposits on marsh dieback and surface elevation change may explain plant community patchwork and species diversity, or the origins and formation of interior ponds, which can be applied in explicit landscape simulation models to improve predictive modeling of future ecosystem consequences under climate change and increasing hurricane frequency and intensity.