Semester of Graduation

Summer 2022

Degree

Master of Science (MS)

Department

Civil and Environmental Engineering

Document Type

Thesis

Abstract

Coastal wetlands provide an abundance of ecosystem services that benefit society, such as essential habitat for commercial species, storm protection, nutrient cycling, and carbon storage. Louisiana faces rapid rates of relative sea level rise (natural subsidence and eustatic sea levels) that threaten wetland survival, which are amplified by a reduction of riverine sediment input. An important determining factor of marsh survival is the formation of wetland platform elevation, known as vertical accretion, which is determined by several processes including sediment deposition & erosion, below ground biomass (BGB) productivity, decomposition of organic matter, shallow & deep subsidence, and soil compaction. Feldspar marker horizon stations were deployed in an active (Atchafalaya Bay) and an inactive (Terrebonne Bay) coastal basin along salinity and hydrogeomorphic (HGM) zone gradients to quantify surface accretion in wetlands exposed to different loadings of riverine sediment. Surface accretion rates were greatest in Wax Lake Delta (3.5-5.7 cm yr-1) in the proximal region of an active basin and lower in the distal region in Fourleague Bay brackish and saline marshes (1.42 cm yr-1). In contrast, surface accretion and inorganic and organic sedimentation rates increased with increased salinity in the inactive Terrebonne Bay. Average BGB ranged from 600-5500 g m-2 while average total root mass ranged from 3000-11000 g m-2 for 50cm deep cores. Belowground necromass had increased lignin content and slower decomposition rates, suggesting necromass is an important component of soil formation.

Filed observations were used to parameterize the NUMAR model (numerical marsh accretion and response), a modification of the NUMAN (Chen and Twilley 1999) and SEMIDEC (Morris and Bowden 1986) models. NUMAR uses surface accretion rates, soil physicochemical characteristics of newly accreted material, and belowground biomass dynamics to estimate decadal (70 yr) sediment accretion rates comparable to 137Cs dated cores. NUMAR was calibrated across salinity gradients in two different coastal basins, however model development is needed to capture important soil building dynamics, such as soil compaction and interannual vertical accretion variability. This model will be applied to an ecogeomorphic landscape model of Atchafalaya and Terrebonne and can help inform wetland restoration and management of coastal deltas in the future.

Date

7-20-2022

Committee Chair

Clinton Willson

Available for download on Thursday, July 20, 2023

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