Graduation Semester and Year
Fall 2024
Language
English
Document Type
Dissertation
Degree Name
Doctor of Philosophy in Civil Engineering
Department
Civil Engineering
First Advisor
Michelle Hummel
Abstract
Historically, coastal salt marshes across the US have been diked to advance agricultural use and land reclamation, leading to significant reductions in their ecosystem services. Over recent decades, restoration initiatives have been designed and implemented to rehabilitate these environments and recover their services. However, decades of restricted tidal inflow have fundamentally changed the hydrological and ecological dynamics of these systems, resulting in degraded topographic conditions through land subsidence, limited vertical accretion, and underdeveloped tidal channels. These changes have complicated the restoration efforts and constrained the adaptability of marsh systems to future water levels. This dissertation aims to enhance our understanding of the feedback between hydrodynamic and marsh accretion processes in diked marsh systems in response to reintroduced tidal flow and current and future water levels. For this purpose, validated hydrodynamic and marsh accretion models are developed and integrated to assess hydrodynamic and ecological feedbacks within the Herring River Estuary in Cape Code, Massachusetts.
Recently, there have been efforts to restore the historic hydrodynamic regime of marsh ecosystems by reintroducing tidal exchange between the ocean and marsh systems. The tidal reintroduction, along with sea level rise, is projected to alter hydrodynamic conditions within these formerly diked systems. In Chapter 2, we investigate the impact of tidal restoration and projected sea level rise on the hydrodynamic behavior of the Herring River Estuary and assess the interactions between key hydrodynamic variables throughout the system. The results demonstrate that the interplay of varying tidal exchange and water levels, integrated with different topographic conditions, human barriers, and tidal channel drainage efficiency, can result in distinct hydrodynamic responses. In the Lower Herring River, substantial tidal exchange coupled with efficient drainage promotes favorable intertidal zone formation, which is essential for successful marsh recovery. In contrast, in the upper parts of the estuary, anthropogenic barriers and subsided land limit tidal flow and reduce drainage efficiency, resulting in prolonged inundation periods and tidal pool formation. This research advances our understanding of hydrodynamic complexities in diked salt marshes.
The interaction between key hydrodynamic variables and marsh accretion dynamics can significantly influence the marsh resilience to reintroduced tidal inflow and rising water levels. In formerly diked salt marshes, prolonged tidal restriction has resulted in reduced vertical accretion and lack of tidal channel enhancement. These impacts, combined with human interventions and land subsidence, further complicate marsh recovery efforts. Therefore, the hydrodynamic and marsh accretion dynamics studied in natural coastal marshes may not be directly applicable to these deteriorated diked systems. In Chapter 3, we examine the feedback between hydrodynamic and marsh accretion processes in the Herring River Estuary through 2100, considering varying scenarios of tidal restoration, sea level rise, and biomass production. The results indicate that sections of the marsh with efficient drainage are likely to maintain their biomass productivity, whereas upper parts of the system located further away from the ocean may transition to open water. In contrast to natural salt marshes, drainage is the main driver of tidal pool formation rather than MHW depth. Further, variations in sea level rise rates have a larger influence on marsh productivity and adaptability compared to biomass production rates. These findings are applicable to similarly diked coastal marshes and emphasize how decades of reduced accretion, land subsidence, and inadequate drainage have diminished marsh resilience to rising water levels.
Previous studies integrating hydrodynamic and marsh accretion processes have mostly assumed existing topographic and tidal channel conditions. In addition, research on marsh restoration actions, such as sediment placement, has been limited to short-term, site-specific observations. In Chapter 4, we evaluate the impact of sediment placement and tidal channel enhancement on improving marsh recovery and resilience in the Herring River Estuary. We find that sediment placement can significantly improve hydrodynamic conditions and marsh recovery within a short timeframe. However, in marsh areas with insufficient drainage, the marsh can transition into open water in response to sea level rise. Therefore, tidal channel improvements, in combination with sediment placement, are crucial to maximize marsh adaptability to future water levels. With these modifications, the marsh system displays resilience to low rates of sea level rise but demonstrates greater vulnerability to high rates of sea level rise. Overall, this study advances our understanding of hydrodynamic processes and marsh dynamics, as well as their interactions, and contributes to the development of restoration strategies that optimize marsh recovery and survival over the coming decades.
Keywords
Hydrodynamic Model, Coastal Salt Marsh, Tidal Restoration, Sea Level Rise, Marsh Accretion, Drainage Efficiency, Sediment Placement
Disciplines
Civil Engineering | Environmental Engineering
License
This work is licensed under a Creative Commons Attribution-NonCommercial-Share Alike 4.0 International License.
Recommended Citation
Naseri, Kasra, "Hydrodynamic and Ecological Response of a Historically Diked Coastal Salt Marsh to Tidal Restoration and Sea Level Rise (Herring River Estuary, MA)" (2024). Civil Engineering Dissertations. 502.
https://mavmatrix.uta.edu/civilengineering_dissertations/502