ALI ABDOLALI
Ph.D. in Coastal Engineering
Tidal Marsh Lands
Fig1. Bombay Hook National Wildlife Refuge, Delaware bay. left) Shearness Gate Right) Dock Creek
The tidal marshes and wetlands provide important ecosystem services and resources for their adjacent water bodies, such as filtering harmful materials and providing a significant ecological resource for terrestrial and aquatic plants and animals. They also function as a major coastal defense against flooding by sea and stabilize water supplies, thus ameliorate droughts and floods. It has been shown that they not only reduce destruction of coastal areas, but also keep costs for coastal structures low. Worldwide 5 to 8% (7 to 10 million km^2) of the land surface is covered with wetlands; approximately 40 million ha in the USA. In the past 50 years alone, the area of coastal and freshwater wetlands declined by about 9%. Most losses occurred due to agriculture, hydrologic modifications, seawall constructions, coastal development, pollution, construction of dams and roads, and river control. Coastal wetlands cover about 2 million ha of the USA, and are the predominant form in the US coasts. The aim of our work is improvement of our understanding about complicated tidal marshes, studies on the past natural/human impacts on the wetlands and give a long-term anticipation of wetland accretion/deterioration in the US coastlines. Our works include numerical modeling of governing phenomena and examine different scenarios to save the tidal marshes.
Fig 2. Aquadopp ADCP Current profiler Programming and Installation.
Our case study is Bombay Hook National Wildlife Refuge where protects one of the largest remaining expanses of tidal salt marsh in the mid-Atlantic region. The refuge, located along the coast of Delaware, is mostly marsh, but also includes freshwater impoundments and upland habitats that are managed for other wildlife. Due to complexity of the tidal wetlands and lack of an appropriate model well validated against in-situ measurements, we have been conducting two parallel works including generation of a model for reconstruction of marsh system driven by current and surface elevations derived from a larger scale model of Delaware Bay, along with local wind input. In addition, and for the first time, we are doing an extensive fieldwork including measurement of tide, wind, wave and current in the system, which later will use for validation of model. At the last stage, when the model performance is validated, we would examine different scenarios for protection of these important regions alongside US coastlines to reach the best solution as a proposal to coastal authorities for the test study and other wetlands. Within the presented project, we are integrating different scientific and technical technologies like Lidar imageries, High Performance Computing (HPC) and data interpretation technique. Based on our expertise, we can improve the mentioned technologies more adaptive to such complicated environment.
Fig 3. Left) RTK survey of tide Gauges. Right) Marsh survey
Technical Summary of Work:
Our mission in this project is numerical modeling of marshland system using a high-resolution 3-dimensional hydro-morphodynamic model capable to reconstruct complex marsh system. The case study is Bombay Hook marshland in the Delaware Bay. The goal was to improve the model abilities for representation of the complex marsh hydrodynamics/sediment transport and exchange processes between marshes and the adjacent bay and to better understand the sensitivity of these complex ecosystems to changes in the bay environment i.e. Storms, river discharges, etc. A high-resolution numerical mesh was required to resolve the complex marsh geometry with sufficient detail for the studies. We have found out the significant role of small creeks, vegetation and topographic/bathymetric resolution/accuracy on model performance. We used existing data including high-resolution Lidar imageries and did modification techniques to exclude wrong data and improve the accuracy compare with existing benchmarks. Then, we conducted extensive field surveys to measure the current velocities, tide and wind waves along channels and tidal marshes. Numerical experiments confirmed that natural and anthropogenic morphological changes are responsible for the alteration of tidal regime during time. Temporal and spatial changes in tidal asymmetry highlight the complex impacts of human interventions on tidal changes and long-term morphodynamics. Our goal was suggesting practical solutions to control marsh accretion/deterioration. The modeling of different scenarios can reveal the system’s sensitivity to changes in tidal channel geometry and to find the best location for a marsh blockage to prevent sediment transport out of the marsh interior that increasingly suffers from losses.
The importance of vegetation on tidal marshes to reproduce realistic erosion/deposition patterns and the influence of suspended sediment concentrations coming in/out of system would cause deterioration/accretion of the marsh as a unit of a larger system of marshes and coastal regions. The long term modeling with our computationally efficient numerical model can arrive into a correct representation of tidal asymmetry, which is a major factor determining the net sediment transport.
The methods developed for model generation in this research can be applied for future studies and help develop and set up numerical models of other marshes more effectively.
Fig 4. Left) Ground truth survey with RTK for LIDAR data correction. Right) Beach erosion in Delaware Bay (Simon River Entrance)
Numerical Modeling:
For the modeling study, we have used the unstructured grid model, FVCOM, which covers the entire marsh system with sufficient grid resolution to resolve small channels and creeks. The grid bathymetric data in channels is based on data collected during an extensive bathymetric survey. The topographic portion of the grid is extracted from LiDAR data sets. We have implemented sophisticated Artificial Neural Network techniques to remove the vegetation bias, well validated against ground truth survey. The model is driven by current and surface elevations derived from a larger scale ROMS model of Delaware Bay, along with local wind input. The coupling between structured (ROMS) and unstructured (FVCOM) models are shown in Fig.5.
Fig 5. Models coupling; (A) ROMS domain (B) FVCOM domain;
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Summary of the Significance of the Work:
Study of tidal marshes has been challenging for decades due to lots of uncertainties about the effective parameters in such complex system. In addition, lack of appropriate numerical tools, which can take the advantage of High Performance Computing (HPC) to reduce the drastic computational time, was the main concern of coastal engineering communities. Our work in this project improved our understanding of marsh performance by mean of field measurements and a well-developed numerical model able to reconstruct the wetland system. The state of the art of our work was integrating variety of sciences and technologies together to provide an efficient numerical tool for marsh systems modeling.