IC15_NZ

Project description
Ebb-Tide Deltas (ETDs) are complex, highly dynamic, morphologic components of the coastline. ETDs serve an active role in the littoral system by providing a mechanism for sediment to bypass tidal inlets. Also, ETDs dissipate wave energy offshore from inlets and redirect waves onto nearby beaches. The shallow sandbars that compose an ETD are coupled to the hydraulic capacity of an ebb-jet and the wave-field of the receiving basin with dynamic feedback. As such, the sandbars move quickly and frequently, making ETDs a significant navigational hazard.
Much is known about the long-term (decadal to centennial timescale) formation and equilibrium size of deltas. However, there is very little in the literature about the short-term (episodic timescale) behavior of ETDs, particularly the movement of shallow bars and channels.
Using analytical methods, video observations, and numerical models, my research builds fundamental understanding of sediment movement at ebb-tide deltas. An analytical model is developed describing the momentum balance between a turbulent-jet and variable wave fields. Analysis of the relationships between the primary forces reveals regimes of geomorphic steady-states inherent to the momentum balance equations. High-resolution video images track wave-breaking patterns to show the evolving position and shape of sandbars at the ebb delta at Raglan, New Zealand. Movement of the sandbars is linked with energetic conditions in order to correlate climate variation with morphological responses. Numerical morphodynamic models are used to highlight processes occurring during state-transitions. This combined approach increases the overall understanding of these complex phenomena.

My research aims to answer the fundamental questions:

At present I am approximately halfway through answering these questions. My progress has been smooth, although not unhampered. I am still on schedule to complete within the original time allowance.

I have successfully developed a (quasi-)analytical model describing the balance of momentum between an ebbing tidal jet and an opposing wave field. It is able to differentiate between scenarios that lead to bar formation and those that do not. Further, it is able to resolve bar movements and predict bar response rates to changes in forcing conditions. The model shows qualitative agreement with video observations of bar movements from the ETD at Raglan, NZ. A publication introducing the analytical model along with its capabilities and implications has been drafted and is currently in review. Following a successful review process, the paper will be submitted. It is intended for publication in the Journal of Geophysical Research - Earth Surfaces.
The field observation component is currently the primary objective. Video datasets of wave-breaking over the ETD at Raglan are used in this study. Two separate datasets exist. The correction, rectification, and mosaicking of the images has been automated. So far, a single measurement technique has been applied to acquire bar position in a dataset. Currently, the higher-resolution dataset is being used to classify and characterize sandbar patterns and movements. Also, representative events are being selected from the full record to be used in conjunction with the analytical model and for the future numerical model study. I hope to have a manuscript drafted describing this work within six months.
The numerical model component has also been initiated. To minimize transportation costs, I planned my stay in Bremen to overlap with the Feb. 2012 INTERCOAST workshop. This timing instigated the initiation of the numerical model work premature to the initial plan. While in Bremen, I worked with Christian Winter’s Coastal Dynamics group at MARUM. There I learned methods and techniques of morphodynamic modeling, primarily using the open source software package Delft3D. The majority of my time was spent working with numerical models of idealized ETDs. Much was learned about the sensitivity of such models to sediment characteristics. I became familiar with the software: how to compile it, install it, and effectively use it for morphodynamic modeling. Now I have an idealized ETD system for testing state-transition conditions. Also, I have written much software to quickly launch, plot, and analyze model results. The numerical model component is primed and ready to become active.
This work fits with the work of Chris Daly in IC1, because it is working on the interaction of the beach shoreline-bar system with the ebb tidal delta. IC1 shows how the beach morphology can achieve stable states, and this works extended this understanding to ebb-tidal deltas using similar models as a basis. It also connects with Cathy Liu (NZIC3) because it studies the fate of sediment that is ejected out of the estuary onto the open coast. Finally, it connects very well with the new IC project of Bryce Blossier because the new IC project will study the stability of rip current channels using similar methodology. The North Sea coast and the Coromandel coast make ideal contrasting studies because they show the differences in dynamics between the estuary-ebb-tidal-beach in closed (compartimentalised) and open systems.