Results

References

1. Fagherazzi, S., & Overeem, I. (2007). Models of deltaic and inner continental shelf landform evolution. Annu. Rev. Earth Planet.

Sci., 35, 685-715.

2. Özsoy, E. (1977). Flows and Mass Transport in the Vicinity of Tidal Inlets. Coastal and Oceanographic Engineering Laboratory,

University of Florida.

3. Thornton, E. B., & Guza, R. T. (1983). Transformation of wave height distribution. Journal of Geophysical Research: Oceans

(1978–2012), 88(C10), 5925-5938.

4. Soulsby, R. (1997). Dynamics of marine sands: a manual for practical applications. Thomas Telford.

5. Ismail, N. M., & Wiegel, R. L. (1983). Opposing wave effect on momentum jets spreading rate. Journal of waterway, port, coastal,

and ocean engineering, 109(4), 465-483

6. Nardin, W., Mariotti, G., Edmonds, D. A., Guercio, R., & Fagherazzi, S. (2013). Growth of river mouth bars in sheltered bays in the

presence of frontal waves. Journal of Geophysical Research: Earth Surface, 118(2), 872-886.

Hydraulic calculations with the model

compare well with published

laboratory observations and numerical

Delft3D model simulations, both with

a steady jet flowing into a directly

opposing, non-breaking wave field

with flat bottom, ℎ = 0.114m and

ℎ = 3m, respectively [Ismail and

Wiegel, 1983; Nardin et al., 2013].

In a series of identical simulations differing only

by wave and jet conditions, sandbars formed

further offshore with stronger ebb currents and

further onshore with larger waves (left).

The model successfully demonstrates observed

sandbar behaviour qualitatively. It is able to

generate and degenerate sandbars in response to

waves and ebb-jet currents. Existing bars can be

moved onshore or offshore by adjusting the

wave/current balance.

The position (distance from inlet) where the convergence point occurs is sensitive to wave

height and initial jet strength. Sandbars form quickly at a convergence point and the rate of

bar response increases with the gradient of convergence.

Acknowledgements

INTERCOAST, The University of Waikato,

Broad Memorial Scholarship, Terry Healey Memorial Award

Model output showing simple jet flow without waves, with 1m, and 2m opposing waves, (left to right).

As expected, with waves the jet width increases and axial velocity decreases relative to the

no-wave case. Further, the existence of a convergence point occurs in cases where

momentum from waves overwhelms the jet.

Morphological evolution of ebb-tide deltas S.R. Harrison1,2, K.R. Bryan1, J.C. Mullarney1, C. Winter2

AGU-OSM 13709

sh229@waikato.ac.nz 1University of Waikato, Hamilton, New Zealand 2Universität Bremen, Bremen, Germany

A semi-analytical model is developed to describe tidal jet flow into a directly-opposing wave

field. We build upon the approach of Ӧzsoy [1977] to include wave influence on the mean-

flow velocity of an ebbing tidal jet. Momentum balance and mass conservation equations for

vertically- and laterally-averaged shallow-water jet equations describe the axial velocity, 𝑢

and jet-width, 𝑏 with distance from the inlet, 𝑥.

Methods

The flow is used as input to sediment flux, 𝑄 calculations. The Soulsby [1997] formulae is

modified to include Stokes drift in the bed transport. Convergences and divergences in

sediment flux are used to update the depth profile, which effectively is the morphological

evolution along the jet axis.

𝐼1 and 𝐼2 are similarity profile functions and 𝐹𝑤 is the excess momentum flux from waves

solved using a Thornton and Guza [1983] type wave dissipation model. Currents are

included in the dispersion relation influencing wave conditions.

In the model, waves attenuate the depth-averaged flow by contributing:

• Excess momentum flux (through radiation stress) as waves shoal and break

• Increased turbulence (transferred into water during wave energy dissipation)

• Increased bed shear stress (from orbital velocities in the bottom boundary layer)

2D Jet Equations: (see diagrams for symbol definitions)

friction turbulence waves inertia non-steady

Introduction

Key Findings • Existing analytic jet model improved by including waves and turbulence

• Simple model able to generate/degenerate, move, and predict growth rate of sandbars

• Sediment response rate increases with waves opposing ebb-jet (convergence)

Ebb-tidal deltas (ETDs) are sandy morphological structures and form at the interface between

a tidal constriction and the open sea where ebb tidal currents and wave action meet. ETDs

serve an active role controlling coastal morphology by dissipating and redirecting wave

energy, storing sediment for beach nourishment, and are a conduit of sediment bypassing.

Deltas serve many critical functions but the processes governing their development and

evolution are poorly understood [Fagherazzi and Overeem, 2007]. ETD morphology is highly

dynamic, with sandbars shifting in response to changing conditions. There is little known

about the short-term episodic movements due to storms and variation in tidal conditions.

ETDs form seaward of tidal constrictions, e.g. barrier islands, harbour inlets. Bathymetry data show abundant

ebb-tide deltas in Germany (kindly provided by Gerald Herrling). The ETD at Raglan, New Zealand is

located seaward of the harbour mouth and is a navigational hazard for boats.

To develop a semi-analytical model in order to gain insight about storm-scale variations of

ETD morphology. Identify what parameters govern the movement of sandbar shoreward

and seaward. Determine what conditions control the dominance of waves over the ebb-jet

in controlling sandbar movement.

Aim

East Frisian Islands, Germany Raglan, New Zealand