Sediment budget of ebb-tidal deltas and their role in the coastal morpho- and sediment dynamics

Ebb deltas at the mouths of tidal inlets are among the key areas in coastal morphodynamics as a majority of years to decades time scale coastal erosion problems can be traced to changes in adjacent inlets. General patterns of sediment dynamics at ebb-tidal deltas are outlined in conceptual models such as the one by Kana (et al., 1999). These models envisage a sediment transport seaward through the tidal channel and deposition at a distal lobe where the ebb current velocity decreases below the sediment transport threshold. Waves will then rearrange the sediment into swash bars on either side of the main channel. The swash bars migrate shoreward and eventually connect to the shoreface.

Four primary inlet domains are considered: (A) main ebb channel where tidally generated ebb currents control sediment discharge, (B) ebb-tidal delta with a broad swash platform that is ultimately in balance between ebb-directed flows and wave- and tide-generated shoreward transport, (C) shoal-bypassing zones at the margins of the ebb-tidal delta where sediment shifts unidirectionally from the delta to the shoreline under wave-generated transport, and (D) recurved spits adjacent to the inlet which receive shoal-bypass sediments. Excess sand accumulating in Domain D becomes subject to longshore advection toward and away from the inlet. A portion nourishes the adjacent beach and the remainder recycles back to the inlet channel (Domain A), completing the inlet transport loop.

The initial focus will be on the Otzumer ebb-delta west of Spiekeroog.

The sediment dynamics at the Otzumer ebb-delta seem to correspond to the conceptual model rather well. However, a strong standing edge wave influence based on the saw-tooth bar dynamics on an upper shoreface is suggested by Flemming and Davis (1994) must also to extend into the ebb-tidal delta region and thus influence the swash bar dynamics. The lack of observations with high spatial and temporal resolution hinders the interpretation of the processes driving the dynamics of the Otzumer ebb-tidal delta.

Aims

Aim of this project is to rationalize the short-term (spring-neap tidal cycle) variations as well as longer-term morphodynamic and sedimentological trends at the Otzumer and Harle tidal ebb-deltas. The initial focus will be on the larger of the two deltas, the Otzumer Balje ebb-tidal delta.

More specifically, the questions to be answered are:

Morphology:

How are the main inlet domains distributed?
What are the general morphological migration patterns at the Otzumer Balje ebb-tidal delta?
What is the morphodynamic response of the swash bars to the hydrodynamic changes within diurnal and spring-neap tidal cycle?

Tidal currents:

What is the tidal flow field at the Otzumer Balje ebb-tidal delta in the various inlet domains (A-D, defined above)?
What is the variability (of the magnitude and duration) in the ebb flow domination over the flood flow? (Storm events supposedly diminish flood current duration by waters piled up in the German Bight pushing back to the sea. Ebb current acceleration of up to 65% occurs due to the same process (Flemming and Davis 1994)).
What is the effect of tidal currents on the morpho- and sediment dynamics, particularly the effect of tidal asymmetry?

Sediment transport:

What is the net sediment transport along the distal lobe of the ebb-tidal delta and across the delta margin?
What is the grain-size variability in the various inlet domains (A-D, defined above)?
What is the variability of the critical bed shear stress (spatial and temporal) in the ebb-delta area?

Progress

First bathymetric survey (Fig.A) and sampling (Fig.B) are planned for the spring-summer of 2004. The bathymetry of the Otzumer Balje ebb-tidal delta will be recorded using the multibeam echosounder in combination with LRK positioning. Sampling will be done using light-weight grab sampler. Three surveys of the delta area are planned to capture the neap-spring-neap or spring-neap-spring tidal cycle.

Additional detailed surveys will then be planned in carefully selected areas at appropriate times to solve specific questions, such as the bedform migration. Tidal flow field will be resolved by conducting the ADCP measurements, first in the main and side channels and then possibly in other areas if bathymetric data warrants this. Overview of the changes in yearly to decadal time scales in the ebb-tidal delta will be obtained by examining the old sounding charts and aerial photographs, and later on, our own data sets as the time series get longer.

References
Flemming, B.W. and Davis R.A.Jr. 1994. Holocene evolution, morphodynamics and sedimentology of Spiekeroog barrier island system (southern North Sea). Senckenbergiana maritime 24, 117-155.
Kana, T.W., Hayter, E.J., Work, P.A., 1999. Mesoscale sediment transport at southeastern U.S. tidal inlets: Conceptual models applicable to mixed energy settings. Journal of Coastal Research 15 (2), 303-313.