Interannual Sandbar Behavior

Sandbars are common nearshore features on sandy coastlines throughout the world and are constantly evolving in response to hydrodynamic forces and gradients in sediment transport. Large waves result in a strong offshore directed current in the lower water column (undertow) that results in the seaward migration of sandbar systems (Hoefel and Elgar, 2003). However, during low wave conditions undertow is weak and under such circumstances non-linear properties of the flow can result in onshore sediment transport. Among the important factors in onshore directed transport are velocity skewness, acceleration skewness, and boundary layer streaming. Driven by the cumulative effects of these competing processes, there is a trend of net offshore subtidal bar migration (NOM) observed around the globe. Depending on the stage of the offshore migration cycle, the number of bars, their location, and their size can vary considerably (e.g., Ruessink et al., 2003). Similarly, there is significant intersite variability in bar behavior. For example this phenomena can occur on time scales of 1 year (e.g., Hasaki, Japan) or up to 15 years (Noord-Holland, the Netherlands).

This same trend of net offshore sandbar migration has been observed in the Columbia River Littoral Cell. A long term dataset of nearshore morphology extending from ~-12m contour to the foredune has been collected annually since 1998. Data from one transect along the Long Beach Peninsula in the CRLC is shown below:

CRLC_Profile

The morphodynamics at Long Beach are particularly interesting as the sandbars are cyclically moving offshore but the net cross shore sediment flux is in the onshore direction, indicating that sediment and bedforms can move in opposing directions.In the picture above you can clearly see the growth of the beach and dunes (anything above the 0 m contour) with time.

 One of my first projects at OSU was applying Unibest-TC, a 1D cross shore wave and sediment transport model, to predict the net offshore bar migration cycle at Long Beach. After calibrating the model to 3 key parameters (angle of repose, the breaker delay parameter, and the current friction factor) we are able to simulate the general bar behavior at Long Beach fairly well. See the video below for confirmation:

 

In the video the blue line is the model simulation and the black and blue lines are real measured morphology in the Oysterville region of Long Beach. After a 1 year simulation, which is being forced by a real time series of waves tides, you can see the model simulation matches up well with the observed 2011 profile.

During the simulation you can see the nearshore sandbars change significantly in terms of their cross shore location and their amplitude. In general the sandbar amplitude (distance from trough to crest) increases during highly oblique waves due to scour in the trough. During periods of low wave energy you can see that the sandbars generally move onshore, but during the winter when waves are larger and storms are more frequent the sandbars migrate offshore.

For next steps we are using this type of modeling to understand more about how sandbars are formed, how and why they migrate offshore, and what the importance of sandbars is on the local sediment budget. Hopefully I will get a paper out on this topic soon…….

In the meantime, check out my AGU poster from December 2012 if you are interested in finding out more:

AGU 2012