Swash Bars

Coastal evolution results from the combined effects of both episodic (e.g., storm scale) and long term trends in sediment transport. After a number of recent extreme events including Hurricanes Katrina and Sandy tremendous focus has been paid to quantifying potential impacts (e.g., erosion and total water levels) from such anomalous events.While event scale erosion indeed plays a major role in the general geomorphic behavior of coastal systems, during periods of lower wave energy beaches can quickly recover due to net onshore directed sediment transport. Despite the importance of onshore sediment transport on beach building, we have a poor characterization of these processes. This is in part because the magnitude of instantaneous onshore fluxes are very difficult to integrate over longer time scales and also because there are few sites globally that exhibit consistent progradation for which we can further (predictably) study onshore transport dynamics.

An exception to this trend is the Pacific Northwest US which has a number of beaches which are naturally prograding. For example the Columbia River Littoral Cell (border of Oregon and Washington) has exhibited positive local shoreline change rates of up to 10 m/yr (see figure below). Furthermore, recent studies of the CRLC have indicated that this additional sediment must in part be derived from offshore sources in order to conserve mass within the system.

CRLCShorelineChangeSouth Beach State Park in Newport, OR has also exhibited large positive shoreline change rates (~6 m/yr) since the installation of the jetties in the 1890s. The south jetty has prevented longshore sediment transport to the north, which has in part led to deposition of sediment at South Beach. However, it is also hypothesized that sediment from the Yaquina River is worked onshore at South Beach and that therefore cross-shore processes are also important for the local sediment budget.

It has also been long proposed that one mechanism by which sediment is exchanged from the surf zone to the beach is via the welding of intertidal sandbars (see example picture of one below). South Beach, just like most beaches in the Pacific Northwest) is characterized by the presence of both subaqueous and intertidal sandbars. In order to explore the role of these sandbar features on the net sediment budget we began a pilot field program in summer 2014.

R:300,G:256,B:406

Using a technology called real-time kinematic GPS (RTK GPS) we are able to measure the vertical elevation of the land surface with accuracies on the order of 5 cm. Our Trimble RTK GPS devices can be setup on a backpack, pole, or all terrain vehicle in order to measure the subaerial beach. Furthermore, our personal watercraft based system outfitted with single beam echosounders can be deployed to measure terrain below the water surface.

At South Beach during 2014 we performed about 25 land based and 5 water based surveys in order to understand the morphologic evolution of the system. Furthermore to understand the physical conditions driving changes to the system we deployed a mooring at 11 m water depth offshore the study site from July-September 2014. The frame was outfitted with an acoustic wave and current profiler (AWAC) which measured the vertical current structure of the water column as well as the full directional wave spectrum. The mooring setup also included a number of turbidity sensors to measure suspended sediment fluxes, thermistors to measure vertical variability in the temperature of the water column, and CTDs to measure the salinity of the water. Below is a picture of Tully and I dropping the lander into the water off the R/V Elakha.

DCIM100GOPRO

Throughout summer 2014 we consistently observed a progradation and aggradation of South Beach based on the morphologic measurements. Below is a cross shore profile documenting some of the changes we observed in throughout the summer at our field site.

SouthBeachTransect21This work is still ongoing, but preliminary analysis suggests that indeed swash bars are instrumental in acting as a delivery mechanism of sediment between the surf zone to the beach. What is unclear at this point is under what wave conditions that swash bars are stable and are able to weld/migrate onshore? This is where there in-situ hydrodynamic measurements will be crucial!

The observations at South Beach also spawned a similar research project at the Sand Motor in the Netherlands. I spent 6 weeks there in September/October 2014 working with folks using similar RTK GPS technology to measure short term sediment dynamics along a massive beach nourishment project. The Mega Perturbation Experiment (MEGAPEX) involved 15 institutions working on understanding specific questions related to nearshore circulation patterns, swash zone processes, ecological responses to shoreline change, remote sensing of the nearshore, and coastal sediment transport. During my time there I was working most closely with folks from the Technical University of Delft (Delft, the Netherlands), exploring variability in intertidal zone morphology and implications of that variability on aeolian sediment transport and backshore sediment supply. I’m still catching up with the analysis of all of that data, but here is a preliminary video of observed morphologic change in the intertidal zone during the MEGAPEX experiments.

Note how the swash bars are generated during periods of low wave energy and the morphology is quickly eradicated during storm events. Its pretty cool data and I am looking forward to making sense of the temporal and spatial variability in the intertidal morphology from these experiments!

These are ongoing projects, so check back for updates! In the meantime check out my AGU poster (from December 2014) on some of the preliminary Newport data if you are interested in learning more:

AGU 2014