TRACKING LITTORAL DRIFT AT BIRCH BAY, NORTHWEST WASHINGTON

Understanding littoral sediment movement is important for seaside communities like Birch Bay Village, WA, because mitigation efforts must account for the sources, rates, and directions of longshore drift in order to preserve human infrastructure. As part of a Geomorphology study, we evaluated sediment transport on a mixed cobble beach in north Birch Bay, Washington, by tracking the movement of cobbles during two tide cycles.

In our study, we collected and painted ~3500 cobbles that encompassed the dominant grain sizes on the beach. We divided the painted cobbles into four clast sizes: <2 cm, 2-4 cm, 4-7 cm, and >7 cm and piled each group on the beach, marking the initial locations as four GPS waypoints using a Garmin GPSMap 60 handheld GPS with WAAS capability (horizontal resolution ~2 m). After two tide cycles (24 hours), we revisited the beach and recorded the locations and sizes of every clast we could find (a total of 500 cobbles). Using ArcMap 10, we determined the maximum and modal distance of travel, as well as transport azimuth, for each of the clast groups. The dominant direction of transport for the three smaller-diameter clast groups was SSE of the original start point, consistent with the prevailing and predominant wind directions at the time of data collection. The clasts were distributed in an elongate oval along the beach, with clasts from the two finer-grained classes traveling as much as 70 m down the beach from their points of origin. Clasts from the coarsest class actually showed a net shift in the opposite direction (N-NW), contrasting the movement of the finer classes (S-SE). The apparent transport distance for the largest grains was small, however, and is only marginally greater than the uncertainty of our GPS measurements. Together, our data indicate that although anomalous, high-energy “rogue” north-verging waves may have affected the area during this initial study, the dominant littoral transport direction to the southeast reflected the prevailing winds that day. We plan to conduct further studies during September 2012 that will measure a wider variety of wind and wave conditions to test the broader significance of our initial findings.