Friday, March 8, 2013

Tracking Change on the Elwha Shoreline (continued)

For the last seven days I've had the opportunity to participate in a variety of field campaigns focused on understanding shoreline change along the Elwha shoreline. We've just wrapped up a USGS-led survey that included everything from aerial imagery (from Andy Ritchie's PlaneCam), terrestrial LiDAR, topographic and bathymetric surveying (the photo above is of the USGS survey boat Snavely, with the new Elwha River mouth in the foreground), and grain size surveys. Almost simultaneously there was an effort by the WA DOE to map the bluffs of both the Elwha and Dungeness shorelines - a repeat survey to look at change since the initial survey in June of 2012.

The best part of the USGS survey work is that it will give us the first comprehensive look at the sub-tidal component of this winters' changes that have been so dramatic and obvious above water. I anticipate more on this to come in the weeks ahead as thousands of individual data points are processed and analyzed.

In the meantime,though, the BIG shoreline changes adjacent to the river mouth just keep getting bigger and better...but the interesting conclusion that I reached after three days of intensive work surveying grain size around the Elwha delta is that the influence of the pulse of sand that hit the shore in December really seems to be focused right around the river mouth - at least as far as its current influence on grain size on the intertidal foreshore. In this photo for example, taken from a point on the tip of the delta looking west, you can see how coarse the beach foreshore is even while, in the far field of the photo you can make out the now sandy river mouth (just beyond the last little person you can see in the photo).

One of the more interesting questions for me continues to be if and how the sediment delivered at the river mouth will nourish "downdrift" beaches - such that we will observe a reversal of chronic erosion and/or a reduction in the grain size on the beach foreshore. Some of the preliminary data from the bathymetric survey suggests that sediment is transporting to the east, but in the shallow water just off-shore. But as of yet, for most of the beach east of the river mouth, the name-of-the-game continues to be some degree of erosion and a beach composed, to an extent, of cobble. Near the river mouth, though, the changes are dramatic. For example, here is a shot taken on May 17, 2011 looking landward from a spot in the low intertidal zone taken on the beach just to the east of the river:

and here is a shot taken two days ago from the same vantage point:

Finally, one of the more dramatic consequences of the pulse of material that was delivered to the shoreline this winter was the formation and evolution of a large sandy bar attached to the west side of the river mouth. This time lapse video, shot from a point to the west of the river mouth (thank you Gene and Shannon Richardson), gives some sense of how that feature has evolved since December...

Note that it appears that you can get a slightly higher resolution version by viewing it directly at youtube.

1 comment:

Scott Sullivan said...

Hi Ian, thanks for posting that time lapse. Really amazing to watch . And its good to hear your findings for the sediment activity as well. I have been spending a lot of time these last 2 months surfing to the east down by PA point and there has been a tremendous effect on the near shore bathymetry with the seditment. All below the high tide line. The quality of the waves has changed throughout the distance from the mouth to the hook. It seems that any sediment that has been being deposited goes away as quick as it comes in. (Moving down to the hook?) One day the shore will be rocks and then 2 days later covered in sand , then 2 more days its gone again. Large amounts. The tides combined with the heavy swells have been dictating what stays or goes. Its been facsinating to watch to say the least. Thanks for all the cool info. Looking forward to going down today since we had these rains and see whats new. Scott