Monday, July 20, 2015

Short-term beach change on Dungeness Spit

The study site - approximately Mile 3 on Dungeness Spit. This view is looking north-east towards the light house.

Our beaches are extraordinarily dynamic, and as I go about doing the work that I do I hear countless stories about big changes to the morphology of beaches that happen over very short time-scales. I don't always get the chance to really address that kind of variability since most of my shoreline monitoring sites are typically re-occupied only once or twice a year. Two of my sites, though, are unique. Both Ediz Hook and Dungeness Spit take two tidal cycles to finish, and so for this year's Dungeness Spit survey I collected some data on both of my survey days (8 June and 1 July) along an area around the 3 mile mark on the spit in order to assess how much change occurred over that time period. The conclusion - more than you might expect for a few relatively calm weeks in the summer.

Lets start with the profile data from three transects. In all cases change is occurring below Mean High Water (MHW). Here is the most landward one first:

Here is appears that the beach has accreted, or grown over that two week period, over almost the entire profile below MHW. Interestingly, though, this accretion doesn't appear to dramatically change the grain size composition of the beach face. Here is an example photo taken at 1 meter above MLLW on 8 June:

and then another taken at the same elevation on 1 July, after beach accretion:

Here is another example from lower down on the beach (at Mean Lower Low Water) that does suggest a sandier substrate associated with the accretion suggested by the profile data. First the photo from 8 June:

and then here is the photo from 1 July, again shot at MLLW:

Moving to the next transect to the northeast, the patterns was reversed, with the profile data suggesting erosion on the lower part of the beach over the three week period:

and finally the next transect to the east (and the last that I overlapped) suggests a bit of a mix of erosion on the lower part of the beach, and some accretion on the upper beach (around 3m):

This seems to be supported by oblique photos collected at that site. Here is the one from 8 June:

and if you compare that carefully to the oblique from 1 July:

you will note the addition of some large woody debris high up on the profile, which is often associated with beach accretion.

What were the factors that conspired to drive these changes? Thats a tough one, but we can be sure that, despite it being summer, there was plenty of energy delivered to the beach. Here is a summary of water level data (bottom panel, from Port Angeles, referenced to MHW) along with significant wave height and dominant wave period from the Hein Bank buoy:

In particular the early part of the period, between maybe June 10 and June 19, was characterized by pretty high water (0.5 m or so above MHW during the high tides), and multiple occurrences of waves with heights > 1 m. The high correlation between the average wind speed (middle panel), and wave heights suggest that these are primarily locally generated wind waves rather than swell energy from ocean - not at all surprising for the summer season in the Strait of Juan de Fuca (see Chapter 3 in my dissertation for a description of seasonal wave patterns from Elwha). If I had to guess, I would think that those profile changes, at least on the upper profile (above MHW) on the last transect, happened in that time period.

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