The composition of beach sediment is dictated by the source material for the beach as well as wave energy. A common beach type in western Washington is gravel and cobble beaches with scattered boulders.
East shore of Lopez Island
The sediment source is from an eroding bluff that extends to the south.
Glacial erratic boulder embedded in a poorly sorted glacial till
The sediment source for this beach is glacial till. The silt and clay content is much greater than the gravel, cobble and boulder content, but the fine grained silt and clay is readily transported off the beach by wave action.
Typically I would call this bluff a "feeder bluff". That is erosion of the bluff provides sediment to the beach, and hence, feeds the beach. MacLennan and others (2013) came up with a mapping scheme to identify and map feeder bluffs.
This particular bluff was mapped as a transport zone, not a feeder bluff. In this case the erosion rate is very slow due to the very moderate wave energy combined with the very hard resistant glacial till. The fact that much of the slope is vegetated and in many areas tree covered further suggests that this shore bluff is providing very little sediment input. Another possibility is that the toe of the slope did not exhibit the erosion shown above when the shore reach was mapped.
Another mapping scheme might be to call this a slow feeder bluff. For rapid erosion bluffs that send lots of sediment to the beach, MacLennan and others (2013) use the term "feeder bluff exceptional", but for slow erosion sites like that shown above they use transport zone. That is a reach where sediment is transported along the beach with very minimal input from the bluff.
Shore reach designations can be important in regards to shoreline regulations. Feeder bluffs in particular are important; if the sediment source from a feeder bluff is stopped, the beach will become starved and the beach will disappear - a loss for recreation and habitat. Hence, the effort to identify and map these shore forms.
No comments:
Post a Comment