On the left is the
1897 USGS map, based on the 1895 survey; on the right is a current
topo, based on 1963 and 1987 data. The maps were shrunken and
enlarged until the distance from the north side of Morro Rock to the
mouth of Shark Inlet was the same, with the largest size, for
detail, reproducible on 8 ½ x 11 stationery. At this size 77
millimeters equals 6076 feet, which equals one nautical mile (nMile),
which shows on the maps as a minute of latitude.
Notice that Shark
Inlet is 0.1 nautical miles shorter than it used to be. Also notice
that the west side of the Sand Spit was farther east than it is now.
Our “northwest”
wind produces blowouts on the Spit that are oriented from
approximately 302 degrees. However, for simplicity of comparison I
chose to measure the west-east dimensions of the Sand Spit every
tenth of a nMile, from 18.8 to 21.2 minutes North latitude.
The difference in
dimensions, from north to south in millimeters, is 6, 2, 6, 7, 7, 8,
3, 5, 4, 2, 3, 7, 6, 2, 2, 4, 4, 4, 4, 3, 2, 5, 2, and 3. The Spit
is getting thick faster to the north. The mean change is 6 for 21.2
to 20.0 minutes, and 3 for 19.9 to 18.8.
3 / 77 x 6076 = 237
feet, and 6 / 77 x 6076 = 474 feet in 68 to 92 years, equals
approximately 100 / 80 x 237 and 474 = 300 and 600 feet of change
per century. That compares reasonably closely with the rate of
advance of the dunes measured by Ernie Eaddy and George Mason at the
tip of Shark Inlet of 10 feet in 2 ½ years, 4 feet per year.
The difference in
west-east position of the western edge of the Sand Spit is 5, 5, 5,
4, 5, 4, 3, 4, 4, 4, 3, 4, 2, 3, 3, 2, 3, 3, 4, 2, 2, 3, 2, 2, 1,
and 0. The means for this data are 4 and 2. Does this mean that 2/3
of the growth in width of the Spit is westerly?
Map and analysis by
Curt Beebe,
curtbeebe@charter.net and 805-772-6013. Non-profit reproduction
permitted.
