The Sand Spit and the Bay

Is the Estuary Filling With Blown Sand?

The Blowouts Are Stable

Addendum

Is The Estuary Filling With Blown Sand?



Click on this image to see a larger version.

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.

 

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