Benchmark P153 is highlighted in the Corps IPET report. This benchmark
is located at the southeast corner of the bridge that crosses the London
Avenue Canal at Lakeshore Drive. The following table shows the
elevations reported for this benchmark in successive surveys between
1951 and 2006.
In 50 years it sank 2 1/4 feet or 27 inches. That's about 1/2 inch per year.
Table III-3
Successive Elevations on Benchmark P 153 from 1951 to 2006
Elevation, ft Datum Adjustment Agency Reference
| Successive |
|
Adjustment |
| Elevation ft |
Datum |
Date |
| 12.087 |
NGVD29 |
19-Mar-52 |
| 11.476 |
NGVD29 |
1951 |
| 11.27 |
NGVD29 |
9-Apr-65 |
| 10.708 |
NGVD29 |
1-Sep-82 |
| 10.623 |
NGVD29 |
30-Jan-86 |
| 10.39 |
NGVD29 |
21-May-91 |
| 10.2 |
NGVD88 |
14-Feb-94 |
| 10.21 |
NGVD88 |
5-Dec-96 |
| 9.79 |
NGVD88 (2004.65) |
10-Feb-06 |
Curiously the Corps can't explain why projects in the 1990's continued
to use the 1965 datum. By assuming that the benchmark was 11.27 above sea level any floodwall built using that
datum is now 1.6 feet shorter than it should be. The same thing happened
at the 17th Street Canal with the difference there averaging 1.9 feet.