were manually removed to maintain a sufficient level of accuracy in the
bathymetry.
There are many significant differences between the DNC and ETOPO5
databases. Significant differences between the two databases often lie along the
continental slopes and in certain shallow regions such as the vicinity of the
Bahamas. Figure 14 is a representation of the fractional differences between the
DNC and the ETOPO5 databases. This comparison indicates that ETOPO5
depths in many regions are extremely inaccurate, with percentage differences
ranging from 20 to 5,000 percent or greater. The Great Bahama Bank, a shallow
region located west of the Andros Islands, extends as far down as the Exuma
Islands in the DNC database. However, the older ETOPO5 database does not
capture this feature. Depths in this region in the ETOPO5 database are on the
order of hundreds of meters, whereas the depths given by the DNC database are
on the order of meters. This updated bathymetry is expected to dramatically
impact the tidal computations since these new features impede the tidal exchange
between the Atlantic Ocean and the Gulf of Mexico, forcing the water to travel
predominantly through much narrower channels between Florida and the Great
Bahama Bank and between Cuba and the Great Bahama Bank. Adding islands
and redefining the bathymetry of the sea mounds located on the eastern edge of
the Caribbean Sea also restricts the forcing flow at the 60W meridian to enter
the Caribbean basin, altering the dynamics of the tides in the Caribbean and in
turn, also affecting the Gulf-Caribbean exchange.
Figure 15 shows fractional differences between the NOS and DNC
bathymetric databases. The majority of the common areas that both databases
cover have a difference of less than 10 percent, with 10 to 30 percent differences
scattered along the continental shelf break. Therefore the DNC database supports
the accuracy of the bathymetric values in the NOS database along the United
States continental shelf.
The DNC database is used in conjunction with the NOS and ETOPO5
databases to create a new composite bathymetry set for the Eastcoast 2001 grid,
Figure 16. This new bathymetry is based on a priority/availability system. In
areas where NOS values, which use a gathering/averaging procedure, are avail-
able, they are used. The secondary database used is the DNC, and the third is
ETOPO5 if no other sources are available, both of which are interpolated
databases. Bathymetry for the southern Louisiana area is provided by regional
bathymetric surveys from the U.S. Army Engineer District, New Orleans
(Westerink, Luettich, and Pourtaheri 2000).
13
Chapter 3 Eastcoast 2001 Grid Development