Bathymetry data for the selected region were obtained from a variety of
sources and adjusted to a mean tide level (mtl) datum. Survey data were used
where available, otherwise elevations were obtained from National Oceanic and
Atmospheric Administration (NOAA) Earth Topography Five Minute Grid
(ETOP05) elevation data. Virtually no data existed for the extensive tidal flats
defined as the area between the mean high water (mhw) and mean lower low
(mllw). Because the available data showed reasonable agreement between mhw
and mllw contours, the tidal flats (for the most part) were established as a linear
interpolation between mhw and mllw.
Horizontal coordinates of the irregularly-spaced bathymetry data were
expressed in latitude and longitude, and the corresponding vertical elevations
were in units of meters relative to mean tide level (mtl). Bathymetry over the
modeled region represented the range between el -184 to el +10 ft relative to mtl.
This total range was equivalent to 5.91 cm (2.33 in.) in the model. It was
necessary to add vertical shoreline banks to the carved model to accomodate
tide levels above el +10 mtl. Relevant scaling factors (prototype-to-model ratios)
for the 3-D model are listed in Table 10 along with approximate model
equivalences.
Table 10
Scale Ratios for 3-D Model
Scale
Scale Value
Model Equivalence
NX = 15,000
1,250 ft ≈ 1 in.
Horizontal Scale
NZ = 1,000
Vertical Scale
83 ft = 1 in.
NV =
NZ = 31.6
Velocity Scale
1.6 m/sec = 5 cm/sec
NQ = NX NZ3/2 = 474,341,650
Discharge Scale
203,000 cu m/sec = 0.43 L/sec
The prototype-scale irregularly-spaced bathymetry was first converted to
State Plane coordinates for Alaska Zone 4: 5004, and then interpolated onto a
240 x 1072 uniform grid containing just over one quarter of a million points. The
data were then scaled to model dimensions (inches) by applying the appropriate
scale and unit conversion factors using a Fortran program. In model dimensions,
the points were spaced at one-fourth inch increments along a line going across
the 1.22-m (4-ft) length, and the 1,072 lines spanning the 3.4-m (134-in.) length
were at one-eighth inch spacings. Figure 59 shows a 3-D rendition of the scaled
model. There is no vertical exaggeration in this image (other than the model
distortion). Digital bathymetry files were transferred to the Model Shop's
computer, converted into 3-D surfaces, and then translated into machine path
instructions.
76
Chapter 7 Three-Dimensional Cook Inlet Model
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