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

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.

Scale

Scale Value

Model Equivalence

1,250 ft ≈ 1 in.

Horizontal Scale

Vertical Scale

83 ft = 1 in.

Velocity Scale

1.6 m/sec = 5 cm/sec

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