Geology Reference
In-Depth Information
TABLE 17.1 Basic Geologic Data Needed for Evaluating Resource Uses in a Region
physically and biologically capable of providing an
identified resource may not be acceptable for other
reasons (such as those listed in 2b above). Some plan-
ners make a distinction between capability and suit-
ability using the term suitability for regions that are
capable (geologically) and also acceptable in terms of
economic, social, political, and cultural factors.
Maps such as geologic maps and topographic
maps are known as basic data maps. Examples of how
geologic variables affect various resource uses are
given in Table 17.1. Note that if septic tanks are to be
used for home sewage treatment, bedrock depth, soil
permeability, slope, flooding, water-table depth, and
stoniness are considered to be important. However,
simply knowing that these are important factors in the
use of the land for septic tanks is not enough if we
want to make a map showing which areas are actually
capable of this resource use. We also need limiting val-
ues for the factors that have an impact on septic tank
use. For example, what bedrock depth would be dele-
terious for septic tank use? Usually such values for
any factor are quantified into a threefold classification
as most capable, capable, and not capable. For exam-
ple, in Table 17.2 water-table depth as it affects resi-
dential development is classified as most capable
(>5ft), capable (2.5-5 ft), and not capable (<2.5ft).
Sometimes these capabilities are depicted by a color
system on a map, with red = not capable, yellow = capa-
ble with some restrictions, and green = most capable.
This color system is known as the stoplight code.
Each factor important in residential development
must be evaluated for each small unit of the map area.
If a unit on the map contains a "not capable" value for
a factor for residential development, then that area has
to be excluded from the capable category of the
resource capability map. Other criteria for residential
development are evaluated on a weighted basis to
determine the overall capability of a site.
TABLE 17.2 Defining Factor Limits for Determining Resource Capability for Residential Development
Factor (Basic data class)
Most Capable
Capable
Not Capable
Water-table depth
>5ft
2.5-5 ft
<2.5 ft
Bedrock depth
> 5 ft
3.3-5 ft
<3.3 ft
Flooding
none
rare
frequent/occasional
Shrink-swell
low
moderate
high
Frost action
low
moderate
high
Groundwater availability
5-10 gpm
>10 gpm
<2-5 gpm
Slope
4-9%
0-3%, 10-15%
> 15%
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