Elevation-Derived Hydrography READ Rules: Underground Conduit
A set of naturally occurring subsurface drainage channels formed from the dissolution of soluble rocks in karst terrain or in terrain similar to karst but formed in non-soluble rocks, as by melting of permafrost or ground ice, collapse after mining, and by outflow of liquid lava from beneath its solidified crust (Figure 73).
Delineation
The limit of Underground Conduit is the virtual line connecting two nonadjacent network segments where the NHD has previously placed an underground conduit, or other data sources have mapped the subsurface flowpath between surface water features in karst terrain. Underground Conduit may also be placed in locations where there is evidence of subsurface flowpath between surface water features in thermokarst terrain.
NOTE: A Sink point feature must be placed at the start of an Underground Conduit.
Representation Rules
Delineate features as points, lines, or polygons based on their area or length along different axes (Table 33).
| Kind of feature object | Area | Shortest Axis | Longest Axis |
| 0-dimensional (point) | -- | -- | -- |
| 1-dimensional (line) | -- | greater than 0 | -- |
| 2-dimensional (polygon) | -- | -- | -- |
Special Conditions
None.
Data Extraction
Capture Conditions
If Underground Conduit is required to identify a known or highly probable groundwater flowpath with verified outflow location,
then capture.
Attribute Information
FClass 1—Hydrography feature defined within the collection criteria of the elevation-derived hydrography specifications.
FCode 42002—Underground Conduit: Indefinite (a set of naturally occurring subsurface drainage channels formed from the dissolution of soluble rocks in karst terrain or in terrain similar to karst but formed in non-soluble rocks, as by melting of permafrost or ground ice (thermokarst), collapse after mining, and by outflow of liquid lava from beneath its solidified crust).
EClass 0— Not used to create elevation derivatives.
Source Interpretation Guidelines
The following conditions indicates when and why the capture of Underground Conduit is required:
• When Underground Conduit is part of a network that is represented as being connected.
• When there is a previously existing groundwater dataset, or other data indicating the presence of karst, thermokarst, mining systems, volcanic terrain, or similar groundwater terrain types disrupting the hydrographic network.
Special Conditions
None.