Elevation-Derived Hydrography READ Rules: Underground Conduit
An Underground Conduit is 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 nonsoluble rocks, as by melting of permafrost or ground ice, collapse after mining, and by outflow of liquid lava from beneath its solidified crust.
Attribute/Attribute Value
Each feature requires domain codes to be entered into the attribute table for the feature class (Elevation-Derived Hydrography Feature type description, associated geometry, and use classification table in the Elevation-Derived Hydrography Data Acquisition Specifications 2023 revision A2). See “Field Definitions and Domain Values for Attributes” section for more information on Elevation-Derived Hydrography code definitions.
Delineation
The limit of Underground Conduit: Indefinite is the virtual line connecting two nonadjacent network segments where the National Hydrography Dataset (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
When delineating a feature, it must be created with the appropriate geometry, either point, line, or polygon, which is determined by the size of the feature or the length along different axes of the feature (table 17).
Special conditions: none.
Table 17. Underground Conduit Representation Rules.
| Kind of feature object | Area | Shortest Axis | Longest Axis |
| 0-dimensional (point) | -- | -- | -- |
| 1-dimensional (line) | -- | greater than 0 | -- |
| 2-dimensional (polygon) | -- | -- | -- |
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 nonsoluble 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 for elevation derivative.
Source Interpretation Guidelines
All
The following list of 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.
