ETL 1110-2-367
31 Mar 95
Existing conditions
Appendix B
The present land use and related conditions occurring under
Glossary of Terms
existing and authorized improvements, laws, and policies.
Exterior stage
Agricultural areas
Water surface level on the unprotected (exterior) side of the
Lands intended primarily for crop production, pastures, and
line-of-protection.
other similar uses, including the closely associated facilities of
on-farm roads, fences, etc.
Feasibility investigations
Planning studies performed in response to specific
Base conditions
Congressional authorization to determine the feasibility of
The land use and related conditions expected to exist at the
adopting Federal projects or modifying existing projects. The
beginning of the first year of project operation.
report is a decision document used to determine the desirability
of authorizing a Federal commitment to a project.
Blocked gravity conditions
Conditions that exist when exterior stages are higher than
Future conditions
interior stages, thus preventing flow of interior flood waters
The most likely land use and related conditions expected in the
through the gravity outlets.
future. Conditions other than those deemed the most likely may
also be considered future conditions.
Coincident probability (Frequency)
Probability of flooding to exceed a given elevation based on the
Gravity outlet rating table
joint probability of flooding from each source.
This table lists the headwater depth required for a range of outlet
flow rates and tailwater depths.
Conditional probability P(a/b)
The probability of flooding from one source given the stage or
Gravity outlets
discharge from another source exceeds a stated level.
Culverts, conduits, or other similar conveyance openings
through the line-of-protection that permit discharge of interior
floodwaters through the line-of-protection by gravity when the
The degree to which flooding from one source occurs or can be
exterior stages are lower than interior stages. Gravity outlets are
predicted from flooding from another source; a mutual
equipped with gates to prevent river flows from entering the
relationship but not necessarily cause and effect.
protected area during time of high exterior stages.
Culvert
Headwater
A relatively short length of closed conduit that connects two
The depth of water at a culvert on the entrance or upstream side,
open channel segments or bodies of water. Culverts are the
as measured from the upstream invert of the culvert.
most common types of gravity outlets for interior areas.
Hypothetical frequency storms
Dependence
Balanced storm distributions with total rainfall amounts
The degree to which flooding of an area from one source is
consistent with specific exceedance frequencies or recurrence
physically and meteorologically related to flooding from another
intervals for each time duration.
source.
Independence
Detention storage areas
Situation in which flooding of an area from one source is
Any low area, often near the inlets to gravity outlets, pumping
unrelated to flooding from another source.
stations, or pressure conduits, used to temporarily store interior
flood waters in excess of the rate at which these flows can be
Index location
passed through the line-of-protection.
A point along the main exterior river where recorded or
computed stage hydrograph data are available.
Discrete events
Flood events in a series that may be considered individually
Inlet control
since they are independent of other flood events in the series.
A condition where flow capacity of the culvert entrance is less
than the flow capacity of the culvert barrel, typically resulting in
Diversions
supercritical flow.
Ditches or conduits designed to bypass flood waters around or
away from a specific area.
B-1
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