FLOOD09463

• Designation Number: 467
• County: Boulder
• Community: Lyons
• Stream Name: North St. Vrain, St. Vrain Creeks
• Basin: South Platte
• Title: Floodplain Information Report - North St. Vrain and St. Vrain Creeks, Floodplain Update, Lyons, Boulder County, Colorado
• Date: 6/1/1998
• Designation Date: 7/1/1998
• Floodplain - Doc Type: Floodplain Report/Masterplan

North St. Vrain & St. Vrain Creeks
Floodplain Update

Town of Lyons

North St. Vrain & St. Vrain Creeks
Floodplain Update

Town of Lyons

GLOSSARY

Flood Profile. A graph showing the relationship of water surface elevation to location, the latter
generally expressed as distance above mouth for a stream of water flowing in an open
channel. It is generally drawn to show surface elevation for the crest of a specific flood,
but may be prepared for conditions at a given time or stage.

Flood Stage. The stage or elevation at which overflow of the natural banks of a stream or body
of water begins in the reach or area in which the elevation is measured.

Intermediate Regional Flood. A type of flood, including the water surface elevation and
territorial occupation thereof, which can be expected to occur at any time in a given area
based upon recorded historical precipitation and other valid data, but with an average
statistical one percent chance of being equaled or exceeded during anyone year. The
term is used interchangeably with a one- percent flood or 100-year flood (see definition
for 100-year flood).

Left Bank. The bank on the left side of a river, stream or water course as the observer looks
downstream.

Probability. The annual chance of occurrence of specific hydrologic events, such as rainfall over
a specified area or peak discharge at a specified location expressed in percent, e.g. 5%
representing one change in 20 of the event occurring in any year or an average
recurrence of once in 20 years.

Recurrence Interval. The average number of years within which a given flood event will be
equaled or exceeded.

Reference Point. A numbered point identifying a specific location for correlating the data
shown in various forms throughout the report.

Right Bank. The bank on the right side of a river, stream or water course, looking downstream.

Runoff. The quantity of rainfall which flows over the surface to enter the stream as discharge
volume. The difference in quantity between rainfall and runoff represents losses in
infiltration and interception.

Stream. Any natural channel or depression through which water flows either continuously,
intermittently or periodically, including any artificial modification of the natural channel
or depression.

Watershed. The drainage area situated above a specified point on a stream including the area
drained by tributary streams, which enter the main stream above this point.

Backwater Effect. The rise in surface elevation of flowing water upstream from and as a result
of an obstruction to flow.

Channel. A natural or artificial watercourse of perceptible extent with definite bed and banks to
confine and convey continuously or periodically flowing water.

Cloudburst. A sudden and extremely heavy downpour of rain that is small in areal extent; of
short duration; and may be accompanied by lightning, thunder, and strong gusts of
wind.

Designated Floodplain. The area designated as a floodplain by official action of the Board of
County Commissioners or City Council with the prior concurrence of the Colorado
Water Conservation Board.

Flood. An overflow on lands not normally covered by water and that is used or usable by man.
Floods have two essential characteristics; the inundation of land is temporary; and the
land is adjacent to and inundated by overflow from a river of stream or an ocean, lake or
other body of standing water.

Normally a flood is considered as any temporary rise in stream flow or stage, but not the
ponding of surface water, that results in significant adverse effects in the vicinity.
Adverse effects may include damages from overflow of land areas, temporary
backwater effects in sewers and local drainage channels, creation of unsanitary
conditions or other unfavorable situations by deposition of material sin stream channels
during flood recessions, use of groundwater coincident with increased stream flow, and
other problems.

Flood. lOO-Year. A type of flood, including the water surface elevation and territorial
occupation thereof, which can be expected to occur at any time in a given area based
upon recorded historical precipitation and other valid data, but with an average
statistical one percent chance of being equaled or exceeded during anyone year. The
term is used interchangeably with a one- percent flood or Intermediate Regional Flood
(see definition of Intermediate Flood).

Flood Crest. The maximum stage or elevation reached by the water of a flood at a given
location.

Flood Frequency. (See Probability.) The average recurrence interval of specific discharges of
water stages which cause flooding.

Flood Peak. The maximum instantaneous discharge of a flood at a given location. It usually
occurs at or near the time of the flood crest.

Floodplain. An area in and adjacent to a stream, which is subject to flooding as the result of the
occurrence of a 100-year flood and which area thus is so adverse to past, current, or
foreseeable construction or land use as to constitute a significant hazard to public health
and safety or to property.

Floodplain Information Report 35
K:\356001004\FLOODPLAIN INFORMATION REPORT.doc

June 1998

Floodplain Information Report 36
K:1356001004\FLOODPLAIN INFORMATION REPORTdoc

June 1998