FLOOD03999

• County: Elbert
• Community: Elbert County
• Stream Name: Arkansas River
• Basin: Arkansas
• Title: Flood-Prone Areas in Elbert County
• Date: 1/1/1982
• Prepared For: State of Colorado
• Prepared By: USGS
• Floodplain - Doc Type: Floodplain Report/Masterplan

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acre-foot.--A volume of water covering I acre to a depth of I foot;
43,560 cubic feet.
cross section.--A representation of the area between a streambed and
an imaginary level string stretched across a stream at right an-
gles to the streamflow. The size and shape of the cross section
is determined by using surveying instruments to measure distances
down to the streambed at various locations along the imaginary
s~ring. Floods usually will change the cross-section configura-
. tlon unless the streambed is composed of stable material.
cub~c foot per second.--A unit of discharge passing a cross section of
a stream. Toe unit of discharge represents I cubic foot of water
passing a given point in I second and is equivalent to approxi-
mately n gallons per second or 450 gallons per minute.
discr.arge.--The water flowing past a particular point, such as a point
at a cross section. The rate of flow generally is measured in
cubic feet per second; the volume of flow, in acre-feet.
drainage area.--A land area. usually measured In square miles in a
norizontal plane, which collects precipitation and allows the
resultant runoff to flow towards the most downstream point In the
area..
envelope curve.--A line drawn above plotted points on a graph to
enclose the largest floods ever measured in a region; the points
show stream discharges for given drainage areas,. Larger floods
are possible; if they occur, the envelope curve is moved upward
to enclose the new, higher values.
f100d plain.--An area that is adjacent to the streambed and is covered
with water when the stream overflows its banks during floods.
f1ood-prone area map.--A map showing areas along the streambanks which
are subject to flooding. Areas expected to be covered by water at
least once in any lOa-year period are shown on the Elbert County
flood-prone area maps.
flood velocity.--Speed with which water moves downstream. A velocity
of 10 feet per second (nearly 7 miles per hour) would move an ob~
ject floating in the center of the stream one township (6 miles)
farther downstream in less than I hour.
floodwater-retarding struature.--A dam placed across a small upstream
water course to provide floodwater storage. Earthen dams con-
,s t ructed in Elbert County, by the U.S. So i 1 Conservat ion Serv i ce
in the 1950's ~ere designed to hold about I inch of runoff from
the basin above and to release it gradually through pipes over a
period of 2 to 3 days. Excess runoff passes over the emergency
spillways at one side of the dams. The structures delay storm-
water traveling towards the larger creek ~hannels and help reduce
consequent flooding.
hundred-year (lOO-year) f1ood.--Peak discharges so large that they may
be expected to be equaled or exceeded on the long-term average
of about once every lOa years and have a I-percent chance of oc-
curring in any given year. The recurrence interval Is an average;
no schedule of regularity is implied.
runoff.--That part of the precipitation that appears in
streams. One inch of runoff is equal to about 27 cubic
second of water flowing for one day from each square
drainage area above the location considered.
stream slope.--The vertical distance a stream drops in traveling a
given horizontal distance. When the stream falls more than about
50 feet per mile, floodflows are shallow and very fast and may
cause considerable channel erosion. As the slope flattens to
less than about 50 feet per mile (a I-foot drop about every
lOa feet), floodflows become relatively deep and spread out as
the water velocity slows.
topographic map.--A map that shows surf"<:eceatures of a given area.

surface
feet per
mi Ie of

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