FLOOD04285

• Designation Number: 270
• County: Adams
• Community: Westminster
• Stream Name: Big Dry Creek & Tributaries
• Basin: South Platte
• Title: Flood Hazard Area Delineation - Big Dry Creek & Tributaries
• Date: 11/1/1986
• Designation Date: 12/1/1987
• Floodplain - Doc Type: Floodplain Report/Masterplan

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BASIN DESCRIPTIONS

FLOOD HISTORY

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The Big Dry Creek Basin originates in unincorporated Jefferson CountYt at the
mouth of Coal Creek Canyon at an elevation of approximately 8,000 feet above
sea leve 1.. The headwa ters a rea is loca ted approxima te iy 5 mi les wes t of the
Rocky Fla ts Plant, 10 miles sou th of the City of Boulder, 20 miles northwes t
of the City of Denver and 10 miles west of the City of Westminster. The basin
drains easterly from the headwaters area across Rocky Flats, where several
tributaries including Walnut, Women and Big Dry Creek form. The basin is
heavily regulated by Great Western and Standley Lake irrigation and water sup-
ply reservoirs which exist in the middle basin. Below Standley Lake, Big Dry
Creek flows in the northeasterly direction, approximately 33 miles to its con-
fluence with the South Platte River near Fort Lupton in Weld County. The
total drainage area at the confluence is approximately 86.3 square miles.
Within the Big Dry Creek study reach from 1-25 to Standley Lake, drainage from
unicorporated Adams and Jefferson Counties, as well as from the cities of
Westminster, Broomfield and Northglenn, enter via a number of tributaries.
The principal tributaries to Big Dry Creek studied in this report are identi-
fied in Table 1 and are shown on Sheet 2 of 83, located in the report appen-
dix.

The Big Dry Creek channel below Standley Lake is a well-defined meandering
channel with a flat sandy bottom and highly erodable side slopes. The typical
tributary channel is broad with native grasses and cattails in the low flow
area. The longer tributary streams, such as Walnut and Airport Creeks which
drain major areas of the basin, have well defined channels with trees along
the channel banks.

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Due to the undeveloped nature of the floodplain areas, little documented his-
tory of flooding exists along the study streams. In May of 1973, flooding oc-
cured along Big Dry Creek in the vicinity of Huron Street and in June, 1984,
flooding occurred along the Middle Branch of Hylands Creek at Sheridan Boule-
vard. Damage was confined to the culvert crossings during the historic
events. Estimates of the historic flood frequencies are not possible, due to
the lack of da ta.

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The surface geology of the area shows that most of the basin is alluvial ter-
race. In addition, there are a number of residual sandstone soils near
Standley Lake, along wi th oelian sand and loess in other areas of the basin.
The bedrock geology of the a rea shows the La ramie forma tion to be genera lly
north of Big Dry Creek and west of Standley Lake. The Denver formation (Ara-
pahoe and Dawson Arkose), is mainly south and east of Big Dry Creek (Reference
2). Soils in the basin consist primarily of the U.S. Department of Agricul-
ture Soil Conservation Service Soil Groups Band C with spotty areas of D soil
upstream of Standley Lake (Reference 3-5). These soils indicate a medium to
low infiltration rate.

The principal cause of flooding results from intense localized thunderstorms
which occur along the Colorado Front Range from April through September.
Snowmelt flooding does not influence the peak discharges due to the relative
low altitude of the basin. During the summer months, warm moist air from the
Gulf of Mexico combines with cool dry air along the eastern plains causing an
upward deflection of the air mass where the unstable air is cooled and preci-
pitation occurs. These storms are characterized by high rainfall intensities
of relatively short duration and produce high peak flows and moderate volumes
of runoff. The largest floods typically occur when heavy rains faU on land
previously saturated by precipitation or on developed urban areas with a high
percentage of impervious land surface.

Potential flood damage a long the study streams is due primari ly to unmain-
tained channel areas with high roughness values, unmaintained culverts full of
sediment, and undersized stream crossings. Damage will be primarily confined
to stream crossings and agricultural lands, however, several areas of residen-
tial development are susceptible to flood damage.

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Land use in the upper basin above Standley and Great Western Reservoirs is
presently undeveloped range land. Due to the expected permanent nature of the
Rocky Flats Plant, the areas tributary to the reservoirs should remain
essentially undeveloped.

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Downstream of the reservoirs, the basin is presently 30% impervious due to re-
sidential, commercial and industrial development. The Big Dry Creek lOa-year
floodplain, as well as the lOa-year floodplains on the principal tributaries,
remain generally undeveloped. Based on future land use projectionstthe ulti-
mate condition land use of the basin from 1-25 to Standley Lake and Great
Western Reservoir will be 50% impervious (Reference 1).

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FLOOD MITIGATION MEASURES

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Existing flood protection measures along the study streams consist primarily
of channelization projects. Middle Cotton Creek, Middle Branch Highlands
Creek, Airport Creek, Countryside Creek and Tanglewood Creek represent some of
the streams involving major channelization projects. Most of the existing
stream channels within the study area are grass-lined with natural channel
bottoms.

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The climate of the study area is Semi-Arid, which is characterized by low rel-
ative humidity, low rainfall, moderate to high wind movement, and a large
daily range in temperatures. A wide range of temperature extremes exist in
the area ranging from -26 degrees to 102 degrees F. The average annual preci-
pitation is 14 to 15 inches. An average of 69% of this precipitation falls
during thunderstorms which occur from April through September.

A number of irrigation ditches cross the channels and floodplains within the
study area. Separation of irrigation and flood flows was assumed for all ir-
rigation structures. In addition, several small privately owned irrigation
and water supply facilities are located within the study area. Examples in-
clude Ketner Lake, Bruchez and Jackson Lakes located within The Ranch area,
Timber Lake along Tanglewood Creek and lakes within the South Branch Highlands
Creek and Airport Creek drainage basins. These facilities were assumed inef-
fective for flood control purposes and inadvertent flood routing was not con-
sidered in the hydrologic analysis of the respective drainage basins.

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