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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Two major water supply reservoirs, Standley Lake and Great Western Reservoir,
were given special consideration with respect to estimating peak flows. These
two reservoirs are located in the center portion of the Big Dry Creek basin on
Big Dry Creek and Walnut Creek respectively. In their current configuTation~
these two large water supply reservoirs have significant surface area and
storage volume which greatly impact peak flows downstream. During the pro~
gress of this study, it was recognized by the Urban Drainage and Flood Control
District and the City of Westminster that Standley Lake and Great Western Re-
servoir provide significant flood control benefits which warrant recognition.
Also, the importance of these major water supply reservoirs lessened the like-
lihood that either facility would be abandoned within the foreseeable future.
Other concerns evolved during the study progress which centered around recent
Colorado State legislation (House Bill No. 1185: codified C.R.S. 37-87-104)
allowing priva te reservoir owners to pass inflows through reservoirs into the
natural stream below such reservoirs without liability. The State statute
further provides that private reservoir owners shall not be held liable for
any personal injury or property damage resulting from water escaping from that
reservoir by overflow. The statute also states that private reservoir owners
shall not be held liable for any personal injury or property damage resulting
from the failure or partial failure of the reservoir unless such failure was
proxima tely caused by negligence of the owners.

Both -reservoirs were assumed full during the occurrence of a IOO-year
runoff event. In 0 ther words, the s ta rting wa ter surface eleva tions
for flood routing purposes were assuming equal to the future spillway
crest elevations.

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The future principle outlet works (pipes and control facilities) were
assumed operating at maximum capacity simultaneous with the occurrence
of a lOa-year flood.

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The principle outlet works were assumed operating at normal capacity
for estimating flood discharges less than the lOa-year flood (i.e. 2-,
5- and 10-year floods).

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HYDROLOGIC ANALYSES

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In addressing the issue of dam owner liability raised by House Bill No. 1185,
the Urban Drainage and Flood Control District (UD&FCD) adopted policy which
essentia lly recognized the recent S ta te legis la tion and required tha t the
flood hydrology and corresponding flood hazard area delineations downstream
from existing water supply and other non-flood control reservoirs ~ consider
the inadvertent flood routing provided by such facilities unless "adequate as-
surances" are obtained to preserve the flood routing characteristics of the
reservoirs. UD&FCD policy further required that, at a minimum, the "adequate
assurances" include mas ter plan recogni tion of the reservoirs flood rou ting
capability and the need to preserve such flood routing capability should re-
servoir owners attempt to make changes which would decrease the reservoirs
flood routing capability. Also, agreements must be executed between the Dis-
trict and the affected local governments which express the intent of the par-
ties to maintain these lIassurances" by whatever means available. The above
policy was adopted by the UD&FCD Board of Directors on June 5, 1986, via Reso-
lution No. 36, Series of 1986.

Since no stream gage data are available for the study streams, a rainfall-
runoff ana lysis was conduc ted on the wa tershed to de termine the flood dis-
charges for the selected 2-, 5-, 10-, and 100-year recurrence intervals. This
analysis was performed in December of 1983 and utilized the most current hy-
drologic data available at that time. The analysis was accomplished by using
the UD&FCD CUHP-D rainfall-runoff computer model to develop the storm hydro-
graphs (Reference 8), and the U.S. Army Corps of Engineers (USACOE) HEC-l
flood hydrograph package (Reference 9) for channel and reservoir routings.

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In July of 1985, the UD&FCD contracted with Muller Engineering Company, Inc.
to develop an Outfall System study for tributaries to Big Dry Creek within the
Westminster area. A hydrologic analysis was also prepared for this study;
however, the UD&FCD's CUHPE program, interconnected with the modified version
of the EPA's SWMM model, was used to develop and route the storm hydrographs.
As a result of the more detailed evaluation, differences in peak flow rates on
the tributaries occurred. Because of the discrepancies and the desire to
maintain consistency in peak flows between the two _studies, it was decided by
the UD&FCD to use the more recently developed flows prepared by Huller
Engineering for the Big Dry Creek tributaries. However, for Big Dry Creek, it
was felt that the differences in peak flows on the tributaries would have an
insignificant impact on the Big Dry Creek flows; and therefore, the original
analysis by Greiner Engineering was maintained for this creek.

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Since the adoption of Resolution No. 36, assurances have been obtained from
the City of Westminster to preserve the flood routing capabilities of Standley
Lake and Great l-iestern Reservoir. In consideration of these assurances, it
was determined appropriate for the flood hazard area delineation presented in
this report to be based on the following conditions:

The basin parameters and peak flows developed by Muller Engineering can be
found in Tables 7 and in the discharge profiles contained within this report.
The basin de linea tions, identification and design points are shown on the
Drainage Basin Boundaries Map, Sheet 4A of 83, located in the Appendix.
For more detailed information on the Muller hydrology, one should consult the

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Future proposed reservoir improvements for both Standley Lake and Great
Western Reservoir were assumed for calculating routed flood flows (Re-
ferences 6 and 7). PrQPosed improvements include increasing reservoir
storage capacity, improving the outlet works and enlarging the emer-
gency spillways. As a result of these future improvements, flow capa-
ci ties and es tima ted lOO-yea r release ra tes were higher than the es ti-
mates calculated for the existing reservoir configurations.

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