FLOOD10854 ‎(2)‎

• Designation Number: 590
• County: Denver
• Community: Denver
• Title: FIS - Denver - Volume 1
• Date: 11/17/2005
• Prepared For: Denver
• Prepared By: FEMA
• Floodplain - Doc Type: Current FEMA Regulatory Floodplain Information

Marston Lake North

The Marston Lake North drainageway improvements extend from Vaile Lake
upstream to the corporate limits. These improvements include construction of an
energy dissipation plunge pool basin and a golf cart crossing in a portion of the
existing Vaile Lake; installation of two 5-foot high by 10-foot wide
reinforced-concrete box culverts under Quincy Avenue; construction of a grass-lined..,.
channel from South Wadsworth Boulevard to just upstream of Quincy Avenue; the
addition of two 72-inch steel pipes under Highway 121; construction of a 40-foot
transition structure directly upstream of Highway 121; addition of 1,532 linear feet of
8'x7' twin barrel box culvert, and 520 feet of open channel; and construction of an
8.4 acre-feet sedimentation control basin with 85 linear feet of drainage channel
which connects to the existing channel (References 33 and 34). The channel and
culvert improvements convey the 1-percent-annual-chance recurrence interval flood,
with the exception of some overland flow at South Wadsworth Boulevard.

3.0 ENGINEERING METHODS

For the flooding sources studied in detail in the community, standard hydrologic and
hydraulic study methods were used to determine the flood hazard data required for this study.
Flood events of a magnitude that are expected to be equaled or exceeded once on the average
during any 10-, 50-, 100-, or 500-year period (recurrence interval) have been selected as
having special significance for floodplain management and for flood insurance rates. These
events, commonly termed the 10-, 50-, 100-, and 500-year floods, have a 10-, 2-, 1-, and
O.2-percent chance, respectively, of being equaled or exceeded during any year. Although the
recurrence interval represents the long-term, average period between floods of a specific
magnitude, rare floods could occur at short intervals or even within the same year. The risk
of experiencing a rare flood increases when periods greater than 1 year are considered. For
example, the risk of having a flood which equals or exceeds the 100-year flood
(I-percent chance of annual exceedence) in any 50-year period is approximately 40 percent
(4 in 10); for any 90-year period, the risk increases to approximately 60 percent (6 in 10).
The analyses reported herein reflect flooding potentials based on conditions existing in the
community at the time of completion of this study. Maps and flood elevations will be
amended periodically to reflect future changes.

3.1 Hydrologic Analysis

Hydrologic analyses were carried out to establish the peak discharge-frequency
relationships for each flooding source studied in detail affecting the community.

The hydrologic analysis for Clear Creek was performed by the USACE, Omaha
District (Reference 23). Their analysis was based on fully developed basin
conditions. Discharge records for the Clear Creek stream gages at Golden and Derby
were analyzed using methods presented in Bulletin No. 17 published by the
U.S. Water Resources Council (Reference 35). The results of these analyses were
used to calibrate to the following runoff models: the Massachusetts Institute of
Technology Catchment Model (MITCAT) (Reference 36), and the

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