FLOOD10386

• County: Jefferson
• Community: Golden, Lakewood
• Stream Name: Upper Lena Gulch
• Basin: South Platte
• Title: FHAD - Upper Lena Gulch
• Date: 1/1/1993
• Prepared For: UDFCD
• Prepared By: Boyle Engineering Corporation
• Floodplain - Doc Type: Floodplain Report/Masterplan

section was identified where flows were combined A profile was then computed along each
flow path for a range of discharges. The energy at an upstream section where flow was once
again combined was computed. When the sum of the two discharges having the same
computed energy head at the upstream section equalled the total discharge, the proper flow
distribution had been identified. This analysis served to identify the discharge down each flow
path and the water surface profile along the path

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Flow through structures is often partially blocked during high flood flows due to debris
accumulation. The obstruction at structures was estimated using a reduction of the cross-
sectional area of the structure opening. For structures less than 3 feet in diameter, the structure
was assumed fully blocked and was not modeled. For structures whose minimum dimension is
between 3 and 10 feet, a 20 percent reduction in opening area was made. Larger structures
were assumed to be capable of conveying all debris and were not modified in the model.
Major obstructions in the floodplain, such as buildings, were incorporated into the model cross-
section data. In developed areas where residences were within the expected 100-year
floodplain, the obstruction resulting from entire blocks of homes was incorporated in the cross-
section. This included sections where no building was physically present but where effects
from buildings at other sections would have an impact.
Overflows left the main channel at several locations and followed an independent flow path
unrelated to the main channel profile. In these cases, the HEC-2 split flow option was used in
which an overflow section was defined and overflow rates were computed. The downstream
discharge in the main channel was reduced and the overflow discharge was diverted into the
alternate channel. Significant overflows were evaluated and a water surface profile established
for the diverted flow using a separate HEC-2 analysis. In cases where overflows were minor, no
specific profile was computed and downstream main channel discharges were not decreased.
In some cases, flow was divided but discharges were related to downstream channel
conditions rather than the upstream control. At those locations, flow splits were the result of
downstream conditions which affected the discharge along each flow path. The analysis for
this condition required the balancing of energy losses along both flow paths. A downstream

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