FLOOD04080

• Designation Number: 226
• County: Jefferson
• Community: Lakewood
• Stream Name: Henry's Lake Drainageway
• Basin: South Platte
• Title: Flood Hazard Area Delineation - Henry's Lake Drainageway
• Date: 7/1/1983
• Designation Date: 1/1/1984
• Floodplain - Doc Type: Floodplain Report/Masterplan

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Wadsworth Boulevard heads north beneath Hampden Avenue. For stonns of greater
intensity, a portion of the flow passes directly over Wadsworth Boulevard into the
Academy Park area. The remaining flow stays within Wadsworth Boulevard and passes
north beneath Hampden Avenue. The flow beneath Hampden Avenue causes some shallow
flooding as the runoff meanders northeasterly towards Bear Creek.

The peak flow diagram for the flow split drainageway is presented on Figure
[11-11. Hydrographs for two hydrologic design points are shown on Figures [11-10
and 11[-11.

The magnitude of the flow split under Hampden Avenue at Wadsworth Boulevard
varied from approximately 165 cfs for the 10-year flood to 180 cfs for the 100-year
flood. Since a majority of the flood peaks for the more intense stonns and most of
the flood volume for all stonns will pass downstream of Wadsworth Boulevard, the
total flood peaks and volumes were considered to be routed downstream into Academy
Park and were used to define the flood hazard areas. The hydrographs for this flow
split condition are presented on Figure [[[-8.

Additional discussion relating to both the flow spl it at Wadsworth Boulevard
and Hampden Avenue can be found in the Addendum to Henry's lake Basin hydrology
report (Reference-l0).

The second flow split occurs upstream of Hampden Avenue at the outfall from the
Academy Park area. This flow split was evaluated assuming the entire peak and
volume from upstream of Wadsworth Boulevard discharges into the Academy Park area.
Surcharging of the 76" x 48" HERCP culvert beneath Hampden Avenue will overtop the
frontage road for all stonns with a return period of 5 years or greater. The flow
overtopping the frontage road travels east out of the Henry's lake Basin to Pierce
Street. At Pierce Street, the flow split combines with the local runoff from the
direct flow area and results in still another flow split. Approximately 50% of the
lOO-year peak flow at Pierce Street and Hampden Avenue will flow beneath Hampden
Avenue to Bear Creek. The remaining 50% will continue to flow east along the South
Hampden Avenue frontage road to the next un-named drainageway.

Since improvements of the culvert beneath Hampden Avenue at Academy Park are
not considered viable, the flow split at Academy Park was evaluated separately as
Addendum No. 1 to the original contract. local runoff was detennined by CUHP
analysis (Table 11[-1 and Figure III-I) and combined with the flow split hydrograph
(Figure 111-9) to detennine peak runoff values along the flow split drainageway.
These values were then used in the backwater analysis described in Chapter IV to
delineate the flood hazard areas.

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