FLOOD04702

• Designation Number: 347
• County: Jefferson
• Community: Unincorporated Jefferson County
• Stream Name: Upper Lena Gulch
• Basin: South Platte
• Title: Flood Hazard Area Delineation - Upper Lena Gulch
• Date: 1/1/1993
• Designation Date: 9/1/1993
• Floodplain - Doc Type: Floodplain Report/Masterplan

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Damages in this reach are primarily to structures in the Heritage Square

development. In addition, damage from erosion is also expected due to the

steep channel slopes and high velocities. Some pavement damage from

roadway overtopping is also expected to occur.

100-year floodplain information presents the thalweg elevation which is the elevation of the

channel bottom at its lowest point across the floodplain at that station. The discharge is also

tabulated for each section. The computed water surface elevation, mean channel velocity and

floodplain top width are also shown in the tables.

Floodplain/Floodwav Table

Alternate Profile information presents the discharge and corresponding water surface elevation
for the to-year, 50-year and 500-year floods.

A table has been prepared which summarizes the results of the hydraulic analyses. This table

relates the cross sectional reference to the mapping, the computed water surface elevations for

several recurrence interval floods, and water surface elevations and limits of allowable

encroachment for two f1oodways. The table is broken down into four main categories of

information:

The Floodway Information presents the computed floodway data and a definition of f100dway

location relative to the channel reference line. The first f100dway limits in the table, the l.foot

f100dway data, are the limits of encroachment which result in an increase of not more than one

foot in the total energy grade line due to an approximately equal reduction in the conveyance

on both sides of the channel. The second f100dway limits in the table, the 0.5-foot f100dway

data, are the limits of encroachment which result in an increase of not more than six inches in

the total energy grade line due to an approximately equal reduction in the conveyance on both

sides of the channel.

1. Reference Information

2. 100-Year Floodplain Information

3. Alternate Profile Information

4. Floodway Information

Reference information relates the various cross-sections to the appropriate channel reach and
the plan and profile sheet number. The section number is related to the stream stationing
which is marked at 500-foot increments along the drainageway baseline or reference line.

The f100dway limits shown in the tables are developed for floodplain managers as a guideline
for limiting future encroachment into the floodplain which would further reduce the drainageway
capacity. Preventing such encroachrnents will help to eliminate future damages in the event of
flooding.

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