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

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the left and

for the channel and

were estimated

IOn

roughness coefficient

Manning's

Values of

Methodolo

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field observations and

Estimates were based on

reaches

each of the study

overbanks for

right

Administration

Highway

Federal

the

outlined in

the procedures

were computed uSing

was

tributaries

its

Gulch and

Lena

Upper

on

floodplain

100-year

the

The delineation of

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Natural Channels and

Coefficient for

Selecting Manning's Roughness

for

publication "Guide

Surface Profiles computer

Engineers HEC-2 Water

the U.S. Army Corps of

accomplished using

I

for a particular

method estimates Manning's "n" by assigning a base value

This

Floodplains

use In computing water

model was developed for

Surface Profiles

The HEC-2 Water

model

the variations in

rregularity of the channel

type and applYing adjustments for the degree of

so

the

The computational procedure used in

man-made channels

surface profiles In natural or

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of vegetation. and the degree of

the amount

the effects of obstructions

channel cross-sections

model capabilities is beyond

A complete description of

the Standard Step Method

model is

from

channels varied

for

the hydraulic model

Manning's "n" used in

of the channe

meander

1) flow is

some basic assumptions used by the program are

report, however

the scope of this

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from 0.020 to 0,050

The values for the overbanks varied

0.020 to 0.035

channel slopes are

is one dimensional: and 4)

flow

3)

is gradually varied

flow

steady; 2)

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relatively smal

Normal

special bridge options

the normal bridge or

Bridges were modeled in HEC-2 using

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the structure. This is most

the principal loss through

friction is

bridge methods assume channel

that the cross-

which means

rigid boundaries

the channel has

The hydraulic model assumes

the section is not obstructed by a large number of

when

very long structures or

for

applicable

bank deterioration

to account for erosion or

rate

time or flow

section shape does not vary with

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openings

irregular

highly

for

appropriate

s also most

The normal bridge method

bents

pier

Any changes in channel geometry are

storm

100-year

for the

This is generally appropriate

transient and are not

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Special bridge options are

section

trapezoidal

representation by a simple

which defy

mits

to significantly affect the floodplain

ikely

I

when

bridge hydraulics or

represent

momentum loss computations best

appropriate when

pressure flow is expected

that flow is controlled by

mplies

This

to be sub-critical

the channel was assumed

Flow in

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reach

the intervening

function of downstream energy and losses In

effects and is a

backwater

coefficient of 2.6 was used

A weir

the possibility of overflows

All bridges were modeled with

the determination of flood

the delineation of floodplains and

This is a common assumption In

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overflows often

weir

extreme undersizing

n cases of

and an overflow section was defined

depths of flow below critical depth are not

Because of the sub-critical assumption

profiles

I

roadway overtopping occurred but

Where

conveyance

of

amount

a significant

accounted for

flood profile

This assumption produces a conservative estimate of the

mathematically possible

further

the bridge overflow options were used without

the main channel

leave

flows did not

the

represents

This assumption also

is deeper than a computed super-critical depth

it

since

I

manipulation

to local anomalies such

s unlikely due

flow

long-term physical condition in which super-critical

as scour-holes and debris snags

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