FLOOD06828

• Designation Number: 177
• County: Jefferson
• Community: Unincorporated Jefferson County
• Stream Name: Weaver Creek
• Basin: South Platte
• Title: Flood Hazard Area Delineation - Weaver Creek
• Date: 5/1/1981
• Designation Date: 9/1/1981
• Floodplain - Doc Type: Floodplain Report/Masterplan

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-7-

-8-

Input data for the
in Table 1. Design
Information," and

unit hydrograph techniques are summarized
rainfall was taken from "Rainfall/Runoff
is summarized in Table 2, while assumed

Channel and overbank vegetal cover ranges from nothing in

areas

of

new

construction,

to

native

grasses in most open
some channel reaches.

areas,

and

to

dense

brush

along
estimated

on

a

reach by
typical.
points.

reach
Peak

flood volumes are summarized in Table 4
select hydrographs are shown on Figure 2.
for other design points are on file at the
Flood Control District.

and

discharges
Figure l,

and

Roughness

coefficients

were

rainfall

losses

are

shown

in Table

3.

Peak

and

basis,

with

a

range of n ~ .03 to n
were interpolated between

.05
design

Flood hydrographs
Urban Drainage and

discharge data

Hydraulic Analysis

The potential for blockage was assessed for each of the 15
major and the many minor crossing structures and the culvert
area was reduced accordingly. Consideration was given to
structure size and shape, and the potential for debris
generation in the reach above each one. Schematic details of
the structures are shown in the profile portion of the Plan
and Profile drawings. Photographs of the major crossing
structures are on file at the Urban Drainage and Flood
Control District.

The lOa-year flood hazard area, and the 10-, 50- and lOa-year
flood profiles were computed with the aid of the U.S. Army
Corps of Engineers' backwater program HEC-2. This was
supplemented with hand computations where required. Input
data for the hydraulic analysis was taken from detailed
topographic mapping, field surveys of crossing structures,
field inspection and engineering judgment. All backup data
are on file with the Urban Drainage and Flood Control
District.

The lOa-year flood hazard area, and the 10- and lOa-year
flood profiles are shown on the Plan and Profile drawings.
The 50-year flood profile could not be distinguished from the
lOa-year profile, so it was omitted. The calculations are on
file at the Urban Drainage and Flood Control District. Cross-
sections used in the analysis are located on the drawings,
and typical sections are illustrated to give information on
channel and overbank flooding depths. Channel velocities
average 7. 1 fps and overbank velocities, where they exist,
average 2.8 fps. Flood hazard data are referenced by
cross-section number and summarized in Table 5.

Except for short reaches immediately upstream of major
crossings, where flood water backs up, most of Weaver Creek's
slopes are "hydraulically steep," for the lOa-year flood.
This results in a condition characterized by relatively
shallow and fast moving water. Flow is classified as mildly
supercritical, a condition which is unstable. Small
obstructions can cause large fluctuations in the water sur-
face profile. Flow can change rapidly from supercri tical to
subcritical, where depths are greater and velocities slower.
For this reason, the flood profiles for Weaver Creek were
computed with the assumption of subcritical flow. In the
HEC-2 backwater program this forces a critical flow profile
solution. The critical flow profile and the resulting flood
hazard area arc more conservative and more realistic than the

In most reaches, flood waters are confined within channel
banks. Tn some areas, however, when the lOa-year flood
overtops a bank or a road, an area of shallow overland flow
is formed. Where appropriate, it has been included within the
flood hazard area. In such cases, the outside limit of the

C\>

technically correct
Leonard Rice ConsultlOg Water EnglOeers, Inc.

supercritical flow profile.

more

C\> Leonard Rice ConsultlOg Water Engineers, Inc

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