FLOOD09310

• County: Adams
• Community: Adams County
• Stream Name: Todd Creek
• Basin: South Platte
• Title: Flood Hazard Area Delineation
• Date: 12/1/1985
• Prepared For: Adams County
• Prepared By: UDFCD
• Contract/PO #: &&
• Floodplain - Doc Type: Floodplain Report/Masterplan

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Cross section data were obtained photogrammetrically by digitizing
sections of the stream and floodplain. These sections were taken from
the same photography flown for the topographic mapping used in this
study (Reference 2). The digitized sections were supplemented with
cross secti ons taken from thi s mappi ng as needed. All bri dge and
culvert data were obtained from actual field measurements.

TABLE IV

2-HOUR DESIGN STORMS
TODD CREEK BASIN, COLORADO
AREALLY ADJUSTED FOR 5-10 SQUARE MILES

Overbank:
Channel:

.04 to .06
.04 to .06

Rai nfa 11 (inches)
(min)
Time lO-year 50-year 100-year
5 .03 .03 .03
10 .06 .08 .08
15 .13 .11 .12
20 .23 .18 .21
25 .38 .33 .36
30 .18 .54 .64
35 .09 .26 .36
40 .07 .18 .21
45 .06 .11 .16
50 .05 .11 .13
55 .05 .07 .11
60 .05 .07 .11
65 .05 .07 .11
70 .05 .05 .05
75 .05 .05 .05
80 .04 .04 .03
85 .03 .04 .03
90 .03 .03 .03
95 .03 .03 .03
100 .03 .03 .03
105 .03 .03 .03
110 .03 .03 .03
115 .03 .03 .03
120 .02 .03 .03
TOTALS 1.80- 2.53- 3.00-

Roughness coefficients (Manning's "n") for the channel and overbank
areas were estimated by field inspection of the study area and
procedures outlined by Chow (Reference 1). Typical ranges for these
values are as follows:

In some areas, the higher On" values were used to account for
obstructions, such as trees, within the floodplain. In other areas,
the effective flow limits were accounted for by uSing encroachment
options with the HEC-2 model.

At certain locations, such as dam embankments, flood flows diverge
from the main flow path of the stream and become hydraulically
disconnected from the flows in the channel. These areas are
identified on the floodplain mapping. The extent of such flooding was
determined by estimating the hydraulic properties of the conveyance
area. A description of the flooded areas and hydraulic assumptions is
detailed later in this report. The computed flood profile and limits
of the 100-year floodplain are shown on the Flood Hazard Area
Delineation Plan and Profile Drawings. Tabulated data regarding cross
sections, stationing, flood elevations, and other pertinent hydraulic
information are found in Table V. Specific information regarding the
hydraulic calculations is available from the Urban Drainage and Flood
Control District.

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