FLOOD09354

• County: Douglas
• Community: Douglas County
• Basin: South Platte
• Title: South Platte River Post Wildfire Floodplain Study, Hayman Burn Area, Douglas County
• Date: 11/24/2004
• Prepared For: CWCB and Douglas County Public Works
• Prepared By: Anderson Consulting Engineers
• Floodplain - Doc Type: Floodplain Report/Masterplan

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along the study reach in different combinations. The first terrain ope consists of a steep,
confined channel bordered by mediwn to dense willows and rocky side slopes, which transition
steeply to either the adjacent county road or to steep forested slopes. The second terrain l)pe
consists of a more open river corridor; the channel is bordered by dense \\iIl0\\'5 with relatively
flat Q\'erbanks of primarily shon grass meadows. which also transition to steep forested slopes.
but at a greater distance from the main channel. Photographic documentation of the study area
is included in Appendix C. A summary of the general owrbank ground cover types and their
associated Manning's n values are included in Table 4.1.

Table ~.l Summar)' of Ground Cover T):pes and j\.tanning's Rou2hness Coefficients
for OHrbank Areas along the Soulh Platte Rin'r.

O\-'erbank Ground COHr Type :\tanning's Roughness Coefficients
County Road (Paved) 0.020
Short Grass Meadow 0.035 to 0.045
Steep Forested Slopes 0.070 to 0,10
Dense Willows 0.10

Included \\ith the general assessment of channel and overbank conditions was the
definition onvlanning's roughness coefficients (n values) for charmels and overbanks throughout
the study reach. The assignment of Manning's n values for the channels was based on the
application of Cowan's Method [Chow, 1959]. Manning's n values for the overbank areas were
defined based on generally accepted roughness coefficients for shallow flow areas given
observed ground cover and approximate flov,,' depths, with adjustments made to account for
obstructions and other ground conditions influencing relative roughness and effective
conveyance. Subsequent to field reconnaissance efforts and the determination of roughness
coefticients, it became evident that the ~'fanning's n values used in the USGSfFIS hydraulic
model were not unifonnly consistent \\ith field conditions observed in 2004. Consequently,
channel and overbank Manning's n values for select cross sections \\ithin the USGSfFIS reach
near Deckers were revised based on field observations of adjacent reaches and an e\"aluation of
color aerial photographs.
It was determined that channel \lanning's n values range from 0.040 to 0.050 within the
study reach: overbank n values vary from 0.035 to 0.10, except for an n value of 0.020 for the
county road which often parallels the river. Horizontal variations in ~lanning"s n values were
detined for cross sections \\ilh overbank areas where more than one type of ground cover is
present. A summary of roughness coefficients defined by the current study for all cross sections
\\ithin the South Plane River study reach is provided in Table 4.2. Detailed information

CQCWCBOJ_South PlaueJepon doc

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