FLOOD00591

• County: Weld
• Community: Greeley
• Stream Name: Cache La Poudre River
• Title: Floodplain and Floodway Analysis
• Date: 2/20/1994
• Prepared For: CWCB
• Prepared By: Brown & Caldwell
• Floodplain - Doc Type: Floodplain Report/Masterplan

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4. MODEl PARAMETERS (from model used to revise 100-year water surface elevation)

1.

Discharges:

Upstream Limit Downstream Limit

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10-year
50- year
I DO-year
500-year

3,500

8,000
10,700

19,000

3,500
8,000
10,700

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19,000

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Attach diagram showing cha.1ges in J DO-year discharge

2.

r:xplain how the starting' water surface elevations were determined Vaules were obtained from
1973 FE~~ data downstream of Ash Avenue for 100-year water surface elevation.

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Also model was started a sufficient distance downstream {1500') to obtain accuratE

water surface elevations at the new Ash Avenue bridge.

3.

Give range of friction loss coefficients (Manning's "N") Channel

0.035 - 0.040

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Overbanks

0.045 - 0.055

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If friction loss coefficients are different anywhere along the revised rea"h from those used to develop the FI RM,
give location, value u~ed in the effective FIS, and revised values and an explanation as to how the revised values
were determined.

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Locution

Frs

Revised

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Downstream Study Lirrit
To Sta 29800

Channel

Overbanks

0.045

0.050

Channel
0.035

Overbanks

0.050

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Explain: River is a natural channel, irregular side slope, even clean and regular

cobblestone bed, and has little variation in cross section. An n value of

0.45 is unreasonable. Chow's "open channel Hydraulics" and normal practice

in region would indicate 0.030 - 0.035.
Describe how the cross section geometry data were determined (e.#., field .'iuruey, topographic map, taken (rom
preuious slud_v) and list cross sections thal were added.

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4.

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Field surveys and digitized aerial mapping, New sections: 25999, 26034,

26054,26074, 26114, 26349, 26787, 27097, 27626, 27427, 27537, 27627, 27647,

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27672, 27697, 27702, 27735, 27825, 27915, 28015, 28105, 28320, 28355, 28755,
29105, 29175

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Riverine Hydraulic Analysn, form

form4

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