FLOOD09807

• County: Statewide
• Community: Nationwide
• Basin: Statewide
• Title: A Uniform Technique for Determining Flood Flow Frequencies
• Date: 12/1/1967
• Prepared By: Water Resources Council - Hydrology Committee
• Floodplain - Doc Type: Educational/Technical/Reference Information

1. Determine extent of reach to which roughness factor will apply.
2. Determine if and where sul:::division between channel and flood plain
is necessary. [etermine how base n will be assigned.

I FlOOD I PLAIN I
FIlli
o

3. Determine channel type, and estimate conditions at time of flow event;
compare the channel with photographs and description of other channels.

By assigning n for individual segments of
channel and deriving an n for the entire
channel.

4. Determine the factors that cause roughness
and how each will be accounted for.

4. Dete~ine the factors that cause roughness
and how each will be accounted for.
5. ~ntally divide channel into segments so
that the roughness factor within a segment
is fairly unifo~.
6. Determine type and size of boundary material
in each segment.
7. Assign a base n for each segment from tables,
formulas, or comparison with other channels
and verification photographs.
8. Apply adjustJrent factors for individual seg-
ments, if applicable.
9. Select the method for weighting n.

6. Determine type and size of bed material.

7. Assign a base n trom tables, formulas, and
conparison with other channels and verifi-
cation fhotografhs.

lOa. Estimate wetted perimeter for ead1
segment of channel.
b. weight the n values by assigning
weighting factors that are pro-
portional to the wetted perimeter.

lOa. Estimate area for each segment of
channel.
b. weight the n values by assigning
weighting factors that are pro-
pJrtional to the area.

11. Adjust for factors not considered in steps 7 and 8, including channel
alinernent, change in channel shape, vegetation, obstructions, and
meander. fbund off as desired for use in the Manning1s equation.
12. Canpare value determined with that for other channels and verification
photographs to test for reasonableness.
13. Fbr sand channels: C1leck flow regime by c:orrp.lting velocity and stream
power for the above n; determine regi.1re from figure 1. '!he n from table
1 is valid only for upper-regime flow.

Figure 21.--Flow chart of procedures for assigning n values.
(Modified from Aldridge and Garrett, 1973, fig. 3)

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