WSP03817

• File Number: 8102
• Description: Arkansas River Basin Basic Hydrology
• State: CO
• Basin: Arkansas
• Water Division: 2
• Date: 5/7/1999
• Author: Inter Fluve Inc
• Title: Fluvial Geomorphic Assessment of Upper Arkansas River Final Report
• Water Supply Pro - Doc Type: Report/Study

1.486AR.1J" s /
0=
- n

(Eqn.7.1)

I
I
I
I
I
I
I
,I
,1
I

Comparing empirical equation methods, Cowan (1956) produced an n-value of 0.13,
Limerinos (1970) produced an n-value of 0.05, and Jarrett (1984) produced an n-value of
0.12. The high n-value produced by the Cowan equation was dependent upon the
appreciable degree of meandering found on the Upper Arkansas River, whereas the high
n-value produced by the Jarrett equation was dependent upon the steep slopes within the
stream. In contrast, the Limerinos equation, based upon particle roughness, generated a
lower n-value.

The slope-area procedure was used for further estimation of n-values by rearranging
Manning's equation (Eqn. 7.1 and 7.2), and solving for n-values for each calculated
discharge (Table 7.5). The area (A), discharge (Q), and friction slope (Sf) were measured
in the field during bed load sampling. The wetted perimeter (P) was determined from the
cross-section survey, and, subsequently, the hydraulic radius (R) was calculated.

1.486AR.1J" s y,
n= f
Q

(Eqn. 7.2)

Table 7.5 Manning's n-value calculated using the slope area procedure.

Cross-section Manninl!:'s n-value
AR-O 0.052
AR-I0 0.069
AR-20 0.061

The calculated n-value for cross-section AR-l 0 was substituted into the HEC-RAS
model. The n-value of 0.069 produced the most representative water surface elevation
for the discharge of interest. Additional in-channel roughness evaluations were
performed using an n-value of 0.07, which produced a water surface elevation difference
of+0.03 feet. This difference was considered well within the ability of the model to
accurately represent hydraulic conditions.

Using an n-value of 0,07, rather than 0.05, produced an average respective increase in
bed shear of 12% for all cross-sections. The larger n-value increased shear at
approximately 80% of the cross-sections, thereby incorporating a small factor of safety
for incipient motion calculations. Because of complexities in backwater calculations, the
larger n-value reduced shear at approximately 20% of the cross-sections. Therefore, a
Manning's n-value of 0.07 can be used in an appreciably sinuous, steep, and rough
channel, providing an additional factor of safety as well as satisfying hydraulic boundary
conditions. An n-value of 0.07 was used for 1M Sites 1 through 4, which surround cross-
section AR-I0. Incipient Motion Sites 5 and 6 are further downstream and used an

May 7, 1999

Fluvial Geomorphological Assessment
Upper Arkansas River

Page 56