FLOOD06381

• County: Otero
• Community: La Junta
• Stream Name: Arkansas River & Tributaries
• Basin: Arkansas
• Title: La Junta, Colorado Local Protection Project Phase I GDM Sediment Investigation
• Date: 9/1/1985
• Prepared For: US Army Corps of Engineers
• Prepared By: The Hydrologic Engineering Center
• Floodplain - Doc Type: Project

where: SR = scour rate in ft/day
Q~ = net bedload transport rate in ft'/day
W = stream width in ft. (1000 ft)
L = reach length in ft. (1800 ft)

4) Critical Velocity - Although computed bed shear stress is the preferred
quantity for dealing with critical conditions to determine the incipient
motion of bed sediments, flow velocities can also be used. The Hjulstrom mean
velocity criterion (11, p. 102) gives the critical velocity for a given
particle diameter (see below).

"'C 6.0
140

~2.0
~
0:: 1.0
~O_6
~
} 0.4

20

~

10

o
o
200 ~

100 ~
60
40 I
~
200

02

10 ~
~

0.1
0.001 0.004 0.01 0.02 0.04 0.1 0.2 0.40.6 I 2 4 6 10 20 40 60 100
Mean sediment size, in millimeters

Hjulstrom Diagram

The Dso grain size (0.3mm) would be removed from the bed at velocities
above 0.9 ftlsec. The 090 grain size (l.Omm) would be removed at
velocities above 1.1 ftlsec. The mean flow velocities for all three of the
single-event floods considered in the analysis were substantially greater than
these critical velocites (they were on the order of 10 or 15 ftlsec). Thus
most of the grain sizes present in the bed would become mobile at the
velocities associated with flood flows.

5.5.4 Hand Computation Results

The calculations (1), (2) and (3) were applied to the computational reach
associated with the cross section at river miles 1197.2 (deposition) and
1198.9 (scour). Calculations were made for existing and project conditions
for each of the three single-event floods. The Dso grain size (0.3mm) was
used to calculate the critical shear stress. Computed results from HEC-6 were
used to obtain the water depths and energy slopes. The results are presented
in Table 5.4 and 5.5.

The Hand computations showed that for existing conditions only a small
amount of deposit is expected at the downstream end of the proposed levees
(river mile 1197.2). Existing conditions indicated a deposition rate of 0.06,
0.08 and 0.09 ft/day corresponding to the 50-year, 100-year and standard
project flood, respectively. Project #3 conditions indicated rates of
depostion of 2.1, 3.3 and 15.0 ft/day respectively.

60