FLOOD07353

• County: Statewide
• Basin: Statewide
• Title: Hydraulic Engineering volume 2
• Date: 1/1/1994
• Prepared By: American Society of Civil Engineers
• Floodplain - Doc Type: Educational/Technical/Reference Information

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HYDRAULIC ENGINEERING '94

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flow, and were designed to model the prototype's minimum Frauds number,
slope. and velocity at the flood peak. This was accomplished using 8 model
scale of 1 :32.1 based on Froudian criteria. Sediment was introduced into the
flume by a constant feed sediment hopper. Water depths in the flume were
measured in stilling wells. The flow exited the flume in free flow.

Rouahness Coefficients. The increase in the Manning's roughness
coefficient due to bed load transport was determined by the slope-area method,
using the known discharge. friction slope, and the hydraulic properties of the
flume. The Manning's n.value of the flume was determined to be about 0.009
based on clear-water flows.

Flume Results. The tests were conducted using two uniform grain sizes
and a gradation that simulates the gravel portion of the creek bed. The uniform
grain sizes simulate the prototype dS4 (76 ~ 153 mm, mean size 108 mm, C1 =
1.4) and the large boulders found in the armor layer upstream from the
proposed project, d_, (153,305 mm, mean size 216 mm, 0; 1,41, Tho
concentrations ranged up to 5000 ppm, which was considered sufficiently high
to simulate the gravel bed load in Mission Creek. Actual prototype gravel
concentrations were determined by the numerical model study.

Results from the flume tests are shown in Figure 1. Tests for the dmu
material were conducted with concentrations varying from 200 to 5000 ppm,
The gravel bed material was observed to move down the flume in bouncing or
rolling motions at velocities slightly less than the water. Between 200 and 800
ppm, there was a minor increase in roughness from 1.4 to 5.7 percent.
Between 1600 and 3000 ppm, the Manning's roughness coefficient increased
from 8.6 to about 16.4 percent. And at 3250 ppm, the flume tests indicated
that the flow regime becomes unstable and bed forms began to develop.

Tests for the dS4 material were conducted with concentrations varying
from 200 to 3000 ppm, Similar to the d_ tests, the gravel bed material
moved down the flume in bouncing or rolling motions at velocities slightly less
than the water. Between 200 and 800 ppm, there was a minor increase m
roughness from 1.4 to 6.4 percent, Between 1600 and 3000 ppm, tho
Manning's roughness coefficient increased from 8.6 to about 16.4 percent. For
the dS4 material. the flow regime became unstable at about 3000 ppm.

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Tests for the bed material gradation were conducted with concentrations
varying from 500 to 3000 ppm, Between 200 and 1500 ppm, there was a
minor increase in roughness from 2.1 to 7.9 percent. Between 2000 and 3000
ppm, the Manning's roughness coefficient increased from 12.1 to about 14.3
percent. A maximum threshold concentration for the bed material gradation
was not determined.

Review of the flume data reveals two points: 1) the results are not very
sensitive to the grain size for the hydrauliC conditions and grain sizes modelled;
and 2) the flow regime would be supercritical for the anticipated range of gravel
concentrations.

..,< "', ,.,.

BED LOAD ROUGHNESS

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0.0175

0.0\70

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& 0.0165
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'00,0160
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'ii 0.0155
.. 0.0153
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I 0.0150
c

-€I-
.. .... ............ Dmall
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0..
. ..... . ..-....................... ----
0..""

0.0145

0.0140
o 500

1000 1500 2000 2500 3000
Grovel Concentration (ppm)

3500

Figure 1. Effect of bed load transport on hydraulic roughness

Conclusions

Pre~icted bed load transport rates from the numerical model have been
couple~ With measured hydraulic roughnesses from the flume study to
determine the effect of the gravel, cobble and boulder load for the proposed
MISSIon Cree~ channel, Based on the numerical modeling completed by the Los
Angeles District, the m~ximum concentration of large-sized material inflowing
:he proposed co~cr~te~hnBd channel would be about 1300 ppm. Results from
he ~lume study I~dlcate that this maximum concentration would increase the
maXimum hydraulic roughness of the bed from 0,014 to 0,01 53, Additionally
res~lts from. ~he flume study reveal that bed forms would not form unde;
prolect conditions for concentrations up to 3000 ppm.

Appendix I. References

us . Army Eng.ineer District. Los Angeles. 1986. ItFeasibility report and

aCnvlronmentallmpact statement. Lower Mission Creek interim Santa Barbara
ounty streams, California. It '

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anon!, . V.A. (1964), ItTransport of suspended sediment by water."
Transactions, ASCE, paper 2267, 1 1 1: 129, 160,

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