3.8.2 PROBLEM 2 Angle of Repose
Determine the angle of repose for well rounded riprap with a D30 of 1 ft = 305 mm.
From Figure 3.4 the angle of repose is 40o.
3.8.3 PROBLEM 3 Resistance to Flow in Alluvial Channels
Manning's n in Sand Bed Streams
(a) Plane bed
Considering only the bed of the stream determine the following:
What is the range of Manning's n values for a plane bed sand channel stream?
Manning's n ranges from 0.010 to 0.013.
What is the Manning n for a plane bed stream with a D50 of 0.32 mm.
Using the Strickler equation as an approximation:
n = 0.0395 D1/6 with D in ft. n = 0.0395 (0.0010 ft)1/6 = 0.013
(b) Antidune Flow
A sandbed channel is observed to have an undulating water surface, a discharge of 850 cfs,
an average velocity of 3.48 ft/sec, a channel width of 105 ft, and a bed slope of 0.003. The
stream bed has a D50 of 0.35 mm. An estimate of the bed form and n-value of the channel is
desired.
Assuming the sieve diameter equals the fall diameter, and the bed slope equals the friction
slope. Determine the stream power.
Stream power equals Vγyo So = (3.48) (62.4) (850/105 x 3.48) (0.003) = 1.53 lb/sec-ft. Figure
3.13 for this stream power indicates that upper flow regime is expected. The bed configuration
should be antidunes with standing waves.
Based upon the bedform the n-value is estimated to be 0.013.
Manning's n in Gravel Bed Streams
Use the following gravel bed material size analysis to estimate the Manning's n value of the
stream.
D16 = 1.8 mm D75 = 4.0 mm D90 = 4.9 mm
D50 = 3.1 mm D84 = 4.4 mm
Using Equations 3.15 through 3.22
3.62
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