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
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
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