chapter 3 hydraulics of open channel flow

3.38 Chapter Three
HYDRAULICS OF OPEN CHANNEL FLOW
Type of Channel Minimum Normal Maximum
D–3 Major streams (top width at flood stage
> 100 ft); the n value is less that for
minor streams of similar description
because banks offer less effective resistance
a. Regular section with no boulders
or brush 0.025 — 0.060
b. Irregular and rough section 0.035 — 0.100
D–4 Alluvial sandbed channels (no vegetation
and data is limited to sand channels with
D50 < 1.0 mm
a. Tranquil flow, Fr < 1
1. Plane bed 0.014 — 0.020
2. Ripples 0.018 — 0.030
3. Dunes 0.020 — 0.040
4. Washed out dunes or transition 0.014 — 0.025
b. Rapid flow, Fr > 1
1. Standing waves 0.010 — 0.015
2. Antidunes 0.012 — 0.020
E. Overland Flow (Sheetflow)
E–1 Vegetated areas
a. Dense turf 0.17 — 0.80
b. Bermuda and dense grass 0.17 — 0.48
c. Average grass cover 0.20 — 0.40
d. Poor grass cover on rough surface 0.20 — 0.30
e. Short prairie grass 0.10 — 0.20
f. Shrubs and forest litter, pasture 0.30 — 0.40
g. Sparse vegetation 0.05 — 0.13
h. Sparse rangeland with debris
1. 0% cover 0.09 — 0.34
2. 20% cover 0.05 — 0.25
E–2 Plowed or tilled fields
a. Fallow—no residue 0.008 — 0.012
b. Conventional tillage 0.06 — 0.22
c. Chisel plow 0.06 — 0.16
d. Fall disking 0.30 — 0.50
e. No till—no residue 0.04 — 0.10
f. No till (20–40% residue cover) 0.07 — 0.17
g. No till (100% residue cover) 0.17 — 0.47
E–3 Other surfaces
a. Open ground with debris 0.10 — 0.20
b. Shallow flow on asphalt or
concrete 0.10 — 0.15
c. Fallow fields 0.08 — 0.12
d. Open ground, no debris 0.04 — 0.10
f. Asphalt or concrete 0.02 — 0.05
Source: From Chow (1959), Richardson et al. (1987), Simons, Li, & Associates (SLA),
1982, and others. * The values in bold are recommended for design
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