K u y1/ 6

o

n=

(3.19)

y

1.16 + 2 log o

D 84

Ku = 0.113

(yo and D84 in meters)

Ku = 0.0927

(yo and D84 in feet)

factor, f, as a function of the hydraulic radius, R, with:

1/ 4

R

1

= 1.9

(3.20)

D

f

84

R

1

= 2.0 log

+ 1.1

(3.21)

D

f

84

Note: R and D84 in same units (meters, feet)

Manning's n can then be calculated from the relationship between R and f :

n = K u R1/ 6 f

(3.22)

Ku = 0.113

(R in meters)

Ku = 0.0927

(R in feet)

This approach should not be applied to river reaches with active beds or significant sediment

transport.

Charlton et al. (1978) and Bray (1979) have proposed the guidelines in Table 3.3 to estimate

D90, D84, and D65, for gravel-bed material when size distribution curves cannot be obtained

from field data.

Table 3.3. Typical Sediment Size Distribution for Gravel-Bed Stream.

Ratio

Mean Value

Standard Deviation

D90/D50

2.1

0.46

D84/D50

1.9

0.36

D65/D50

1.3

0.08

3.31

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