In applying the Colby (1964) method, the required data are: the mean velocity, the depth,

the median size or size distribution of the bed material, the water temperature and the fine

sediment (wash load) concentration. The mean velocity and the depth of flow may be

obtained by hydraulic calculations, as in Problem 3. However, to obtain best results from

calculations, directly measured values of velocity and depth are required.

For purposes of comparison, the sample problem used to illustrate Einstein's method is

solved using the Colby method. The required data are taken from Table 4.8 established by

hydraulic calculations. In addition, the water temperature and the fine sediment concentration

are assumed to equal 70F and 10,000 ppm, respectively. For convenience, the calculations

are summarized in tables. Two procedures are presented. Table 4.10 gives the calculations

using the mean diameter of bed material, whereas Table 4.11 gives the calculations for

individual fractions using the bed material size distribution. Note that depths, y, of 5.07 ft and

6.03 ft from Table 4.8 are not shown in Tables 4.10 and 4.11 since the corresponding

velocities are greater than 10 ft/sec. The Colby curves (Figure 4.13) are limited to 10 ft/sec.

Table 4.10. Bed Material Load Calculations for Sample Problem by Applying the Colby

Method (Median Diameter).

y

W

V

qn

k1

k2

k3

qT

QT

1.36

103

2.93

14.5

0.92

1.20

0.99

16

1,648

1.76

136

4.44

50.0

0.91

1.21

0.99

56

7,616

2.50

170

6.63

135.0

0.91

1.22

0.99

150

25,500

3.30

194

8.38

220.0

0.90

1.23

0.99

243

47,142

4.14

234

9.92

325.0

0.90

1.25

0.99

365

85,410

Notes:

(1)

y, ft (mean depth), taken from Table 4.8

(2)

W, ft (surface width), taken from Table 4.8

(3)

V, fps (average velocity), taken from Table 4.8

qn tons/day-ft (uncorrected sediment discharge per unit width of channel), taken from Figure 4.13

(4)

for the given V, y, and D50

(5)

k1 = f (y, T) (correction factor for temperature), given in Figure 4.14

k2 = f (y, Cf) (correction factor for fine sediment concentration), given in Figure 4.14

(6)

(7)

k3 = f (D50) (correction factor for sediment size), given in Figure 4.14

qT = [1+(k1k2 -1) k3]qn tons/day-ft (true bed material discharge per unit width of stream), given by

(8)

Equation 4.46

QT = WqT tons/day (bed material discharge for all size fractions for entire cross section)

(9)

4.50

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