Encyclopedia of Environmental Science and Engineering, Volume I and II

SEDIMENT TRANSPORT AND EROSION 1077

b  0.9 P (48)

b  0.92 B –2.0 (49)

R  K 2 Q 0.36 (50)

y  1.21 R for R  7 ft. (51)

y  2 0.93 R for R  7 ft. (52)

v  K 3 ( R^2 S )^ m (53)

C

g

v

gyS

K

vb

v

22

4

037

 =

⎛

⎝⎜

⎞

⎠⎟

.

(54)

in which C  Chézy coefficient; B  surface width; the

values of the K ’s and m are defined in Table 5.

Schumm 17,45 studied the Great Plains rivers in the US

and similar rivers in Australia; he correlated stream geom-

etry with discharge (mean annual flood Q ma ) and the percent

silt-clay, M, in the channel boundary. Some of his correla-

tions are:

Full Bank width  W 

23058

037

.

()

.

.

Q

M

ma

(55)

Width: depth ratio 

W

D

Q

M

(^21) ma
018
074
.
(). (56)
Sinuosity  0.94 M 0.25 , (57)
The high correlation of stream geometry and M led
Schumm to classify channels (see Table 6) using M as an
index to the ratio of coarse load to total load.
Schumm 45,17 associated meandering channels with high
values of M and low bed load and braided channels (a relatively
straight, steep main channel consisting of a maze of sub-channels
sometimes separated by bars or islands) with low values of M
and high bed load.
RESERVOIR SEDIMENTATION
A common objective of many sediment transport studies is
the prediction of reservoir “siltation” rates. Reservoir siltation
depends on: (1) the average annual sediment load entering
the reservoir; (2) the grain size distribution of the sediment
load; (3) the reservoir trap efficiency; (4) the bulk dry specific
weight of the deposited sediments in the reservoir.
The determination of annual sediment yields was dis-
cussed in Sections Erosion and The Mechanics of Sediment
Transport in a Stream. It is important to predict the possible
effects of land development and
or sediment control mea-
sures on future sediment yields.^46
An estimate of the percentage sand, silt, and clay for the
incoming sediment can be obtained on the basis of grain size
analyses of the existing load.
Brune in 1953 47,8 presented the trap efficiency curve
shown in Figure 15, which applies to reservoirs which nor-
mally ponded. Detention type reservoirs would have lower
trap efficiencies.
Since annual sediment yield is usually determined in terms
of weight it is necessary to know the bulk dry specific weight,
and the trapped sediments, in order to estimate the volumet-
ric decrease in reservoir storage. To accomplish this Lane and
Koelzer 48,8 developed an equation to describe the change in bulk
dry specific weight,  , of reservoir deposits with time, i.e.
gg
g
()

• ()


• 
  

  ( log )
  ( log )
  
  
  sand sand sand
  silt silt si
  110
  110
  KTX
  KTXllt
  ( cla() claylog ) clay
  g
  y 11 KTX 0
  (58)
  in which T  time in years; X sand  fraction of sand in deposit;
   sand(1) etc. are the respective bulk dry specific weights at T 
  1 year (see Table 7); K sand etc. are constants (see Table 7).
  After a period of N years a reservoir will contain depos-
  its varying in age from less than one year to N years with
  the consequence that the true bulk dry specific weight,  * ,
  for the entire deposit should be found by averaging Eq. (58)
  TABLE 4
  Canal classification21,44

  
  

1. Sand bed and banks.


2. Sand bed and cohesive banks.


3. Cohesive bed and banks.


4. Coarse noncohesive material.


5. Same as for 2, but with heavy sediment loads, 2000–8000 ppm.
   TABLE 5
   Constants in Eqs. (47) to (54) (ft-sec. units)
   Coefficient
   Channel type
   1234 5
   K 1 3.5 2.6 2.2 1.75 1.7
   K 2 0.52 0.44 0.37 0.23 0.34
   K 3 13.9 16.0 — 17.9 16.0
   K 4 0.33 0.54 0.87 — —
   m 0.33 0.33 — 0.29 0.29
   TABLE 6
   Schumm’s classification
   Stream type Bed load Mixed load Suspended load
   M Braided Meandering
    5% 5–20% > 20%
   Coarse load
   Total total
    11% 3–11% < 3%
   C019_001_r03.indd 1077C019_001_r03.indd 1077 11/18/2005 11:06:02 AM11/18/2005 11:06:02 AM