Encyclopedia of Environmental Science and Engineering, Volume I and II

SEDIMENT TRANSPORT AND EROSION 1079

in which z  bed elevation; q TB  volumetric bed material

load; m  bed porosity; C ′ and p are constants for a given

bed material and bed form.

As the bed is soured, an armouring process occurs due to

the fact that the finer bed material is more readily removed

than the coarser particles in the bed (See Figure 1). This may

be accounted for by adjusting  c in Eq. (62).

SEDIMENT CONTROL

The adverse effects of excessive sediment loads in reser-

voirs, navigation channels, harbors, and aquatic life may be

alleviated by a sediment control program which may include

measures such as prevention of over-hand erosion or con-

tainment of eroded soil near its source. On the other hand

the removal of an established sediment load from a stream

may also lead to undesirable consequences such as channel

degradation or changes in the established aquatic life.^2

The ASCE Sedimentation Tank Committee^46 has clas-

sified sediment control measures under: (1) land treatment,

(2) structural.

Land treatment measures are used to reduce wash load

(fines) resulting mainly from sheet erosion. Structural mea-

sures are most effective for reducing sediment load derived

from stream-channel erosion, gully erosion, and sediment

associated with mining and construction work.

The main land treatment measures are summarized

below:^46

1) Vegetative treatment includes changes in the exist-

ing land use towards more: use of cover crops and

crop rotation, maintenance of effective vegeta-

tive cover in critical areas, leaving of straw and

stubble in the field, use of long-term hay stands,

mulching, pasture planting, and re-forestation.

2) Protecting existing vegetative cover involves pro-

tection of existing forest sands from excessive fire

losses and the protection of all vegetated areas

from over grazing.

3) Mechanical field practices are used in conjunc-

tion with (1) and (2) and include contour farm-

ing, contour furrowing of range land, contour

strip-cropping, use of gradient and level terraces,

use of diversions to divert runoff away from crit-

ical areas, use of grassed waterways and ditch

and canal linings, and the use of grade stabiliza-

tion structures in areas subject to possible gully

erosion.

The ASCE Task Committee^46 outlined three commonly

used structure measures:

1) Reservoirs, either detention or multi-purpose,

decrease flood stages and consequently may

decrease downstream damages due to deposition

of flood borne sediments; however it should be

remembered that reservoirs themselves create

sediment problems which may be just as serious

as the problem being solved.

2) Stream channel improvement and stabilization —

these measures may include straightening, clean-

ing, deepening, and widening of existing channels

in order to decrease local flood stages and related

sediment damages; again, complications, such

as increased channel erosion, can be expected.

Other channel improvement methods are: lining

the channel on bends and other erodible areas,

and providing spur dykes to deflect the flow away

from erodible banks.

3) Debris and sedimentation basins are usually rela-

tively small reservoirs designed to trap debris and

sediment near its source. This method is particu-

larly useful in controlling sediment yields from

construction sites or mining operations.

MILES DAM

DEGRADATION

CLAYS

FEET

DENSITY

CURRENT

SILTS

DELTA

GRAVELS

SANDS

FIGURE 16 Distribution of deposits in a reservoir.

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