Encyclopedia of Environmental Science and Engineering, Volume I and II

1066 SEDIMENT TRANSPORT AND EROSION

Some important descriptors of the frequency distribution

are: (1) the median size or d 50 , that is, the size for which 50%

by weight of the sample has smaller particles; (2) the scatter of

particle size as indicated, for example, by the standard devia-

tion or perhaps the geometric deviation; (3) the characteristic

grain roughness which has been associated with the d 65 ; 5,7,8

(4) the d 35 has also been used as characteristic sediment size.^9

Bulk Properties The determination of bulk, in place

specific weights of sediments is discussed under Reservoir

Sedimentation.

EROSION

Most of the sediment in streams is produced by the following

processes:^1

1) Sheet erosion,

2) Gully erosion,

3) Stream channel erosion,

4) Mass movements of soil (e.g. landslides and soil

creep),

5) Erosion to construction works,

6) Solids wastes from municipal, industrial, agricul-

tural and mining activities.

Morisawa,^10 using a system diagram, similar to Figure 2, sum-

marizes the inter-relation of climatic and geologic factors that

influence soil erosion and runoff. Figure 2 also shows man’s

influence on the system.

Langbein and Schumm 3,17 proposed the correlation

shown in Figure 3 between annual sediment yield and effec-

tive annual precipitation for the United States. The effec-

tive precipitation is the adjusted precipitation which would

have produced the observed runoff for an annual mean

temperature of 50°F.

A recent paper by Saxton et al.^11 relates total runoff, surface

runoff and land use practices to the sediment yield from loessial

watersheds in Iowa. This paper compares erosion and surface

runoff from contoured-corn watersheds and from pastured-

grass and level-terraced areas. In a 6-year study the contoured-

corn areas yielded, annually, about 19,000 tons
sq. mile of

sheet erosion plus 3000 tons
sq. mile of gully erosion while the

level terraced and grassed watersheds yielded about 600 tons


sq. mile. Similarly the surface runoff from the contoured-corn

areas was approximately 5 inches compared with 1.5 inches

for the level-terraced and grassed areas. The experimental

watersheds were of the order of 100 square miles.

Other land use factors are discussed in a paper by Dawdy^12

who presents sediment yields for the state of Maryland. The

annual sediment yield from heavily wooded areas is about

15 tons
sq. mile compared with 200 to 900 for crop land.

The annual sediment yields from urban development areas

(usually only a few acres) varied from about 1000 to 140,000

tons
sq. mile.

Curtis^13 obtained annual sediment yields of 390 and 290

for two watersheds (264 and 651 square miles) in the Miami

Conservancy District, Ohio.

A number of empirical formulae have been devel-

oped 1,14,15 to permit estimation of rates of overland erosion.

The US Department of Agriculture developed the universal

soil-loss equation (for upland areas),

E  RKLSCP, (1)

where E  soil loss
unit area; R  rainfall runoff factor;

K  soil erodibility factor; L  slope length factor; S  slope

steepness factor; C 1  crop management factor; and P 1 

erosion control practice factor. Details for estimating the

above factors are given by Meyer.^15

MEASUREMENT OF SEDIMENT DISCHARGE

Samplers have been developed to measure both suspended

and bed load in streams. However bed-load samplers are not

temp, rain rock type topography

excavations

fills

reservoirs

CLIMATE GEOLOGY

SOIL CHARACTER

SOIL EROSION

(RUNOFF)

RAINFALL

VEGETATION

MAN

farming

lumbering

amount

intensity

duration

MAN

eg.

eg.

FIGURE 2 The relationship of climate and geology to soil erosion (adapted from Morisawa).

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