Agriculture Reference
In-Depth Information
30 m
Fig. 6.4
Schematic of
contour method
32 m
31 m
be done by hand with a planimeter, electronically by a digitizer, or directly with a
CADD program. If the horizontal areas enclosed by each contour line are large rel-
ative to the elevation difference between the two contour elevations, averaging the
two areas and multiplying the average by the height difference can determine vol-
umes. However, for relatively small earthworks (like spoil piles and borrow areas),
volumes can be calculated based on the formula for the volume of a truncated
pyramid:
(
B
1
+
B
2
)
(
B
1
×
h
3
V
=
B
2
+
(6.6)
where
volume (m
3
)
B
1
,
B
2
=
V
=
areas of the contour elevation lines (m
2
)
h
=
elevation difference between the contour elevations (m)
6.5 Runoff and Sediment Yield from Watershed
6.5.1 Runoff and Erosion Processes
Surface runoff is the result of precipitation and is the amount of water which appears
in the stream channel network during and after precipitation. Surface runoff, as
direct flow of water over the soil surface and in small, definable channels, is termed
overland flow. Overland flow is not necessarily sheet flow, although it may be under
idealized conditions and on a sufficiently small scale. It consists of flow to, into, and
within small concentrated flow channels or rills. Overland flow is thus sheet flow
on the interrill areas and channel flow in the many small rills. For surface runoff to
be classified as overland flow, it must be that the mean flux per unit width of the
flow area cross-section is proportional to the storage in an incremental area. When
surface flow cannot be hydrologically or hydraulically treated as overland flow, it
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