FLOOD04263

• County: Statewide
• Basin: Statewide
• Title: Design of Open Channels
• Date: 10/1/1977
• Prepared By: USDA Soil Conservation Service
• Floodplain - Doc Type: Educational/Technical/Reference Information

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Occasionally the character of certain stratigraphic units differs widely
from the type normally expected for a particular environment. For
example, the occurrence of a very dark, highly coherent unit in a semi-
arid environment near the sea coast. In known instances where these
disparities occurred, this coherent unit was interpreted to have accumu-
lated during a rise in sea level. Fine sediment deposition in an embay-
ment followed by the growth of marsh vegetation was probably the origin
of this stratigraphic unit.

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Another example of seeming disparity is the presence of an old fan of
gravelly deposits encroaching on a valley but lacking a watershed of a
size and geology capable of furnishing these materials. Inspection of
geologic maps indicates that the gravelly fan deposits accumulated prior
to stream piracy which transferred most of the watershed and also the
source of coarse sediment to another drainage system.

Stratigraphic Units Without Internal Continuity

In certain geomorphic environments, the sediment deposited varies widely
in texture within a short distance. If there is any unit identification,
it is to the effect that there is a range in particle sizes from silt to
gravel, all noncoherent, except for possibly some lens cementation. Such
deposits are extensive in arid, semiarid and glaciated regions where in a
network of ill-defined channels, the deposits were reworked during flood
flows. Variability on a smaller scale may occur in humid climates where
deposits accumulated during braided flows, but weathering and other factors e
contributing to development of coherent soil may be inhibited by such
environmental factors as a high water table. In arid or semiarid climates,
caliche by cementation can provide a resistant stratigraphic unit in an
otherwise noncoherent alluvial deposit.

In the instances cited above, the absence of continuity, as well as the
lack of coherence, should be established in the investigations stage
preliminary to sampling. Then procedures to establish continuity can be
modified.

Conducting the Site Investigation

Data Requirements and Observations Preliminary to Site Investigations

A longitudinal profile of the channel reaches to be improved, with survey
stationing and profiles of the existing and proposed channel invert and
top of bank is needed. A profile of the top of one bank is usually suf-
ficient in alluvial valleys where the flood-plain surface is about the
same on both sides of the channel. In addition, a flood-plain map is
needed that shows the existing channel, the proposed channel, and the
survey base line. The location of test sites and other pertinent geo-
logic features are to be shown on the map.

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