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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The tentative position of stratigraphic units within the profile and
their extent along the channel alignment should form the basis for
selecting soils investigation sites. The unit delineation must be
informative enough to indicate the location of sites required to
provide adequate data in the detailed investigation.

Determining the Intensity of Investigations

Four factors control the intensity of needed site investigations: (I)
design requirements that may be independent or partially independent
of the soil materials in the study reach; (2) hazard to life or
property in the event of failure or partial failure; (3) complexity
of the geomorphology in the site area; and (4) possible effects on
quality of environment.

Concerning design requirements, for example, restrictipns in availability
of rights-of-way and capacity for each earth channel may dictate that
the channel be a lined one, irrespective of,the kind of materials in
the reach involved. In this instance the investigations need only
obtain the information that may affect construction such as classifi-
cation of materials to invert grade, location of the water table and
of seeps or springs. The ~resence of rock outcrops, layers of hard
caliche, etc. srould be carefully identified by location and
description.

Where the water table is above proposed invert grade, its elevation
should be identified in the logs of test holes or pits. If the water
table fluctuates from season to season so that in one part of the year
its elevation is below invert grade, the average time period this
occurs and the dates of occurrence should be estimated. A high water
table may interfere substantially with construction or affect the
stability of an earth channel after excavation. If drop structures
or other installations requiring data on bearing strength are being
considered, investigations should be made in accord with recommendations
in NEH, Section 8, Engineering Geology.

Channels located in urban or intensively developed agricultural areas
differ widely in damage potential if a failure occurs than do channels
in rural areas. In an urban area, the channel is usually designed to
retain the one percent event with even minor erosion generally not
tolerable. In both instances, the erosion characteristics of the bed
and bank materials must be identified by procedures which include
stratigraphic unit delineation and representative sampling and testing.
Stratigraphic units must be more thoroughly delineated and variations
within the units more closely identified by sampling and testing in
intensively developed areas than in agricultural areas.