Fundamentals of Engineering Design
Sediment sampling provides information on the composition of the sediments derived from each
source. In general, the channel bed material samples are taken at the thalweg in order to obtain a
representative sample. Analysis of these samples provides information on the spatial variations of grain size
within the channel system. Samples of channel bank material and, if applicable, each stratigraphic layer,
are collected. Sediments in tributary mouth bars are used to determine if tributary sediments are radically
different from the channel sediments. Samples taken at surveyed cross sections can be correlated to the
channel hydraulics, geometry, and geomorphology.
e. Sediment Depth. The depth of sediment in the channel bed can be useful in determining the
stability status of the channel bed. For most streams, an average sediment depth of 3 to 4 feet or greater
is an indicator of an excess sediment supply and, hence, aggradational tendencies for the reach. Likewise,
an average sediment depth of 1 foot or less indicates an excess of transport capacity and possible
degradational tendencies for the reach.
Depth of sediment is easily determined in the field by probing the channel bed with a metal rod.
Probing indicates the presence of clay outcrops or coarse material below the surface, and is done
frequently along a reach to find the average sediment depth. Although probing of point bars or even
berms can be beneficial, probing should be concentrated at the channel thalweg. Correlating sediment
probing with survey cross sections is recommended.
f. Bank Heights and Angles. Heights and angles of the channel banks are field-determined to assist
in the geotechnical stability assessment. These data can be determined from survey cross sections, but field
verification is recommended since survey cross sections may not be representative of the entire reach.
Bank heights and angles are used to establish geotechnical stability criteria for the channel reach. Field
measurements include measurement of bank height with a survey rod or cloth tape, and of bank angle with
an inclinometer. Measurement at locations where bank failure is impending or has recently occurred is a
bonus. These measurements are used to empirically define stability criteria for the channel reach.
Also, observe tension cracks in the upper bank and mode of bank failure. Tension cracks can
indicate a stressed condition in the upper bank which can lead to slab type failure. Slab failure is the failure
due to gravity of large mass blocks of the upper bank along a near vertical plane. The classic rotational
failure is rotation of the bank mass along a circular arc.
g. Bank Stratigraphy. Proper identification of bank stratigraphy and its role in channel bank
stability is probably best determined by an investigator with a background in geology. A classification of
the general composition of the observed layers and the percent of the total bank composed by each layer
are made in the field investigation. If the strata indicated bank instability, then the field data can be analyzed
by a geologist at a later date.
h. Vegetation. The spacial distribution, size, and approximate age of the vegetation existing within
and along a channel are recorded in the field investigation. Vegetation colonizing with the channel and along
berms are evaluated with respect to growth and whether or not it may be removed by the next flood flow.
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