Geomorphic Assessment of Channel Systems
Vegetation. The spatial distribution, size, type, and approximate age of the
vegetation existing within and along the channel should be recorded in the field investigation.
Vegetation colonizing the channel and along berms should be evaluated with respect to
growth and whether or not it may be removed by the next flood event. Not only is substantial
in-channel vegetation an indication of lateral stability, but it also impacts the hydraulic
efficiency of the channel, and plays an important role in establishing the overall stability of the
channel. Simon and Hupp (1992) provide a detailed discussion of the use of vegetative
indicators of channel morphology.
Sediment Data. Major sediment sources supplying material to the main channel
should be recorded during the field reconnaissance. These sources may include the bed and
banks of the channel, tributaries, gullies, drainage ditches from roads and highways, and
watershed (upland) erosion. In many unstable streams, the bed and banks are a major source
of sediment. In this case, the sediment is introduced into the system over a sometimes lengthy
reach of channel. Tributaries that are undergoing similar instabilities may be sources of heavy
sediment input. During the field reconnaissance, the amount and size of sediment deposited
at and just downstream of tributary confluences should be noted.
Sediment sampling provides information on the composition of the sediments derived
from each source. In general, channel bed material samples should be 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, including if applicable, each stratigraphic layer, should be collected. Sediments in
tributary mouth bars are used to determine if tributary sediments are radically different from
the main-stem channel sediments. It is also helpful to periodically collect bed material
samples at several locations across a cross section in order to determine the lateral variability
of sediment size in a section.
Hydrologic Factors. During the field investigation, estimates of channel roughness
should be made for various reaches of the channel. These data are important for calibrating
water surface profiles in the detailed assessment phase of the investigation. Roughness
(Manning's `n') should be estimated for the active channel, berms, and the floodplain.
Vegetation and trash frequently preserve evidence of water surface elevations during
floods. Debris transported during floods is often trapped in the vegetation. The highwater
marks should be recorded, even if the method of measurement is crude. Any evidence of
frequent overbank flows such as sand splays, overbank erosion, and crop damage, etc., should
also be noted during the field investigation.
Existing Structures. The location of all existing structures along the channel should
be recorded during the field reconnaissance. A partial list of common man-made features
found in streams includes bridges, bank protection, drop inlet structures, culverts, grade
control structures, water intakes, and pipelines. An assessment of the structure condition, and
the impact on the local channel morphology should be made during the field investigation.
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