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 Geomorphic Assessment of Channel Systems
3.1.3.3 Comparative Surveys and Mapping
One of the best methods for directly assessing channel changes is to compare channel
surveys (thalweg and cross section).
Thalweg surveys are taken along the channel at the lowest point in the cross section.
Comparison of several thalweg surveys taken at different points in time allows the engineer
or geomorphologist to chart the change in the bed elevation through time (Figure 3.4).
There are certain limitations that should be considered when comparing surveys on
a river system. When comparing thalweg profiles it is often difficult, especially on larger
streams, to determine any distinct trends of aggradation or degradation if there are large scour
holes, particularly in bendways. The existence of very deep local scours holes may completely
obscure temporal variations in the thalweg. This problem can sometimes be overcome by
eliminating the pool sections, and focusing only on the crossing locations, thereby, allowing
aggradational or degradational trends to be more easily observed.
While thalweg profiles are a useful tool it must be recognized that they only reflect
the behavior of the channel bed and do not provide information about the channel as a whole.
For this reason it is usually advisable to study changes in the cross sectional geometry. Cross
sectional geometry refers to width, depth, area, wetted perimeter, hydraulic radius, and
channel conveyance at a specific cross section.
If channel cross sections are surveyed at permanent monumented range locations, then
the cross sectional geometry can be compared directly at different time periods. At each
range, the cross section plots for the various time periods can be overlaid and compared.
However, it is seldom the case that the cross sections are located in the exact same place year
after year. Because of these problems it is often advisable to compare reach average values
of the cross sectional geometry parameters. This requires the study area to be divided into
distinct reaches based on geomorphic characteristics. Next, the cross sectional parameters are
calculated at each cross section, and then averaged for the entire reach. Then the reach
average values can be compared for each survey period. Cross sectional variability between
bends (pools) and crossing (riffles) can obscure temporal trends, so it is often preferable to
use only cross sections from crossing reaches when analyzing long-term trends of channel
change.
Comparison of time sequential maps can provide insight into the planform instability
of the channel. Rates and magnitude of channel migration (bank caving), locations of natural
and man-made cutoffs, and spatial and temporal changes in channel width and planform
geometry can be determined from analysis of historical maps. With this type of data, channel
response to imposed conditions can be documented and used to substantiate predictions of
future channel response to a proposed alteration. Planform data can be obtained from aerial
photos, maps, or from field investigations.
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