Fundamentals of Engineering Design
188.8.131.52 Comparative Thalweg Analysis
One of the best methods for directly assessing historical channel response is to compare thalweg
surveys. This consists of comparing thalweg surveys at different time periods. Comparison of surveys can
give a good indication of the historical response of the channel. For instance, a thalweg comparison can
show whether the channel bed was degradational or aggradational during the time period between surveys.
Thalweg surveys are taken along the channel at the lowest point in the cross section. Thus, a
thalweg survey provides a profile of the bed elevation along the channel at a specific point in time.
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. Whereas, a specific gage record
simply reflects changes in the water surface stage, analysis of thalweg surveys can indicate if these changes
are due to changes in bed elevation.
The first step in comparing thalweg surveys is to gather all the existing surveys on the channel reach
being studied. In most cases, the surveys will be in a cross sectional format. If this is the case then the
thalweg elevation must be obtained from the cross section survey. This is not necessary in situations where
an actual survey of the thalweg is made by the survey team. The thalweg profiles for each time period are
then plotted on the same graph for comparison.
An example of a comparative thalweg survey for Long Creek in north Mississippi between 1977
and 1985 is shown in Figure 5.16. As indicated by this thalweg comparison, the bed of the channel was
approximately 10 feet lower in 1985 than in 1977 below about station 320+00. Thus this plot indicates
that the channel was degradational at some time during the period 1977 to 1985. However, it provides
no information on the stability of the channel bed in 1985. Although the bed was 10 feet lower in 1985 than
it was in 1977, this does not necessarily mean that the channel bed was actively degrading in 1985. In fact,
it is possible that the channel could have degraded 15 feet between 1977 and 1980, but then began to
aggrade after that so that by 1985 the bed was only 10 lower. Therefore, caution must be used when
interpreting comparative thalweg profiles. If the surveys are only a few years apart, there may be
reasonable confidence that the surveys are depicting what is currently happening in the river system.
However, if the time of the surveys are far apart (say 10, 20, or maybe 50 years) then there would be some
uncertainty as to whether the surveys reflect the ongoing process.
There are certain limitations that should be considered when comparing surveys on a river system.
When looking at thalweg profiles it is often difficult, especially on large river systems, to determine any
distinct trends of aggradation or degradation if there are large scour holes, particularly in the bendways.
These local scour holes may completely overwhelm the variations in the thalweg. For instance, on a large
river system such as the Mississippi River, scour depths may be in excess of 60 feet, but variations or
changes in the overall bed elevation may be an order of magnitude less. In this situation, it would be very
difficult to note any aggradational or degradational trends because of the scale effects. This problem can
sometimes be overcome by eliminating the pool sections, and focusing only on the crossing locations.