Fundamentals of Engineering Design
sediment load from the degradational reach upstream. Thus, the specific gage records indicate that two
entirely different modes of channel response were occurring: degradation at Shreveport and aggradation
at Alexandria.
An excellent example of the use of specific gage records to illustrate the varied response along a
river system is shown in Figure 5.13. Figure 5.13 shows specific gage records for seven gaging stations
along the Mississippi River from Columbus, Kentucky to Red River Landing, Louisiana for near bankfull
conditions for the time period 1860 to 1975 (Winkley, 1977). During the period 1933 to 1942, 16
manmade cutoffs were constructed on the river from just below Natchez to just above Helena. These
cutoffs shortened the river approximately 160 miles. The immediate effect of these cutoffs is clearly shown
(Figure 5.13) at the Natchez, Vicksburg, Arkansas City, and Helena gages where stages were lowered
considerably. The most dramatic lowering occurred at Vicksburg and Arkansas City where stages were
lowered approximately 15 and 12 feet, respectively. Since about 1950, the Natchez and Vicksburg
gages have reversed their degradational trend and appear to be aggradational, while the stage at Arkansas
City appears to have stabilized. In the upper reaches, Helena and Memphis are continuing on a downward
trend. Further upstream, at the Columbus gage there has been no observed lowering that could be
associated with the cutoffs. Thus the specific gage records provide a record of the complex response of
the channel and a means of assessing the relative stability between various locations along the river. One
of the most common mistakes in the utilization of specific gage records is to place too much emphasis on
a short time period. The specific gage records on most rivers exhibit considerable variation about a mean
value. There may even be cyclic patterns in the record. Therefore, localized trends in the specific gage
record over relatively short time periods may not reflect a true long-term progression of the river. This is
illustrated by examining the specific gage record for the Mississippi River at Arkansas City for the time
period 1940 to 1974 (Figure 5.14). Looking at Figure 5.14, one can see how an engineer in 1974 might
be tempted to conclude that degradational trend that had existed since 1940 had ended about 1967, and
that the channel was now in an aggradational mode. The engineer might then use this aggradational
assumption as the basis for design of channel improvement features such as levees, revetments dikes, or
for making projections about long-term dredging quantities and channel response. However, when the
period of record is extended to 1990 (Figure 5.15) it becomes apparent that the 1967 - 1974 period
was just a short-term phenomenon, and that the channel is still on a degradational trend. Thus the design
assumption mentioned above would have been in error.
Specific gage records are an excellent tool for assessing the historical stability at a specific location.
However, specific gage records have two limitations. First, a specific gage record only indicates the
conditions at a particular gaging station and does not necessarily reflect river response upstream or
downstream of the gage. Second, a specific gage record does not provide any indication about future
degradation or aggradation trends. Extrapolation of specific gage records into the future is extremely risky
and is generally not recommended. Therefore, even though the specific gage record is one of the most
valuable tools used by river engineers, it must be coupled with other assessment techniques such as slope
analysis in order to assess reach conditions, or to make predictions about the ultimate response on a river.
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