Fundamentals of Engineering Design
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.
A simple example of the use of comparative surveys to document historical channel changes can
be seen by observing the behavior of the Simmesport discharge range on the Atchafalaya River in Louisiana
at six points in time between 1931 and 1977 (Figure 5.17). As shown in Figure 5.17, the channel at this
location has been progressively deepening and widening since the early 1930s.
The above example is a very simple case where the channel changes at a specific location were
analyzed. However, the response at this particular location may not be representative of the morphology
of the rest of the channel system. For this reason, it is usually best to compare cross sections along the
entire length of the study reach. Unfortunately, it is very rare for cross sections to be located in the same
place from survey to survey. This presents a problem in making direct comparisons of cross section
dimensions. One solution to this problem is to develop and compare reach average values of cross
sectional parameters. Biedenharn (1983) used reach average values to document the channel response
on the Little Tallahatchie River below Sardis Dam in north Mississippi. The study area below Sardis Dam
was divided into two geomorphic reaches. Reach 1 extended from the dam to Floyd's Island, a distance
of 2.7 miles. Reach 2 extended about 7 miles below Floyd's Island to the Railroad Bridge. Channel
surveys were available for the study area from 1940 to 1980. Unfortunately, the locations of the cross
section ranges varied somewhat on each survey. Therefore, the average depth, width, and area from all
the cross sections in each study reach were calculated. Figures 5.18 and 5.19 show the average cross
section geometry changes from 1940 to 1980 for reach 1 and reach 2, respectively. These reach averaged
values were used in conjunction with specific gage analysis to document the morphologic response of the
channel to the construction of Sardis Dam.
188.8.131.52 Aerial Photography
A comparison of historical and present aerial photography can identify areas that have been
channelized or realigned. Aerial photography also provides some information on land use changes. With
good quality photography, it is possible to locate knickpoints in the channels as well as areas of instability
(bank caving and channel widening) and their progressive growth.
Vertical changes in the river system are also determined using stereoscopic means. This is
especially useful in a large river system where vertical changes in the point bars or middle bars can be
identified. The average height of the point bars or middle bars can be measured for different time periods
using stereoscopic pairs. A decrease in the overall height of the bars indicates a degrading river system.
The level of confidence of these analyses depends on the time between surveys.