Aerial Photography

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.

5.1.6.4 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.

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