Fundamentals of Engineering Design
records clearly indicate the immediate lowering in water levels after the barrage was built. Figure 5.10 also
indicates that the stages began a slow increase following the initial drop. Thus, the specific gage record
provides a clear representation of the historical long-term channel response on this system.
On many river systems the response of the channel at low and high flows is entirely different. For
this reason, it is often advisable to develop a specific gage record that covers the entire range of flows that
the river encounters. The following example is presented to illustrate how the specific gage record can
document the difference between the low flow and high flow response. Biedenharn (1983) used specific
gage records to document the channel response of the Little Tallahatchie River below Sardis Dam in north
Mississippi. Figure 5.11 shows the specific gage record for the Little Tallahatchie River at Belmont Bridge
which is located approximately six miles below Sardis Dam in north Mississippi. The specific gage record
was developed for the entire range of flows encountered on the river, ranging from a low flow of 500 cfs
to a near bankfull flow of 5,000 cfs. As shown in Figure 5.11, the constant level of the specific gage record
prior to 1939 indicates that both the low and high flows were fairly stable. With the closure of Sardis Dam
in 1939 and the construction of five cutoffs immediately below the dam in 1941, the high flow stages
showed an immediate and dramatic lowering. Between 1943 and 1950, the high flow stage began to
increase as the channel began to aggrade. After about 1950, the stages appeared to have stabilized
somewhat until about the late 1960's when the stages began to increase again. The response of the low
flow stage was entirely different. Following the closure of Sardis Dam and the construction of the cutoffs,
the low flow stage do not show any change. In fact, the low flow stages do not show any significant change
until the late 1960s when they began to increase dramatically. Thus the most significant changes in the low
and high water stages occurred during two different time periods and in different directions: the high flow
stages lowered about 3 to 4 feet between 1939 and 1943 while the low flow stages rose about 3 feet
during the period 1967 to 1980.
By definition, a specific gage record represents the variation in stage for a given discharge over time
at a specific location on the river. Therefore, a specific gage record provides a picture of the river behavior
at one point on the river, and does not necessarily reflect how the river is behaving upstream or
downstream of that location. For this reason it is helpful if specific gage records can be developed at
various locations along the river in order to illustrate how the overall system has responded. This often
allows the engineer to develop an understanding of the connectivity of the system. This point is illustrated
in Figure 5.12 which shows the specific gage records for the Red River at Shreveport, Louisiana and
Alexandria, Louisiana for a discharge of 100,000 cfs for the time period 1895 to 1985. A discharge of
100,000 cfs is a fairly high flow with a return period just under 2 years. Shreveport is located
approximately 160 miles upstream of Alexandria. Throughout the 1800's, the Red River was blocked by
a huge log jam which extended about 80 miles upstream and downstream of Shreveport. This log jam,
which was known as the Red River Raft, was finally removed in the late 1800's. With the removal of this
blockage, the stages at Shreveport were lowered dramatically. According to the specific gage record
(Figure 5.12), the stages were lowered approximately 15 feet by 1940. However, during this same time
period (1895 to 1940), the stage at Alexandria actually increased approximately 4 feet (Figure 5.12),
possibly as a result of the increased
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