ASSESSMENT OF SEDIMENTATION OF THE DREDGED CHANNEL IN
Department of Earth Sciences,
The Dog River Pilot Project-Environmental Restoration was a one-time dredging project carried out by the Corps of Engineers roughly two years ago. It was done to enhance the river’s bottom by restoring it to depths of five feet or more as it once was in the 1950’s. However, since the project’s introduction, there has been a considerable amount of controversy over its funding and efficacious worth. My project provides information concerning the effectiveness of the Pilot Project as well as a plausible estimate of shoaling along the dredged channel. By taking a series of bottom core samples throughout the channel with a piston-corer, I have collected enough evidence to draw an empirical conclusion. According to my samples, the amount of soft sediment that has accumulated on top of the firm sediment ranges from 15.5 inches up to 35.5 inches, which shows undeniably that the channel has been filling-in over the past two years. This ultimately suggests that the Pilot Project was a misconstrued and ineffectual enterprise.
In order to provide a better understanding of the scope of this study, I will explain some of the causes of deposition associated with this project. Once these have been distinguished, it will be easier for the reader to interpret how streams work, therefore allowing a clearer conceptualization of how and why Dog River has become so shallow.
of the most important causes of deposition pertaining to this
project include flocculation and diminished flow speed.
As the river widens and the gradient becomes
less steep, as occurs at sea level, the water velocity decreases and in
the river loses its energy or ability to carry the stream load thus
the sediment to slowly settle on the bottom (Leopold, Wolman and
1992). In addition to this, flocculation
also contributes to the deposition process within
response to the excessive sedimentation of
Although my research is not directly concerned with the controversy surrounding the politics of the Dog River Pilot Project, it is necessary to briefly explain a few of the details to provide some background information dealing with the situation. However, ultimately this project will serve as an overall assessment of collected data concerning the present state of the depth of the dredged channel, which will in turn, help to alleviate any reservations as to the effectiveness and efficiency of the one-time environmental restoration project carried out by the Corps of Engineers.
About two years ago, the Mobile District Corps of Engineers undertook a dredging project that resulted in a dredged channel approximately 100 feet wide, 8 feet deep and 3,600 feet long (Corps of Engineers, 2002). It was federally funded and cost taxpayers roughly $4.1 million (Reilly, 2002). Along with the project came opposition resulting in a debate as to whether the project was meant to assuage the siltation of Dog River from a standpoint of environmental concern, or if the motive was based more on the wishes of recreational boaters and Congressman Sonny Callahan’s personal aspirations to appease his neighbors on Dog River. Congressman Callahan has a home on the river and at the time of the Pilot Project, he headed-up the congressional subcommittee that was responsible for allocating funds to the Corps of Engineers (Reilly, 2002). With this in mind, it is interesting that the project, which was once deemed undeserving of federal attention, suddenly became a prioritized endeavor.
The significance of my research shall stand to serve two purposes. The first concerning the one-time dredging project and its overall effectiveness from the time it was completed until the present. When I mention “overall effectiveness,” I mean to say whether or not the dredged areas have retained their intended depths and if they still serve their intended purposes. In so doing, the question of the project’s monetary worth as well as its practical function may be answered. When I say its “practical function,” I mean its ability to correct the environmental issues it was supposedly designed to fix. Some of the issues included: improved water quality, meaning clarity, the removal of sediment to restore the river bottom to its historically characteristic state, improved water flow, enrichment of the aquatic environment and an improved tidal exchange (Army Corps of Engineers, 2001).
Therefore, the first purpose of my research shall be to answer the question of, how much fresh sediment, on average, has filled the dredged channel since its time of completion. This, in turn, should resolve whether the cost and effort of the project were worth the benefits gained over the past two years.
The second purpose of this research shall be to provide information concerning the present sub-aquatic depths and an estimated rate of deposition along specific areas of the dredged channel. The data obtained may be useful to support or dispute any similar cases that may arise in the future. After all, the bill for the Pilot Project was paid for by taxpayers, therefore making it everyone’s business.
order to obtain the data needed to measure the exact amount of soft
that has collected on top of the firm, compacted sediment within the
had to first acquire the necessary equipment and precisely locate the
channel. On two separate occasions, at least one assistant and I went
The equipment I used included a series of five piston-corers and long aluminum poles that were connected together, which enabled my assistant and I to reach the bottom surface (Figure 1). On the submerged end of the poles was a piston-corer, which was
securely attached. As the piston-corer was forced down into the sediment in the channel bed, the piston remained stationary within the corer, creating a vacuum. This suction, along with the downward force, produced a sediment sample within each corer anywhere from 38.5 inches up to 57 inches.
A total of five independent sites were chosen throughout the dredged channel (Figure 2). I used a Garmin etrex legend GPS unit to accurately record the longitude and latitude of each site. Three of the sites were chosen because of their proximity to converging streams, namely Halls Mill Creek, Rabbit Creek and Moore Creek. The remaining two sites were indiscriminately chosen within the main channel.
Once I had the sediment samples, it was necessary to measure the total length of each sample. After extracting the samples from the piston-corer, I measured the amount of compacted sediment, which was much more firm and very distinguishable from the soft alluvium (Figure 3). Calculating the amount of soft sediment was a simple process of subtracting the amount of firm sediment from that of the total length of the sample.
Once the new, soft sediment from each site had been recorded, I could determine how much fresh silt and clay had been deposited within the dredged channel over the past two years. Since it is the measure of the fresh, soft sediment that this project is concerned with, I will only list those quantities for each of the five sites.
1, which was located in the center of
careful examination of each of the collected samples, it became
the formerly dredged channel within
As for the Dog River Pilot Project-Environmental Restoration being an effective means of alleviating the siltation, it did achieve that goal, but only on a temporary basis. As this study suggests, it is only a matter of time before the channel is back to its previous shallow levels. According to Bray, Bates and Land in their book, Dredging: A Handbook for Engineers (1997), it is the natural process of equilibrium that causes a, “submerged excavated hole to fill quickly with material,” at a rate that far surpasses an excavation of equal size that exists on land.
way to assist in preserving the channel would be to implement more
BMP’s and to reduce the fluvial erosion process throughout
When the Corps of Engineers proposed the Pilot Project, they claimed that dredging the channel would, “restore the river bottom to coarser sediments that were historically characteristic,” however, after thoroughly inspecting each sediment sample, I made no observations of any coarse sediment. The contents of the samples that were extracted were almost entirely clay and fine silt, which goes against their assertion.
conclusion, based on the information and data retrieved from the
channel, it is evident that the one-time Pilot Project was an expensive
only a temporary solution to a much larger problem.
In addition, this project may serve as a
strong argument against its effectiveness and overall efficiency to
problems it was ostensibly intended to correct.
The significance of this research may also stand to provide data
future projects concerning the channel and its depth, or it may merely
an informative piece for anyone concerned with the state of
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Department of the Army. Mobile District, Corps of Engineers. 2002. “Dredge Sediment
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2002. “Dredging and
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