ASSESSMENT OF SEDIMENTATION OF THE DREDGED CHANNEL IN DOG RIVER

 

 

Gabriel Dean, Department of Earth Sciences, University of South Alabama, Mobile, AL  36688.  E-mail:  gkd302@jaguar1.usouthal.edu.

 

            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.

            Keywords:  Dog River, dredging, shoaling

 

 

Introduction:

            Since the 1950’s, West Mobile has been subject to an ever increasing amount of urban expansion.  Along with this development has come a constant increase in fluvial erosion.  Due to the large areas of exposed soil, a significant amount  of alluvium, which is stream-deposited debris consisting mostly of clay and silt, has found its way into Dog River through several tributaries such as Halls Mill Creek, Rabbit Creek, Eslava Creek and the Montlimar Canal, also known as Moore Creek (McKnight and Hess, 2000).  These tributaries are fed by a conglomeration of minor streams and ditches dispersed throughout the Dog River watershed.  Since Mobile, Alabama has an average annual precipitation rate of sixty-seven inches, fluvial related erosion is quite common and probably the single most ubiquitous process that has contributed to the in-fill of Dog River (Coastal Weather Research Center, 2003).  Aside from the overland run-off that has been exacerbated by urban advances in West Mobile, another significant consequence emerged from the destruction of vital wetland areas, which acted as natural filtering systems throughout the Dog River drainage basin.  In addition to improving the water quality, the former wetlands also acted as retainers by trapping sediment that would otherwise end up in Dog River (Ford, 2002).

            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. 

            Two of the most important causes of deposition pertaining to this particular 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 effect the river loses its energy or ability to carry the stream load thus allowing the sediment to slowly settle on the bottom (Leopold, Wolman and Miller, 1992).  In addition to this, flocculation also contributes to the deposition process within Dog River.  This occurs when seawater comes into contact with clay particles suspended in fresh or brackish water.  As a result, the clay particles tend to coagulate and settle rapidly (Witherick, Ross and Small, 2001).  Since fresh water is less dense than saline water, it flows on top of a wedge-shaped volume of saltwater that enters Dog River from Mobile Bay.  This saline wedge is, in part, responsible for the silting patterns associated within Dog River (Bray, Bates and Land, 1997). 

            In response to the excessive sedimentation of Dog River over the decades, the U.S. Army Corps of Engineers carried out an assessment of the river, but ultimately found it to be primarily a recreational waterway that provided no commercial activity, therefore declaring it unworthy to receive federal funding and maintenance (Editorial, 2001).  However, despite this assessment, the dredging project moved forward under a newly revised environmental restoration venture that required no commercial or economic gains (Reilly, 2001).  Herein lies the controversy that has accompanied the project, known as the Dog River Pilot Project-Environmental Restoration, since its beginning.

            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.

 

Research Question:

            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.

 

Methods:

            In order to obtain the data needed to measure the exact amount of soft sediment that has collected on top of the firm, compacted sediment within the channel, I had to first acquire the necessary equipment and precisely locate the dredged channel. On two separate occasions, at least one assistant and I went out into Dog River to retrieve core samples.  The first outing took place aboard a vessel provided by the Auburn Marine Extension and Research Center (AUMERC), and the second excursion was made possible by a member of the Alba Club who volunteered to help.  On each occasion, electronic depth finders were mounted to the vessels, which were used to assure an accurate position over the channel. 

             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.

 

 Results:

 

            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. 

            Site 1, which was located in the center of Dog River approximately 400 meters northwest of the Alba Club, yielded 15.5 inches of fresh sediment.  Site 2 was in the dredged channel near the mouth of Halls Mill Creek.  This area yielded the second highest amount of fresh sediment at 25 inches.  Site 3 produced 19 inches and was located at the confluence of Rabbit Creek, Halls Mill Creek and Dog River.  Site 4 had the greatest accumulation of soft sediment, producing 35.5 inches.  It was located at the confluence of Moore Creek and Dog River.  Finally, Site 5, which was in the center of the channel roughly 400 meters north of the Scenic Yacht Basin, yielded 18 inches.  Figure 4 shows the amounts of soft and firm sediment that were recovered from the bed of the dredged channel.  It also indicates the total lengths of each core sample and their individual relation to the original depth of the dredged channel, which was 8 feet. The total lengths of each of the five samples were as follows:  Site 1, 52 inches;  Site 2, 46.5 inches;  Site 3, 57 inches;  Site 4, 38.5 inches;  and Site 5, 52.5 inches.

                  

Conclusion:

           

            After careful examination of each of the collected samples, it became apparent that the formerly dredged channel within Dog River is, in fact, being filled-in at a relatively constant rate.  According to the data, the channel has collected an average of 22.6 inches of alluvium, which is approximately two feet over the past two years.  It is also worth mentioning that the greatest amounts of fresh sediment were found at the three confluence sites.  This suggests that West Mobile has been producing the highest concentrations of sediment run-off.  The most probable cause of this erosion may be attributed to an increase in the number of construction sites as well as a negligence to enforce Best Management Practices (BMP’s), which are intended to reduce the cumulative amount of overland run-off by means of better management, structural and cultural practices (Shaw, 2001). 

            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. 

            One way to assist in preserving the channel would be to implement more stringent BMP’s and to reduce the fluvial erosion process throughout West Mobile.  Improved structural engineering and increased vegetative covering would be a good start to help mitigate erosion (Leopold, 1997). 

            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.

            In conclusion, based on the information and data retrieved from the dredged channel, it is evident that the one-time Pilot Project was an expensive and 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 correct the problems it was ostensibly intended to correct.  The significance of this research may also stand to provide data for any future projects concerning the channel and its depth, or it may merely serve as an informative piece for anyone concerned with the state of Dog River as it presently exists. 

 

 

Works Cited:

 

Bray, R.N., A.D. Bates and J.M. Land.  1997.  Dredging:  A Handbook for Engineers.  2nd ed.  New York: John Wiley & Sons, Inc.

 

Coastal Weather Research Center, University of South Alabama.  2003.  Accessed: 8 Apr., 2005.  Available online:  http://www.southalabama.edu/cwrc/mobileclimate.html.

 

Department of the Army.  Mobile District, Corps of Engineers. 2002. “Dredge Sediment

Deposition Basins, Dog River, Rabbit Creek, and Halls Mill Creek Mobile, Alabama.”  Joint Public Notice.  No. AL02-00600-U.

 

Department of the Army.  Mobile District, Corps of Engineers and Alabama Department of Environmental Management.  2001. “Dog River Pilot Project-Environmental

Restoration:  Mobile County, Alabama.”  Joint Public Notice.  No.  FP0-DR0-10.

 

Dog River tells the tale of bloated federal budget.”  Editorial. 2001.  Mobile Register, 19 Oct., sec. 1A.

 

Ford, David W.  2002.  “Dredging and Dog River.”  Available online: Dog River

Watershed:  Geography 480.  Accessed:  11 Apr., 2005.

<http://www.southalabama.edu/geography/fearn/480page/dogriver.html>.

 

Leopold, Luna B.  1997.  Waters, Rivers and Creeks.  Sausalito, CA:  University Science Books.

 

Leopold, Luna B., M. Gordon Wolman, and John P. Miller. 1995.  Fluvial Processes in

Geomorphology.  New York:  Dover Publications.

 

McKnight, Tom L., and Darrell Hess. 2000.  Physical Geography: A Landscape Appreciation. 6th ed.  New Jersey:  Prentice Hall.

 

Reilly, Sean. 2002. “Documents Show Twists and Turns Behind Dog River Dredging Project.”  Mobile Register, 21 July, sec. 1A.

 

Reilly, Sean. 2001.  Dog River dredge project raises questions.”  Mobile Register, 14

Oct., sec. 1A.

 

Shaw, Joshua S.  2001.  “Factors Affecting the Occurrences of Sand Deposits in a Sub-Basin of Dog River Watershed.”  Available online:  Dog River Watershed:  Geography 480.  Accessed:  16 Feb., 2005.

<http://www.southalabama.edu/geography/fearn/480page/dogriver.html>.

 

Witherick, Michael, Simon Ross, and John Small. 2001.  A Modern Dictionary of Geography.  4th ed.  New York:  Oxford Univ. Press.