Field Evaluation of the Dog River Tributary System and
the Types of Reinforcement Used
Ashley E. Turton, Department of Earth Sciences, University of South Alabama, Mobile,
AL 36608. E-mail: aet401@jaguar1.usouthal.edu
Mobile has grown around Dog River’s tributaries over time which has led to the
channelization and armored reinforcement of streams. The predominant methods for
ditch reinforcement are concrete, gabion, and piled rocks which are sporadically
intermingled with large segments of natural flowing water. As the city grows, more and
more canals are in need of repair to maintain the boundaries of business and home owner
properties. As the percentage of manmade drainage work increases, will there be an
environmental impact? The purpose of this project is to categorize the tributaries and
differentiate their make-up which will be made into a map that is available to the public. I
hoped to answer questions concerning what percentage of Dog River’s tributaries are
concrete, gabion, and natural. It was easiest to actually walk the ditches with a buddy and
carry a map to distinguish where the materials change. The result will be a relatively
large scale map showing different colors that correlate to the material that compose the
tributaries. This will be most beneficial for students that hope to learn about the
watershed in relevance to the location of natural vs. concrete or gabion reinforcement.
Moreover, after calculating the statistical coverage of the different tributaries it will be
possible to approximate the total area that each reinforcement type consumes.
Keyword: gabion, canals, armored ditches
Introduction
The past hundred years has had a phenomenal impact on the rate of urban growth
and development in the United States. The population of Mobile County has grown
substantially in the past twenty years with a 5.0 percent gain which is a growth rate of 8.5
percent. This was the highest of any of Alabama's metropolitan areas (U.S. Census
Bureau 2006). As population increases, urban sprawl dominates the inner and outer city
limits by spreading away from the heavily traveled business districts into suburbs of
endless proportions. Mobile sits at the mouth of the bay which has many inlets that
influence the city’s layout. Tributaries meander through neighborhoods, parks, and
business districts. As the rate of building increases around these creeks and streams, it has
become necessary to harness the water and maneuver it around our homes and shopping
centers.
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The idea of moving the rain and groundwater away as fast as possible has led to
the channelization and armoring of many waterways in the Dog River Watershed.
Storage and conveyance canals and structures are used to manipulate the flows in these
canals. Low-land areas, especially around the Gulf Coast, sometimes have to deal with
major storm events. Some reinforced ditches allow for more control over water levels.
Canal operators can use these waterways in some areas to increase and decrease the water
levels to avoid damage, although this method is not used in Mobile (Overloop et al.
2006). The tributaries of the watershed carry groundwater, rainwater, and any other
substances that can flow into them. Due to the fact that Dog River drains most of Mobile,
Alabama, the tributaries range in load, cleanliness, and types of reinforcement from
relatively untouched streams to concrete lined drainage ditches to sluggish, sediment
choked, tidal channels (Fearn 2009).
The flow of natural waterways makes
it harder to maneuver and manage the water
around developing areas. Depending on the
state of the vegetation, meaning the density
and type, streams and creeks tend to move
around on their floodplain changing the
Figure 1: Image taken in a natural tributary
direct path that the water flows. Stream-bank
vegetation heavily influences the
morphology of streams (Hession et al. 2003). “Natural-like” waterways that have dense
vegetation act as buffers along the banks and filter sediment that could enter the streams
(Figure 1). The slight shift of a natural creek could be enough to wash away a house
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under the right circumstances. Parking lots and foundations prevent rainwater penetration
into the ground, which increases runoff that is swept into concrete canals that goes
barreling down to the nearest water body. Urbanization has changed natural landscapes
over time into “anthropogenic impervious surfaces” (Xian et al. 2007). The increasing
percentage of impervious surfaces leads city engineers to be concerned with the
placement of new “permanent” ditches.
The tributaries of Dog River have been reinforced using four basic methods that
have been observed. These methods are concrete, gabion, natural, and piled rock. City
officials initially used concrete to armor drainage system walls. Surprisingly, much of the
inner city, meaning Mid-Town Mobile, remains natural throughout neighborhoods with
the use of piled imported rocks placed every few hundred feet presumably to slow
erosion and reinforce the natural walls. In recent years, Mobile County has begun
reinforcing some of these tributaries with gabions rather than concrete.
Figure 2: Basic gabion structure
used for channelization.
Figure 3: Installed gabion terraced wall.
Merriam Webster defines a gabion as a basket or cage filled with earth or rocks
and used especially in building a support or abutment (Figure 2). They are typically
fashioned from wire mesh and can be stacked or layered for the terrace look (Figure 3).
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Gabion-lined ditches tend to be overgrown and act to slow water and reduce turbidity, but
enable quick growth of water loving plants (Ellison 2004).They are structurally sound
due to the interwoven mesh material, yet they are permeable and yield to earth movement
while allowing natural groundwater to flow into them. Due to the fact that this method of
reinforcement has seemingly more benefits to the watershed, it is important to know
where they are located in our area.
The map that results from this project can be used for any project that requires
samples from gabions, concrete, and natural canals. Also it would be most interesting to
see the changes that have occurred over time when compared to this map. An evaluation
of each of the options based on cost, constructability, aesthetics, and channel width
showed that the gabion wall method is considered most economically feasible. The heavy
gage wire baskets can be stacked in a configuration that produces a rock-lined channel
that is resistant to erosion and maintains an attractive appearance (Paden 2003).
Research Question
What percentage of the Dog River Watershed’s tributaries is gabion, concrete,
piled rock and natural ditches? I predicted that most of the inner city would be concrete
reinforced due to the fact that most of it was constructed earlier than West Mobile. Also,
it was hypothesized that West Mobile would have more gabion emplaced because of the
increase in construction in the past few years. Where exactly are these different channel
types used and is there a noticeable transition zone that separates their usage? Also,
calculating the statistical coverage of the different tributaries will enable the
approximation of the amount of each type of reinforcement used for the entire watershed.
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Methods
After some collaborating with my mentor, it was decided that it would be most
beneficial to create an actual map showing the different types of tributary reinforcement,
or lack thereof. Advice on methods for gathering data were discussed and decided upon
in order to create an accurate interpretation of the findings.
First and foremost, I required a
simple map of the watershed that showed
some cross streets and the tributaries
(Figure 4). It was best to have an enlarged
map that could be laminated so it would
not be damaged in the reconnaissance. I
started with the water passageways that
are closer to Downtown Mobile
Figure 4: Map of the Dog River Watershed.
considering that most of the creeks farther
out are mostly natural. The origin of the
individual creeks was the easiest place to start considering that it was a walk down
stream. I took note of the changes in the makeup of the ditches and categorized them as
natural, piled rock, concrete, or gabion. However, there were numerous subcategories
depending on the situation in each ditch. Some were categorized as more than one type
considering that multiple methods were used in a single location.
After the fieldwork was complete, I was able to measure the total distance of each
tributary and the percentage of each that is natural, concrete or gabion reinforced. This
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data told exactly what percentage of the total stream length is constructed using the
different methods.
Creating the final map will require a base map using ArcGIS that can be color
coded for the different types of ditches. A grey-scale map will enable an easier
visualization of the border of the watershed. The streams and creeks can be made bolder
and colored to display the results. The resulting map is a complete picture of the Dog
River Watershed and the make-up of its waterways. All of the data recorded for this
project will soon be made into an actual GIS map. For this reason, I did not complete a
final area view because my GIS skills are lacking and the reader will benefit from seeing
it completed by a more professional computer technical person.
Results
All of the total lengths, including smaller tributaries were inserted into Excel in
order to portray the final results (Table 1).
Table 1: Percentages calculated for the total coverage of each reinforcement type.
Creek Name
1.Milkhouse
2.Woodcock
3.Eslava
4.Montlimar
5.Bolton
6.Rabbit
7.Halls Mill
8.Second
9.Bolton West
10.Spencer
11.Spring
12.Campground
13.Eslava West
Natural
%
98.0%
0.0%
51.2%
49.4%
10.0%
100.0%
100.0%
99.1%
0.0%
0.0%
28.1%
100.0%
37.9%
Natural w/
Piled Rock
%
0.0%
40.2%
17.9%
32.8%
39.0%
0.0%
0.0%
0.0%
6.4%
0.0%
37.3%
0.0%
0.0%
Gabion
%
Concrete
%
Bulkhead
%
0.0%
22.2%
0.1%
0.0%
17.7%
0.0%
0.0%
0.9%
28.6%
11.8%
7.5%
0.0%
0.0%
2.0%
37.0%
30.8%
17.9%
31.4%
0.0%
0.0%
0.0%
63.6%
88.2%
23.4%
0.0%
62.1%
0.0%
0.0%
0.0%
0.0%
2.1%
0.0%
0.0%
0.0%
1.3%
0.0%
0.0%
0.0%
0.0%
Brick
%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
3.7%
0.0%
0.0%
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The total length of each major waterway was measured including several of the
smaller off-shoots. For example: Milkhouse Creek has several large unnamed tributaries
that were also measured and included in the percentage calculations. So the original
lengths that were recorded in the field book are much larger when all of the data is
compiled.
Some areas had bulkheads used for reinforcement. All of the area south of
Interstate 10 within the watershed is considered natural for the purposes of this project.
However, Bolton Branch West on Azalea Road between Pleasant Valley and Michael
Boulevard and Bolton Branch on Navco Road and North McVay Drive do have
bulkhead reinforcement used. These places were recorded due to their rarity.
In the neighborhoods at Cottage Hill
Road and Knollwood Drive there is a two block
span of Spring Creek that is a checkerboard
layered brick reinforcement that lines the
drainage ways between backyards (Figure 5).
This was the only area that was observed to
Figure 5: Checkerboard brick pattern
reinforcement.
maintain this unique method.
The average percent coverage for all the different types was calculated and
recorded. The total average for the use of concrete reinforcement is approximately 27.5
percent. Natural with piled rock composed about 13 percent, while gabion made up only
6.8 percent. Natural tributaries dominate our watershed, composing about 51.9 percent of
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the ditches that were observed. Meanwhile, bulkhead and brick only make up about 6
percent combined (Figure 6).
Figure 6: Bar graph illustrating the reinforcement type used for each tributary that was observed.
Discussion and Conclusion
The results of this project have disproven all hypotheses that were made prior to
starting the assignment. Much of the area within the neighborhoods of Mid-Town has
natural flowing streams as well as many areas of West Mobile. There are a large
percentage of the tributaries that have remained natural, which is surprising to the
researcher.
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There are some things that could have been done differently in order to improve
this project. The use of a hand-held GPS unit would increase the overall accuracy of the
locations of reinforcement change. There are also a percentage of the smaller tributaries
that did not get done and therefore were not calculated in the results. Moreover, given a
bit more time it would have been possible to research the times that some of the
reinforcement was constructed which would have resulted in a change detection map of
the entire watershed.
The completed map of the Dog River Watershed showing the different types of
ditches will be very helpful in doing future projects to better our city. It would be
interesting to see the areas that are affected by these methods considering that there is no
division in usage around the watershed. Upon its completion it can be used for multiple
projects that are focused on differences between gabions, concrete, and natural ditches in
our watershed.
Acknowledgements
A special thanks to Jennifer Youngblood, Jesse Wheat, Chris Metzger, David
Cacamis, and Andrew Turton. Without a buddy, ditch walking would not be the same.
References Cited
Ellison, P. 2004. Turbidity Levels of Moore Creek’s Tributaries in Relation to Rainfall
Events. University of South Alabama.
Fearn, M.L. 2 February 2009. Welcome to the Dog River Watershed in Mobile, Alabama.
Accessed on March 5, 2009.
http://www.southalabama.edu/geography/fearn/480page/dogriver.html
Hession, W.C., Pizzuto, J.E., Johnson, T.E., Horwitz, R.J. February 2003. Influence of
Bank Vegetation on Channel Geomorphology in Rural and Urban Watersheds.
Geological Society of America Bulletin. January 1, 2005; 117(1-2): 229 - 243.
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Overloop, P.J., Weijs, S., Dijkstra, S. 2006. Multiple Model Predictive Control on a
Drainage Canal System. Delft University of Technology. IOS Press Amsterdam,
Netherlands.
Paden, J. 28 July 2003. Glenwood/Railroad Drainage Project Report. Development
Services Department. Minutes from Stillwater, Oklahoma City Commission’s
Regular Meeting.
U.S. Census Bureau. July 2006. Alabama Quick Facts. Accessed on April 25, 2009.
http://www.census.gov
Xian, G., Crane, M., Su, J. December 2007. An Analysis of Urban Development and its
Environmental Impact on the Tampa Bay Watershed. Journal of Environmental
Management.
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Bloopers
>>> Image taken of Woodcock Branch
facing Crenshaw Street. There is a
disturbing amount of erosion in close
proximity to houses.
>>>This was also taken facing Crenshaw
Street in Woodcock Branch. This was the first
time I had to back-track to find my “lost
creek” because there are several man-made
ditches that link up to the ditches that are
mapped. It is easy to get confused but the first
hint is when the water starts flowing the
opposite direction.
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>>>Fallen tree from past storm damage that
slows the flow of water and debris.
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>>>Some ditches were dry in areas then
flooded in the next. I found that groundwater
was actually flowing into the concrete ditches
through cracks made by slow erosion
processes.