TURBIDITY LEVELS
OF MOORE CREEK’S TRIBUTARIES
IN RELATION TO
RAINFALL EVENTS
Preston Ellison Department of Earth Sciences, University of South Alabama, Mobile AL. Email: ape301@jaguar1.usouthal.edu
The foci of this study
area, is the tributaries of Moore Creek within the Dog River Watershed
(DRW). The DRW drains most of the city
of Mobile; in fact, 60% of the
watershed lies within city limits.
Consequently, Moore Creek’s tributaries have undergone many
transformations including channelization and streambed engineering. Another tremendous influence is the increase
of impermeable surfaces, such as road and parking lots. The growing area of impermeability has led to
increased runoff rates and high turbidity levels, including the siltation of
many of Dog River’s
Tributaries. This research studies the
problem of turbidity in conjunction with significant rainfall events. Turbidity is a measurement of the cloudiness
or murkiness of a stream due to suspended and colloidal materials. Specifically, the central question determined
how long it took certain tributaries to regain normal or “background” levels of
turbidity after various intensities of rainfall. This question is also be correlated to
certain types of tributaries (natural, gabion lined, and concrete armored) and how they influence turbidity fluctuations
over time. Results show that among the
three main types of drainages; turbidity reached higher levels but also regained
background levels much quicker in concrete armored channels.
Keyword: turbidity, stream engineering, Dog River
Introduction
The focus of this study area is the tributaries of
Moore Creek within the Dog River Watershed (DRW). The map in
Figure 1 displays
the study area as well as the sampling sites. The DRW drains most of the city
of
Mobile; in fact, 60% of the
watershed lies within city limits (ADEM 1994). Moore Creek and its tributaries,
which include
Montlimar Canal,
is one of the larger contributing sources to
Dog
River. Moore Creek’s watershed,
excluding the southernmost extent, drains primarily the developed areas of
Mobile.
Consequently, Moore Creek’s tributaries have undergone many transformations
such as streambed engineering and channelization. Another tremendous influence
is the increase of impermeable surfaces including roads and parking lots, which
have led to increasingly high runoff rates. The changes brought about by
urbanization have undoubtedly led to the overall degradation of the Moore Creek
Tributary as well as many areas of the DRW.
This research is
intended for the community of
Mobile
and anybody concerned with the health of the DRW. Its purpose serves the
community in testing selected areas of Moore Creek’s watershed for turbidity
after significant rainfall events. Turbidity is a measurement of the cloudiness
or murkiness of a stream due to suspended and colloidal particles composed of
algae, detritus (decaying organics), and sediment (Caplinger 2002).
Many
of
Dog River’s
tributaries are sources of sediment and remain somewhat turbid, which alone can
greatly degrade the overall water quality. Turbid waters tend to be warmer as
suspended particles absorb heat from sunlight, causing oxygen levels to fall.
Less light penetration also results in less photosynthesis, which further
decreases oxygen levels. Turbidity can not only affect plants but also can clog
fish gills and prevent egg and larval development (fivecreeks.org 2004). In
addition to the reduction in a stream’s health, turbid waters detract from the
aesthetic qualities of our streams.
The fact that a
large majority of Moore Creek and
Dog
River is an urban watershed has led
to such problems as high turbidity that has contributed to the overall decline
in water quality. It is evident that channelization and high runoff rates have
increased siltation in the watershed, from the costly dredging projects in the
past.
The dredging project of 2001 cost
taxpayers $1,900,000 (Ford 2004).
Dredging exemplifies the consequences facing an urban watershed,
although the project may have reflected a political agenda rather than a
genuine effort to improve the quality of
Dog
River.
The Alabama
Department of Environmental Management (ADEM) has established standards for
overall water quality, which includes turbidity. ADEM states, “there shall be
no turbidity other than natural origin” furthermore, “in no case shall
turbidity exceed 50 Nephelometric Units (NTU’s) above background”. ADEM defines
“background” as the natural condition of the stream without the influence of
man-induced causes (McIndoe 2002). This research will therefore be useful in
determining whether Moore Creek’s tributaries comply with ADEM standards.
Research Question
The
goal of this study is to research the problem of turbidity in conjunction with
significant rainfall events.
Specifically,
the central question is to determine how long it takes certain tributaries to
regain normal or “background” levels of turbidity after various intensities of
rainfall? This will also be correlated to the types of tributaries (natural,
gabion lined, and concrete armored) and how they compare in turbidity
fluctuations over time. Presumably, concrete armored drainages will regain
background levels much quicker and have a relatively shorter and greater spike
in turbidity during a rain event.
This research will
add to the growing body of knowledge on the overall health of
Dog
River. The research could even
broadly reflect the type development within the Moore Creek Watershed, from the
overall amount of turbidity in the streams. This could be compared to the turbidity
in Halls Mill Creek, where expanding growth has created large amounts of
sediment run-off.
A good knowledge of
background levels would also aid in recognizing turbidity sources whether it be
poor BMP’s, failing engineering or even excessive areas of algal growth.
Methods
In
order to complete the proposed research, two major components were necessary.
Proper data called for significant rainfall events of approximately .5 inches
or greater within a 24/hour period.
Secondly, periods of drier weather were required to establish
backgrounds. Background turbidity levels were collected at various times but
were allowed at least 3 days without rainfall. Accurate precipitation events
were measured for the study area using the online NOAA regional radar at the
Mobile
Regional Airport.
During the
precipitation event, samples were taken before, during and after at roughly 2-3
hour intervals.
Longer intervals were
planned for up to 72 hours after an intense amount of precipitation, however
only one significant rain event of .34 in. occurred during the study and
required no additional samples.
The
following sites were used in this study: Montlimar Canal
(site 1) and West
Eslava Creek
(site 2) confluence, West Bolton Branch
(site 4) at Montlimar Rd.,
the Spencer Branch
(site 5) and Montlimar Canal
(site 6) confluence and West
Bolton Branch
(site 7) at Azalea Rd.
Samples taken from
the field were brought back to the lab at the
University
of South Alabama. They were
measured using a standard Turbidity monitor and are represented in NTU’s.
The raw data was then combined with the
timing of sampling, as well as intensity of rainfall.
This resulted in a correlation of turbidity
levels versus rainfall and duration for the selected tributaries.
Results
Background
turbidity levels were established by sampling all six sites at least 4 times
during an extremely dry period.
Total
precipitation for each day during the study is located in Appendix A. The four
samples for each site were then calculated to give the average background for
turbidity levels.
The results are listed
in Table 1. Values of over 10 NTU’s can begin negatively affecting the aquatic
environment within several hours. As little as five NTU’s can increase coughing
rates in fish if it persists for several days (www.Waterontheweb 2004). In
comparison, all the tested sites were relatively low, although site 1 may have had
high enough turbidity rates to cause minor stress to the surrounding
environment.
Table 1. Average background turbidity for all sites
|
1
|
6.4
|
2
|
2.6
|
4
|
1.5
|
5
|
2.2
|
6
|
2.6
|
7
|
1 |
Once background
levels were established I could then determine the time it took for the
tributaries to regain background levels after a rain event.
This would also allow me to verify whether
the selected tributaries comply with ADEM standards.
On April 11,
Mobile
received .34 in. precipitation that lasted roughly two hours.
Sampling occurred 7-8 times throughout the
following 36 hours.
This rain event
although under .5 in. produced sufficient data for analysis. A picture of the site
and a graph displaying time vs. turbidity are as follows: Montlimar Canal
(site 1), West
Eslava Creek
(site 2), West Bolton Branch
(site 4) at Montlimar Rd.,
the Spencer Branch
(site 5), Montlimar Canal
(site 6), and West
Bolton Branch
(site 7) at Azalea Rd.
Another aspect of
the study was to determine if the type of drainage had any influence on the
amount of time it took for turbidity to regain background levels. It was
presumed that West Bolton Branch at Azalea (site 7) and
West Eslava
(site 2) would reach relatively quicker background levels than other sites. The
reason for this assumption was due to the type of channel, both site 7 and 2
are concrete armored, which allows for increased flow rates.
This presumption
was tested by taking samples at site 7 and West Bolton Branch at Montlimar
confluence (site 4).
Site 4 is
downstream from site 7, but becomes more of a natural drainage that is gabion
lined and overgrown with vegetation after
Azalea Rd.
This allowed sampling of two different types of drainages, one concrete armored
and the other gabion lined that were roughly the same distance. The sites were
sampled during the same rain event of April 11. Site 7 resulted in reaching
background levels within 10 hours of initial precipitation, while site 4 did
not regain background for another 26 hours.
The same test was
applied to W. Eslava (site 2) and
Montlimar
Canal (site 1). The drainage for
site 2 is mostly concrete armored and the drainage for site 1 is a straight
channel with dense vegetation along the banks for most of its length. This
again gives turbidity information on two different types of drainages that are
roughly the same length. Background levels at site 1 were only similar at 36 hours
after the initial precipitation, while Site 2 reached its background in roughly
8 hours.
The other proposed
question was whether concrete armored drainages recorded a greater spike in
turbidity through the duration of the rain event.
To test this, sites 7&4 and sites 2&1
were again compared.
Samples were taken
at equal intervals and resulted in a far greater peak in NTU’s among the
concrete armored drainages.
Figure 6 shows that site 1 (natural drainage)
reached a gradual peak of 11 NTU’s within roughly 9 hours, while site 2
(concrete armored) quickly peaked at 80 NTU’s in just over 2 hours. Testing
sites 4 and 7 resulted in a similar trend.
Figure 7 shows site 7 (concrete armored) had a rapid peak and decline in
turbidity downstream at site 4 (gabion lined), turbidity was relatively lower
and declined gradually.
Testing both
sites proved the presumption
that the concrete
armored drainages regain background levels quicker and have a relatively
greater spike than more natural drainages.
The reason for the difference in times between the two drainages is a
result from different discharge rates.
According
to (Jones 2004), concrete channels have increased
stream velocity compared to vegetative drainages, which act to retard
water flow and can be explained as the difference in the roughness coefficient
Discussion
One
question that was proposed from this study was whether the selected tributaries
complied with ADEM regulations; which was not to exceed 50 NTU’s above background.
The answer is yes, however ADEM requires
sites to be sampled 72 hours after a rain event.
This does not effectively address the issue
of reducing the overall human induced turbidity.
This is where regulations meet reality due to
the engineering marvels, which allow drainages to rapidly move run-off
downstream.
This is the case for sites 2
and 7 that potentially could release exceeding amounts of sediment but still
comply with ADEM regulations.
Evidence
of this was observed during the study when site 2 went from a background of 2.6
to 80 NTU’s within about 2 hours and then dropped off to 4.2 NTU’s in under 5
hours.
Data from site 6 revealed that
the spike in turbidity at site 2 had little overall influence downstream.
However, keep in mind that data was collected
during a .34 in. rain event that lasted 2 hours and effects could be
exponential given a characteristic
Mobile deluge of 2+
inches. Continuing research could build on this idea by studying the effects
greater amounts of precipitation have on varying drainages.
Conclusion
The differences in
drainages observed in this study point to one obvious conclusion; drainages
with vegetation have less turbidity for longer durations and have less dramatic
reactions to a rain event. “Natural-like” streams such as
Montlimar
Canal that have dense vegetation
along the banks act as buffers, which filter sediment entering streams.
Even overgrown gabion-lined drainages such as
site 4 acted to slow and reduce turbidity. However, in one aspect these
drainages may not necessarily be better for the aquatic environment. Drainages
that release somewhat turbid waters for several hours may impede water quality
as much as drainages that spike high turbidity in short durations.
The problems and
transformations facing an urban watershed like
Dog
River are many, and some are
ultimately unavoidable.
However,
restoration of a stream’s health can be attained if proper policies and
standards are met. Further, some regulations may need to be addressed again by
setting standards for specific tributaries based on discharge rates.
Researching the specific problem of increased
turbidity due to rainfall events could provide standards for various drainages,
which could aid in pinpointing siltation problems.
Cited Sources
ADEM. Alabama
Department of Environmental Management (1994) A Survey of the Dog
River Watershed Mobile.
Caplinger, Jessica A. and David N. Mott Water Quality Assessment of Springs and Perennial
Streams within the watershed of Buffalo
National River. National Park Service Sep. 10, 2002.
Ford, David Wesley. Dredging
and Dog River Department of Earth Sciences, University South
Alabama. http://www.usouthal.edu/geography/fearn/480page/dogriver.html
Accessed Apr. 13 2004.
Jones, R. Phillip.
Arguments Against Concrete Armored Channels in Dog
River Watershed. Department of Earth Sciences, University
of South Alabama. Accessed Apr. 13 2004. http://www.usouthal.edu/geography/fearn/480page/dogriver.html
McIndoe, James, M. Alabama Department of Environmental
Management Water Division-Water Quality Program Chapter 335-6-10 Water
Quality Program Sep 2002.
www.fivecreeks.org/monitor/turbidity.html Water Quality Monitoring: Turbidity 2004.
www.waterontheweb.org/under/waterquality/turbidity.html Turbidity 2004.
