AERIAL ANALYSIS OF SEDIMENTATION IN DOG RIVER WATERSHED

James G. Carver, Department of Geography and Geology, University of South Alabama, Mobile Alabama 36688. INTERNET: st94090@jaguar1.usouthal.edu.

Evidence of sediment buildups in Dog River and its tributaries is widely reported. Sediment accumulations that occur over this widespread region have many different sources. Although studying the sedimentation by foot and boat is possible, it is very difficult and time consuming. The sedimentation in the river can be analyzed through the use of aerial photography. This study applies various remote sensing techniques to Dog River Watershed to determine the location and source of sediment buildups.

Standard 35mm photographs taken from a Cessna 182 airplane provide the foundation for a photo collection throughout the entire watershed. Analysis of these photographs shows sand deposits in the tributaries as well as plumes of suspended clay in the river. Erosion, construction sites, and other point sources are identified from aerial photos demonstrating their effectiveness in pinpointing sources of sediment buildup in the river.

Keyword: sediment accumulations, remote sensing, Dog River Watershed
 

Introduction

Sedimentation is the "process of deposition of a solid material from a state of suspension or solution in a fluid (usually air of water)" (Goetz, 1991). It is a problem that plagues many river systems throughout the southeastern United States. The buildup of sediments not only affects fish, plants, and other wildlife, but it also affects human life. Rivers provide humans with many valuable uses. They give us drinking water, water for crops, a source of power, transportation, fish and wildlife, and recreation. The disruption that sedimentation causes prevents us from utilizing these resources to their potential. Also, sedimentation causes instability in channel beds and banks, clogging of culverts, and a never ending need for dredging. Although sedimentation is a natural occurrence, it can be exacerbated by mankind through activities such as those involved with construction. The consequences may be disastrous, ranging from fish kills to flooding.

The Dog River Watershed is an area that covers approximately 95 square miles in Mobile County. Over half of this area is within the Mobile City limits, with the rest lying just to the south. The watershed not only has Dog River in its boundaries, but it also includes all of Dog River's tributaries. These include Alligator Bayou, Rabbit Creek, Rattlesnake Creek, Halls Mill Creek, Montlimar Creek, and Eslava Creek. At the mouth of Dog River is Mobile Bay.

Excessive sedimentation is a major problem in Dog River. Through individual and team field work experiments in GEO 480, sediment buildups were found at many road crossings and construction sites in the watershed. In spite of these findings, potential point sources could not be identified. Before something can be done to stop these buildups, possible sources must be identified.

Although there are several ways to detect sediment accumulations and identify their sources, one proves to be superior when compared to others. An aerial analysis of sedimentation is the best method possible. It only takes a few hours and a minimal amount of money to complete the study. Any other method would take weeks, have inaccurate results, and would cost an enormous amount of money. Obviously, remote sensing techniques would be the best economically and environmentally.
 

Research Questions

With sedimentation in Dog River Watershed being the object of study, a few questions must be answered. Where are the buildups? If the locations of the buildups are detected, then the sources can be identified. Another question that must be answered is what are the sediment sources? If this problem is going to be properly addressed, the type of sources must be identified. The final question that will be answered is about the location of sources. Where are they? If the location is obtained, then it can be determined which sources are associated with different tributaries.
 

Methods

To perform this analysis, many methods will be used together to determine the results. In order to obtain any aerial photographs efficiently, an airplane, camera, film, pilot, passenger, detailed map, and a flight plan are needed.

The aircraft best suited for an independent study of this nature is the Cessna 182. It is big in size and provides much needed room for movement, which will be a necessity to take the photos. It is also a high performance aircraft (200 horsepower) that has good acceleration, climbing, and turning abilities for its cost. These capabilities will give the pilot and passenger a higher degree of safety while flying at the low altitudes required for the study. It is also inexpensive and can be rented from almost any local airport.

The camera, a 35mm Olympus, will be used to take the photographs. The camera must be handheld and durable since the photos will be taken in winds in excess of 100 m.p.h. With limited funds, camera mounts and aerial photography equipment could not be obtained.

The best film for an aerial study of sedimentation is Fuji I.S.O. 400 speed color film. Fuji film is inexpensive and provides good detail. The speed must be at least 400 because of the rapid movement and vibration of the aircraft. "Advantages of color over black and white photography for this application include better discrimination of smaller objects at a given scale and the fact that identification of the objects is more positive and more rapid on the color photography" (Colwell, 1983).

A passenger will be needed along with a pilot. The need for a pilot is self explanatory. The passenger is needed to take the photos and help guide the pilot along the flight path. Since the Cessna 182 has 4 seats, additional passengers could be used to record important data that might be missed by the pilot and cameraman.

A flight plan will also be needed to ensure that the flight will be productive and efficient. Within the flight plan, 5 details must be accounted for:

1. Flying height must be determined so that the scale will remain the same throughout the entire watershed. Also, the flying height must be in accordance with F.A.A. Rules for the safety of the people onboard and other flyers in the area. Two flying heights for this study will be used, approximately 1200 and 2100 feet above ground level. How descriptive the pictures can be without jeopardizing the safety of the crew, other people, and aircraft in the area, helped determine the flying height.

2. Scale must be as large as possible to get the most detailed photographs. Usually, for sedimentation studies, the smallest scale recommended is 1:10,000. Flying at such a low altitude will keep us well below this limit.

3. Ground distance must be approximated so that we will know how many exposures it will take to successfully complete the survey.

4. Flight lines must be calculated so that the pilot will know where to fly and when to turn. This will help better organize the actual shoot in the watershed because they will be taken in straight lines and not spots from different locations.

5. Photos needed will have to be figured out so that costs for the study can be projected. Also, if the number of photos needed exceeds the monetary limit for buying the exposures, a different scale may be used by adjusting our altitude to help reduce the number.
 

Results

One hundred thirty-two photographs were taken throughout the watershed, from 1:00 P.M. to 2:30 P.M., after significant rainfall had produced runoff and erosion. Fifty-eight were taken at an altitude of 1200 feet A.G.L. on September 4, 1998, one day after a cold front that had stalled over the area and dumped rain for three straight days, moved out. Seventy-four photos were taken on October 4, 1998, after Hurricane Georges flooded the Gulf Coast with over 18 inches of rainfall and a storm surge over 3 feet high. Of the 74 photos, 26 were taken at 2100 feet A.G.L., while the remaining 48 were taken at the standard 1200 feet. I was able to film most of Dog River Watershed in under 3 hours total.

The differing dates proved to be very useful after I had examined the photos. The September photographs showed that there was a significant amount of sediment being deposited in Dog River, most of which was coming directly from Halls Mill Creek. The October pictures were the most spectacular of the two dates. A giant sediment plume was photographed at the mouth of Montlimar Creek, clearly showing that it was the major source of deposition throughout the watershed. An analysis of the photographs from both dates has been made for each individual waterway.

When examining these photos for sedimentation, I looked for these things:

1. A milky-brown to orange discoloration of the water
2. Differences in color of the water at the mouths of the creeks
3. Differences in color of the water within the creeks
4. Sediment buildups within the streams
5. Erosion on creek and river banks
6. Point sources immediately upstream from any discoloration, such as construction sites

Analysis of September photos :

1. Rattlesnake Bayou:

There was evidence of a slight water discoloration, but only in and near the western tip of the bayou. No point sources for sedimentation could be found in the western bayou, leading me to believe that the discoloration is due to shallow waters that have been easily stirred up from the rain. No sediment can be seen in the eastern portions or being deposited in Dog River. Most of the river is shallow enough for the bottom to be seen in the photos.

(Figure 1 - meander in eastern Rattlesnake Bayou)

2. Halls Mill Creek:

This was the major source of sediment deposition into Dog River. Water discoloration is very evident throughout the entire Creek extending into Dog River. A number of construction sites can be found along the banks. Although construction sites could be contributing to the problem, the worst discoloration occurs farther west where the creek meanders through a relatively undeveloped area, mostly a wooded area. No point sources could be found through the thick foliage west of railroad tracks.

(Figure 2 - water discoloration west of railroad tracks in Halls Mill Creek)

3. Montlimar Creek:

Montlimar Creek ran a close second to Halls Mill Creek in terms of sediment being deposited into Dog River. All along its banks, construction sites are present. Erosion can be seen in large stretches, sometimes over 100 meters long (Fig. 3). Sediment buildups within the stream are present. There are many drainage systems dumping excess water and sediment into the creek. The water discoloration is not as bad north of where the Eslava Creek runs into it north of Airport Blvd.

(Figure 3 - Erosion next to Industrial Park Circle in Montlimar Creek)

4. Eslava Creek:

There are two Eslava branches to note: the first connecting to Montlimar Creek just north of Airport Blvd; the second beginning east of Bel-Air Mall and running into Dog River. The first branch shows potential for being the major contributor of sediment into Montlimar Creek. Beginning at the intersection of Eslava and Montlimar, sediment is built up along its bed as it extends westward past Lleyn Ave. (Fig. 4). The large deposition of sediment in this stream seems to be coming from a site on the southeastern corner of the Mobile Country Club. The second branch also has sediment buildups along its bed. Erosion, construction site runoff and drainage seem to be the main point sources in the northern parts of the creek. The farther south Eslava travels towards Dog River, the less developed the area gets. Also, the water appears to become milkier, although the color is not even comparable to Halls Mill or Montlimar Creeks.

(Figure 4 - sediment buildups at Lleyn Ave. in Eslava Creek)

5. Dog River:

Once we reach Dog River, the water seems to turn towards a more natural looking color, not as milky-brown looking. This continues until we come upon the mouth of Montlimar Creek. From this point on, sediment can be seen flowing downstream along the western bank of DogRiver. Once the mouth of Halls Mill Creek is reached, the sediment from Montlimar seems to stop and the sediment from Halls Mill Creek starts being deposited directly into the center of the stream.

(Figure 5 - sediment being deposited into Dog River from Halls Mill Creek)

6. Alligator Bayou:

The bayou has almost no development along its banks and is relatively untouched because of this. The water quality is the best that I have seen in the watershed.

7. Rabbit Creek:

A few construction sites are present along its banks. In spite of this, water quality seems to be in good condition with only one spot in the water where sediment is evident.

Analysis of October Photos :

1. Rattlesnake Bayou:

The water quality does not seem to be any different than in the September photos. The bottom can be seen as it was before. I did notice sediment in Dog River near the mouth, but it did not seem to be deposited from this bayou.

(Figure 6 - sediment in Dog River at mouth of Rattlesnake Bayou in upper left corner)

2. Halls Mill Creek:

This time around, Halls Mill is a distant second to Montlimar. Water in Halls Mill Creek seemed milky, but apparently not as discolored as it was in the September photos. Construction sites are still present. Ironically, most of the discoloration begins east of the railroad tracks. Water quality west of the tracks seems to be better than anywhere else in the creek.

(Figure 7 - comparison of water color from September west of tracks in Halls Mill Creek)

3. Montlimar Creek:

The water in Montlimar is the worst of the two sets of photos. Nothing in any part of the entire watershed even compares to it. At the mouth, a giant sediment plume can be seen from miles away (Fig. 8). All along the creek, the same conditions apply to this set as to the September set. There are differences though. The water color is much brighter than before. There is a lot more erosion along the banks than previously recorded. Some streets that run alongside the creek have caved into the stream. Also, some construction sites that did not appear to have much runoff before, can be easily spotted from the air because of the significant amount of runoff (Fig. 9). Just as before, once the creek reaches north of the Eslava intersection, the water changes color from bright orange to brown (Fig. 10).

(Figure 8 - giant sediment plume at mouth of Moore Creek)

(Figure 9 - construction site runoff on Airport Blvd. and Montlimar Creek)

(Figure 10 - water discoloration at Eslava and Montlimar intersection)

4. Eslava Creek:

The first branch, that runs into Montlimar Creek, appears to be the major source of sediment for Montlimar. The point source is identified at the end of the creek as a possible construction site on the southeastern corner of Mobile Country Club Golf Course (Fig. 11). The second branch of it is basically the same as before with slightly more sediment accumulated on the creek bed.

5. Dog River:

Water quality in the north seems to be better than in September photos. This continues downstream until the mouth of Montlimar Creek is reached. At this point, sediment is deposited into Dog River and downstream towards the mouth of Dog River (Fig. 8). Near the mouth, sediment is not as evident as it is near Montlimar, but is still noticeable along banks and in center of river.

6. Alligator Bayou:

There were no noticeable changes in water quality from September to October.

7. Rabbit Creek:

Water quality appeared to be the same as it was in September.
 

Discussion and Conclusion

The purpose of this study is not to heal the wounds that society has inflicted on our environment, but rather to identify what we are doing and find out where we are doing it. Sedimentation is a major problem that can be amplified through human intervention. When a heavy rain comes and washes clay from a construction site or erosion worsens because a stream=s natural flow has been altered to fit our needs, sediment flows downstream wreaking havoc on native plants and wildlife. "Changes in the character of sediments thus force changes in the bottom fauna, compel expulsion of those organisms adapted to the previous bottom, and permit entrance of a new association" (Twenhofel, 1950). The turbidity that is produced can reduce the amount of oxygen fish need for respiration. When the sediment settles to the bottom, plants are smothered because they are covered by it. Not only does it affect wildlife, but it also affects humans as well. The quality of drinking water is reduced. Recreational activities, such as fishing, are halted because the fish have either died or have moved to a more suitable environment. "Sedimentation (and scour) of channels affect (1) flood damages and flood control by increasing the size of stream channels and floodways..." (Trask, 1950). If there are any advantages to sedimentation, the negative impacts heavily outweigh them.

In this study, finding any sedimentation within Dog River Watershed was performed by analyzing data collected from air photos and notes taken while in flight over the area. As I have previously stated, there are several necessities in performing this operation. Most of which could be borrowed, cutting down the overall price. Everything else that did come with a pricetag was relatively inexpensive. This method proved to be the most cost efficient way to collect the data needed.

Even though my survey was a success, there were many problems and a lot of aggravation I had to deal with to complete it. The biggest problem associated with this project was the infamous "El Nino". To really get quality information about sedimentation, an adequate amount of rainfall must occur to produce runoff, erosion, and any other sources related to it. When Spring rolled around and it was time to fly and take some pictures, we were stuck in a drought that lasted throughout the entire summer. Rain did fall, but not enough to cover the entire watershed or produce runoff, etc... The first "real rains" did not fall until late August and early September. Next, scheduling the airplane with another pilot became very difficult because of conflicting schedules. As if those two weren=t enough, the flight plan had to be thrown out of the window (not literally). Dog River Watershed sits between two airports with control towers. During the entire flight, our aircraft was in one of the two airspaces. This means that any changes made in flight would have to be okayed by one of the control towers. This made it difficult considering the fact that neither the creeks nor the river run on a straight course. To top things off, as I stated earlier, the watershed is in two different airspaces, both having control towers. The northwestern portion of the watershed falls into Mobile Regional Airport's airspace and the rest of the area is in Brookley International Airport's. While flying from one airspace to another, radio frequencies must be changed back and forth in order to communicate with each tower. This is a painstaking process. Not only did this aggravate myself and the pilot, it touched the nerves of the people in the air traffic control tower who began to be rude after a while. While we were flying, the tower was constantly having to turn us and deny our requests for course deviations because of safety concerns due to the high frequency of aircraft around the two airports. Not everyday is that bad, we were just very unfortunate to have been in the air on those particular days.

All in all, I was able to locate sedimentation throughout Dog River Watershed. There were plenty of buildups, along with their possible sources, that were identified and located. Now, possible point sources may be examined thoroughly on site. Researchers can go directly to these sites without having to search the entire watershed for weeks or possibly months. This analysis saved a lot of money that would have been lost to manpower and other additional costs. Other than having to wait on rain, everything went very quickly and smoothly. Most of the photographs turned out to be really detailed and useful. An added plus is that they can be examined at a later date by others, who were not on the flight, that may be able to provide some good insight on the area.

Acknowledgments

I would like to thank Hugh Avery C.F.I. and Louise Showalter C.F.I. for taking time out to fly with me on the photo shoots and for putting up with A.T.C.

I would like to thank Woody Wooden for providing me with valuable knowledge concerning photography.

I would like to thank Dr. Roy Ryder for sharing his expert knowledge of remote sensing with me.
 

References Cited

Colwell, Robert N. 1983. Manual of Remote Sensing. Gene A. Thorley. Second Edition. Volume II. Falls Church, Virginia. American Society of photogrammetry.

Goetz, Philip W. 1991. The New Encyclopaedia Britannica. Volume 10. Chicago, Illinois. Encyclopaedia Britannica.

Trask, Parker D. 1950. Applied Sedimentation. New York. John Wiley & Sons, Inc.

Twenhofel, W. H. 1950. Principles of Sedimentation. Second Edition. London, New York, and Toronto. McGraw - Hill Book Company Inc.