EROSION AND STREAM BANK PROTECTORS

Samuel O. Wilson IV, Department of Earth Science, University of South Alabama, Mobile AL, 36688. EMAIL: wilsonis@bellsouth.net.

The degradation of stream banks is an environmental concern for many people, especially property owners along stream banks. These property owners are faced with decisions on how to protect their banks from erosion. This research project focuses on erosion and the ways the property owners on Halls Mill Creek in Mobile, AL are currently protecting their stream banks. The purpose of the project is to show the different types, costs, life spans, functionality, and environmental impacts of stream bank protectors. I also explored alternative methods used to protect stream banks, and compared my study area to a previous study done by Sherry Allison on stream bank protection for Rabbit Creek. A map was created to visually show the types of protectors current residents along Halls Mill Creek and Rabbit Creek are using. Pie charts were established using data gathered from both studies showing percentage of armored stream bank protectors. With my research done a local environmental group called Dog River Clearwater Revival (D.R.C.R.), that has members living along the Halls Mill Creek and Rabbit Creek areas, can distribute the information to its members and the public.

Keyword: Stream Bank Protectors, Map.

Introduction

Riverbank erosion for property owners along Dog River, and other bodies of water is a major concern. These residents have come up with ways to protect their properties, but are these methods the best way to solve the erosion problem? Dog River is located in the southwestern portion of Alabama in the city of Mobile. The river functions as the main outlet for the Dog River Watershed in Mobile County. The area studied is a major tributary creek, Halls Mill Creek, connecting to Dog River. Halls Mill Creek contains residential property on both sides of the creek. My study focused on understanding stream bank erosion, bulkhead types, average cost of each type, and alternative methods for stream bank protection along Halls Mill Creek. The study will help residents along this specific creek and other streams make an informed decision on what type of stream bank protection to use, if any, in the future.

Understanding local conditions and stream bank erosion factors is critical for determining which of many types of protectors to use. Dog River is an alluvial river basin lying adjacent to Mobile Bay, itself a submerged river valley. The tidal cycle is diurnal (one low and one high tide in a day), and has moderate seasonal changes. The waters vary from fresh to brackish. Land surrounding Dog River is sandy and has slightly acidic soils.

Several tributaries, including Halls Mill Creek, connect to Dog River to help make up its average yearly volume of 6,417 acre-feet of water flow (Fuller, Lau, Stowe, 1978). Dog River’s average depth in 1972 was 4.5 feet with a diurnal tide range of about 1.5 feet. The tide change is important to the estuary because of its effect on water exchange and the vertical range of the tide determines the extent of tidal flats.

Dog River’s biological productivity depends on a combination of: submerged plants; tidal marsh plants; floating plants; fresh water marsh plants; and phytoplankton. Phytoplankton provide oxygen in the water and are a good food source for filter feeders such as some fish and clams.

There is a wide variety of flora and fauna in the area, which has been influenced by man over the years. We have introduced new species and affected population numbers of many types of plants and animals. The river is a popular area for commercial, residential, and recreational uses. The latter (recreational uses) is seriously contributing to an increase in erosion.

Research Question

What is stream bank erosion, how is it caused, and what type of stream bank protectors are used on Halls Mill Creek? What are alternative methods for stream bank protection?

Methods

I took many steps to gather and analyze information on Halls Mill Creek. First, I created a base map of the property lines in the area using ArcView. ArcView is a computer program designed specifically for mapping. Next, I determined a counting method for the protectors, and went out in a boat armed with my base map, and global positioning system. I started in the southern portion of Halls Mill Creek and worked my way northward. Using the GPS, I took 70 waypoints over a three-day period. I would go from protector to protector marking the corners with my GPS. I placed each waypoint number on my hard copy base map and documented the type of bulkheads/protectors. I used a digital camera to document each site. The digital photograph received a number designation. At the end of each day I would connect the GPS to my computer and upload the points into ArcView, which displays the latitude and longitude of each waypoint as a dot. I was then able to calculate the linear footage of each type of protector and put the information into an Excel spread sheet and create a pie chart.

A similar study done on Rabbit Creek by Sherry Allison (dogriver.southalabama.edu) was used to figure bulkhead costs and environmental impacts. I also visited the library genealogy center downtown on two occasions researching erosion and the history of Dog River and Halls Mill Creek.

Results

Understanding stream bank erosion requires knowledge of stream patterns and functions. The Dog River Watershed has a dendritic pattern, which means it consists of random merging streams, with tributaries joining larger streams irregularly but always at an angle smaller than 90˚. The pattern of the Watershed can help predict where erosion will occur.

These streams erode the banks through the “Fluvial Process.” The Fluvial Process produces one set of landforms by erosion and another by deposition. Two types of erosion are Overland Flow erosion and Stream Flow erosion. My research focused more on Stream Flow erosion. This type of erosion occurs when surface flow is channeled and its ability to erode is increased with increased volume of water. Erosion is accomplished by hydraulic power. The moving water can excavate and transport material at the bottom and sides of the stream. Stream banks can be undermined by stream flow, particularly at times of high water, dumping more loose material into the water to be swept down stream (McKnight and Hess 2002). On Halls Mill Creek the natural erosion processes are occurring every day with an added unnatural factor.

An added unnatural factor for erosion, in this specific creek is from the wakes of boats. These vessels are producing wakes, which generate waves up to two feet in height. The waves crash into the banks causing a severe increase in erosion. When the waves hit bulkheads 85 percent of the waves power is deflected upward, to the sides and toward the creek bottom. The reflective energy scours away sand from the foot of the bulkhead. This scouring action can cause the bulkhead to fall forward and collapse (Raines, 2003) (Fig. 1).

Bulkheads are placed in a vertical position and usually do not have enough structure at the bottom to support any type of life. Armoring the banks this way can cause certain portions of Halls Mill Creek to become incapable of supporting a complete ecosystem. Dr. Scott Douglas, a University of South Alabama civil engineering professor who specializes in coastal erosion, recently stated in the Mobile Register, “With a bulkhead, it’s like the tide doesn’t go out anymore. If you build a bulkhead along a receding shoreline you will loose any beach in front of it. Then you have lost all the intertidal zone, the shallow water and the water in front of the wall will get deeper and deeper over time.” So what can be done about erosion without affecting the environment or restricting boaters?

Alternative Methods

Currently research is being conducted on experimental erosion control devices. Four methods are being studied in our area. Biotube, Sandbags, Anchored Logs, and Wave Fences are all designed to slow down the wave energy before it hits the bank and protect the intertidal zone (Raines, 2003) (Fig. 2). Becky Roland, a graduate student at South Alabama, has these experiments underway on Dog River and in Mobile Bay. Dog River Clearwater Revival, which is also concerned with Halls Mill Creek works with her.

Stream Bank Protector Types

Some residents along Halls Mill Creek have decided to armor their creek bank properties. These bulkheads provide protection from natural and unnatural erosion. Do these residents know the environmental impact of their bulkheads on the creek?

My study area started in the southern portion of the creek at the confluence of Halls Mill Creek and Dog River and went northward until I reached the CSX railroad track. I found five different types of stream bank protectors. They are Natural Vegetation, Timber, Aluminum, Concrete, and Rip Rap, (Fig. 3). Each of the stream bank protectors has unique qualities.

Natural vegetation was the largest stream bank protector in my study area (Fig. 4). It protects 69% of the stream banks, and the vegetation provides natural habitat for animals and plants. The vegetation also protects the bank from erosion and being natural there is no cost involved. Natural vegetation is not a viable stream bank protector for some homeowners whose houses are too close to the creek.

Timber occupied the second most space on Halls Mill Creek with 24% coverage. Timber costs between $70 and $100 per linear foot. Timber falls in-between Aluminum and Vinyl for total cost, but timber has the lowest lifespan of the three. Timber bulkheads are chemically treated with a wood preservative and can cause environmental problems (Allison 1999).

Aluminum bulkheads occupied only .47% of the total linear footage recorded on Halls Mill Creek. Aluminum corrodes from the inside out, which may cause its unpopularity among residents. The corrosion will go unnoticed until it is too far gone for repairs. Aluminum costs between $50 and $93 per linear foot.

The third highest percentage stream bank protector is concrete. There were 994 linear feet of this along the Halls Mill Bank. Concrete costs four times as much as the other methods, but if installed properly it will out last all of them, except natural vegetation.

Finally, I recorded 516 linear feet of rip rap. Rip rap’s cost is debatable, because you can pay a lot for certain types of stones to put along the bank, or use concrete construction debris that is less expensive. The construction debris can be contaminated, and possibly have an adverse effect on the environment. Overall rip rap is a better habitat for plant and animal life than the other protectors, except natural vegetation. Rip rap is not vertical and absorbs 70 percent of wave energy. Rip rap can house several small bait fish, whom will attract larger game fish.

Rabbit Creek vs. Halls Mill Creek

Halls Mill and Rabbit Creek are very similar in nature and armoring. When comparing armored vs. nonarmored the two creeks have virtually the same percentages. The Rabbit Creek study area is protected by 71% natural vegetation while Halls Mill Creek has 69%. Timber, aluminum, vinyl, and rip rap were a little more prevalent on Rabbit Creek, while concrete was 1% higher in Halls Mill Creek.

The two study areas were combined to form a final map displaying the different types of protectors for Halls Mill Creek and Rabbit Creek (Fig.5).

Discussion and Conclusion

In conclusion, I provided a basic understanding of how and why erosion occurs and described the types of protection for stream banks on Halls Mill Creek and Rabbit Creek. I was honest with evaluations and diverse with research so other people can form an opinion based on their needs. I listed types, costs, life span, functionality, and environmental impacts of stream bank protectors. Dog River Clearwater Revival can take my study and pass it along to its members. They can use the study to determine, in the future, how to approach stream bank protection.

To many people rivers are the most important and extraordinary part of the environment. People are at work every day protecting them and the environment for future generations to enjoy.

References

Allison, Sherry G. 2001. Analysis of Streambank Protectors on Rabbit Creek .

Fuller, R. Lau, S. and Stowe, N. 1978 Archaeological Investigations on Dog River: An Experiment In Public Archaeology Mobile, Al. University Of South Alabama Archaeological Research Laboratory

McKnight, T. and Hess, D. 2002. Physical Geography: A landscape appreciation seventh edition. Upper Saddle River, New Jersey: Prentice Hall

Palmer, T. 1986 Endangered Rivers and The Conservation Movement University Of California Press Berkeley and Los Angeles, California.

Raines, B. 2003. Rivers of Concern. Mobile Register, April 6, 2003.

U.S. Army Corps of Engineers Mobile District 1984 Detailed Project Report on Navigation Improvements on Dog River Channel

U.S. Army Corps of Engineers Mobile District 1985 Detailed Project Report on Navigation Improvements on Dog River Channel

Witherick, M. Ross,and S. Small, J. 2001 A Modern Dictionary of Geography Fourth Edition Oxford University Press Inc., New York