Sherry G. Allison, Earth Sciences Department, University of South Alabama, Mobile, AL 36688. E-Mail: firstname.lastname@example.org.
This investigation looks at the type of streambank protectors used by residents along Rabbit Creek, Alabama. Located in south Mobile County, Rabbit Creek is a major tributary flowing northeast into Dog River. Rangeline Road bisects Rabbit Creek into an eastern and western side. An inventory shows an extensively developed residential area on the eastern side of Rangeline Road with a significant number of bulkheads. A newly constructed subdivision on the western side exhibits waterfront property without bulkheads and natural vegetation in place for protection against wave action, allowing access to the waterfront by use of wharfs and boat docks built over a wetland environment.
Under section 10 of the Rivers and Harbors Act, placement of structures in or over navigable water body including piers, boat docks, boat ramps, bulkheads, and riprap, requires a permit from the United States Corps of Engineers. An outline of the process, type of permit, and time it takes to obtain a permit is described.
Various types of streambank protection methods are available including bulkheads, natural vegetation and riprap. Timber, concrete, aluminum, and more recently, vinyl are used to construct the more popular bulkheads. An analysis of these manmade devices comparing service life, maintenance, installation costs and effects to the environment should provide waterfront property owners options to protect their shoreline with the appropriate streambank protector.
Keyword: streambank protectors, Rabbit Creek, bulkheads
Vegetation is nature’s way of protecting a shoreline from erosion, but observation within the Dog River Watershed indicates a large number of manmade streambank protectors installed on waterfront property. This study focuses on the shoreline on Rabbit Creek, an area of residential development with a significant number of bulkheads and/or riprap in place to protect property from water action. Rangeline Road bisects Rabbit Creek into an eastern and western side (Fig. 1). On the east is an extensively developed residential waterfront. Timber, concrete, and riprap are the more popular types of manmade structures, but investigation indicates vinyl sheet bulkheads will soon replace the timber structures. With the exception of one residence, newly constructed homes in Riverchase Estates subdivision located on the western side of Rangeline Road are not using manmade protectors and have built boat docks or wharfs over a natural wetland environment to allow access to the water.
Erosion is the removal of soil particles from a bank surface primarily due to water action. A major cause of streambank erosion is wave motion (Corps of Engineers 1981). Installation of manmade protectors is a method of protecting property from erosional processes. Evidence of wave attack by recreational watercraft appears to be one of the primary culprits of erosional problems to waterfront property on Rabbit Creek. In addition, observation of vegetated shorelines adjacent to properties with manmade structures showed signs of erosion. Interviews with residents indicate both the use of personal watercraft and recent hurricanes as primary reasons of erosion. Three of the more common types of streambank structures used in this area are vegetation, riprap and bulkheads.
The function of natural vegetation for streambank protection is to provide a riparian buffer. Without this buffer of trees, shrubs and other vegetation, the shorelines erodes and becomes unstable. When a site is cleared for building, natural vegetation is often removed and new grasses and woody plants are replanted. Grasses do not require as much time to be established as the woody plants, and grasses do not provide much protection during a storm surge; therefore, property owners install manmade structures. (Keown, Oswalt, Perry, Dardeau, Jr. 1977). Riprap is a layer, facing or protective mount of rubble or stones randomly placed to prevent erosion, scour, or sloughing of a structure or embankment (Corps of Engineers 1981). The property bank usually has a slope and a bed of stone placed on the slope to allow seepage but prevent erosion of the bank. The cost of installing riprap is inexpensive and is a popular choice for shoreline protection. If damaged, it can be easily repaired. Vegetation will often grow through the rocks adding structural value to the bank (Keown 1977). Mounds of stone or concrete can be found along many property shorelines along Rabbit Creek (Fig. 2).
Bulkheads protect banks by separating land from water. Acting as a retaining wall, bulkheads are installed to keep the earth behind them from crumbling or slumping, but these structures do not protect the shoreline in front of them. If there is significant wave action, such as motor boats, jet skis, or storm surges, erosion is accelerated at the toe of the structure. Walls tend to divert wave energy downward, resulting in undermining and eventual collapse of the bulkhead (Corps of Engineers 1981). The most popular bulkhead used on Rabbit Creek is timber. Concrete stream protectors are also present, but a new vinyl sheet bulkhead product is becoming a favorite along the waterfront.
What type of streambank protector is best at controlling erosion along Rabbit Creek, and is natural vegetation the better option to prevent property erosion especially with the high impact use of personal watercraft? A comparison of the types of streambank protectors, service life, maintenance and costs are analyzed to help homeowners with decisions about protecting their shorelines.
Rabbit Creek is a tributary in the Dog River Watershed of Mobile County, Alabama. Flowing northeast, the creek empties into the lower part of Dog River. Approximately four thousand meters is navigable with a large portion on the eastern side of Rangeline Road. During the first part of February, visits to governmental offices were made to obtain property maps, aerial photographs and data regarding streambank protectors. On February 11, an initial drive through the subdivisions enveloping the banks of Rabbit Creek was made in order to provide an overview of the area. With the aid of a section property map from the Mobile County Revenue Commissioner’s office, streets and roads leading to water access were noted.
Three separate canoe expeditions were made while inventorying the types of streambank protectors over a period of six weeks providing an opportunity to see the creek at different tidal levels. With a field assistant, the first trip to Rabbit Creek was made during a neap tide on the afternoon of February 12. The air temperature was above normal (25°C) and watercraft activity was light. Using a 1:24,000 topographic base map, a section property map, and a Global Positioning System (GPS) the length of Rabbit Creek was navigated to its confluence with Dog River in order to establish a baseline map of streambank protectors.
On Tuesday, March 6 at sunrise, a second canoe trip was made during a low tide of -0.7 feet below normal levels. The morning air temperature was 5°C with clear conditions. The toe of bulkheads was displayed showing deterioration of wood structures, as well as plants attempting to grow indicating a loss of some valuable submerged aquatic vegetation (Fig. 3).
A third canoe trip was made on April 1 during a high tide of nearly two feet above normal levels in the late afternoon. Ideal weather conditions (25° C) and the beginning of daylight-saving time, yielded a large number of recreational watercraft on Rabbit Creek creating extensive wave action against the bulkheads. Due to high tide conditions, overbanking was observed as waves splashing over the top of the bulkheads particularly on property slopes of only a couple of feet above the waterline. Many waterfront property owners were on their boat docks and piers, and several interviews were recorded. Initial interview questions are as follows: How long has your current bulkhead been in place? Have you observed erosional problems to your property?
With nearly 200 parcels of waterfront property inventoried a baseline map has been established showing the types of bulkheads and/or vegetated areas (Plate 1). Brown color indicates timber bulkheads, yellow shows concrete manmade structures, red displays riprap, blue demonstrates aluminum bulkheads, purple documents vinyl, and green designates natural areas or no manmade structures.
A significant number of waterfront property owners use timber bulkheads. Since wood is susceptible to rot and marine life infestation, structures are usually treated with a wood preservative that can cause environmental problems. Recent studies indicate that toxic metals such as copper, chromium, and arsenic leach from the pressure-treated wood and can accumulate in nearby sediments and biota (Weis and Weis, 1996). Pentachlophenol, creosote and chromated copper arsenate (CCA) are the types of wood preservatives used on timber bulkheads.
Creosote is a sticky tar-like substance used in prior years on dock pilings and telephone poles. Applied as a wood preservative, it can help timber bulkheads last longer by protecting the wood from insects and other organisms. Creosote is soaked into wood using heat and pressure. It is toxic to aquatic life and even humans who come into direct contact with it (Focus, 1993).
Another type of pressure treatment is chromated copper arsenate (CCA), which is replacing pentacholphenol and creosote. It is the more favored method of timber preservation. The metals are leached from the wood and absorbed onto fine-grained sediment particles, from which they can be accumulated by benthic organisms. In Weis and Weis paper presented in 1996, it was concluded that the extent and severity of effects in any area will depend on the amount of wood present, its age, and the degree of dilution by water movements. Wooden structures require significant maintenance with the preservative treatment and have a relatively short life span of only ten to twenty years.
Few concrete bulkheads were observed on Rabbit Creek. The ones identified were in fair condition, but erosion was observed behind one of the concrete structures on the south side of Rabbit Creek. A resident near the confluence of Rattlesnake Bayou and Rabbit Creek stated his concrete bulkhead had been in place more than 20 years, and his neighbor’s concrete bulkhead has been in place before Hurricane Camille in August 17, 1969. Located on a headland, there is evidence the bulkhead is crumbling. This situation could be the result of its service life and/or the heavier watercraft traffic in front of the property.
Only two aluminum bulkheads were noted, and both are located on the northern side of Rabbit Creek. Aluminum corrodes from the inside out, therefore, damage cannot be observed until it is too late. Contacts with area contractors indicate opposition to installing aluminum bulkheads just for the reasons mentions previously.
An environmentally-friendly alternative to aforementioned bulkheads is a newly engineered vinyl sheet piling. According to the width of the Z-shape panel, it interlocks either on the face of the panel or along the vertical slope of the angled sides, away from stress (Crane Plastics 1992). It is not necessary to use wooden face pilings with the installation of vinyl panels because the panels are held in place by wales that are anchored to the shoreline (Fig. 4). Wales are the frameworks which hold the individual sheets of vinyl sheet piling installation in place. Wood is the most readily available material to construct wales, but steel tubing can also be used for this purpose (Crane Plastics 1998). The performance of a bulkhead is affected by the type of backfill used during installation because backfill determines the amount of force acting on a streambank protector. Granular soil, such as sand, is the suggested backfill because granular soils allow drainage. Clay or clay content soils are not recommended as backfill due to fact that clay does not yield to proper drainage and is a natural reservoir for moisture.
Vinyl comes in various thicknesses and some of the brand names are as follows:
C-LOC, Shoreguard, and Northstar. Advantages to using vinyl are as follows: 1) 50 year-limited warranty; 2) guarantee not to rust, rot, crack or crumble; 3) impervious to sunlight, salt water and marine borers; 4) environmentally safe; 5) maintenance-free; and 6) cost-effectiveness (Crane Plastics, 1992). Local contractors indicate that a soil probe analysis must be completed before installing vinyl bulkheads. The probe is used to determine where solid ground begins because drive ability of the sheet piling is critical. If tree stumps, debris or rocks are present, it becomes too costly to install vinyl.
Ben Brooks, Audubon Drive, recently installed a vinyl streambank protector on a portion of his waterfront property to replace a section of a timber bulkhead that was decaying. Mr. Brooks indicates that he will be replacing the remaining wooden structure with vinyl as necessary (Fig. 5) as he is pleased its performance. In response to erosional problems on his property near the bulkhead, none were noted; but he has observed land subsiding in places of removed trees as stumps and roots of the removed trees decay.
Developer, Ron Johnson, reports that he will be replacing timber bulkheads with vinyl in the near future. As to the erosional problems, he stated that Hurricane Danny in July 1997 and Hurricane Georges in September 1998 have caused the most erosional damage to the properties along Rabbit Creek. Shorelines along the 4800 section of Audubon Drive have lost approximately ten feet of property during these past two storms.
With the exception of one property owner on the western side of Rangeline Road bulkheads are not in existence. The manmade structure located in Riverchase Estates has a bulkhead that wraps around the property at a 90-degree angle. All other waterfront property in this newly constructed residential area is fronted entirely by wetland vegetation. A conversation with Arthur Hammontree in the Riverchase Estates subdivision indicated that real estate owners in his subdivision do not require bulkheads due to the natural slope of their property. Neighbor, Ron Morrow, said he requested a permit to build a bulkhead, but the Corps of Engineers did not authorize due to the fact he could not show evidence of erosion. According to Mr. Morrow, it is only a matter of time before he will be granted a permit due to the increasing number of watercraft speeding through the area. He also said a homeowner just west of his property had a bulkhead. Observation indicated a manmade structure in place approximately 75 feet back from the shoreline with a wetland environment in front of the bulkhead (Fig. 6). From the author’s perspective this is an ideal solution. The homeowner should be commended in his landscaping techniques. The home is protected from a storm surge by a manmade horizontal structured embankment, but at the same time the owner is allowing nature to do its job of protecting the shoreline with the use of natural wetland vegetation to absorb incoming waves.
Telephone interviews were conducted with two area contractors for costs. Bo Phillips of Gulf Coast Construction and David Jones of Gulf Shores Builder Supply indicated that installation costs for a bulkhead varies according to type. In comparing costs, all quotes were competitive. It was difficult to obtain a firm amount because many variables go into the cost of building a bulkhead such as length of property, height needed, type of soil and debris encountered during installation, as well as the material and equipment required. A timber bulkhead generally costs less beginning at $70 per foot and can go as high as $100 per foot, but its service life is shorter and requires a regular application of wood preservative to minimize decay. Vinyl begins at $75 to $110 per foot. Aluminum bulkheads begin at $50 to $93 per foot (American Hatz Association, 2001). Concrete is four times the cost of timber bulkheads but can provide long service if failures due to cracking or pressure from the fill behind it can be avoided (Keown, Oswalt, Perry, Dardeau, Jr. 1977).
Anyone planning to install a bulkhead, pier, or other structure within Rabbit Creek or any navigable water must obtain a regulatory permit from the district United States Corps of Engineer’s office. Under Section 10 of the Rivers and Harbors Act, placement of structures in or over navigable water requires permission, and the Corps of Engineers was assigned the responsibility for issuing permits to prevent alteration or obstruction of navigable waterways within the United States (Corps of Engineers 2001).
Four types of regulatory permits are available as follows: 1) Nationwide Permits (general permits) are the simplest permits designed to regulate activities of small scope with minimum impact. 2) Regional General Permits are similar to Nationwide Permits because they too authorize minor activities and are developed in cooperation with State agencies. 3) Letters of Permission (LOP) are permits in coordination with Federal and State environmental agencies and a public interest evaluation but without the publishing of an individual public notice. 4) Standard Individual Permits requires publishing of a public notice, opportunity for a public hearing and receipt of comments from environmental and governmental agencies and the public. This type of permit is used for any activity which does not qualify under other permit processes (Corps of Engineers, 2001). According to a spokesperson at the Corps of Engineers Mobile District office, most residents of Rabbit Creek qualify for a Nationwide Permit or Regional General Permit as long as no wetland, including bottomland hardwoods are filled. Protection must be made along the existing shoreline at the plane of an ordinary high tide, the plane of ordinary high water or landward of all jurisdictional wetlands (including bottomland hardwoods) at the time of construction, unless otherwise specifically authorized (Corps of Engineers, 1999).
A Nationwide Permit or a General Permit can be issued within four to six weeks. Standard Individual Permits require a minimum of 60 to 90 days. No fee is charged for a Nationwide Permit, General Permit, or Letter of Permission. Fees are required for Standard Permits which require a Public Notice. Ten dollars is charged for noncommercial activity and $100 for commercial or industrial activity (Corps of Engineers, 2001).
This report and base map on streambank protection methods along Rabbit Creek should provide Dog River Clearwater Revival with sufficient information to make informed decisions as to the commonly used streambank protection methods. Location, types, costs, advantages, disadvantages, and service life of manmade structures as well as vegetated areas have been documented. As future students sign up for the Geography 480 Field Methods class, the completed base map can be used to compare changes in streambank protectors. It will be interesting to compare the use of streambank protectors on the eastern side of Rangeline Road with the property owners on the western side of Rangeline Road who are not using these structures but taking advantage of the wetland environment. Vegetation is an effective and inexpensive way to protect property while enhancing the natural beauty of the landscape and providing protective cover for small animals.
Keown, Malcolm P. 1984. Streambank Protection Guidelines. Vicksburg, MS: U. S. Army Engineer Waterways Experiment Station.
Keown, M.P., Oswalt, N.R., Perry, E. B. Dardeau, Jr. E.A. 1977. Literature Survey and Preliminary Evaluation of Streambank Protection Methods. Technical Report H-77-9. Vicksburg, MS: U. S. Army Engineer Waterways Experiment Station.
U. S. Army Corps of Engineers 1981. Low Cost Shore Protection. Philadelphia, PA: Rogers, Golden & Hamilton, Inc.
Weis, Judith S. and Weis, Peddrick 1996. The Effects of Using Wood Treated with Chromated Copper Arsenate in Shallow-Water Environments: A Review. Estuaries 19 (2A) 306-310.
Crane Plastics 1997. C-Loc: The long-lasting erosion control alternative. CPL-1105-1.
Crane Plastics 1998. Design and Installation Information for the C-Loc Vinyl Sheet Piling System. CLP-1111.
Focus 1993. Creosote-Treated Wood in the Aquatic Environment. Washington State Department of Ecology, F-TC-93-122.
U. S. Army Corps of Engineers. Introduction to the Corps Regulatory Process, April 4, 2001. http://www.sam.usace.army.mil/op/reg.permty.htm
American Hatz Association. Bill of Materials - Wood & Aluminum. April 4, 2001. http://www.weebeastie.com/hatzcb1/Bomwood.html