Careful studies of an eroding island help uncover solutions to preserving habitats.

By Yu-Chun Su, Paul Jensen, Marty Heaney, Bob Gearhart,
Kay Jenkins, and Charlie Belaire

An eroding shoreline and potential loss of fish and wildlife habitat initiated a call to action to protect a popular Texas state park. Over the past 35 years, Goose Island State Park in Aransas County has lost more than 25 acres of land, and the rate of erosion is increasing. Not only is a popular recreational destination threatened, but continued erosion also could degrade oyster reefs, seagrass beds, and coastal marsh habitats that are valuable to the finfish, shellfish, and avian species living in the Aransas Bay system.

A partnership of federal and state resource agencies as well as nonprofit and private organizations answered the call by overseeing the development of shoreline-stabilization and marsh-restoration plans that would enhance fish and wildlife habitat. A key to the success of the project is the comprehensive marine, shoreline, and habitat analysis conducted upfront for the creation of feasible, cost-effective shoreline-protection and marsh-restoration alternatives.

Shoreline Erosion

Goose Island State Park comprises 321 acres located on Aransas Bay on the southern tip of the Lamar Peninsula, 9 miles north of Rockport, TX. It is a popular fishing and birding destination for visitors from nearby Corpus Christi and Victoria as well as for out-of-state visitors who reside in Texas during the winter. Nearly 300,000 people visit Goose Island Park in a typical year, and more than 13,000 vehicles each year use the park's public boat ramp for access to St. Charles and Aransas Bays.

Park facilities located on the eastern half of Goose Island include camping sites with shade shelters, parking areas, restrooms, and a fishing pier. The shoreline of this developed portion of the island has been stabilized with bulkheads constructed in 1974 and 1980.

The western half of the island is not protected and has suffered significant erosion. An analysis of aerial photographs and recent onsite surveys indicates that approximately 17.1 acres of Goose Island were lost due to erosion between 1969 and 1995. An additional 8.5 acres were eroded between 1995 and 2002. These land-loss rates equate to an average of 0.66 ac/yr before 1995 and 1.21 ac/yr between 1995 and 2002, which indicates the shoreline erosion rate is increasing. Since 1969, several sections of the shoreline have eroded back as much as 400 feet, and several breaches in the island have formed.

Habitat Impacts

The developed eastern side of Goose Island has been stabilized with bulkheads constructed in 1974 and 1980.

Smooth cordgrass marsh is a natural defense against erosion and provides a vital nursery habitat for many fishery species.

Continued shoreline erosion threatens the remaining undeveloped part of Goose Island and its valuable fish and wildlife habitat. The island's wetlands include approximately 10 acres of intertidal emergent marsh dominated by smooth cordgrass and another 8 acres of adjacent high marsh dominated by saltwort. Estuarine marshes like these are important nursery habitats for commercial and recreational fisheries, providing both protection and food for the aquatic species living there. They provide nutrients for the flora and fauna in the entire bay system, and their vegetation provides food, shelter, perching, and roosting areas for many birds, particularly waterfowl and wading birds. Furthermore, these wetlands provide excellent water-quality functions for the adjacent bay by removing sediments and contaminants from runoff leaving adjacent uplands. The smooth cordgrass is also the first line of defense in protecting the shoreline against erosion.

Aquatic habitats associated with Goose Island include 15 acres of seagrass beds, which provide extremely valuable habitat for finfish, shellfish, and invertebrates and serve as a critical nursery habitat for many fishery species. Seagrasses are a direct food source for fish, sea turtles, and waterfowl and contribute organic matter to the bay ecosystem. Seagrasses also bind sediments to stabilize the bay bottom, dampen wave action, and prevent erosion. Scattered live oyster clumps and oyster reefs in the area offer a valuable habitat for aquatic plants and animals by providing food or nutrients, hiding places, and attachment sites.

In 1999, the Audubon Outdoor Club of Corpus Christi listed 315 species of birds that have been observed in Goose Island Park or its immediate vicinity. Included are several federally listed endangered or threatened species, such as the whooping crane, and some state-listed avian species. Fifty-two of these species have been documented to nest or breed in the park or nearby, and many of the birds observed depend on the island's wetlands and aquatic habitats for some stage of their life cycle.

Erosion Response

The Texas Parks and Wildlife Department (TPWD) entered into a cooperative agreement with the Texas General Land Office (GLO) and the Coastal Bend Bays & Estuaries Program (CBBEP) to evaluate shoreline-stabilization and marsh-restoration alternatives for Goose Island. Funding for the project was provided by the Coastal Erosion Planning and Response Act (CEPRA) program administered by GLO with matching funds provided by CBBEP.

The project's advisory team consisted of staff from federal and state resource agencies, including TPWD,  the Texas GLO, the US Army Corps of Engineers, the National Marine Fisheries Service, the US Fish and Wildlife Service, EPA, and the Texas Commission on Environmental Quality (TCEQ). The team sought methods to avoid, slow, or remedy coastal erosion as well as preserve and restore wetland habitats by creating additional smooth cordgrass marsh. The preferred solution would use dredge material from nearby boat channels to create estuarine emergent marsh. This beneficial use of the dredge material would also help local communities meet their navigational needs. All goals were to be met while avoiding negative impacts to existing seagrass beds and oyster reefs as much as practicable.

The engineering consulting firm PBS&J was selected to conduct the alternative analysis and feasibility study and to prepare the preliminary engineering design of the selected alternatives. Belaire Environmental Inc., a subconsultant, assisted PBS&J in assessing and developing the marsh-restoration alternatives.

To develop and assess workable solutions for Goose Island, complete and accurate information was needed about the island and surrounding bay waters. The engineering and consulting firms conducted a number of surveys and data-collection efforts to characterize the situation at Goose Island. Several alternatives to stabilize the shoreline and enhance and restore adjacent aquatic habitats were then developed. Resource agency staff also conducted biological surveys to aid the project team in determining the best locations for the project features that would avoid or minimize potential effects on existing aquatic habitats. Finally, PBS&J evaluated the feasibility and cost-effectiveness of the selected alternatives.

Data Collection on the Bay

Data on wind, tides, water depths, area bathymetry, soils, and the location of pipelines, seagrass beds, and oyster reefs were collected using bathymetric surveys, shoreline mapping, side-scan sonar, magnetometer readings, and other detailed surveys.

A highly specialized 20-foot aluminum survey boat was put to work for data collection. The boat featured Hydropro 2.0 navigation software and a Satloc SLX global positioning system (GPS) with an Omnistar differential correction receiver. Data-collection tools onboard the boat included the following:

• EdgeTech 272-TD sonar towfish
• CODA Technologies DA50 side-scan sonar data acquisition system, with side-scan sonar range of 98 feet on each side of the towfish
• EG&G Geometrics G-881 cesium magnetometer, taking one reading per second
• Odom Hydrotrac echo sounder with narrow beam (four-degree), 200-kilohertz transducer
• TSS DMS-05 heave compensator to correct for wave actions
• Valeport VTM740 automatic tide recorder
• Sonar transducers, at a depth of approximately 2 feet
• Echo sounder transducer, at 1.1 feet beneath the surface

With near-shore waters of approximately 3 feet deep at high tide, the boat survey took place mostly in deeper water, but attempts were made to get as close to the shoreline as possible. A near-shore survey was also conducted along transects perpendicular to the shoreline at 500-foot intervals. Survey data were corrected for tides using Texas Coastal Ocean Observation Network (TCOON) gage records. So all data could be referenced to the same vertical datum, the surveyed bathymetric data were converted to NAVD88 using two benchmarks located at the Copano Bay State Fishing Pier.

The side-scan sonar survey mapped any substrate anomalies that might have represented oyster or seagrass beds. A ground-truthing survey was also conducted to verify anomalies and to survey areas not accessible by sonar. A magnetometer survey identified magnetic anomalies that might have represented pipelines, oil well heads, shipwrecks, and other objects. Identified anomalies were verified through crosschecking with the Texas Railroad Commission's database.

Soils, Winds, and Waves

Using dredge materials from nearby boat channels, workers will build a levee here.

Soils information was needed to develop erosion control and habitat-creation alternatives compatible with the area's composition. A 1974 Natural Resources Conservation Service (NRCS) soil survey indicated that soils in the area were 60% Tatton loamy sand and 40% other soils. In addition, PBS&J conducted a geotechnical coring survey taking 3-inch-diameter, 3-foot-long core samples. Laboratory testing indicated that the top 16 inches of collected samples consisted of clayey sand of nearly 55% sand, approximately 40% clay, and the remainder silt. At a depth of between 16 and 36 inches, the sample composition was silty sand with over 80% sand and the remainder silt. The results of this survey helped determine whether suitable foundation for particular alternatives could be expected at the site.

Wind and wave analyses were important steps in the development of preliminary shoreline-protection alternatives because the wind-generated waves are the driving forces eroding the shoreline. For wind analysis, PBS&J collected and analyzed more than 10 years of wind data from the TCOON 009 gage at Port Aransas. The analysis showed dominant winds for the project area to be SE, SSE, and S winds varying in speed from 15 to 22.5 knots. Wave analysis was conducted using the dominant-wind and water-depth data. Wave analyses using the Coastal Engineering Design and Analysis program, version 2.01E, yielded anticipated wave heights and periods.

Although the Texas coastal area experiences hurricanes and tropical storms bringing high winds and seas, the team decided not to design for these extreme events because of cost concerns. The relatively shallower water in front of the island and the presence of an offshore barrier island (San Jose Island) contributed to the decision. Local knowledge of the island indicates that most of the shoreline loss is caused by the eroding forces of the wind and waves under typical weather conditions.

Developing Preliminary Solutions

Data collected on water depths, location of seagrass beds, oyster habitats, and manmade structures plus geotechnical, wind, and wave conditions all factored into the development of shoreline-protection and marsh-restoration alternatives. Five preliminary alternatives were developed

These alternatives were assessed on criteria that included structure location, size and footprint, effectiveness against design waves, construction methods, costs and timelines, availability of adequate quantities of dredge materials, aesthetics, longevity, safety to park visitors and boaters, maintenance requirements and costs, and potential impacts or benefits to existing seagrass, marsh, and oyster reef habitats.

Alternatives were then evaluated for how well they met project objectives identified earlier by the project team. The selected alternatives had to

• address shoreline protection against erosion;
• preserve or increase the quantity, quality, and diversity of habitats and resources in Aransas Bay;
• be environmentally acceptable with the ability to secure regulatory agency permits, including the US Army Corps of Engineers' 404 permit and the TCEQ's 401 Water Quality Certificate; and
• be acceptable to funding partners, resource agencies, park users, and local citizens.

Building a Breakwater

After reviewing the alternative analysis report, the project advisory team determined that the construction of an offshore breakwater was the best approach for addressing shoreline erosion. Four types of materials for breakwater construction were considered: geosynthetic-material containment systems, rock, articulated concrete blocks, and gabions. The materials were evaluated for cost, safety, suitability for marine environment, availability, longevity, and aesthetics.

On the suggestion of staff from two of the funding agencies participating on the project advisory team, geosynthetic-material containment systems and gabions were removed from further consideration because of past problems encountered with these options. From the original five alternatives, three were further analyzed to determine the most feasible alternative for Goose Island's shoreline.

The first alternative featured onshore rock or articulated concrete block revetment. While this alternative offered the advantages of low cost and ease of construction, it would only stabilize the shoreline and not restore wetland habitats. It could also negatively affect seagrass beds and smooth cordgrass marsh located on the shoreline.

The second alternative proposed an offshore rock breakwater or articulated concrete block on a soil levee located at what had been identified as the 1969 shoreline—that is, the shoreline location prior to erosion occurring over the past 35 years. This solution offered the opportunity to restore or enhance wetland and aquatic habitats between the breakwater and current shoreline. However, the breakwater's footprint could potentially impact up to 2.75 acres of seagrass beds found along the proposed breakwater's alignment.

The third alternative also consisted of an offshore breakwater, this one located approximately 500 feet from the current shoreline to avoid interfering with existing seagrass beds. The breakwater, constructed of either rock or articulated concrete blocks on a soil levee, would create a lagoon effect between the breakwater and the existing shoreline to enhance seagrass beds and reestablish the marsh habitat. This alternative was found to have the fewest negative environmental impacts. However, construction costs were higher because of the breakwater's location in deeper waters, which required more construction materials.

Moving Ahead

This breakwater exemplifies what will be built at Goose Island.

Results of the engineer's evaluation were presented to the local community at public meetings in January 2003 and to the project team. The project team reviewed all the engineer's findings and public comments in February 2003 and selected the third alternative: an offshore rock breakwater and the creation of a 22-acre smooth cordgrass marsh north of Goose Island.

An additional shoreline survey conducted by resource agency staff in February 2003 mapped the existing seagrass beds. PBS&J used the survey results to refine the breakwater alignment, which was also revised to tie into the existing bulkhead and to further avoid impacts to existing seagrass beds. A vane-shear and plate load test was conducted in April 2003 along the revised breakwater alignment to evaluate the bearing capacity of the breakwater foundation. The test used a 6-inch round steel plate with a maximum 165 pounds of weight for a 15-minute loading time to evaluate potential settlement, which was measured to be mostly less than 1 inch with a maximum of 4 inches of settlement. The survey also identified significant sand and shell composition indicating good bearing capacity of the foundation.

Marsh restoration would be accomplished by placing dredge material from nearby boat channels into marsh cells at selected locations north of Goose Island. The dredge material would be contained with levees constructed from insitu material at the marsh creation site. The locations were selected based on proximity to boat channels, water depths, and geotechnical conditions of the sites. The dredge materials would come from boating channels accessing nearby Neptune Harbor and the Goose Island State Park boat ramp. Community volunteers and students would help plant the vegetation.

Construction Considerations

A follow-up shoreline survey refined the breakwater alignment in the selected alternative to avoid impacts to the seagrass beds.

From the beginning of project development, the project sponsors were aware that the site included valuable aquatic habitats, including seagrass beds, protected by federal and state laws. Oyster reefs were found along the channels to be dredged. The project was routinely coordinated with federal and state regulatory and resource agencies through interagency meetings. Input and concerns brought up during these coordination meetings were addressed through modifications in the project design and in the proposed construction methodology. The project sponsors will provide onsite biological monitors during construction of all project features to help contractors avoid and minimize impacts to these habitats.

During the US Army Corps of Engineers' internal review of the permit application for the project, archeological concerns were noted. More information was requested by the corps before a permit could be issued. Using information collected during the side-scan sonar and magnetometer surveys, the firm's naval archeologist recommended an access channel for work barges that would avoid impacts to anomalies of possible archeological significance. All other anomalies noted on surveys within the project site were analyzed and determined not to be of archeological concern.

The estimated project construction cost is $1,794,000 for the breakwater phase and $426,000 for the marsh-restoration phase. Approximately $1 million in federal, state, and local government funds and private donations have been raised. Approximately 2,500 linear feet of the desired 4,400 linear feet of breakwater will be constructed based on this cost estimate. The marsh-restoration phase is on hold until more funds can be raised for the project. Construction is planned to begin in fall 2004 for a winter 2004 completion date.

Yu-Chun Su, Ph.D., P.E., CPESC, CPSWQ, CFM, is a project manager and Marty Hearney is a senior scientist, both with PBS&J in Houston, TX. Paul Jensen, Ph.D., P.E., is a vice president and Bob Gearhart is a senior scientist, both with PBS&J in Austin, TX. Kay Jenkins is a coastal biologist with the Texas Parks and Wildlife Department office in Corpus Christi, TX. Charlie Belaire is the president of Belaire Environmental Inc. in Rockport, TX.

EC - September October 2004