A Decade of Beneficial Use, Brazos Island Harbor, Dredging

T. Neil McLellan, P.E. 1 , Herbie Maurer, P.E. 2 , Bob Fudge, P.E. 3 , Dan J. Heilman, P.E. 4

ABSTRACT

The Brazos Island Harbor Channel, entrance channel to the Brownsville Ship Channel, requires maintenance dredging every two years. Much of the sediment dredged is beach quality sand. Prior to 1988 dredging operations, sediments dredged from the channel were discharged at a designated offshore placement area. Depths in the placement area are 40 ft or greater, removing the sediment from the littoral system. To reintroduce the dredged sediments into the littoral system, a nearshore berm was constructed with 220,000 cubic yards of dredged material in 1988. The berm placement continued through 1995. With the cooperation of the Town of South Padre Island, the Galveston District was able to place approximately 490,000 cubic yards of sandy dredged material on the badly eroding beaches of the Town in 1997. Beach placement was repeated in 1999 and 2000. To date over 2.5 million cubic yards of dredged material have been used to benefit the beaches of South Padre Island. This project represents the only long-term beach placement project in the State of Texas. Making this project a success has been the positive cooperation between the Corps and Town of South Padre Island. The Corps and the Town have conducted monitoring of the nearshore berm and beach. This paper covers the design, construction and monitoring of the berm and beach, and includes a discussion of the cooperative efforts between the Town and the Corps. Partnering between the Corps, Town and Contractor has enable the group to address project concerns such as clay balls, project noise and safety issues without project interruption. The partnering sessions allowed the Corps, Town and Contractor to express concerns, establish communication lines and develop operating condition everyone could understand and support. Beach monitoring has aided in the long-term management of the material to determine the best location to place the material for maximum benefit to the beaches of South Padre Island. Berm migration as well as shoreline modification are provided and discussed.

Key words: Dredging, beach nourishment, partnering, and beneficial uses

INTRODUCTION

The Brazos-Santiago Pass (BSP) Inlet is located just north of the Texas-Mexico border on the southern tip of the Padre Island, Texas (Figure 1). Early attempts to maintain a navigable channel at BSP were unsuccessful (Morton and Pieper, 1975) until the construction of the present day jetties in 1935. The entrance channel is maintained at a 44 ft depth, deepened from 38 ft in 1992, and a 300 ft width. South Padre Island has had a long-term trend of shoreline erosion with the exception of the extreme southern tip located next to the BSP jetties with erosion rates up to 8 ft/yr. Morton and Pieper (1975) show a predominate littoral drift from the south to the north, with some seasonal reversals during the winter months, October through February. Tides near South Padre Island are predominantly diurnal with a frequent superimposed semidiurnal component, which produces two low waters of different elevation each day, resulting in a mixed cycle. Winds are predominately south to southeast through out the year with the exception of winter months when the winds are out of the north-northwest.

Sediments dredged from the entrance channel ranges from high-grade sands (95% or better) to a low-grade sand ( <50%). The entrance channel requires maintenance dredging every two years. Most of the higher-grade sand is dredged between the jetties and the finer materials are dredged from the offshore section of the BSP entrance channel. In the late 1980' s the Galveston District Corps of Engineers began planning and design of a nearshore berm (McLellan and Burke, 1988). Nearshore berms are normally constructed with beach quality sand from adjacent navigation channels. Berms are a cost-effective method for retaining beach quality sand within the littoral system, McLellan 1990a. The first berm construction was conducted in 1989 placing 220,000 cubic yards of dredged material in the berm location, Figure I, McLellan, 1990b. The berm site was used twice more in 1991 and 1995, the deepening in 1992 dredged mostly clays and the sediment was placed in a designated offshore area.

In 1996 the Town of South Padre Island express an interest to the Galveston District in placing the suitable dredged material directly on the beach to reduce erosion affects on the north section of the town. To accomplish this task the Town and the District had to coordinate extensively on location, costs, construction methodology, safety and timing. South Padre is destination for vacationers at all times of the year. Particular concern was expressed to limit construction to avoid winter visitors and spring breakers. Successful negotiations resulted with placing 489,111 cubic yards of sandy dredged material on the beaches of South Padre in February 1997. Building on the lessons learned and success of the 1997 project, dredged material was again used on the beach in February 1999 and December 2000. This paper describes the nearshore berm construction and performance, the beach placement operation and the coordination efforts executed to establish the successful project.

NEARSHORE BERM

To determine the best location for the construction of the nearshore berm the Galveston District conducted an analysis, (McLellan and Burke 1988) of the area to determine the optimal location to place the dredged material. Economical haul distances, depth of dredge operation, current patterns and suitable depth had to be determined. To aid in the effort McLellan and Burke conducted a seabed drifter study to evaluated distance from the jetties the material needed to be placed to remove the berm from the affects of the jetty. In addition, grain size analyses, wave evaluations and dredging methodologies were reviewed. Economical haul distances were determined to be within 3 miles north of the jetties, Medina 1987, and the minimum economic depth the dredges could safely operate was determined to be 20 ft.

The water depth at which sediment is an active part of the littoral system is dependent on wave height and period, and sediment composition. The complex littoral system extends from the back beach and dune to some distance offshore. The distance offshore is depth dependent and determination of that depth will define a boundary of the active portion of the beach profile where sediment may be introduced into the littoral system. Hallermeier (1981) utilized lab data to develop an equation to predict the depth at which significant amounts of sand are moved and can be considered part of the littoral system. The Hallermeier equation estimates the maximum depth at which changes in the beach profile occur for large storm events based on the wave period and the significant wave height which is exceeded 12 hrs a year. Utilizing the significant wave height and period from the Wave Information System, a 20- year hindcast data base (McAneny 1985), depth of closure for the south Padre Island area can be estimated at 28 ft.

The study resulted with the identification of the placement area as shown in Figure 1. The site is approximately 1 mile north of the jetties, located 1/4 to 3/4 miles offshore, between the 20 and 28 ft contour. The initial berm construction was conducted in December 1988 with North American Trailing Company's hopper dredge MANHATTAN ISLAND placing 220,000 cubic yards. The MANAHATTAN ISLAND is a 3600 cubic yard hopper dredge, drafting 19 ft when loaded. The material was placed along the 26 ft contour developing a well defined mound, Figure 2. The berm was up to 4 ft high and 3500 ft long. Pre-and post-construction monitoring of the site, Figure 2, shows the definition of the berm and its migration post-construction. The berm migrated approximately 300 ft shoreward between January and March 1989. Because of the success of the berm the placement process was repeated in 1990 and 1995. Table 1 provides the volumes and dates of each of the berm placement operations.

BEACH NOURISHMENT

As the Town of South Padre Island continued to grow and increased as a destination for national and international tourism, the potential impacts from hurricanes and tropical storms increased to the commercial development. To reduce the trend of erosion and protect the shore against storm damage, the Town of South Padre Island began to negotiate with the Galveston District to place suitable dredged materials directly on the beach in 1996. Several studies, Kraus et al 1996, Heilman and Kraus 1995, were conducted by the Town to determine the best course of action. The studies indicated the highest erosion rates, average of 8 ft/yr (Morton, ~993) along the Town's beachfront was occurring in the northern section of the Town. Based on these results the Town entered into negotiations with the Galveston District to place dredge materials between 3.5 to 4.5 miles north of the BSP jetties. These areas were denoted as the most severely eroding within the Town's border. As negotiated the Town would provide the additional funds (approx. $700,000) for the additional cost over historical offshore placement and provide the beach design and monitoring required by the State of Texas for a beach nourishment project. Only that material suitable for the beach would be placed onshore, sediment from the offshore section of the channel would be placed in the historical off shore placement area. T.L. James's dredge the Tom James was used to remove and place the sediments on the beach. Beach nourishment was accomplished in 1999 and 2000; see Table 1, with placement locations at slightly different sites. Funding constraints limited the pumping distance to 1.5 miles north of the jetties in 1999, in 2000 a grant from the State of Texas' newly developed Coastal Erosion and Response Act (CEPRA) allowed for sand to be pumped further to the north.

Table 1: Beneficial Uses South Padre Island

Survey stations numbers represent distance in feet from Brazos Santiago Pass north Jetty (see Figure 1).

Beach Response

The analysis presented herein is intended to provide insight as to how the combined projects have functioned as opposed to focusing on any single nourishment episode. Key findings of this monitoring effort includes the following:

A total of 13 project surveys, have been conducted between Jl991 and 2001. Since August 1996 surveys have been collected approximately every 6 months. The surveys consist of lines of data aligned approximately perpendicular to the shoreline and typically extending from the back of the dune or location of a seawall to a depth of about 3 ft. The nominal spacing of the survey lines was 500 ft alongshore.

To define the beach affects the +3.5 MLT, Galveston District Datum, contour was plotted as the shoreline for each of the surveys Figure 3. This contour is considered representative of the approximate shoreline position during higher seasonal tides and is less dynamic than lower contours within the swash zone that are exposed to more wave energy. Available data for these two survey dates extends from Station 48+00 at Isla Blanca Park to Station 260+00 at Andy Bowie Park. Between these two stations, which are roughly 0.9 and 4.9 miles north of the Brazos Santiago Pass north jetty, respectively, the shoreline advanced seaward! at an average rate of about 6 ft/yr, as calculated from the data plotted in Figure 3. However, as can be seen in the figure, local rates of shoreline change can be considerably larger than the average, with rates of advance at some locations as high as 13 ft/yr due to placement of fill material.

Also plotted in Figure 3 is the historic average rate of shoreline change. as measured by the University of Texas - Bureau of Economic Geology through analysis of aerial photography for the period from 1937 to 1991. Note that the trend from December 1993, to March 2001, differs from the average historic trend. Prior to construction of the jetties at Brazos Santiago Pass in 1935, shoreline position along South Padre Island exhibited great variability with an overall trend of retreat since at least the late 1800's (Morton and Pieper. 1975). After construction of the jetties, the trend of shoreline retreat along the southern end of the island turned to one of shoreline advance, while the shoreline at the northernmost end of the Town continued to recede at a long-term average rate of approximately 5 ft/yr. The trends measured during the December 1993, to March 2001, monitoring period differ from these long- term trends since the entire reach experienced net shoreline advance.

Analysis of the data from the December 1993, to March 2000 indicated the shoreline should be divided into three regions that each exhibited distinct shoreline movement trends. The first region consists of the southern 500 ft of the survey reach, which experienced average net advance of about 1 ft/yr, in contrast to the long-term trend of advance of 10 to 15 ft/yr. It is unknown whether this region of shoreline retreat extended farther south than the north end of Isla Blanca Park, since the area from Isla Blanca Park to the jetty was not surveyed. However, anecdotal information provided by personnel at the Cameron County Parks System suggests that the area from Isla Blanca Park to the jetty has been eroding in recent years. The second region consists of the central 3.1 miles of the survey reach, which experienced average net advance of about 5 ft/yr, except within the area nourished during the December, 2000 beach fill, which advanced at higher rates. This trend differed from the long-term trend in that the shoreline has historically experienced average recession of up to 3 ft/yr beyond the approximate location 3.3 miles from the jetty. The third region consists of the northern 0.7 miles of the survey reach, where average net rates of advance decreased with distance north to a minimum of about 1 ft/yr at Andy Bowie Park, in contrast to the long- term trend of retreat of 4 to 5 ft/yr.

Local shoreline undulations caused by seasonal variations in tide and waves can cause rates of shoreline change to be much larger than the average rates calculated for longer monitoring periods. As can be seen in Figure 4, the rate of shoreline change is highly variable with high rates of accretion followed by high rates of erosion at the same location depending on the interval of time examined. Because of this high degree of variability, the average rate for the total survey period should not be the only rate considered in beach nourishment planning and design. These short-term undulations can cause the shoreline to periodically move within closer proximity to beachfront structures. The structures are then exposed to increased risk of storm damage, particularly along northern areas where the beach is narrower and there is less of a buffer to dissipate wave energy.

Local shoreline undulations caused by seasonal variations in tide and waves can cause rates of shoreline change to be much larger than the average rates calculated for longer monitoring periods. As can be seen in Figure 4, the rate of shoreline change is highly variable with high rates of accretion followed by high rates of erosion at the same location depending on the interval of time examined. Because of this high degree of variability, the average rate for the total survey period should not be the only rate considered in beach nourishment planning and design. These short-term undulations can cause the shoreline to periodically move within closer proximity to beachfront structures. The structures are then exposed to increased risk of storm damage, particularly along northern areas where the beach is narrower and there is less of a buffer to dissipate wave energy.

PROJECT COOPERATION

Padre Island and in particular the Town of South Padre Island is a national and international tourist destination. Concern during the beach nourishment project was expressed by the Town to ensure safety and continued use of the beach throughout the construction process. In addition, the first beach nourishment project was the first maintenance cycle after deepening of the BSP channel. The maintenance or shoaled material is high quality sand, but the underlying sediment within the channel footprint is consolidated clay. There was considerable concern about clay balls being pumped onto the beach. To address and alleviate these problems the Galveston District and Town of South Padre Island entered into a partnering arrangement to address foreseeable and unforeseen problems that may occur. The Partnering Session included public meetings, definition of lines of communication and determination and limitation of expected impacts. Following the close of the project a lessons learned meeting was conducted to review what could have been handled/designed better and modified in later projects.

1997 Beach Nourishment

Several public meetings were held within the Town of South Padre Island that had participants from the Corps of Engineers, Town of South Padre Island, and local interested groups. Concerns included type of material, construction techniques, public safety, noise and contamination issues. Most concerns could be addressed with information available at the time. Because the channel had just been deepened the amount of clay that may end up on the beach was of concern for the entire group. When T L. James was selected as the dredging contractor, a pre- construction meeting was held with participants from the Corps of Engineers, T.L. James and the Town of South Padre Island. At the pre-bid meeting a construction schedule, construction techniques (i.e. ingress and egress locations) were developed and a list of contacts was developed to address issues as the project progressed. The construction was initiated in January 1997 and completed in February the same year. The project was completed without major incident, primarily because of the coordination between the Town, Corps and Contractor. Some complaints were registered based on the noise, locations of equipment and clay balls that did occur from the dredging operation. During the post-construction lessons learned meeting the following topics were identified:

Many of the problems in the first contract were addressed at the time they occurred. The Town and T.L. James coordinated to stockpile the clay balls and remove them from the beach, ingress and egress locations were coordinated designated during the construction process. These issues were addressed quickly mainly because of the trust built up during the partnering session between the participants.

1999 and 2000 Nourishment

Lessons learned in the initial contract were implemented in following contracts. Increased signage, increased safety and a clay ball handling plan was developed for the contract in 1999. The project was constructed in January and February of that year when considerable tourists are enjoying the beaches of South Padre. Because of limited funding the distance pumped was decreased to 1.5 miles north of the jetty. Although not placed at the 'hot spot' of erosion the nourishment was positive affect on the beach. When State of Texas CEPRA funds were made available for the December 2000 project the sand was again pumped to the northern section of the Town. The December 2000 project was constructed prior to influx of tourists after the Christmas holiday.

CONCLUSION

A decade of beneficial uses of dredged material has occurred in and along the Town of South Padre Island. Since 1988 a total of 1,550,000 cubic yards have been place on within a the littoral system in the form of a nearshore berm and 1.350,000 cubic yards of sandy dredged sediments have been placed directly on the beaches of south Padre Island. Evaluations to date have shown that the beneficial use has had direct positive impact to the beaches of South Padre Island and over the course of the last 10 years have decreased the erosion rate along the beach. The Town has been fortunate in that no major storm has hit the beach over the course of the project. Since 1993 the beach line has moved seaward an average of 6 ft/yr in some locations that had a long-term erosion trend of up to 8 ft/yr. The project owes part of its success to the partnering efforts between the Corps of Engineers and the Town of South Padre Island. Discussion early and often though the planning. construction and post-construction phases have decreased the problems associated with each successful beach nourishment project. The Town and the Corps have developed a partnership process were problems are handled quickly and incremental improvements are implemented in each project.

REFERENCES

Hallermeier, R.J., (1981), "Seaward Limit of Significant Sand Transport by Waves: An Annual Zonation for Seasonal Profiles," costal Engineering Technical Aid No. 81-2, Coastal Engineering Research Center, Vicksburg, MS

Heilman, D.J. and Kraus, N.C. (1995), "Beach Fill Functional Design, Town of South Padre Island, Texas," Technical Report CBI-95-14, Conrad Blucher Institute for Surveying and Science, Texas A&M University-Corpus Christi, Corpus Christi, Texas.

Krause, N.C. et al. (1996) "South Padre Island, Texas Section 933 Beach Fill Study, Technical Report CBI-95-05, Conrad Blucher Institute for Surveying and Science, Texas A&M University-Corpus Christi, Corpus Christi, Texas

McAneny, D.S. (1986), "Sea-State Engineering Analysis System (SEAS), Supplement f' WIS Report 10, US Army Engineer Waterways Experiment Station. Vicksburg, Mississippi

McLellan, T .N. (1990a), "Engineering Design Consideration for Nearshore Berms," Dredging Research Technical note DRP-5-0 1, Coastal Engineering Research Center, US Army Engineer Waterways Experiment Station, Vicksburg, MS

McLellan, T.N. (1990b), "Nearshore Berm Site Selection and Construction: South Padre Island, Texas," Proceedings The Coastal Society 12th International Conference, San Antonio, Texas

McLellan, T .N. and Burke, C.E., (1988), "Site Selection for Neashore Berm Construction South Padre Island, Texas," Waterways Experiment Station, Corps of Engineers, Vicksburg Mississippi

Medina, R. (1987), Personal Communication, US Army Corps of Engineers, Galveston District

Morton, R.A. and Pieper, M.J., (1975) "Shoreline Changes on Brazos Island and South Padre Island (Mansfield Channel to the Mouth of the Rio Grande), As Analysis of Historical Changes of the Texas Gulf Shoreline," Bureau of Economic Geology Circular 75-2, University of Texas at Austin, Austin, Texas.

Morton, R. A. (1983), "Shoreline Movement Along Developed Beaches of the Texas Gulf Coast: A User's Guide to Analyzing and Predicting Shoreline Changes." Open-file Report 93-1, Bureau of Economic Geology, University of Texas at Austin, Austin, Texas

1 Sr. Project Manager, Shiner Moseley & Assoc. 3300 S. Gessner, Suite Ill, Houston, Texas 77063
2 Deputy District Engineer, Galveston District, 2000 Ft. Point Blvd, Galveston, Texas 77550
3 Director of Public Works, Town of South Padre Island, P. O. Box 3410, South Padre Island, Texas 78597
4 Project Engineer, Shiner Moseley & Assoc. 555 N. Carancahua, Suite 1650, Corpus Christi, Texas 78478



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