Gulf Shoreline Movement between Sabine Pass and the Brazos River, Texas: 1974 to 1996

GC9703. Gulf Shoreline Movement between Sabine Pass and the Brazos River Texas: 1974 to 1996, by R. A. Morton. Assisted by S. J. Clift, E. S. Angle, and E. M. Boghici. 46 p., 10 figs., 4 tables, 3 appendices, 1997. ISSN: 0082-3309: Print.


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ABSTRACT
Changes in shoreline position along the southeastern Texas Gulf coast between 1974 and 1996 were documented by conducting a kinematic real-time differential global positioning system (DGPS) field survey in 1996 and comparing that shoreline with other shorelines archived in a geographic information system. Results show that (1) beach morphology, shoreline movement, and the regional geologic framework are closely interrelated, (2) Gulf beaches are generally retreating, and (3) the rates of retreat have accelerated locally. Between Sabine Pass and Rollover Pass, almost all of the Gulf beaches are retreating. Exceptions are the locally stable beach segments at Sea Rim State Park and southwest of High Island. On Bolivar Peninsula, beaches between Caplen and Rollover Pass are rapidly retreating, whereas those farther southwest are either slowly retreating or are relatively stable; beaches at the southern end of the peninsula are stable or advancing, and they benefit from the sand supplied by updrift erosion. The shoreline on East Beach of Galveston Island undergoes minor fluctuations, but the beach position is relatively stable. West Beach of Galveston lsland continues to retreat, but rates of retreat depend on location. The beach segment between the Jamaica Beach and Sea Isle subdivisions has the lowest long-term average retreat rate, whereas retreat rates generally increase to the northeast toward the seawall and to the southwest toward San Luis Pass. These three segments having different beach morphologies and retreat rates were also identified in prior studies. Beaches from San Luis Pass to the mouth of the Brazos River also are generally retreating, and rates of retreat near Surfside, Quintana, and Bryan Beach are influenced by the jetties at Freeport Harbor and the diversion of the mouth of the Brazos River. The study also demonstrated that the high-water line mapped on aerial photographs undergoes large seasonal fluctuations and therefore is a less reliable indicator of shoreline position than the vegetation line, berm crest, or back-beach erosional scarp. DGPS field surveys are a rapid, relatively low cost method of acquiring accurate shoreline positions and have many advantages over interpreting shoreline positions from aerial photographs.



Keywords: beach morphology, coastal geology, environmental geology, GPS, shoreline changes, Texas




CONTENTS

Abstract

Introduction

Shoreline monitoring features

Berm crest

Erosional scarp
Vegetation line

Crest of washover terrace

High-water line

Coastal structures

Beach morphology

Beach and substrate composition

Sediment supply and beach stability

Adjacent elevations and storm-wave heights

Mapped shoreline positions

Field surveys

Classification and distribution of shoreline features

Global positioning system

Beach profiles

Geographic information system

Sources of error and estimated magnitudes

Analysis of net shoreline movement, 1974-1996

Shoreline features

Spatial analysis

Sabine Pass to Bolivar Roads

Galveston Island

San Luis Pass to the Brazos River

Temporal analysis

Sabine Pass to Bolivar Roads

Galveston Island
San Luis Pass to the Brazos River

Influence of human activities on shoreline movement
Coastal structures

River diversion

Beach nourishment

Conclusions

Acknowledgments

References

Appendix A. Shoreline history plots, Sabine Pass to Bolivar Peninsula (SP-BP)

Appendix B. Shoreline history plots, Galveston Island (GI)

Appendix C. Shoreline history plots, San Luis Pass to the Brazos River (SLP-BR)

Figures

1. Locations of shore-normal monitoring transects used to analyze recent movement of the Gulf shore

between Sabine Pass and the Brazos River

2. Generalized beach profiles illustrating typical beach morphologies and associated shoreline

features observed in the study area

3. Surveyed profiles selected from the four morphocompositional beach sections

4. Washover sand and shell deposited by Hurricane Carla and exposed by erosion of a scarp

5. Distribution of shoreline types between Sabine Pass and the Brazos River

6. Real-time DGPS equipment mounted on a four-wheel-drive all-terrain vehicle
7. Ten-year evolution of an advancing beach profile, Follets Island, near transect 3

8. Eight-year evolution of a retreating beach profile, West Beach, Galveston Island, near transect 3

9. Summary diagram illustrating recent (1974-1996) shoreline movement between Sabine Pass and the Brazos River

10. Locations of coastal structures and beach nourishment activities in Galveston and Brazoria Counties

Tables

1. Index of human impact on dunes and beach vegetation of the southeastern Texas coast

2. Net shoreline changes between Sabine Pass and Bolivar Roads, 1974-1996

3. Net shoreline changes along Galveston Island, 1974-1996

4. Net shoreline changes from San Luis Pass to the Brazos River, 1974-1996

Citation
Morton, R. A., 1997, Gulf Shoreline Movement between Sabine Pass and the Brazos River Texas: 1974 to 1996: The University of Texas at Austin, Bureau of Economic Geology, Geological Circular 97-3, 46 p.