Title: EXTENDED ABSTRACT: Models for Beach Renourishment of a Back-Barrier Shoreline, Padre Island National Seashore, Texas
Abstract: Padre Island National Seashore, located along the southern coast of Texas, protects the largest portion of undeveloped barrier island in the world. Bird Island Basin, which lies in the developed area of the park, is a popular park destination for day visitors and longer-term campers; primary recreational activities include camping, fishing, boating, and windsurfing (Figs. 1 and 2). Continued recovery of the barrier island from impacts of past overgrazing and several major droughts occurring prior to National Seashore establishment has reduced the eolian sand supply to this back-barrier shore in recent decades. This has led to accelerated erosion and eliminated much of the beach. The purpose of this project was to provide Padre Island National Seashore with the necessary information to understand rates and causes of the shoreline erosion processes so that effective measures can be applied to restore the Bird Island Basin shoreline. Prouty and Prouty (1989a, 1989b, 1989c, and 1991) provided past erosion rate estimates and summarized causes of shoreline erosion at the site. For the present study, October 2005 digital aerial photography of the site was obtained. Other imagery (1964 and 1982) was digitized by the researchers and rectified to a 1995 color-infrared (CIR) Digital Ortho Quarter Quad (DOQQ) with a 1-m ground resolution and referenced to the NAD83 datum (DOI-USGS, 1998), and then the shoreline was delineated as the wetdry line on the 2005, 1982 and 1964 digital images. Researchers compared the relative positions of shorelines between the 1982, 1995, and 2005 images along shoreperpendicular profile lines (Fig. 3) located in part to correspond with profile lines formerly established by Prouty and Prouty (1989a, 1989b) to estimate lateral shoreline retreat rate from 1982-2005. Researchers also estimated the amount of erosion or accretion using 1964 imagery, the best available aerial imagery dating closest to the park’s creation in 1968. There is an inherent degree of imprecision in using the wet-dry line to establish trends and rates of shoreline change (White et al., 1978, their Appendix A), but the total length of time elapsed between images was substantial enough that these observations should have produced accurate conclusions about long term trends. Researchers also field-surveyed the shoreline in November 2005 along the nine shore-perpendicular profile lines (Fig. 3), and collected nearshore surficial sand samples along those profile lines for dry-sieving via Ro-Tap and grain-size analysis using methods of Folk (1974). Volumes of sand needed for nourishment for a 5-, 10and 20-yr lifespan were estimated by translating the 2005 shoreline profiles lagoon-ward using current lateral shoreline retreat rates along each profile multiplied by the desired fill lifespan. Volumes were interpolated between profiles by the average end area method (U.S. Army Corps of Engineers, 1995). From 1982-2005, the back-barrier shoreline eroded at average rates of -3.28 ft/yr (-1.0 m/yr), and from 1995-2005, at -3.44 ft/yr (-1.05 m/yr). Gently-sloping shallow submerged sand lobes of fine sand extend 300 ft (100 m) or more from shore, terminating in
Publication Year: 2007
Publication Date: 2007-01-01
Language: en
Type: article
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