Protection Manual (SPM) (U.S. Army, Corps o£ Engineers, Coastal Engineer- 

 ing Research Center, 1977). These RSA data, as well as sieve data from 

 one sample too coarse to process by RSA, are presented in Appendix B. 

 Cumulative distribution curves are also presented. 



All of the sand sample sizes are described in both millimeters and 

 phi (())) units where 4> = -log.D. D is the grain-size diameter in milli- 

 meters. In the RSA analysis the sand sample falls through a tube of 

 water and a pressure transducer is used to determine the fall velocity 

 of the sand grains. The SEDANL computer program is then used to compute 

 moments to convert fall velocity to hydraulic grain-size diameter. The 

 RSA method is fast and reliable, but it is limited to analyzing very fine 

 to medium sands. Any fine-grained material present with the sand often 

 remains in suspension in the tube when the measurements are stopped. 

 Thus, the silt and clay fraction in a muddy sand sample is often omitted 

 from the size analysis results, making the sample appear better sorted 

 than it actually is. Most researchers agree that RSA values are consist- 

 ent and slightly coarser than sieve values for identical samples. Ramsey 

 and Calvin (1977) suggest adding 0.33 phi to the RSA mean to obtain the 

 equivalent sieve mean; another formula, with a similar constant, is shown 

 in Appendix B. 



2. Geographic Setting . 



The study area is part of the Texas coast of the Gulf of Mexico, a 

 large geosynclinal basin of active deposition that receives sediment from 

 nine major rivers draining from most of Texas and part of New Mexico. 

 The entire Texas coast from the Sabine River at the Louisiana border, 

 south to the Rio Grande River marking the border with Mexico, is composed 

 of long and narrow sandy barrier islands broken by tidal inlets which 

 lead to lagoons and estuaries between the barrier islands and the main- 

 laTid. The islands are generally of low relief, except for vegetated sand 

 dunes, and most remain in their natural state, relatively undisturbed by 

 human activity. All the barrier island peninsulas are attached to main- 

 land areas on their northeastern ends with the unattached ends extending 

 southwest (Bullard, 1942). Also, the northeast ends of the islands ex- 

 hibit maximum width and the islands taper toward the southwest. In the 

 study area, these conditions are best exemplified by Bolivar Peninsula 

 (Fig. 3). 



The back-barrier lagoons and barrier islands of the Texas coast are 

 geologically very recent features which have formed during the past 5,000 

 years in response to complex coastal processes as well as the gradual 

 worldwide rise in sea level. The barrier islands in the study area, and 

 for much of the Texas coast, are constructional landforms built up over 

 thousands of years by sand transported to the coast by rivers and then 

 westward along the coast by wave-generated longshore currents. 



Galveston Bay is a shallow estuary extending about 32 kilometers (20 

 miles) inland from the gulf and consists of East Bay behind Bolivar Penin- 

 sula and Trinity Bay to the north. Galveston Bay occupies the ancestral 



23 



