each survey line, the ISRP-formatted data were used in the Beach Morphology 

 Analysis Package (BMAP), a program developed by CERC to inspect and 

 analyze two-dimensional beach profiles and beach profile changes 

 (Sommerfeld et al. 1994). Although the impoundment basin profiles do not 

 resemble typical beach profiles, BMAP was able to calculate volumetric change 

 in successive surveys along the range lines. Example survey data are plotted in 

 Figure 20. Figure 20 shows successive surveys along range line 33600 as the 

 basin infilled. From bottom to top, the four surveys were taken in March 

 1990, August 1990, April 1991, and September 1992. Northeast is on the 

 left-hand side of the plot, southwest on the right. The large flat depression and 

 V-shaped depression are the impoundment basin and navigation channel, 

 respectively. Volume changes along each range line are given in Table 9. 



Sediment Grain Size 



Sediment grab samples were taken on the beach, in the surf zone, and in the 

 impoundment basin at the mouth of the Colorado River on October 18, 1989, 

 and February 11, 1994. The samples were sieved using standard techniques to 

 characterize the sediments in this area. The results are shown in Table 10. An 

 example distribution curve is shown in Figure 21. These sediments are fme 

 sands with median sizes in the range 0.17-0.22 mm, and contained a small 

 percentage of silts and clays. 



The samples are much coarser than the material carried to the sea by the 

 Colorado River. The results of a 1965 analysis of 113 sediment samples taken 

 from the river in the vicinity of the GIWW show that 63 percent of the 

 river-borne sediments are fmer than 0.002 mm. Thus, the majority of the 

 material consists of clay-sized particles. Only about the coarsest 1 percent are 

 fme sands or coarser (U.S. Army Engineer District, Galveston 1977). 

 Additional sediment samples were taken by SWG on August 16, 1994, in the 

 river at and near the mouth and in the impoundment basin. All of these 

 samples showed greater fractions of silt and clay than any of the monitoring 

 effort samples discussed above. Five samples in the entrance chaimel at the 

 mouth had median grain sizes of 0. 10 to 0. 18 mm and silt and clay fractions of 

 12 to 34 percent. River samples 427 and 731 m inland from the mouth showed 

 a mixture of beach and river sediments. These samples had mean grain 

 diameters of 0.04 mm and contained 60- to 80-percent silt and clay fractions. 

 The one impoundment basin sample had a mean grain diameter of 0.04 mm and 

 contained 88-percent silt and clay. This shows that riverine sediments were 

 reaching the mouth even after the river was diverted into Matagorda Bay and is 

 consistent with casual observations that the impoundment basin sediments 

 contained a patchy mix of river- and beach-derived sediments. 



Chapter 4 Data Observations and Analyses 37 



