Shoreline and Nearshore Bathymetry Changes 



Beach profiles were taken northeast and southwest of the inlet in 1984 by 

 SWG and in 1990, 1991, and 1992 by this monitoring effort on the range lines 

 shown in Figure 9. Profile data are available in Liang (1995). Several 

 problems with the surveys limited their usefulness in making quantitative 

 assessments of beach change volumes. The 1984 data extended only 125 m 

 offshore. Later data sets showed that the depth of closure was substantially 

 seaward of this distance. 



Because access to the southwestern side of the inlet was limited, no surveys 

 were taken there in 1990, and later surveys were somewhat abbreviated. This 

 beach was used as the deposition site for the material dredged from the 

 impoundment basin in 1990 and 1992. Unfortunately, as a result of the 

 dredged material deposition, several principal 1984 benchmarks southwest of 

 the inlet were lost, resulting in a poor match in range locations and reference 

 elevations between the 1984 survey and later surveys. The later surveys also 

 either did not extend far offshore or did not cover all the range lines. 



An analysis by Liang (1995) showed that the beach northeast of the inlet 

 accreted between 1984 and 1990, and then lost some of that amoimt by 1992. 

 Between 1984 and 1990, the average accretion was 19.2 m or 3.2 m/year, 

 while the 1984 to 1992 average accretion was 8.8 m or 1.1 m/year. Figure 19 

 shows the changes in shoreline position during this time interval. Out to a 

 distance of 125 m, the 3-km-long section northeast of the river mouth gained 

 about 135,000 cubic meters between 1984 and 1990. From 1990 to 1992, the 

 same section lost about 19,000 cubic meters out to the 125-m line. However, 

 using the entire set of 1990 and 1992 profiles (extending about 700 m offshore) 

 the total volimie loss in the 3-km-long beach section was about 85,000 cubic 

 meters, roughly four times the loss to 125 m. 



Limited analysis could be made from the data collected southwest of the 

 inlet because surveys were not well-timed to monitor the beach nourishment 

 events. However, general trends were similar to that on the northeast beach. 

 The shoreline prograded between the 1984 and 1992 surveys. This was 

 probably principally due to the fact that large amounts of fill (1.36 x 10* cubic 

 meters, see Table 8) were placed on this beach during that time. In the 6 

 months between the 1991 and 1992 surveys, the first 900-m section southwest 

 of the inlet lost a total volume of about 22,100 cubic meters indicating that this 

 beach is still erosional in namre. 



The Matagorda Peninsula shoreline in the vicinity of the mouth of the 

 Colorado River was modeled by Heihnan (1995) (see also Heilman and 

 Edge 1996) using a GENESIS type numerical model. For an overview of the 

 GENESIS modeling program, see Hanson and Kraus (1989). This numerical 

 modeling effort used the wave and profile data collected as part of this 



Chapter 4 Data Observations and Analyses 35 



