Follelf s 



Figure 30. San Luis Pass thalwegs. 



(2) Flood Tidal Delta. The boat sheets and aerial photos examined 

 provided little quantitative data for the flood tidal delta. Only about 60 

 percent of the area surveyed in 1933 had been surveyed in 1853 and 1867, and 

 then widely spaced survey lines were used. Therefore, the accuracy of the 

 second method is not as great as the method used for the ebb tidal delta. An 

 833-foot grid square was superimposed on each boat sheet and the depths at 

 each grid point were noted. The difference between the 1853 and 1933 depths 

 at each grid point was averaged for each square. This difference was then 

 multiplied by the grid square area to yield the volume difference. The volume 

 difference for the flood tidal delta area (about 8.7 x 10^ square feet) was 

 178,000 cubic yards. However, accounting for the relative sea level rise of 

 1.4 feet, the total volume deposited was 4,690,000 cubic yards with an annual 

 accretion rate of 59,000 cubic yards per year, which is about the same as the 

 ebb tidal delta rate of 63,000 cubic yards per year. Since the sum of these 

 rates (122,000 cubic yards per year) differed from other computed estimates, a 

 more accurate method was then applied to both the ebb and flood tidal deltas. 



(3) Ebb and Flood Tidal Deltas . The third method consisted of again 

 using 625-foot grid squares, but this time applied over the entire area of the 

 1853' survey (2.3 x 10^ square feet). At each grid point, the 1853 and 1933 

 depths were recorded and the 1933 value subtracted from the 1853 value. The 

 results of the differences were then contoured (Fig. 31). To obtain the 

 volume change, the areas within each contour line were measured, using a 

 planimeter, and then multiplied by the average depth change within the area. 

 These volumes were then summed, arriving at values of 2,540,000 cubic yards of 

 deposition on the flood tidal delta and 11 million cubic yards of erosion on 

 the ebb tidal delta. If the 1.4-foot sea level rise is multiplied by the ebb 

 and flood delta areas, and the products added to these values, deposition was 

 7,700,000 cubic yards on the flood tidal delta and erosion was 4,330,000 cubic 

 yards from the ebb tidal delta, for a net deposition of 3,370,000 cubic yards 

 at an average annual deposition rate of 42,000 cubic yards per year. This 

 third method was also used by Morton (1977) to estimate volume changes at 

 other Texas inlets. 



37 



