The Effects of Channel Migration on Point Bar Development in the Lower Trinity River, Texas

Elizabeth Rinehart

The goal of this study is to gain an understanding of the geomorphological effects of channel migration, specifically meander cutoffs, on point bar development. Sand-bed rivers are physically dynamic and undergo lateral migration. In areas of sediment transport equilibrium, often associated with higher rates of lateral migration, healthy point bars are built. Because the lower Trinity River from Lake Livingston Dam to Galveston Bay is an excellent example of such a sand-bed coastal river and because it experiences approximate sediment transport equilibrium in the middle section, it was chosen as the field site. This middle section, or Zone 2, is bounded upstream by a zone of dam influence and bounded downstream by a zone of backwater influence (Fig 1).  The Trinity River is important as a field site because it is a protected species habitat, it helps build the 7^th largest esuary in the US, and because it is a water source for several population hubs, including Ft. Worth, Dallas, and Houston.

Four sets of aerial photos corresponding to the years 1952, 1968/1972, 1996, and 2009, were used to study the migration rate and point bar shape in the Lower Trinity River. This was done by tracing centerlines for all four years of aerial photography in GIS, Figure 2. Field work was then completed to gain understanding of the physical characteristics of the point bars to relate to the aerial photo analysis. In one field campaign two point bars from Zone 2 of the Trinity were trenched and surveyed. There was also bathymetric data taken around each point bar and photographs and sediment samples taken from each of the trenches.

Using the aerial photographs, average rates of lateral migration for the channel around both point bars in zone two were calculated. I also analyzed the shapes of the two point bars; the downstream point bar, Bar 2, has the classic crescent shape (Fig. 3), while the upstream point bar, Bar 1, which experienced a progressive, meander-loop cutoff between 1996 and 2009 (Fig. 4), deviates from the crescent shape. This deviation in planform shape is likely because of the aforementioned cutoff.

The survey data showed a very steep slope at the upstream end of Bar 1. I attribute this high bar slope to the steep bank of the channel sidewall itself, a consequence of the meander cutoff. The grain size data from Bar 1’s trench one, the most upstream trench, showed a trend of decreasing grain size from the river’s edge to the bar top. The other trenches showed no clear trends in grain size. This is likely connected to the considerably shallower bar slope topography associated with the downstream end of Bar 1 and all of Bar 2. Finally, the sedimentary structures observed in trench one of Bar 1 included a unique zone of climbing ripples at the bar top with an underlying zone built entirely of dune strata. This zonal change in structures and the lack of gravel size material on the bar top at trench one is attributed to the uniquely steep topography of the upstream end of Bar 1. All of this data indicates that channel migration does play a role in point bar development, particularly on bar shape and style of sediment deposition associated with bar growth.

Advisor: David Mohrig