EXTENT OF ARMOR LAYER DEVELOPMENT IN THE SAND-DOMINATED SAN ANTONIO RIVER

Armor layers are commonly found in gravel-bed channels but the extent of armor development in sand-bed channels with gravel content is not well documented. The aim of this study was to quantify the extent of armor layer development in the lower San Antonio River, Texas, which is a poorly sorted sand-bed channel. In the three reaches investigated, between 14% and 30% of the bed material consists of gravel sizes. In the head area of channel bars, 12 paired surface and subsurface sediment samples were extracted based on a 2 m x 3 m grid. Samples were sieved to derive grain size distributions from which armor ratios were computed to quantify the extent of armor layer development. Preliminary results indicate that armor ratios range from 0.49 to 66 with 50% exceeding 1.5. Surface sediment exhibit a wider range in median diameter compared to subsurface sediment, which helps explain the large ratios. Overall armor layer development is present but highly variable, which indicates a limit to the effectiveness of size selective sorting that develops coarser surface layers.