ABSTRACT

The Hill Country near Austin is characterized by a stepped microtopography formed by alternating beds of hard limestone and soft marl in the Glen Rose Formation. These microlandforms control the distribution of soil, with thick residual soils (up to 10 ft deep) lying over the risers and thin transported soils (less than 1 ft deep) on the treads. Detailed field studies in the Barton Creek watershed reveal that hillslope processes are governed by these features. Infiltration rates on risers typically exceed 4 in/hr, and overland flow rarely extends beyond one tread-riser-tread sequence. Sediment transport is limited by this system, and sinks, including juniper stands, are abundant on hillslopes. Small drainage basins function as partial-area stormwater systems, with much more water and sediment held in storage than is generally thought to be so by city planners and engineers. These discoveries provide new opportunities for improved site planning and local stormwater management in areas undergoing development. Recommendations include landscape restoration, which promotes trees on high-infiltration riser soils, reducing the scale of development to fit the fabric of the microtopography, and protection of the partial-area system from grafting of additional drainage areas during development of roads and stormwater systems.