Ephemeral Drainage-Line Assessments: Indicators of Stability

Abstract

In chapter 15 we describe eight indicators that inform restoration practitioners (RPs) about hether a drainage line is still active or is becoming stabilized. Here, we are referring to drainage lines that flow occasionally, that is, ephemeral drainages, such as gullies running down a hillslope rather than permanently flowing creeks, streams, or rivers. Ephemeral drainage lines on hillslopes in undisturbed landscapes are smoothly concave in cross-section (no sharp edges), receive diffuse overland flow, and gently drain water from the surrounding slopes. (See figure 15.1.) These undisturbed landscapes do not have high rates of runoff or erosion. Landscapes that have a history of disturbance, however, often have hillslopes here drainage lines have become incised. (See figure 15.2.) If erosion cuts a channel deeper than 0.3m, the drainage line is defined as a gully; if the cut is less than 0.3m, it is called a rill. Open image in new window Figure 15.1. A hillslope on which the natural ephemeral drainage line shows no sign of an incised channel. Lateral water flows into the drainage line are diffuse and slow and the cross-section of the line is smoothly concave. Open image in new window Figure 15.2. A channelized ephemeral drainage line on a hillslope shows signs of rapid over-bank flow (bare side-walls) but a stabilizing grassy floor. This implies that the restoration solution involves reducing flow rates from the hillslope above the channel, rather than within the channel itself.