What is an entrenchment ratio and bankfull widthdepth ratio

What is an entrenchment ratio and bankfull width/depth ratio? How are these metrics used in stream management?

Solution

The term \"entrenchment ratio,\" which is the vertical containment of the river, has been quantitatively defined (Rosgen 1994) to provide a consistent method for field determination. The entrenchment ratio is the ratio of the width of the flood-prone area to the surface width of the bankfull channel. The flood-prone area width is measured at the elevation that corresponds to twice the maximum depth of the bankfull channel as taken from the established bankfull stage.

A channel type unit length with the same channel type existing for a length over twenty Bankfull channel widths (Rosgen). The length of channel uniform with respect to discharge, depth, area, and slope. The length of a channel for which a single gage affords a satisfactory measure of the stage and discharge.

Many classification systems have been used in stream management and developing monitoring strategies (Omernick 1986; Rosgen 1994; Wolock et al. 2004). For example, the Rosgen classification has been widely applied in the US as a detailed template to guide stream restoration practices. For example, stream channel classes differ in entrenchment ratio, width/depth ratio, sinuosity, bed slope, and substrate coarseness. These classes represent expected conditions for the morphological condition of streams; thus, the expected values guide specific restoration practices of impaired systems, such as channel reconfiguration to match sinuosity conditions or placing grade control structures to stabilize systems back to typical bed slopes.

Another important consideration of classification systems is developing spatially contiguous layers. In many cases, classifications are created by clustering discrete observations. As common examples, hydrologic classifications have been developed by summarizing discharge patterns at stream gaging stations into statistics, which are then used as input to multivariate clustering algorithms (Olden et al. 2012). The result is that hydrologic classes are represented as groups of gaging stations that represent similar patterns in hydrology; however, this leaves large sections of rivers unclassified according to their hydrologic behavior. Predictive models can be developed using climate, landscape, and soil variables can be used to extrapolate hydrologic classes to ungaged streams (e.g., Liermann et al. 2012). In doing so, every stream reach will have a respective hydrologic class to readily inform spatial assessments.