From the August 2013 issue of LAM:
By Adam Regn Arvidson, FASLA, editor of Now
Of the professionals who deal with stream restoration—civil engineers and landscape architects—the latter, according to Tim Keane, generally have little understanding of how streams actually function. To remedy that, Keane, a landscape architecture professor at Kansas State University, with Chris Sass, Associate ASLA, who teaches landscape architecture at the University of Kentucky, and dozens of students through the years have been literally getting their feet wet in Kansas streams. They’re not inventing new ways of looking at watercourses, but gaining an understanding of how accepted stream science actually works.
And there is plenty of science. The primary guide is Applied River Morphology by Dave Rosgen. It sets forth protocols for assessing and classifying watercourses. Students learn these protocols by doing hands-on assessment. They survey stream cross sections and the streambed by wading in with transit poles. They install study banks: delineated sections of bank that are surveyed in successive years and compared to assess erosion or deposition. They take pebble counts to determine the dominant size of sediment. They dig holes in the streambed, lower chains vertically into the holes, fill the holes, and cut the chains off at the bed surface, then return in a year to see how much sediment has departed or arrived (these are called scour chains). A survey of a 2,500-foot length of stream can take a crew of four people two days each year.
With this data, Sass, Keane, and their team can classify streams according to the oft-cited “Rosgen (1996).” Classification is based on a combination of longitudinal slope (how many feet of elevation change per mile of run), stream width-to-depth ratio, sediment size, entrenchment ratio (how wide a stream is compared to its floodplain), and sinuosity (wiggliness). Rosgen identifies watercourse types A through G, then adds a number from 1 through 6 based on sediment size.
Then comes the Level 3 Assessment (also from Rosgen, which incidentally is a big compilation of previous scientific literature in the field). The focus at this stage is on sediment and water flows. If a stream has less power than it should according to its classification and sediment size, gravel and sand will build up and create stagnating pools. If it has too much power, that’s what’s called “hungry water.” This is common in urbanized areas and results in severe downcutting. Sass and Keane’s work in northeastern Kansas has also found this condition in the agriculturally flanked waterways there.
Disturbance, of course, affects how watercourses work, but in a dynamic way. “Streams will go through a series of three or four adjustments in form to achieve a new stability,” says Sass. “It is important to know what the stream’s succession is going to be and where you are in that succession in order to do restoration.”
Landscape architects frequently work with streams and rivers, and these scientific tools can help. Maybe a stream should not be restored (in the strictest sense of the word) at all. Maybe it should be designed to a new classification. And what might the implications be downstream? Could your restoration project create hungry water in the next reach? Stream assessment science can teach landscape architects how stream systems react when the landscape around them changes.