Understand How the River Works...Before You Begin Work

Those of us who work with rivers and natural ecosystems need to approach our work carefully, with a measure of ignorance, remembering that rivers are complex. We must come with a clear set of objectives, and seek to understand how the river is working before we can begin our work. I’ve read many books and journal articles but Luna Leopold’s A View of the River ( http://www.amazon.com/View-River-Luna-B-Leopold/dp/0674018451 ) has been most helpful. Leopold theorizes that the river system has two tendencies: to minimize total work done in the system, and to equalize the power per unit bed area. He ties these tendencies back to entropy as used in the second law of thermodynamics, indicating that as energy becomes more evenly dispersed in the river system that the possibility for energy to be used for mechanical work is decreased, thus increasing entropy.

The river system, with both its natural and man-made obstructions, and its sediment load are significant aspects that play into the dynamic equilibrium of the river as expressed in Lane’s relationship.

Qs ds ∝ Qw So

Lane theorized that the product of sediment discharge (Qs) and sediment size (ds) is proportional to the product of river discharge (Qw) and channel slope (So). The side by side photos of a riverbank restoration project are a story in how rivers respond to changes to these variables.

The photo on the left shows an area of significant bank erosion between the downstream end of an earlier bank restoration project and the upstream side of a bridge from which the photo is taken. The missing piece of information is that the former single span truss bridge was replaced with a two span deck girder bridge at about the same time as the first riverbank restoration project was constructed. I have never been a fan of two span bridges over water because the center pier ends up as an obstruction located in the highest velocity portion of the river causing a rise in backwater from losses caused by the pier, and bank instabilities caused by changes in sediment carrying capacity. In this case, during larger floods, the pier presented a greater obstruction, raising the water surface upstream of the bridge, reducing channel velocity, and depositing sediment that created a center bar. The center bar became armored with cobbles, causing flow to become divided and directing the flow to the unprotected bank, downstream of the first riverbank restoration project, which contained finer particle soils. Scour at the toe of the bank resulted in a translational failure of the upper bank. The restoration, shown in the photo on the right, included removing the center bar and extending the bank protection (riprap to the 2-year flood level and dormant live stakes on the upper slope) to the bridge’s abutment.

As this example shows, understanding how the river system works on both a macroscopic and within the river reach being considered is essential before beginning our work.