Wednesday, May 6, 2009
Grade control Part 1.
I've been working on an incised urban channel needing grade control, and took the opportunity to review what's out there, and also my own thick files. Here I'll talk a bit about theory, practice, and the politics of using grade control in channel management.
First, a skydiver. On leaving the plane he has huge potential energy relative to the ground thousands of feet below. After opening his chute, he slowly dissipates that energy by converting it to kinetic energy using a parachute.
Water works the same way. Especially in urban channels, I use this simple conceptual approach: We have a certain volume of water entering our area of interest at a certain elevation. As this flow moves down the channel, potential energy (from the elevation difference) is converted to kinetic energy and dissipated. How that conversion and dissipation happens largely controls channel erosion and sediment transport.
Grade control structures are a way to manage this. At the top of this figure we see a smooth channel. Conversion of potential (PE) to kinetic energy (KE) is more or less evenly distributed throughout the reach. Below we've added grade controls. What's important here is that we now see 1) most of the energy is now dissipated only at discreet locations (the GC structures) and 2) we pick and armor those locations.
Pretty simple. And it works. In theory, you don't have to armor the channel between these structures. One of the many things about river mechanics that is counterintuitive to most people.