Natural and human activities can cause unstable changes in stream channel slope. Natural stable stream channels utilize riffle pool sequencing to armor against erosion and dissipate energy. Riffles armor the bed from swift flows while pools dissipate the energy and slow the flow. This is called dynamic equilibrium which can take centuries to evolve. When man made activities interrupt this sequence, a chain reaction begins. Slope change in stable streams can most often result in channel incision when bedrock or similar control is not present. The end result is a which is a streambed and base flow that is significantly lower than its natural floodplain. This in turn produces bank erosion, collapse and deposition when the deeper channel is forced to convey larger storms.
Engineered grade control measures are typically used to mitigate this interruption in stability. Grade control structures can be composed of a variety of materials and configurations such as rock, wood and earth structures placed across the channel and anchored in the streambanks. We have learned to design like nature, mimicking natural conditions and processes, like bedrock outcrops, felled trees, riffles and pools.
Part one of this two part webinar presents typical examples where grade control measures are required, the types of grade control structures, how these structures dissipate the water’s energy, their pros and cons. Part two presents research and practice of pool design for culvert outlets, erosion control and aquatic passage.Part 1 presented by David Williams
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