First of all, think about it…aren't most of your stormwater outlets designed for less than 10 feet per second, ...for use with the traditional rock rip rap design curve? 10 feet per second is easily managed by known soft armor protection; it just won't handle this scour force accelerating immediately out of a pipe - ScourStop™ far exceeds this critical force!
Additionally, the ScourStop engineered design takes into account traditional apron calculations and resultant forces; providing established data for determining the appropriate downstream soil protection.
Independent research in 2005, at Colorado State University in Fort Collins, Colorado; confirmed 50% reduction in velocity at the standard design velocity of 8 fps within a standard apron width and length.
Still, the question from engineers lingers "what about energy dissipation"?
Additionally, the ScourStop engineered design takes into account traditional apron calculations and resultant forces; providing established data for determining the appropriate downstream soil protection.
Independent research in 2005, at Colorado State University in Fort Collins, Colorado; confirmed 50% reduction in velocity at the standard design velocity of 8 fps within a standard apron width and length.
Still, the question from engineers lingers "what about energy dissipation"?
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Designers use the formula "Q" (flow) equals "V" (velocity) times "A" (area). By this equation, velocity is significantly reduced by expanding the area. In the example above, 12 cubic feet per second of water is flowing out of the pipe (with an area of 2 square feet) at 6 feet per second; but downstream of the scour area, the same 12 cubic feet per second is moving at only 2 feet per second because the area has expanded to 6 square feet. The forces are clearly much lower downstream as the water is shallower and slower; both factors critical in shear force computation. |
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We have specific, independent scientific data from the Colorado State University research project, where tests demonstrated similar data – the velocity decreased about 50% at the termination of the 17 foot apron area - in the tests where the apron and the velocity were properly matched. This ‘properly matched’ apron came from the traditional rock rip rap design table/curve.
Where they were not matched (the apron was not wide enough and the forces became channelized), the velocity did not significantly slow down. We are in the middle of research for derterming exactly where the hydraulic jump is happening. Until the research is completed, we will continue to over-design to ensure protection with a safety factor.
Federal Highway specifications recognize both velocity and shear for design options, but indicate shear may be a stronger model. In that regard, using rock is actually counter-productive because roughness causes a significant increase in shear as the water height rises over the rocks. If expansion is not utilized in the rock scenario, the high shear forces continue downstream and require heavy armor because the volume has not changed. Why do you think there are so many rock rip rap failures?
Where they were not matched (the apron was not wide enough and the forces became channelized), the velocity did not significantly slow down. We are in the middle of research for derterming exactly where the hydraulic jump is happening. Until the research is completed, we will continue to over-design to ensure protection with a safety factor.
Federal Highway specifications recognize both velocity and shear for design options, but indicate shear may be a stronger model. In that regard, using rock is actually counter-productive because roughness causes a significant increase in shear as the water height rises over the rocks. If expansion is not utilized in the rock scenario, the high shear forces continue downstream and require heavy armor because the volume has not changed. Why do you think there are so many rock rip rap failures?
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Where does it occur, and how do we protect from it. Again, 90% of designs do not have a significant hydraulic jump to deal with, but for those that do have higher velocities, we know from the research and field experience that we are designing appropriate soil protection far enough down the scour area to be effective (we are protecting the scour area computed from known scour size formulas). We use professional channel protection software to indicate what soil protection is needed downstream.
With ScourStop, you get mechanical protection from the high energy forces at the scour area, and soil protection with the soil cover BMP under the ScourStop. The stormwater spreads out quickly in a uniform laminar flow in most instances. The hydraulic jump occurs downstream where the water has expanded into a much shallower and thus significantly less erosive force.
With ScourStop, you get mechanical protection from the high energy forces at the scour area, and soil protection with the soil cover BMP under the ScourStop. The stormwater spreads out quickly in a uniform laminar flow in most instances. The hydraulic jump occurs downstream where the water has expanded into a much shallower and thus significantly less erosive force.
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