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    "Trapezoid critical depth: Fr=1 with A=(b+z yc) yc and T=b+2 z yc so Q² T = g A³; SI; default g=9.80665; z>0 is run/rise",
    "Critical speed vc = Q/A = √(g A/T) is reported, not an input; Fr=1 is the definition, not a Froude wrap",
    "Not rectangular yc=(q²/g)^{1/3}; not normal-depth yn; not specific energy E; not Manning n; not Q=Av"
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    "Trapezoid critical depth: Fr=1 with A=(b+z yc) yc and T=b+2 z yc so Q² T = g A³; SI; default g=9.80665; z>0 is run/rise",
    "Unknown is yc (depth mode) or Q (discharge invert); vc=√(g A/T) is reported; Fr=1 is the definition, not a Froude wrap",
    "Not rectangular yc=(q²/g)^{1/3}; not normal-depth yn; not specific energy E; not Manning n; not Q=Av"
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      "url": "https://www.iso.org/standard/31895.html",
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      "section": "Critical flow in a trapezoidal section",
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