At McAlpine L & D, normal upper pool elevation is 420.0 feet MSL, equal to 12.0 feet on the upper gage. The vertical clearance at the Clark Memorial Highway bridge is 72.6 feet above normal pool. What is the clearance if the gauge reads 27.2 feet?
ā¢ Bridge vertical clearance is measured above a specific water level (here: normal pool / normal gage reading) ā¢ A higher gage reading means water is closer to the bridge, so clearance decreases ā¢ You need to compare the change in gage reading to the original clearance
ā¢ First, find how many feet higher the water is now compared to the normal gage reading of 12.0 feet ā¢ Then, think: if the water comes up by that many feet, what must happen to the vertical clearance from water to bridge? Increase or decrease, and by how much? ā¢ Apply that change to the original 72.6-foot clearance to get the new clearance
ā¢ Be sure you subtract the normal gage reading (12.0 ft) from the new gage reading (27.2 ft) to find the water level rise ā¢ Confirm that you then subtract the water rise from the original 72.6 ft clearance, not the other way around ā¢ Check that all your values are in feet, and your final answer is a reasonable, smaller clearance than 72.6 feet because the water has risen
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